Solid State Communications 145 (2008) 502–506
www.elsevier.com/locate/ssc

Analysis of relationship between magnetic property and crystal structure of
La1−x Srx CrO3 (x = 0.13, 0.15)
Yuhta Matsunaga a , Fumihiko Nakamura a , Hiroki Takahashi b , Takuya Hashimoto a,∗a Department of Integrated Sciences in Physics and Biology, College of Humanities and Sciences, Nihon University, 3-25-40, Sakurajousui,Setagaya-ku, Tokyo 156-8550, Japanb Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40, Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan

Received 20 April 2007; received in revised form 3 December 2007; accepted 10 December 2007 by T. Ando
Available online 15 December 2007

Abstract
Dependence of dc magnetization, M, of La1−x Srx CrO3 (x = 0.13, 0.15) polycrystalline specimens on external magnetic field, H , wasmeasured at various temperatures. At higher temperatures above 274 K and 265 K for the specimens with x = 0.13 and 0.15, respectively, almost
linear relationships were observed, indicating paramagnetic property. At the lower temperatures, hystereses, indicating canted antiferromagnetic
property, were observed in M–H curves. The temperatures where the magnetic phase transition was observed showed agreement with those wherebase line shift was observed in differential scanning calorimetry (DSC) measurement. In the M–H curves of La1−x Srx CrO3 (x = 0.13, 0.15),
saturation magnetization, Ms , was not observed at external magnetic fields as high as 70 kOe. At 226 K and 189 K for the specimens with x = 0.13
and 0.15, respectively, structural phase transition from orthorhombic- to rhombohedral-distorted perovskite was observed in DSC curves. At thetemperature, abrupt decreases of residual magnetization, Mr , and variation of temperature dependence of coercive force, Hc , were detected. We
regard that the abrupt decrease of Mr and the variation of temperature dependence of Hc at the structural phase transition temperature can be
ascribed to discontinuous variation of Cr–O–Cr angles.
c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.30.Kz; 75.20.Ck
Keywords: A. Perovskite chromites; C. Crystal structure and symmetry; D. Magnetic property; D. Phase transitions

1. Introduction
It has been reported that oxides with distorted perovskite
structure including 3d transition metal, La1−x Aex MO3 , (Ae =Ca, Sr; M: 3d transition metal), have high functions such as highelectrical conductivity at high temperatures under various gas
atmospheres, catalytic property for decomposition of pollutiongas, colossal magnetic resistance (CMR) and so on [1–3].
Especially, CMR of La1−x Aex MnO3 (Ae = Ca, Sr) attracts
much interest toward new switching device and/or large scale
memory by magnetic field [3]. For the application, informationon the effect of oxygen nonstoichiometry, chemical state of
the elements and crystal structure on magnetic property are∗ Corresponding author. Tel.: +81 3 3329 1151x5516; fax: +81 3 5317 9432.

E-mail address: takuya@chs.nihon-u.ac.jp (T. Hashimoto).
0038-1098/$ - see front matter c 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.12.007

essential. However, large difficulty could be prospected forexperimental analysis for the effect of crystal structure on
magnetic property of La1−x Aex MnO3 , because not only crystal
structure but also oxygen nonstoichiometry i.e. chemical stateof Mn should be affected by not only Sr content but also
preparation conditions [4]. In order to clarify the relationshipbetween crystal structure of La1−x Aex MO3 (Ae = Ca,Sr, M: 3d transition metal) and magnetic property, oxidewith no oxygen nonstoichiometry under normally employedpreparation conditions, such as in air or in O2 , is desirable.
La1−x Aex CrO3 , (Ae = Ca, Sr) is the suitable candidate sinceno oxygen ion deficiency has been reported at temperature forpreparation under log P(O2 ) of 0 to −5 [5–7], i.e. chemical stateof Cr and concentration of holes and spins are determined by
Ae content only. Moreover, Tezuka and coworkers carried out
neutron diffraction of La0.85 Sr0.15 CrO3 and observed magnetic
Bragg peaks indicating that magnetic moment configuration

Y. Matsunaga et al. / Solid State Communications 145 (2008) 502–506

of the specimen was G-type originating from superexchangeinteraction between electrical spin on neighbouring Cr ions [8],which is analogous to that of La1−x Aex MnO3 .
In the preceding paper [9], we have reported analysis ofphase transition of La1−x Srx CrO3 by using differential scanning calorimetry (DSC), dilatometry, temperature-controlled
X-ray diffraction and magnetic measurement using SQUID.
Combining obtained data, structural and magnetic phase
diagram of La1−x Srx CrO3 , which includes paramagnetic orthorhombic, canted antiferromagnetic orthorhombic, canted antiferromagnetic rhombohedral and paramagnetic rhombohedralphase, has been proposed. In particular, two kinds of canted
antiferromagnetic phase have been observed in the specimen
with x larger than 0.12; one is orthorhombic-distorted, theother rhombohedral-distorted perovskite, showing agreementwith the results of neutron diffraction of La0.85 Sr0.15 CrO3 , bywhich the electron spin configurations of both phases were proposed [8]. However, residual magnetization (Mr ), and coercive
force (Hc ), which might be affected by crystal structure, has notbeen investigated because dependence of magnetization at constant external magnetic field on temperature was mainly measured by using SQUID and few measurements were carried out
on variation of magnetization on external magnetic field in theprevious studies.
In this paper, dependence of magnetization of La1−x Srx CrO3
(x = 0.13, 0.15) on external magnetic field were reported.
The measurements were carried out at various temperaturesand effect of structural phase transition on Mr and Hc has
been investigated combining with DSC measurements anddiscussion on local structure around Cr3+ .
2. Experimental
The polycrystalline specimens La1−x Srx CrO3 (x = 0.13,0.15) were prepared by Pechini method [10]. The starting
materials were powdery La2 O3 (99.9% Furuuchi Co., Ltd.),
SrCO3 (99.9% Furuuchi Co., Ltd.) and Cr(NO3 )3 ·9H2 O(99.9%, Wako Chemical Co., Ltd). Prior to the preparation,
La2 O3 was heated in air at 1200 ◦ C for 12 h to decompose
small amount of impurities such as La(OH)3 and La2 (CO3 )3 .
SrCO3 was dried in air at 120 ◦ C for 6 h. These materials
in a stoichiometric ratio were completely dissolved in dilutenitric acid. Then, excess citric acid was added to the solution
to obtain the chelate of each metal. Posterior to addition of
ethylene glycol, the solution was heated at about 300 ◦ C. The
solution changed to a polyester resin, which was burned to a
powder. The obtained powder was calcined at 700 ◦ C for 7 h
in air and pressed into pellets of 20 mm diameter and 2–3 mm
thick, which were successively sintered at 1400 ◦ C for 24 hin air. After cooling, the obtained pellets were reground into
powder in alumina mortar and pressed into pellets with 5 mmdiameter again, which were sintered again at 1400 ◦ C for 24 hin air. For powder X-ray diffraction measurement (Cu Kα;
50 KV, 250 mA, Rigaku RINT-2500) and difference scanningcalorimetry (DSC; DSC8230, Rigaku Co., Ltd.), some of theobtained pellets were reground again into a powder in alumina
mortar. DSC was carried out by using about 40 mg of powder503

Fig. 1. Dependence of dc magnetization, M, on external magnetic field, H , of
La0.87 Sr0.13 CrO3 at various temperatures.

specimen crimped in Al pan as a sample at a heating rate of
10 ◦ C/min in Ar atmosphere. The measurement temperature
range was −150–450 ◦ C. Al2 O3 powder also crimped in Al
pan was used as a reference. X-ray diffraction measurement
at room temperature indicated that the obtained specimens
were single phase of rhombohedral-distorted perovskite. In theobtained DSC curves of La1−x Srx CrO3 (x = 0.13, 0.15),
endothermic peak and baseline shift were observed at thetemperatures, which showed correspondence with the phase
diagram proposed in the previous study [9].
DC magnetization, M, of the sintered La1−x Srx CrO3 (x =0.13, 0.15) were measured with a SQUID magnetometer(Quantum Design MPMS model). The temperature of thesample was controlled to measurement temperature underzero magnetic field and then susceptibility was measured byapplication of external magnetic field, H , between −70 and
70 kOe. The measurement temperature was 130–300 K.
3. Results
Fig. 1 shows representative M–H curves of La0.87 Sr0.13
CrO3 . Hysteresis originating from canted antiferromagnetic
property was observed in M–H curve at 148 K. The DC
magnetization, M, monotonically increased with increase of
external field. Saturation of M was not observed up to 70 kOe,
indicating canted antiferromagnetism.

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Y. Matsunaga et al. / Solid State Communications 145 (2008) 502–506

Fig. 3. (a) DSC curve, temperature dependence of (b) residual magnetization,
Mr , and (c) coercive force, Hc of La0.87 Sr0.13 CrO3 . Dotted lines represent
phase transition temperatures observed by DSC. Inset shows magnetization oftemperature dependence of Mr around the second order phase transition.
Fig. 2. Magnified M–H curves of La0.87 Sr0.13 CrO3 at the temperatures above
233 K. Further magnifications are shown in insets.

With increase of temperature, hysteresis became smaller.
The hysteresis apparently disappeared in Fig. 1 at thetemperatures between 233 and 260 K, however, those withsmall Mr and Hc were observed as shown in Fig. 2,
which is magnification of Fig. 1. The hysteresis completely
disappeared at 300 K, indicating that phase transition fromcanted antiferromagnetic to paramagnetic occurred below
300 K. Fig. 3(a) shows DSC curve of La0.87 Sr0.13 CrO3
observed in this study. Endothermic peak, corresponding
to the structural phase transition from orthorhombic torhombohedral which had been already confirmed by theprevious study [9], was observed at 226 K. The base line
shift, which was identified as magnetic phase transition fromcanted antiferromagnetic to paramagnetic, was detected at 271
K, showing agreement with disappearance of the hysteresisin M–H curve at 300 K. Three phases observed in the
DSC measurement, i.e. canted antiferromagnetic orthorhombic,
canted antiferromagnetic rhombohedral and paramagnetic
rhombohedral are called phase [I], [II] and [III], respectively,in this paper. Fig. 3(b) and (c) shows dependence of Mr andHc measured from the data depicted in Figs. 1 and 2 ontemperature, respectively. Within the phase [I] and [II], both
Mr and Hc decreased with increase of temperature, suggestingthat entropy of electronic spin configuration prevailed at hightemperature. At the boundary of phase [I] and [II], drastic
decrease of Mr and variation of temperature dependence of Hc
have been observed, originating from structural phase transition

which will be discussed later. For the paramagnetic phase [III]above 271 K, Mr and Hc should be equal to 0. At the boundaryof phase [II] and [III], discontinuity has been observed in both
Mr and Hc as inset of Fig. 3(b) shows.
Fig. 4 shows representative M–H curves of La0.85 Sr0.15 CrO3 .
Parts of the curves depicted in Fig. 4 were magnified as shownin Fig. 5. The hystereses indicating canted antiferromagnetic
property were observed below 250 K. The hysteresis disappeared at 290 K, indicating transition to paramagnetic. However, small deviation from linear relationship was observed, thereason for which has not been identified. Fig. 6 shows (a) DSC
curve, (b) variation of Mr on temperature and (c) dependenceof Hc on temperature of La0.85 Sr0.15 CrO3 . Although, transition
temperatures differ from those of La0.87 Sr0.13 CrO3 , the similartendency was observed in dependence of Mr and Hc on temperature. Abrupt decrease of Mr and variation of temperaturedependence of Hc at the phase boundary between [I] and [II]have also been detected.
4. Discussion
It can be concluded that Mr and Hc of orthorhombic
canted antiferromagnetic phase of La1−x Srx CrO3 is larger
than those of rhombohedral one, indicating that some effectsof crystal structure on magnetic property exist. Canted
antiferromagnetism of LaCrO3 was explained as follows. Cr in
LaCrO3 takes trivalent state with 3d3 electronic configuration.
In [CrO6 ] octahedral in perovskite structure, energy of 3dorbitals splits into two state, one is composed of three kindsof 3d orbitals with lower energy called t2g , the other iscomposed of two kinds with higher energy, called eg . The

Y. Matsunaga et al. / Solid State Communications 145 (2008) 502–506

505

Fig. 5. Magnified M–H curves of La0.85 Sr0.15 CrO3 at the temperatures above
210 K. Inset shows further magnification of M–H curve at 290 K.
Fig. 4. Dependence of dc magnetization, M, on external magnetic field, H , of
La0.85 Sr0.15 CrO3 at various temperatures.

three 3d electrons in Cr3+ occupy t2g state with parallel spin
according to Hund’s rule and generate large spin on Cr3+ ion. If
temperature is low enough, superexchange interaction between
the large spins on the nearest Cr ions sandwiching O2− iongenerates, resulting in antiparallel spin configuration. Since thesuperexchange interaction is the highest if angle of Cr–O–Cr
bond is 180◦ , hysteresis in M–H curve of LaCrO3 should not be
observed if the crystal structure of LaCrO3 was ideal cubic. Dueto orthorhombic distortion of LaCrO3 structure, Cr–O–Cr bondangle slightly deviate from 180◦ , resulting in residual electronic
spin originating from imperfect superexchange interaction andhysteresis in M–H curve. Therefore, it can be deduced thathigher Mr and Hc can be expected in the phase with large
deviation of Cr–O–Cr bond angle from 180◦ .
It has been already confirmed that the space groups oforthorhombic and rhombohedral LaCrO3 are Pnma (No. 62)
and R3c (No. 167), respectively, by convergent beam electron
diffraction [11]. Octahedral tilting and rotational mode of Pnma
and R3c distorted perovskite were analysed by Glazer [12],Woodward [13], Howard and Stocks [14]. They concluded
that Pnma phase has lower symmetry than that of R3c one.
Therefore, it can be prospected that distortion of orthorhombic
La1−x Srx CrO3 phase from ideal perovskite structure is larger
than that of rhombohedral phase, resulting in larger deviationof Cr–O–Cr angle from 180◦ in orthorhombic phase than inrhombohedral one.

Fig. 6. (a) DSC curve, temperature dependence of (b) residual magnetization,
Mr , and (c) coercive force, Hc of La0.85 Sr0.15 CrO3 . Dotted lines represent
phase transition temperatures observed by DSC.

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Y. Matsunaga et al. / Solid State Communications 145 (2008) 502–506

neutron diffraction at wide range of temperatures are necessary
and they are now in progress.
5. Conclusion

Fig. 7. Cr–O–Cr bond angle of La0.85 Sr0.15 CrO3 calculated by using atomic
parameters obtained by the powder neutron diffraction measurements [8].
Cr–O1 –Cr and Cr–O2 –Cr represent bond parallel and perpendicular to [010],
respectively, in orthorhombic Pnma phase.

To confirm this prospect experimentally, information on
atomic parameter of La0.85 Sr0.15 CrO3 is necessary, which
can be obtained from neutron diffraction measurements.
Tezuka and coworkers [8] reported atomic parameters of
La0.85 Sr0.15 CrO3 at 50, 250 and 300 K obtained by neutron
diffraction measurements. Fig. 7 shows Cr–O–Cr angles
calculated from their reported atomic parameters. In phase [I]with space group of Pnma, there are two kinds of Cr–O–Cr
bond. One is parallel, the other is perpendicular to [010].
The angles of former and latter Cr–O–Cr bond were 160.22◦(depicted as Cr–O1 –Cr in Fig. 7) and 163.31◦ (depicted as
Cr–O2 –Cr in Fig. 7), respectively, at 50 K. In phase [I] and [II]with rhombohedral phase, only one kind of Cr–O–Cr angle isobserved. The angles were 163.48◦ and 163.80◦ at 250 K and
300 K, respectively. Cr–O–Cr angles depicted in Fig. 7 shows
agreement with the prospect that the deviation of Cr–O–Cr
angle from 180◦ is larger in Pnma phase than that in R3c one.
Cr–O–Cr angles in orthorhombic and rhombohedral LaCrO3
estimated by using neutron diffraction were also reported [15].
Since deviation of Cr–O–Cr angle from 180◦ in rhombohedral
LaCrO3 is smaller than that in orthorhombic phase, which wassame tendency as La0.85 Sr0.15 CrO3 , it can be expected that thistendency should be also observed regardless of Sr content in
La1−x Srx CrO3 .
The above discussion of crystal structure shows qualitative
agreement with larger Mr and Hc in canted antiferromagnetic
orthorhombic phase of La1−x Srx CrO3 than in rhombohedral
one. The abrupt decrease of Mr and variation of temperaturedependence of Hc can be qualitatively explained by using theabove discussion on crystal structure. In order to quantitative
confirmation, direct measurements of Cr–O–Cr angle using

The effect of crystal structure on M–H curves of
La1−x Srx CrO3 (x = 0.13, 0.15) was investigated. At hightemperatures, where paramagnetic phase was stable, linear
relationships were observed. At low temperatures, where canted
antiferomagnatic and orthorhombic-distorted perovskite phaseis stable, residual magnetization, Mr , decreased with increaseof temperature. At the temperature of the structural phasetransition from orthorhombic to rhombohedral, discontinuous
decrease of Mr was observed. The dependence of coercive
force, Hc , on temperature also varied at the structural phasetransition temperature. Larger decrease of Hc on temperature
was observed in rhombohedral phase than in orthorhombic
phase. We regard that this variation of Mr and Hc could be
attributed to difference of deviation of Cr–O–Cr angle from180◦ between both structures.
Acknowledgments
Part of this work was supported by the “High-Tech ResearchCentre” Project for Private Universities and a matching fundsubsidy from the Ministry of Education, Culture, Sports,
Science and Technology, 2000-2004.
References[1] N.Q. Minh, J. Amer. Ceram. Soc. 76 (1993) 563.
[2] N. Sakai, H. Fjellvåg, B.C. Hauback, J. Solid State Chem. 121 (1996) 202.
[3] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido,
N. Furukawa, J. Phys. Soc. Japan 63 (1994) 3931.
[4] J. Mizusaki, N. Mori, H. Takai, Y. Yonemura, H. Minamiue, H. Tagawa,
M. Dokiya, H. Inaba, K. Naraya, T. Sasamoto, T. Hashimoto, Solid State
Ionics 129 (2000) 163.
[5] T. Nakamura, G. Petzow, L.J. Gauckler, Mater. Res. Bull. 14 (1979) 649.
[6] J. Mizusaki, S. Yamauchi, K. Fueki, A. Ishikawa, Solid State Ionics 12(1987) 119.
[7] S. Onuma, K. Yashiro, S. Miyoshi, A. Kaimai, H. Matsumoto, Y. Nigara,
T. Kawada, J. Mizusaki, K. Kawamura, N. Sakai, H. Yokokawa, Solid
State Ionics 174 (2004) 287.
[8] K. Tezuka, Y. Hinatsu, A. Nakamura, T. Inami, Y. Shimojo, Y. Morii, J.
Solid State Chem. 141 (1998) 404.
[9] F. Nakamura, Y. Matsunaga, N. Ohba, K. Arai, H. Matsubara, H.
Takahashi, T. Hashimoto, Thermochim. Acta 435 (2005) 222.
[10] M.P. Pechini, US Patent 330 (3) (1967) 697.
[11] T. Hashimoto, K. Takagi, K. Tsuda, M. Tanaka, K. Yoshida, H. Tagawa,
M. Dokiya, J. Electrochem. Soc. 147 (2000) 4408.
[12] A.M. Glazer, Acta Cryst. B 28 (1972) 3384.
[13] P.M. Woodward, Acta Cryst. B 53 (1997) 32.
[14] C.J. Howard, H.T. Stokes, Acta Cryst. B 54 (1998) 782.
[15] K. Oikawa, T. Kamiyama, T. Hashimoto, Y. Shimojo, Y. Morii, J. Solid
State Chem. 154 (1998) 524.

PHYSICAL REVIEW B 72, 054520 共2005兲

Magnetic field effect on the pressure-induced superconducting state in the hole-doped
two-leg ladder compound Sr2Ca12Cu24O41
T. Nakanishi,1,2,* N. Motoyama,3,† H. Mitamura,1 N. Takeshita,1,‡ H. Takahashi,2 H. Eisaki,3,§ S. Uchida,3 and N. Môri1,储1Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8581, Japan
2

College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
3Department of Superconductivity, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
共Received 16 May 2005; published 24 August 2005兲
We report electrical resistivity on a single crystal of the hole-doped two-leg ladder compound
Sr2Ca12Cu24O41, which becomes superconducting with Tc ⬃ 5 K only at pressures above ⬃3.0 GPa. Measurements were performed at nearly hydrostatic pressures up to 5.7 GPa and low temperatures down to 100 mK
under static magnetic fields up to 20 T parallel to the a axis 共along the ladder rungs兲 and up to 7 T parallel toboth the b axis 共perpendicular to the ladder plane兲 and the c axis 共along the ladder legs兲. A clear difference inthe resistive upper critical field Hc2共T兲 is observed among these three directions, confirming that this system
has a highly anisotropic superconducting ground state. Also, Hc2共T兲 parallel to the ladder plane is found to
exceed the Pauli limit by a factor of more than 2, suggesting either a strong spin-orbit scattering or spin-triplet
pairing. Furthermore, it is implied, from measurements of resistivity versus angle of magnetic field in thebc plane, that another superconducting phase is stable below around 3 K only when the magnetic field isapplied exactly along a certain direction that is ±35° from the ladder direction.
DOI: 10.1103/PhysRevB.72.054520

PACS number共s兲: 74.62.Fj, 74.72.Jt

I. INTRODUCTION

ladder system is not well understood yet. There are only few
measurements concerning the upper critical field on this system, for example, ac susceptibility9 at a pressure of 4.0 GPa
up to 10 T and resistivity10 at a pressure of 4.8 GPa up to
7 T.
In this paper, we present the results of precise magnetotransport measurements of a Sr2Ca12Cu24O41 single crystal
at three different pressures where dTc / dP ⬎ 0, dTc / dP ⯝ 0,
and dTc / dP ⬍ 0 in magnetic fields applied along all three
crystallographic axes. In these measurements, the magnetic
field range is extended up to 20 T parallel to the a axis
共along the ladder rungs兲 and up to 7 T parallel to both the baxis 共perpendicular to the ladder plane兲 and the c axis 共along
the ladder legs兲.

A concept of S = 21 Heisenberg-type spin-ladder has offered predictions1–3 of two features: the existence of a spin
gap and superconductivity by doped hole-pairs with the spin
gap. These are closely related to features of high-Tc cuprates
共HTSC兲. The most important point in the predictions is thatthe pairing interaction is supposed to be caused by purely
electronic origin, namely, magnetic interaction. At present, acompound Sr14−xCaxCu24O41 共Refs. 4–6兲 is the only known
candidate as a real system having the theoretically predicted
properties of the doped spin-ladders as mentioned above, andalso is the first nonsquare lattice superconducting cuprate.
This spin-ladder system becomes superconducting withTc ⱗ 12 K only in a pressure P above ⬃3.0 GPa when x
艌 10 and the Tc shows the bell-shaped curve against appliedpressure.4–6 These features are quite similar to those of
HTSC, having two-dimensional 共2D兲 CuO2 planes in which
the spin gap behavior has been observed so far and the superconducting phase is stabilized only in a certain range ofcarrier doping. The essential difference is that this laddersystem has conducting layers composed by weakly coupled
one-dimensional 共1D兲 Cu2O3 two-leg ladders with a spin gap
and shows 1D charge transport along the ladder at low pressure below ⬃2 GPa.7 Furthermore, the anisotropy ratio of
the resistivity in this system is quite similar to that of aquasi-1D organic superconductor 共TMTSF兲2PF6 共Ref. 8兲,which has conducting layers composed by weakly coupled
1D chains. On the other hand, the anisotropic electrical resistivity measurements under high pressure5 indicate thatpressure causes a dimensional crossover from 1D to anisotropic 2D charge transport in the normal state and then superconductivity is suggested to occur in the Cu2O3 two-leg
ladders as in the anisotropic 2D system. However, the superconducting property, especially under magnetic field, of this1098-0121/2005/72共5兲/054520共6兲/$23.00

II. EXPERIMENT

A single crystal of Sr2Ca12Cu24O41 was grown by thetraveling-solvent floating zone method using an infrared furnace under oxygen gas.7 Using a newly developed selfclamped high-pressure cell11 that employs Bridgman anvils
with a Teflon capsule filled with a pressure transmitting liquid medium 共a 1:1 mixture of Fluorinert FC 70 and FC 77兲,
the absolute values of electrical resistivity by a four-probe
technique were measured under nearly hydrostatic pressurein the temperature range down to around 100 mK by a dilution refrigerator. A sample with dimensions of 0.3 共a axis兲
⫻ 0.2 共b axis兲 ⫻ 0.7 共c axis兲 mm3 was used in the resistivity
measurements with an ac current 共10 or 100 ␮A at 15.9 Hz,
ohmic response without self-heating effect was confirmed
within this current range兲 along the c axis. We used two
superconducting magnets: a Helmholtz-type horizontal rotatable 7 T parallel to the bc plane and a cylindrical 20 T parallel to the a axis. Although it was required to warm thecryostat with the pressure cell up to around 100 K in order to054520-1©2005 The American Physical Society

PHYSICAL REVIEW B 72, 054520 共2005兲

NAKANISHI et al.

FIG. 1. 共a兲 ␳c共T兲 of Sr2Ca12Cu24O41 at several pressures. 共b兲
Pressure dependence of Tc that was determined by several criteria
as illustrated in 共c兲 for the case at 4.0 GPa as an example where we
define Tc, which satisfies a condition that ␳c共Tc兲 is equal to a fixed
percentage of the resistive maximal value before the transition.
Lines in 共b兲 are guides for the eyes.

change a magnet to another at a fixed pressure, this thermal
cycle did not cause any change in the pressure because Tc did
not change in this process. This ensured that these experiments were done under exactly the same pressures for theidentical sample.
For pressure calibration at low temperatures in this pressure cell, we have measured the temperature dependence ofac susceptibility of lead to determine the superconducting
transition temperature. In this measurement, at ambient pressure, the superconducting transition temperature Tc is observed at 7.2 K, with a transition width ⌬Tc of 0.03 K. It isnoted that the ⌬Tc under each pressure still remains 0.03 K,
which is at ambient pressure. This result clearly indicatesthat the generated pressures in this pressure cell are close to
the hydrostatic one although the pressure transmitting medium is solidified in this temperature region. From this result,
we construct the pressure calibration curve of this pressure
cell by using the data of Bireckoven and Wittig.12 A reproducibility of this curve is confirmed with a precision of±0.1 GPa at 2.55 GPa by four different measurements whichmonitor the resistive transition of Bi due to the I-II structuralphase transition at room temperature. More detailes are described elsewhere.11
III. RESULTS AND DISCUSSION
A. ␳c under magnetic fields

Figure 1共a兲 shows the temperature dependence of resistivity ␳c along the c axis of Sr2Ca12Cu24O41 up to 5.7 GPa. At

ambient pressure, ␳c is ⬃3 m⍀ cm at room temperature andshows T-linear metallic dependences upon cooling. These
facts indicate that this ␳c clearly shows the transport properties of the c axis, namely, the ladder direction at ambientpressure. At room temperature, ␳c decreases with pressure
continuously. Above 3.5 GPa the sample becomes superconducting at around 5 K and the Tc exhibits the bell-shaped
curve against pressure as shown in Fig. 1共b兲 where Tc isdetermined in several ways as shown in Fig. 1共c兲. These
pressure effects on the transport and superconducting properties in the zero field are consistent with the previously reported results on the x = 11.5 single crystal.5
Figure 2 shows the temperature dependence of ␳c of
Sr2Ca12Cu24O41 at 3.5, 4.0, and 4.5 GPa in various magnetic
fields applied along the a axis up to 20 T and the b axis up to
7 T. It is clearly observed that, with increasing pressure from
3.5 to 4.5 GPa, the pressure influences the qualitativechanges in the behavior of resistive-transition in the magnetic fields. For the fields up to 7 T along the b axis, at 3.5
and 4.0 GPa the superconductivity is not completely destroyed while at 4.5 GPa ␳c共T兲 at 5.5 and 7 T shows almost
the same curve at least down to around 1 K. These qualitative changes in the behavior of resistive transition in themagnetic fields are also observed in field sweep measurements at fixed temperatures. The results are shown in Fig. 3,
where the fields were applied along the a axis up to 18 T andthe b axis up to 7 T at pressures of 4.0 and 4.5 GPa. As seenin Fig. 3, for the fields up to 7 T along the b axis, at 4.0 GPa
the resistivity has not reached a constant value while at
4.5 GPa the resistivity has reached a constant value at 7 T.
These facts indicate that normal state resistivity is recoveredat 4.5 GPa under the magnetic fields along the b axis above
5.5 T at temperatures down to about 1 K. Thus the ␳c共T兲
curve at 4.5 GPa above 1 K under 7 T along the b axis refers
to normal state resistivity.
B. Resistive upper critical field

The resistive upper critical field Hc2共T兲 is defined as themagnetic field where the normal state resistivity is recoveredin the measurement of field sweep at a fixed temperature. In
this sense, we can determine Hc2共T兲 only for data at 4.5 GPa
where the normal state resistivity is recovered within themeasured magnetic field range. However, due to the difficulty of applying this criterion to the data perfectly, we determined critical magnetic fields Hn where ␳c共T兲 is equal to a
fixed percentage 共n = 10%, 50%, and 90% and n = “ ␳ = 0” for0%兲 of the normal state resistivity, that is ␳c共T兲 curve above
1 K at 4.5 GPa and 7 T as denoted ␳c共H = 7T / / b兲 as shownin Fig. 4. From the field dependence of the resistive transition as shown in Figs. 2 and 3, together with the results along
the c axis not shown here, the critical fields at 4.5 GPa for all
crystallographic axes are determined and summarized in Fig.
5. Figure 6 shows a summary of critical fields H␳=0 at several
pressures. As we can see in Figs. 5 and 6, it is found that
there is no significant discrepancy between the results fromtemperature sweep and field sweep data. Also, it is noted that
Hc2共T兲 ⬎ H90%共T兲 and H␳=0共T兲 corresponds to the irreversibility line in the case of HTSC. As it has been observed and054520-2

PHYSICAL REVIEW B 72, 054520 共2005兲

MAGNETIC FIELD EFFECT ON THE PRESSURE-…

FIG. 2. ␳c共T兲 of Sr2Ca12Cu24O41 at 3.5, 4.0, and 4.5 GPa invarious magnetic fields along 共a兲 the a axis and 共b兲 the b axis. Inset
in 共b兲 shows a schematic structure of the two-leg ladders viewed
from the b axis.

discussed in HTSC, it is difficult to interpret the resistive
upper critical field because in some cases the motion of flux
may affect the shape of the resistive transition. However, atpresent we cannot distinguish a difference between the resistive upper critical field and the thermodynamic upper criticalfield for the present ladder system because of the technicaldifficulty of performing thermodynamic experiments, such as
specific heat, at hydrostatic pressures above 3 GPa undermagnetic fields.
As observed in Fig. 5, there is the clear difference in thecritical fields, also implied in Hc2共T兲 as well, among thesethree directions. This fact clearly indicates that this system
has a highly anisotropic superconducting ground state. It is

FIG. 3. Magnetic field dependence of ␳c of Sr2Ca12Cu24O41 at
4.0 and 4.5 GPa and at several temperatures for the fields along 共a兲
the a axis and 共b兲 the b axis.

seen in Fig. 6 that the anisotropy of H␳=0共T兲 is unchanged atthree different pressures where dTc / dP ⬎ 0, dTc / dP ⯝ 0, anddTc / dP ⬍ 0 within measured temperature and magnetic field
range, implying that the anisotropy of Hc2共T兲 is also unchanged in the whole pressure range. The most important
fact seen in Fig. 5 is that Hc2 along the a axis, parallel to the
conducting ladder plane, exceeds the value of Pauli paramagnetic limit 关as indicated by arrows in Fig. 5 for a criterion of
Tc共50% 兲兴 by a factor of more than 2, which is given by
H p共T = 0兲 = 1.84 Tc共H = 0兲 for isotropic singlet s-wave pairingwithout spin-orbit scattering.13
As reported by the anisotropic electrical resistivity measurements under high pressure,5 superconductivity is suggested to occur in the Cu2O3 two-leg ladders as in the anisotropic 2D system. This implies that coherence length along
the b axis ␰b becomes shorter than a distance between the054520-3

PHYSICAL REVIEW B 72, 054520 共2005兲

NAKANISHI et al.

FIG. 4. Examples of criteria to define Tc, where we define Tc
which satisfies a condition that ␳c共Tc , Hn兲 is equal to a fixed percentage n of the normal state value ␳n = ␳c共H = 7 T 储 b兲 at 4.5 GPa
for 共a兲 the temperature sweep data and for 共b兲 the field sweep data.

ladder plane. In fact, a value of ␳c ⬃ 0.5 m⍀ cm gives amean free path along the c axis lc ⬃ 10– 100 Å by assuming
a simple free electron model with a carrier density estimatedfrom a Hall coefficient of ⬃2 ⫻ 10−3 cm3 / C.14 We would

FIG. 5. Critical fields H90%, H50%, H10%, H␳=0 of
Sr2Ca12Cu24O41 at 4.5 GPa for H 储 a axis, H 储 b axis, and H 储 c axis.
Data for ␳c共H兲: 90%, 50%, and 10% and ␳c共H兲 = 0 are from the fieldsweep data 共Fig. 3兲. Lines are guides for the eyes. Arrows indicate
H p共T = 0兲 for Tc共50% 兲 共see text兲.

FIG. 6. Critical fields H␳=0 at 3.5, 4.0, and 4.5 GPa for H 储 aaxis, H 储 b axis, and H 储 c axis. Lines are guides for the eyes.

expect ␳b to be lower than ␳c by two orders of magnitude,7this means lb to be ⬃0.1– 1 Å. Also, this system is supposed
to be a dirty superconductor due to being a highly doped
system; thus we expect that coherence length ␰ is shorter
than l; then ␰b would be less than ⬃0.1– 1 Å. This fact implies that ␰b seems to be shorter than the distance between
the ladder planes about 7 Å.15 Therefore ␰b is expected to
become smaller than the ladder spacing at a certain temperature. Below this temperature orbital effects as a mechanism
for the quenching of superconductivity might be suppressed,then Hc2 parallel to the ladder plane would be infinite in theabsence of both Pauli limit and spin-orbit scattering that reduces Pauli paramagnetic effect, as predicted for the upwardcurvature of Hc2共T兲 for 2D layered superconductors.16 Thus
one possible explanation for the observed absence of Paulilimit is that there is a spin-orbit scattering that is strong
enough for the quenching Pauli paramagnetic effect, as observed in layered superconductors TaS2−xSex 共x = 0 and
0.4兲.17 Besides such a strong spin-orbit scattering, one may
speculate that another possible explanation for the absence ofPauli limit might be due to spin-triplet pairing. In fact, very
recently, Fujiwara et al.18 have shown, from the highpressure NMR experiment at 3.5 GPa, that the Knight shift
does not change at Tc within their experimental accuracy.
This fact indicates a possibility of triplet superconductivity
in this system. Furthermore, they have also observed thepeak of T−1
1 at Tc. These behaviors are quite similar to thoseof a quasi-1D molecular superconductor 共TMTSF兲2PF6 inwhich a large evolution of Hc2 parallel to conducting layers
at low temperatures, exceeding the Pauli limit, has been
observed.19 This might be due to spin-triplet pairing that has
been shown recently.20 In contrast, the Pauli limit is observed

054520-4

PHYSICAL REVIEW B 72, 054520 共2005兲

MAGNETIC FIELD EFFECT ON THE PRESSURE-…

FIG. 7. ␳c vs angle ␪ of a magnetic field H = 7 T in the bc planeof Sr2Ca12Cu24O41 at pressures of 3.5, 4.0, and 4.5 GPa in the twodifferent temperature regions of 1.7 to 1.8 共open circles兲 and 3 K
共closed circles兲. Lines are guides for the eyes.

in YBa2Cu3O7−␦ 共Ref. 21兲 for Hc2 parallel to the conducting
CuO2 layer, in which a singlet d-wave superconducting statemight be realized.
C. ␳c vs angle of magnetic field in the bc plane

Figure 7 shows the results of ␳c versus angle of magnetic
field of 7 T in the bc plane in the two different temperature
regions, 1.7 to 1.8 K and 3.0 K, at each pressure. As seen in
the figure, apart from resistive drops at the ladder direction
共c axis: ␪ = 0° and 180°兲 due to the anisotropy of the criticalfield, other extra resistive drops are clearly observed at theangles of ±35° from the ladder direction symmetrically. This
angle is independent of both current and magnetic field,
which indicates that these resistive drops are not caused by
the motion of flux due to the Lorentz force, which depends
on both current and magnetic field. The angle is also independent of applied pressure. On the other hand, the sharpnessof this resistive drops at ±35° seems to be similar to that at c
axis, namely, ␪ = 0 and 180°. Therefore this resistive anomaly
is supposed to be caused by superconductivity, possibly another superconducting phase. Also, it is difficult to explainthat the symmetrical appearance of this additional superconducting phase at the angles of ±35° from the ladder direction
would be caused by the inhomogeneity of a pressure-induced
superconducting region in the sample. Figure 8 shows theresults of ␳c共T兲 curves at 4.5 GPa under three different magnetic fields, 1, 3, and 7 T, applied along the directions withthree different angles of −30°, −35°, and −40° from the ladder direction. At 1 T the shift of resistive transition in thesecurves seems to be in accordance with its anisotropy from−30° to −40°. With increasing fields, the shift of the transi-

FIG. 8. ␳c共T兲 of Sr2Ca12Cu24O41 in fields H = 1, 3, and 7 T in
the bc plane with angles of ␪ = −30°, −35°, and −40° from c axis
共ladder direction兲.

tion goes to lower temperature; then, just after passing
through around 2 to 3 K, the shift of the transition in the
curve with an angle of −35° starts to deviate from the two
curves with angles of −30° and −40°. This behavior is also
observed in field sweep measurements at 1.8 K with different field angles, as shown in Fig. 9. These facts indicate that
this additional superconducting phase becomes to develop
below around 3 K only when the magnetic field is applied
exactly along a certain direction that is ±35° from the ladderdirection. Although, at present, a physical meaning of ±35°from the ladder direction is unknown, we might say at leastthat the appearance of this additional superconducting phaseclearly depends on both the temperature and the direction of
the applied magnetic field. Also, it is noted that this addi-

FIG. 9. Magnetic field dependence of ␳c共H兲 of Sr2Ca12Cu24O41
at 4.5 GPa and 1.8 K for the fields in the bc plane with angles of␪ = −30°, −35°, −40°, 90° 共H 储 b兲, and 0° 共H 储 c兲. Lines are guides forthe eyes.

054520-5

PHYSICAL REVIEW B 72, 054520 共2005兲

NAKANISHI et al.

tional superconducting phase appears at three different pressures. Additionally, we would like to point out a possibility
that a physical meaning of ±35° is related to a mathematical7
兲 = 34.99°. However, in order to confirm
relation: tan−1共 10
whether or not the additional superconducting phase exists inthe pressure-induced superconducting state in the
Sr2Ca12Cu24O41, precise specific heat measurements at pressures above 3 GPa under magnetic fields are required for
future study.

superconducting ground state. Also, the absence of Paulilimit in Hc2 parallel to the conducting ladder plane is found,which suggests either a strong spin-orbit scattering or spintriplet pairing. Furthermore, it is implied, from measurements of resistivity versus angle of magnetic field in the bc
plane, that an additional superconducting phase is stable below around 3 K only when the magnetic field is applied exactly along a certain direction that is ±35° from the ladderdirection.
ACKNOWLEDGMENTS

IV. CONCLUSION

In summary, we report the results of precise magnetotransport measurements of a hole-doped two-leg ladder
Sr2Ca12Cu24O41 single crystal in fields applied along all three
crystallographic axes. The clear difference in the resistive
upper critical field Hc2共T兲 is observed among these three directions, indicating that this system has a highly anisotropic

We thank T. Nagata and J. Akimitsu for their collaboration in the early stage of this study. One of us 共T.N.兲 would
also like to thank G. Sparn and F. Steglich for their kind
hospitality during his preparing the manuscript. This work
was supported by a Grant-in-Aid for Scientific Researchfrom the Japanese Ministry of Education, Culture, Sports,
Science and Technology.

Physica B 281&282, 957 共2000兲.

*Present address: Max Planck Institute for Chemical Physics ofSolids, Nöthnitzer Strasse 40, D-01187 Dresden, Germany. Electronic address: tks@cpfs.mpg.de†Present address: Department of Physics, Aoyama-Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan.
‡Present address: Joint Research Center for Atom Technology, 1-1-4
Higashi, Tsukuba-shi, Ibaraki 304-0046, Japan.
§Present address: Nanoelectronic Research Institute, AIST, 1-1-1
Umezono, Tsukuba 305-8568, Japan.
储Present address: Department of Physics, Saitama University, 255
Shimookubo, Saitama-shi, Saitama 338-8570, Japan.
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054520-6

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 69 (2008) 358–361
www.elsevier.com/locate/jpcs

Optical properties of Sr1xEuxGa2S4 mixed compounds
Chiharu Hidaka, Takeo Takizawa
Department of Physics, College of Humanities and Sciences, Nihon University, Sakura-josui 3-25-40, Setagaya-ku, Tokyo 1568550, Japan

Abstract
The mixed crystal system of CaGa2S4–EuGa2S4 has already been investigated and conﬁrmed to form the solid solution. In this report,
the same procedure of the previous report is applied to the SrGa2S4–EuGa2S4 system using differential thermal analysis and X-ray
diffraction. It is shown that the solid solution is also formed in this system. Photoluminescence spectra of the present system have abroad-wavelength peak characteristic of the phonon-terminated 5d–4f transitions. The peak energy of the emission shifts to the lowenergy side as x increases. The maximum intensity of the emission is attained at 10 mol% as about four times larger in magnitude thanthe emission of a stoichiometric EuGa2S4 compound.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds; B. Chemical synthesis; D. Luminescence; D. Optical properties

1. Introduction
Alkaline earth thiogallates (MGa2S4, M ¼ Ca, Sr, Ba)
doped with Ce3+ and Eu2+ exhibit broad emissions due tothe phonon-terminated transition between 5d and 4f levels[1]. As the emissions show high luminance in the blue toyellow region, the compounds are expected to be used indisplay and lighting applications. Many works concerning
electroluminescence devices and light emission diodes havebeen reported [2–6].
Moreover, laser action was observed in Dy3+ or Eu2+doped CaGa2S4 [7,8]. Thus, the compounds above are also
attractive as host crystals of laser materials. Especially, thebroad emission arising from the 5d–4f transition may allow
us to use Sr- and Ca-thiogallates doped with Ce3+ andEu2+ to realize for tunable laser devices in the visible region.
At present, the photoluminescence (PL) quantum efﬁciency
of CaGa2S4:Eu2+ (1 at%) was estimated to be 30% [9]. On
the other hand, that of a stoichiometric compound of
EuGa2S4 in which all Ca atoms were substituted by the Eu
atoms was obtained as 21% [10]. This suggests that a
Ca1xEuxGa2S4 compound with an appropriate composiCorresponding author. Tel./fax: +81 3 5317 9772.

E-mail address: takiz@chs.nihon-u.ac.jp (T. Takizawa).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.07.016

tion may have the maximum PL quantum efﬁciency. Until
now, we have studied the single crystal growth and opticalproperties of Ca1xEuxGa2S4 (x ¼ 0–1) toward the realization of a laser [11]. And we have reported that
Ca1xEuxGa2S4 compounds form the solid solution and
the PL intensity had maximum at x ¼ 0.25.
Here, we focus our attention on the Sr thiogallate, i.e.,
Sr1xEuxGa2S4 to ﬁnd the best value of x for PL, since thecompound has been known as an excellent green phosphor
with good purity of color and high brightness [1]. Peter andBaglio brieﬂy described in their paper that Sr1xEuxGa2S4
compounds formed the solid solution [1]. However,experimental data as well as optical properties concerningthese facts have not been reported.
EuGa2S4 compounds also have the same crystal structure as that of SrGa2S4, leading us to expect that the Srions can be substituted by the Eu ions at any amount in
Sr1xEuxGa2S4. The maximum quantum efﬁciency is also
expected at an appropriate x in the chemical formula
above. In this work, we report on the solid solution in the
SrGa2S4–EuGa2S4 system in terms of the differential
thermal analysis (DTA) and powder X-ray diffraction.
We discuss the PL and PL excitation (PLE) spectra of thecompounds along with the related levels of Eu2+ ions in
the mixed compound.

ARTICLE IN PRESS
C. Hidaka, T. Takizawa / Journal of Physics and Chemistry of Solids 69 (2008) 358–361

To investigate the phase diagram of the SrGa2S4–
EuGa2S4 system, DTA was performed. Compounds of
SrS, EuS and Ga2S3 were weighed to about 0.3 g in total in
Ar atmosphere at an appropriate composition of Sr1xEux
Ga2S4 (0pxp1). These compounds were evacuated in aquartz ampoule of 7 mmF  40 mm for DTA measurements. DTA curves were obtained at a speed of 2 1C/min
by scanning twice; whole of the chemical reaction wascompleted in the ﬁrst scan and the compounds with thevalue of x were synthesized. The transition and melting
points can be determined in the second scan. In the coolingprocess of DTA, the supercooling sometimes occurred,
where the solidifying points were not determined.
After DTA measurements, powder X-ray diffraction
patterns were measured at 300 K by using the CuKa line
(40 kV, 30 mA). A single phase of the orthorhombic
structure belonging to D24
h -Fddd was found. As Eu
concentration increases, diffraction lines shift to the highdiffraction angle. The lattice parameters of SrGa2S4 andEuGa2S4 are calculated as 20.85, 20.51, 12.22 Å and 20.73,
20.41, 12.20 Å, respectively. These values are comparedwith the reported data [1]. In the composition of 0pxp1,
the a- and b-axes decrease and the c-axis stays constant as x
increases. Based on the results of DTA and XRD
measurements, the phase diagram of the SrGa2S4–EuGa2S4
system is shown in Fig. 1. It is considered that the solid
solution was formed in the SrGa2S4–EuGa2S4 system. Thisassures that a compound of Sr1xEuxGa2S4 can beprepared at any x.

ments were used for the measurements. The products wereground and ﬁlled in a quartz tube of 3 mmF  30 mm.
The PL spectra of Sr1xEuxGa2S4 (0oxp1) were shownin Fig. 2. The spectra were similar to those of Eu2+-doped
SrGa2S4 and understood as due to the 5d–4f transition of
Eu2+ ions [1,4]. Fig. 3 shows the integrated PL intensity as
a function of Eu concentration. The emission intensity has
the maximum value at 10 mol% (x ¼ 0.1). The reduction in
PL intensity above this value is due to the so-called
concentration quenching. The energy of emission peakshifts to the lower energy side with increasing Eu
concentration. The ionic radii of Sr and Eu are 1.26 and1.25 Å, respectively [12]. The lattice volume decreases withincreasing Eu concentration x. Thus when Eu concentration continuously increases, the crystal ﬁeld around a Eu
ion becomes stronger as the number of the surrounding

Intensity [arb.units]

2. Phase diagram of the SrGa2S4–EuGa2S4 system

x=0.2

0.3
0.4
0.5
0.6
0.7

1.0
1.8

3. PL spectra of Sr1xEuxGa2S4
PL spectra were measured using a spectrometer equippedwith a CCD detector (HAMAMATSU, PMA11) at room
temperature. A He–Cd laser (325 nm) was used as an
excitation light source. The products after DTA measure-

359

2.0

2.2
2.4
2.6
Photon energy [eV]

0.9

2.8

3.0

Fig. 2. Photoluminescence spectra of Sr1xEuxGa2S4 (0pxp1) at 300 K.
The emissions are due to 5d–4f transition of Eu2+ ions.

1260
Liq.

1220

PL intensity [arb.units]

Temperature [°C]1240

Liq. + S.S

1200
1180

S.S

1160
1140
0

20
40
60
80
Eu concentration [mol%]100

Fig. 1. Phase diagram of the SrGa2S4–EuGa2S4 system. Solid solution(S.S) is formed in the Eu concentration between 0 and 100 mol%.

0.0

0.2

0.4
0.6
Eu concentration x

0.8

1.0

Fig. 3. The PL integrated intensity as a function of Eu concentration. The
maximum intensity is obtained at 10 mol%(x ¼ 0.1).

ARTICLE IN PRESS
C. Hidaka, T. Takizawa / Journal of Physics and Chemistry of Solids 69 (2008) 358–361

360

ions increases. Then, the split of excited states of 5d levels
broadens and the energy difference between the lowest
excited state of 5d levels and 4f levels decreases.

5d

Conduction Band
4. PLE spectra
PLE spectra were measured at 300 K. A xenon lamp of
300 W together with a monochromator (Bunkoh-KeikiSM-25) was used as a light source. PLE was detected bymonitoring at 2.3 eV with a photomultiplier (HAMAMATSU, R562) and a monochromator (Nikon, G250).
Powdered samples as previously mentioned were used.
Fig. 4 shows the PLE spectra of Sr1xEuxGa2S4 (x ¼ 0.01,
0.1, 0.5, 1). The spectra were normalized at 2.5 eV. In a low
Eu concentration, three excited levels are found at 2.5, 3.1
and 3.8 eV. The Sr atoms in SrGa2S4 are surrounded byeight sulfur atoms forming a square anti-prism. When thecompound was doped with Eu, the Sr atoms are substitutedby the Eu atoms and the Eu atoms are exposed to the
crystal ﬁeld of the D4d symmetry. The 5d states of the Eu
ions are splitz into three levels corresponding to the values
obtained in the excitation spectra.
5. Energy levels of Eu in Sr1xEuxGa2S4
An energy diagram of Eu2+ in SrGa2S4 was reported byChartier et al. [13]. Fig. 5 shows the energy levels of Eu2+
in Sr1xEuxGa2S4. The value of Evc for SrGa2S4 can be
estimated as 4.4 eV from the optical band gap of CaGa2S4(4.2 eV) [14]. The 4f state of Eu2+ locates at 1.75 eV from

4.4eV

3.1eV
2.5eV
4f
1.75eV

Valence Band
Fig. 5. Energy levels of Eu2+ in Sr1xEuxGa2S4. The 4f level locates at
1.75 eV from valence band. The upper level of 5d states sites in conduction
band.

the valence band [15]. Thus from the peak energy of PLE,
the difference between the lowest 5d level and the 4f level
was determined to be 2.5 eV. As a result, the middle andhighest levels of the 5d states locates within the conduction
band. From PLE, the peaks at 3.1 and 3.8 eV decreased
and became blurred with increasing Eu concentration. It isconsidered that the band structure changes with increasing
Eu concentration. When the number of the Eu atoms in
Sr1xEuxGa2S4 increases, the d-band of Eu2+ ions
gradually broadened, and the two upper levels of 5d statesof Eu2+ ions could be no more observed.

6. Summary
We have conﬁrmed the solid solution in the SrGa2S4–EuGa2S4 system. It is shown that the Sr atoms canbe continuously replaced by the Eu atoms. The integrated
PL intensity has maximum at 10 mol% (x ¼ 0.1). The
PL intensity of the optimum compound is 3.5 times
stronger than that of EuGa2S4. The quantum efﬁciency
will approximately reach 75%. The PL peak energy shifts
to the lower energy side due to the stronger crystal ﬁeld
when Eu concentration increases. We have determined theenergy level of Eu2+ in the band diagram of Sr1xEux
Ga2S4.
The compound with 10 mol% Eu2+ is suitable for thelaser material and its crystal growth is now under way.

Intensity [arb.units]

1mol%

10mol%

50mol%

100mol%Acknowledgments
2.0

2.5
3.0
3.5
Photon energy [eV]

4.0

4.5

Fig. 4. The excitation spectra of Sr1xEuxGa2S4 with 1, 10, 50 and
100 mol%. The spectra are normalized by the intensity at 2.5 eV. A peak at
3.8 eV decreases with increasing Eu concentration.

This work is partly supported by the grant for the hightechnology research center project for private universities
from the Ministry of Education, Sports, Science, Culture
and Technology, Japan.

ARTICLE IN PRESS
C. Hidaka, T. Takizawa / Journal of Physics and Chemistry of Solids 69 (2008) 358–361

References[1] T.E. Peter, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230.
[2] W.A. Barrow, R.C. Coovert, E. Dickey, C.N. King, C. Laakso,
S.S. Sun, R.T. Tuenge, R. Wetross, J. Kane, in: SID International
Symposium Dig., 1993.
[3] Y. Inoue, K. Tanaka, S. Okamato, K. Kobayashi, I. Fujimoto, Jpn.
J. Appl. Phys. 34 (1995) 180.
[4] I.R. Limousin, A. Garcia, C. Fouassier, C. Barthou, P. Benalloul,
J. Benoit, J. Electrochem. Soc. 144 (1997) 687.
[5] Y. Nakanishi, N. Uekura, F. Nakano, Y. Hatanaka, J. Lumin. 72(1997) 373.
[6] T. Yang, B.K. Wangner, M. Chaichimansour, W. Park, Z.L. Wang,
C.J. Summers, J. Vac. Sci. Technol. (1996) 2263.
[7] M.C. Nostrand, R.H. Page, S.A. Payne, W.F. Krupke,
P.G. Schunemann, Opt. Lett. 24 (1999) 1215.

361[8] S. Iida, T. Matsumoto, N.T. Maruyama, A.I. Bayramov,
B.G. Tagiev, O.B. Tagiev, R.B. Dzhabbarov, Jpn. J. Appl. Phys. 36(1997) 857.
[9] H. Najafov, A. Kato, H. Toyota, K. Iwai, A. Bayramov, S. Iida, Jpn.
J. Appl. Phys. 41 (2002) 1424.
[10] S. Iida, A. Kaoto, M. Tanaka, H. Najafov, H. Ikuno, J. Phys. Chem.
Solids 64 (2003) 1815–1819.
[11] C. Hidaka, E. Yamagishi, T. Takizawa, J. Phys. Chem. Solids 66(2005) 2058–2060.
[12] R.D. Shannon, Acta Crystallogr. A32 (1976) 751.
[13] C. Chartier, C. Barthou, P. Benalloul, J.M. Frigerio, J. Lumin. 111(2005) 147–158.
[14] P. Dorenbos, J. Crystal Growth 237–239 (2002) 2009–2013.
[15] P. Dorenbos, J. Lumin. 104 (2003) 239–260.

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 69 (2008) 400–403
www.elsevier.com/locate/jpcs

Crystal growth study of CaGa2S4 Co-doped with Ce3+ and Na+
Shinya Oikawa, Chiharu Hidaka, Takeo Takizawa
Department of Physics, College of Humanities and Sciences, Nihon University 3-25-40 Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
We have examined the effects of co-doping of Ce3+ and Na+ in CaGa2S4. Differential thermal analysis and X-ray diffraction analysis
were carried out on the compounds of Ca(1x)Cex/2Nax/2Ga2S4 (0pxp1) to obtain the necessary information for their synthesis. It isshown that Ce3+ and Na+ can be simultaneously doped into CaGa2S4 up to about 30 mol% (x ¼ 0.6). The melting point wasdetermined as a function of x. Photoluminescence (PL) was measured with these products, where the maximum intensity was found at
10 mol% (x ¼ 0.2). Single crystals of Ca(1x)Cex/2Nax/2Ga2S4 (x ¼ 0.02 and 0.2) were grown from melt by the horizontal Bridgman
method. Light absorption was measured near the fundamental absorption edge using a high quality part of the single crystals.
Photoluminescence excitation (PLE) spectra were also measured using compounds of Ca(1x)Cex/2Nax/2Ga2S4 with x ¼ 0.02, 0.2 and 0.5
and a Ce only-doped CaGa2S4:Ce+ (0.8 mol%).
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds; D. Luminescence; D. Optical properties

1. Introduction
Alkaline earth thiogallates (MGa2S4, M ¼ Ca, Sr, Ba)
have wide band gaps appropriate for host materials of
luminescent devices. Especially it is known that thecompounds doped with rare-earth elements radiate lightof various colors with high efﬁciency [1,2]. These compounds were ﬁrst studied by Peters and Baglio [1]. An
electroluminescence (EL) device made of CaGa2S4:Ce3+
compounds was ﬁrst fabricated by Barrow et al. [3]. Since
then, CaGa2S4 and SrGa2S4 have attracted much attention
as host materials of EL devices, and many researches onphotoluminescence (PL) and cathode luminescence (CL)have been carried out toward their application to full color
displays.
On the other hand, a laser oscillation in a CaGa2S4:Eu2+
single crystal was reported by Iida et al. [4]. A laser action
was also observed in a CaGa2S4:Dy3+ single crystal at
4.3–4.4 mm [5]. In view of the laser action, CaGa2S4 single
crystals doped with Eu2+ or Ce3+ could be a goodcandidate for the wavelength tunable laser in the blue-green
region because of the luminescence with a wide wavelength
Corresponding author. Tel./fax: +81 3 5317 9772.

E-mail address: takiz@chs.nihon-u.ac.jp (T. Takizawa).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.07.003

band. However, it is considered that the Ce3+ ion cannot be
fully incorporated into CaGa2S4, since the charge balance is
not established due to the difference in the valence of Ce andCa ions. We have already reported that the single crystals
grown from Ce3+ only-doped melt became blackish at Ceconcentration of more than 0.8 mol% and became worse in
crystal quality [6], which may arise from such charge
imbalance.
In order to achieve a high concentration doping of Ce3+,
co-doping with alkaline ions such as Na+ may be one of
the best methods in view of the charge compensation with
Ce3+ ions. It should also be noted that the average ion
radius of Ce3+ and Na+ (1.16 Å) in eight coordinations isalmost as large as that of Ca2+ (1.14 Å) in the same atomic
arrangement.
In what follows, the effects of co-doping of Ce3+ andNa+ on the PL intensity are examined. Single crystal growthwith a composition giving the maximum PL intensity has
been tried.
2. Differential thermal analysis and X-ray diffraction ofCa(1x)Cex/2Nax/2Ga2S4
Differential thermal analysis (DTA) measurement was
carried out using mixtures of CaS, Ce2S3, Na2S and Ga2S3.

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S. Oikawa et al. / Journal of Physics and Chemistry of Solids 69 (2008) 400–403

These compounds were weighed in Ar atmosphere to about
1 g in total in the stoichiometric composition. The weighed
mixtures were put into quartz ampoules of 7 mmF (innerdia.)  40 mm (length), the inner walls of which were
coated by carbon, and sealed under the vacuum of
103 Torr. DTA measurements were run twice up to1140 1C at a speed of 2 1C/min in both heating and coolingprocesses.
After DTA measurements, powder X-ray diffraction(XRD) patterns (Cu-Ka, 40 kV, 30 mA) were measured
using the products prepared during the DTA measurements. As a result, it is found that the solid solution wasformed between CaGa2S4 and Ce1/2Na1/2Ga2S4 in theconcentration range from x ¼ 0 to about x ¼ 0.6 inCa(1x)Cex/2Nax/2Ga2S4. However, new diffraction lines
which could not be assigned to any known compounds
appeared above x ¼ 0.6.
The melting points of the compounds of Ca(1x)Cex/2
Nax/2Ga2S4 were determined by the second run of the DTA
measurements. Fig. 1 shows the melting points in theconcentration range from x ¼ 0 to x ¼ 0.5. The melting
points of the compounds lowered as the concentration of
Ce3+ and Na+ increased.
The lattice constants and the unit cell’s volume wereshown to be almost independent of the Ce3+ and Na+concentrations. This may result from the fact that ion
radius of Ca2+ (1.14 Å) is nearly equal to the average ion
radius of Ce3+ and Na+ (1.16 Å).
Though the phase diagram was not fully completed, we
have tried to grow single crystals as described in Section 4.
3. PL of Ca(1x)Cex/2Nax/2Ga2S4
To check the effect of co-doping on PL intensities, wemeasured PL spectra using the products after DTA under

excitation of 325 nm light from a He–Cd laser (Omnichrome 3056-15M, 23 mW) together with a photonic multichannel analyzer (PMA11, HAMAMATSU) at room
temperature. All the samples used were powdered andﬁlled into quartz ampoules of 3 mmF (inner.dia)  25 mm(length).
Fig. 2 shows the PL spectra of Ca(1x)Cex/2Nax/2Ga2S4(0pxp0.6) and CaGa2S4:Ce (0.8 mol%). Two well-known
emitting bands originating from the radiative transition
between the 5d(T2g) excited-state level and either 4f(2F7/2)
or 4f(2F5/2) ground-state level of Ce3+ [1] found in the PL
spectra. The integrated PL intensity as a function ofcomposition x is shown in Fig. 3. It is interesting to notethat the PL intensity of Ca0.98Ce0.01Na0.01Ga2S4 is stronger
than that of CaGa2S4:Ce3+ (0.8 mol%). The maximum
PL intensity of Ca(1x)Cex/2Nax/2Ga2S4 was attained at
x ¼ 0.2 (10 mo%). Fig. 4 shows the peak energy shift ofboth bands as a function of the concentration of Ce3+ andNa+ where the peaks energies of both bands shifted
toward the lower energy side, indicating the strengtheningof the crystal ﬁeld with the doping concentration. This is
explained by the lowering of the upper state d-band underthe effect of the increase in the crystal ﬁeld with the dopant
concentration.
4. Crystal growth of Ca0.98Ce0.1Na0.1Ga2S4
The compounds having two kinds of concentration of
Ce3+ and Na+ were tried for the crystal growth. One is
10 mol% (x ¼ 0.2) at which the PL shows the highest
intensity and the other 1 mol% (x ¼ 0.02) for comparison
to the former one and also to the 0.8 mol% (Ce onlydoped) one already grown. CaS, Ce2S3, Na2S and Ga2S3
were weighed to about 3 g in total in Ar atmosphere inorder to prevent these compounds from oxidation.
Weighed compounds were put in a quartz ampoule of1150

30x103

1100

25
PL Intensity [counts.]

Temperature [°C]

1200

1050

1000

401

10mol

20

Ce and Na
Ce

5mol
25mol

15
1mol

10

0.8mol95059000(x = 0)

5

10

15

20

25
(x = 0.5)

0.4mol

0
1.5

2.0
2.5
3.0
Photon energy [eV]3.5

concentration [mol%]Fig. 1. Melting points of CaGa2S4-Ca(1x)Cex/2Nax/2Ga2S4 (x ¼ 0.5)
system obtained by DTA measurements.

Fig. 2. Emission spectra of CaGa2S4:Ce (0.8 mol%) and CaCaGa2S4
doped with various Ce3+ and Na+ concentration under excitation of
325 nm light from a He–Cd laser at room temperature.

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S. Oikawa et al. / Journal of Physics and Chemistry of Solids 69 (2008) 400–403

402

PL Integrated Intensity [arb. units]: Ce and Na
: Ce

0

5

10
15
20
25
concentration [mol%]

30

Fig. 3. Integrated emission intensity of Ca1xCex/2Nax/2Ga2S4 and
CaGa2S4:Ce3+ 0.8 mol% as a function of x (x ¼ 0–0.6). The concentration should be read as (x/2)  100.

ampoule was elevated up to a temperature a little below themelting point. The precursors of 1 mol% (x ¼ 0.02) and10 mol% (x ¼ 0.2) were prepared by the solid-state
reaction; the synthesizing temperatures were determinedbased on the data of Fig. 1 as 1120 and 1070 1C,
respectively. Each product after solid-state reaction was
powdered and ﬁlled in another carbon boat of 8 mmF(inner dia.)  100 mm (length) which was again sealed in alarger quartz ampoule. The ampoule was set in a furnace
and a single crystal was grown from melt (one is kept at
1135 1C and the other at 1085 1C) by the horizontal
Bridgman method. The speed of the growth was 0.25 cm/h.
A single crystal of the low dopant concentration was
successfully grown, but no good crystal was obtained forthe higher one (x ¼ 0.2). This is because the compoundwith x ¼ 0.2 exists just in the middle of the solid solution
and its melting point sensitively changes with the dopant
concentration. In addition, the Horizontal Bridgman
method has a drawback that the composition of the grown
crystal changes according to temperature variation. Thus
for the success of the latter crystal growth, the strict control
of both the melting temperature and the composition of thecompound will be required.
5. Absorption and excitation spectra: high energy: low energy

2.6

200
2.5

Ce,Na 1mol%

2.4

0(x = 0)102030(x = 0.6)

concentration [mol%]Fig. 4. Peak energy shift of PL spectra as a function of Ce3+ and Na+concentration. The energy shifts to the low energy side with increasing
Ce3+ and Na+ concentration. The A and B bands show the high and low
energy parts of the Ce3+ emission.

7 mmF (inner dia.)  100 mm (length) evacuated to about
1.0  106 Torr, inside of which was coated with carbon
ﬁlm as mentioned previously. The ampoule was set inanother larger ampoule of 13 mmF (inner dia.)  150 mm(length) and evacuated to about 1  106 Torr. The whole

Absorption coefficient [cm-1]

Peak energy [eV]2.7

Absorption spectrum was measured using a grown single
crystal of Ca0.98Ce0.01Na0.01Ga2S4. Light from a 300 W Xe
lamp after passing through a monochromator (BunkohKeiki, SM-25) was used for both absorption and excitation.
Transmitted light was detected by a photomultiplier(HAMAMATU R562). Fig. 5 shows the absorption spectraof a Ca0.98Ce0.01Na0.01Ga2S4 single crystal. The absorption
peak due to Ce3+ was obtained at 2.9 eV. A shoulder which

150

100

50

0
2.0

2.5
3.0
3.5
4.0
Photon energy [eV]4.5

Fig. 5. Absorption spectrum of a Ca0.98Ce0.01Ga0.01Ga2S4 single crystal.
A peak and a shoulder appear at 2.9 and 3.6 eV, respectively.

ARTICLE IN PRESS
S. Oikawa et al. / Journal of Physics and Chemistry of Solids 69 (2008) 400–403

Intensity [arb.units]

Ce PL
0.8 mol%

PLE

Ce, Na1mol%

Ce, Na10mol%

403

at 2.65 eV of the PL spectrum using a monochromator(Nikon G250). Fig. 6 shows the PL and PLE spectra. It is
known from this ﬁgure that the band widths and peak
positions of PLE spectra did not depend on the concentrations of Ce3+ and Na+. Several structures were foundin PLE at 2.8, 3.0, 3.6 and 4.0 eV. The former three
ones corresponded to the strong absorption bands dueto the doped Ce3+ ions and the latter one (4.0 eV) to thefundamental absorption of the host compound. In
reference to Fig. 6, we propose the energy diagram of
Ce3+ in CaGa2S4 as shown in Fig. 7 [7].
6. Summary

Ce, Na25mol%

2.0

3.0
4.0
Photon energy [eV]Fig. 6. PL and PLE spectra of a Ce3+ only-doped and Ce3+ and Na+ codoped powdered compounds. PLE was monitored at the peak energy ofPL (2.65 eV).

The compounds of Ca(1x)Cex/2Nax/2Ga2S4 (0pxp1)
were prepared to check the effect of co-doping of Ce3+ andNa+. XRD patterns of these products showed that Ce3+
and Na+ could be doped into CaGa2S4 up to about
30 mol% (x ¼ 0.6). The melting point was determined foreach compound, and was shown to lower more than 100 1C
as the concentration of Ce3+ and Na+ increased.
From PL measurements, the maximum intensity was
found to be attained at 10 mol%. Two single crystals of
Ca0.98Cex0.01Na0.01Ga2S4 and Ca0.9Ce0.1Na0.1Ga2S4 wereprepared by the horizontal Bridgman method, and theoptical absorption was measured using a Ca0.98Cex0.01
Na0.01Ga2S4 single crystal of better quality. PLE spectra ofvarious concentrations of Ce3+ and Na+ were alsomeasured. Based on these data, the electronic energy
diagram of Ce3+ in CaGa2S4 was proposed.
Acknowledgments
This work was partly supported by Research Grant from
Nihon University.
References

Fig. 7. Energy levels of Ce3+ in CaGa2S4 obtained from PL, PLE andabsorption spectra.

was also due to the same ion was found at 3.6 eV just below
the fundamental absorption edge of CaGa2S4.
For photoluminescence excitation (PLE) measurement,
powdered samples of Ce 0.8, (Ce, Na) 1, 10 and 25 mol%were used. Measurements were carried out by monitoring[1] T.E. Peters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230–235.
[2] B.G. Tagiev, M.G. Shakhtakhtinskii, V.A. Dzhaliov, T.A. Gyul’maliev, B.M. Izzatov, G.K. Aslanov, O.B. Tagiev, Ya.G. Talybov, Inorg.
Mater. 29 (10) (1993) 1238–1240.
[3] W.A. Barrow, R.C. Coóvert, E. Dickey, C.N. King, C. Laalso,
S.S. Sun, R.T. Tuengo, R. Wentross, J. Kane, SID Int. Symp. Dig.
(1993) 761.
[4] S. Iida, T. Matstumoto, N.T. Mamedov, A.N. Gyejong, Y. Maruyama,
A.I. Bairamov, B.G. Tagiev, O.B. Tagiev, R.B. Dzhabbarov, Jpn.
J. Appl. Phys. 36 (1997) L857.
[5] M.C. Nostrand, R.H. Page, S.A. Payne, W.F. Krupke, Opt. Lett. 24(1999) 1215–1217.
[6] C. Hidaka, T. Takizawa, J. Cryst. Growth 237 (2002) 2009.
[7] A. Bessière, P. Dorenbos, C.W.E. van Eijk, E. Yamagishi, C. Hidaka,
T. Takizawa, J. Electrochem. Soc. 151 (2004) H254–H260.

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 69 (2008) 347–352
www.elsevier.com/locate/jpcs

Single crystal growth of RE doped thiogallates
and their optical properties
T. Takizawa, C. Hidaka
Department of Physics, College of Humanities and Sciences, Nihon University, Sakura-Josui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
Rare earth (RE) elements of the lanthanide series are well known as light emitting sources when doped into transparent host materials.
The mechanism of light emission due to these centers is rather well understood from both experimental and theoretical, but the detailed
information concerning emitting efﬁciencies, spectral widths and quenching characteristics due to the RE ion concentration has not been
fully obtained yet. To improve the emission efﬁciency and to control the functionality, now become the main issues to be addressed fortheir practical application to the light emitting devices. As the host materials for the RE centers above, alkaline earth thiogallates havelong been expected as promising ones. However, most of the researches on these materials are carried out using polycrystalline thin ﬁlms
or polycrystals because of the difﬁculty in the single crystal growth arising from the high chemical reactivity along with high melting
points of these compounds, so that the physical characteristics fundamental for the requirements mentioned above has not been fully
clariﬁed. Here, ﬁrst we would like to introduce the details of our single-crystal growth method applied to Ca, Sr thiogallates, and thenpresent the results of optical assessments for CaGa2S4 single crystals doped with several RE elements (Ce3+, Pr3+, Eu2+, Tb3+, Er3+,
Tm3+). The optical transition levels of these ions are revisited in view of the calculated RE 4fn energy level now easily available throughthe Internet (http://www.phys.canterbury.ac.nz/crystalﬁeld/).
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds; B. Chemical synthesis; D. Luminescence; D. Optical properties

1. Introduction
Rear-earth element (REE) doped compounds haveattracted much attention because of their usefulness tothe ﬂuorescent applications. Actually, REEs are employed
as luminescent centers in various materials for ﬂuorescenttubes and also for LED light devices. Recently, therequirement for the illumination is strengthened and widenin accordance with the spread of our interest to various
kinds of cultural ﬁelds. If much more proper materials arefound, new devises and techniques for illumination as well
as for light communication are expected to emerge to fulﬁll
the broadened requirements of the illumination world.
From a viewpoint of the material science, we have to ﬁrst
search for proper materials and then pursue the research on
their practical synthesis and crystal growth. Among thetremendous amount of candidates, we have conﬁned ourCorresponding author. Tel.: +81 3 5317 9772; fax: +81 3 5317 9772.

E-mail address: takiz@chs.nihon-u.ac.jp (T. Takizawa).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.07.017

attention to the wide gap compounds belonging to the(Ca,Sr)–III2–S4 system [1,2]. These are simply contrived bychanging the IIb element to the IIa one in the IIb–III2–VI4
system derivative of the adamantine compounds. Though
the physical characteristics of the former are very different
from those of the latter, especially in the electric properties,
the wider band-gaps of the former thiogallates well suit for
ﬂuorescent host crystals.
The REE electronic energy levels important for luminescence have been well studied theoretically and are now
available even through the web sites [3]. Actually, theenergy level scheme of REE 4fn electron conﬁguration inseveral crystal lattices has been investigated, and an
example is summarized as shown in Fig. 1 which is anenergy band diagram of doubly and triply ionized REE
ions in the CaGa2S4 lattice [4]. This ﬁgure shows that thiscompound may be a good candidate for luminescentdevices, since many excited levels of 4f electrons are justwithin the band-gap. Another merit of these compounds isthe surrounding of the doped REE ions. The crystal latticeARTICLE IN PRESS
T. Takizawa, C. Hidaka / Journal of Physics and Chemistry of Solids 69 (2008) 347–352

348

forms in a complicated D24
2h –Fddd structure consisting of 32
formula units in a unit cell, and the doped ion occupies thecenter of a decahedron having eight sulfur ions at eachcorner. Thus, the doped REE ion is rather isolated withother like ions, leading to the stability of luminescenceagainst both temperature and doping concentration [5].
In general, powdered compounds are enough for the use
in a passive manner by painting them on luminescent
panels or walls inside of bulbs. Thus, almost all of theﬂuorescent compounds is synthesized and used in a form ofpolycrystalline powder doped with luminescent centerssuch as Mn2+, Ce3+, Eu2+ and Tb3+. Regarding thesingle crystals of ﬂuorescent compounds, much attention
has not been paid so far. However, single crystals areinevitably needed for the physical characterization of thecompounds. In addition, a new possibility to use them as
an active device, e.g., wavelength variable laser devices,
also emerges [6]. A laser action was already observed
10
8
6
4
2
0
-2
-4

Fig. 1. Energy level scheme of divalent (m) and trivalent (.) lanthanides
in CaGa2S4. The lowest 5d levels are also shown by the dotted (3+) andbroken (2+) lines. The thin solid line indicates the fundamental opticalabsorption edge.

in CaGa2S4:Eu2+ [6], though an emission induced decay
was detected in a different species of the same compound [7].
Here, we present our method of synthesis of Ca and Sr
thiogallates and their crystal growth. Optical properties of
the compounds doped with various kinds of REE are also
discussed.
2. Single crystal growth
Ca, Sr and Ba thiogallates are well known as ﬂuorescentmaterials with high efﬁciency of light emission when dopedwith Ce and Eu [1]. However, their synthesis was not soeasy because of their high melting points and chemical
reactivity. As a characteristic feature of sulﬁdes, a sealed
quartz ampoule of the products sometimes explodes at ahigh temperature. To escape from the explosion, we
adopted the sulfurization technique when reacting starting
materials such as CaS, SrS, Ga2S3 with each other. The
purity of the starting materials was 99.9% for Ca and Sr
shots, and 6% for Ga pellets and S powder. Ca or Sr
sulﬁdes of the same purity were also used. These were
weighed in Ar atmosphere to about 5 g in total according
to the chemical formulae, and then put into a carbon
crucible and sealed in a quartz ampoule under vacuum of
1.0  10 4 Pa. If S powder must be added, the S part wasalways set apart from the remaining materials in theampoule to ensure the sulfurization. The temperature of
the S part was kept at less than 450 1C, while the other end
near the melting point of Ga2S3, where the whole chemical
reaction took places, that is, all S was absorbed in thisprocess. An appropriate amount (0.1–1 mol%) of REE was
added in the synthesized compounds. Single crystals weregrown in the horizontal Bridgman method using temperature proﬁles designed according to the binary or pseudobinary phase diagrams already published for the systems of
CaS–Ga2S3 and SrS–Ga2S3 [8,9]. Photos of the grownFig. 2. REE doped single crystals of CaGa2S4. The concentration is 0.8 mol% for Ce and Eu and 0.5 mol% for Pr, Tb, Er and Tm.

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349

Fig. 3. The pseudo-binary phase diagrams: (a) CaGa2S4–EuGa2S4 and (b) CaGa2S4–YbGa2S4.

Fig. 4. Optical absorption of (a) doped Ce3+ ions (sample thickness of 60–120 mm) and (b) Eu2+ ions at different doping concentrations in CaGa2S4 single
crystals.

Fig. 5. Optical absorption of (a) Pr3+, (b) Tb3+ and (c) Tm3+ ions in CaGa2S4 single crystals. The doping concentration is all 0.5 mol%. The temperature
variation is small for these ions as shown in (a).

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crystals doped with six kinds of REE are shown in Fig. 2,
each of which was slightly colored but transparent and has
cleaved surfaces.
Supercooling sometimes happened when a melt wascooled down to be solidiﬁed. In this case, a special care had
to be taken to initiate the crystallization [10,11]. One
method was to adopt a crucible with part of a small crosssection in one end and a larger one in a needed size in theother end [12].

3. Stoichiometric compounds and co-doping of REE
In the case of stoichiometric compounds such as Ca1 x
EuxGa2S4, Eu content can be freely varied. In Fig. 3(a), thephase diagram of the pseudo-binary system of CaGa2S4
and EuGa2S4 is shown, indicating a solid solution at any
Eu content. We have conﬁrmed that SrxEu1 xGa2S4
formed a similar solid solution. However, in the case of
CaxYb1 xGa2S4, no solid solution was found but theFig. 6. Photoluminescence spectra of (a) Pr3+, (b) Tb3+, (c) Er3+ and (d) Tm3+. The spectral assignments were performed in reference to the diagram ofCarnall et al. [16].

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eutectic reaction took places as shown in Fig. 3(b). Since
the ion radii of Ca2+ and Yb2+ are very close to each other(1.12 and 1.14 Å in the eighth coordination, respectively)
and Yb2+ is belonging to the same family of Eu2+, thisseparative character is difﬁcult to understand.
In the growth process of single crystals, REE doping
could not sometimes exceed some amount. For example,
Ce could not be added more than 1 mol% in the Ca or Sr
thiogallate without degrading the quality of the crystal. If
one increased the amount of Ce more than 1 mol%, thegrown crystal became black and a lot of cracks weregenerated inside. In this case, the so-called co-doping withan alkaline element was effective [1,13]. We have tried theco-doping of Ca and Sr thiogallates with alkaline metals of
Li, Na and K. In each case, it was conﬁrmed by powder
X-ray diffraction that up to 20 mol% Ce doping was
successfully achieved [14].
4. Optical properties
The electronic energy levels of REE ions have been wellstudied since several decades ago [15]. Recently, thecalculation of energy levels between f-electron conﬁgurations is available by the ‘‘SPECTRA’’, being supplied andopen through the web site (http://www.phys.canterbury.
ac.nz/crystalﬁeld/) [16]. Such calculation is now extended
to the UV–VUV range [17]. Calculation in terms of thecluster model (DV-Xa) is also open for public, enabling us
to get the energy levels of REE ions embedded in modelclusters [18,19].
Among the 14 lanthanide elements, Ce, Eu and Yb show
broad phonon terminated 4f–5d transitions, and the others
show emission spectra consisting of narrow 4f–4f lines.
Especially, the broad emission of Ce and Eu is of highquantum efﬁciency, and the thiogallates doped with theseelements are regarded as promising for electroluminescenceas well as for wavelength variable lasers when used in aform of single crystal [6,7,20].

351

One of the merits of single crystal is to measure theoptical absorption of doped REEs directly. The opticalabsorption of these two elements is shown in Fig. 4(a) and(b); for the detailed information of Ce3+, see the literatures
already reported [9,20]. The broad absorption of Eu2+ ionsbelow 4.2 eV (CaGa2S4’s band-gap) in Fig. 4(b) shows aninteresting feature, indicating the band structure variationfrom Ca- to Eu-thiogallates. The optical absorptionsof Pr3+, Tb3+ and Tm3+ in CaGa2S4 are shown in
Fig. 5(a)–(c) respectively. In comparison with the energy
diagram in Fig. 1, the absorptions of Ce3+, Eu2+ andTb3+ were considered to be due to the optical transition
from the ionized 4f ground states to the corresponding
lowest 5d excited states. However, those of Pr3+ and Tm3+
were assigned to the so-called charge transfer from thevalence band to the lowest 5d level for the former ion andto the ground state of 4f level of the latter Tm2+ ions. The
absorption spectra of other ions have not been measured
except for those of powder samples. This is because goodsingle crystals for which the thickness can be made thinnerthan 0.05 mm have not been grown at present. The
measurements of absorption spectra for the remaining
lanthanides are now in progress.
The f–f optical transitions of REE in CaGa2S4 werealready reported partly by Georgobiani et al. [21].
However, their data of Pr, Er and Tm differ from ours,
so that we again show our data obtained by using single
crystals in Fig. 6(a)–(d); for other ions a similar research is
now under way. The energy level assignments weretentatively done in comparison of the REE 4fn energies in
LaF3 given by Carnall et.al. together with the calculating
results using ‘‘SPECTRA’’ [3,14]. The assignments are
summarized and shown in Fig. 7. In our study, the PL
spectra of Pr, Tb and Tm showed little temperature
variation, but those of Er did a large temperaturedependence as shown in Fig. 6(c). This may be caused by
the variation of electron population due to temperature,
but the detailed mechanism is not yet clariﬁed.

Fig. 7. Assigned energy levels for each ion.

ARTICLE IN PRESS
352

T. Takizawa, C. Hidaka / Journal of Physics and Chemistry of Solids 69 (2008) 347–352

5. Summary
The crystal growth method of REE doped Ca and Sr
thiogallates were described. To escape from explosion
sometimes occurring in the synthesizing process, thesulfurization method was adopted. In the case of supercooling, a specially devised carbon crucible was shown tobe effective. Triply ionized REE could be incorporated toan appropriate extent into a mother crystal by co-dopingwith alkaline metals, while doubly ionized ones except Yb
formed the so-called stoichiometric compounds. It seems
strange that Yb thiogallates could not mix with Ca ones.
The complex f–f transition energy levels can now be
calculated using a program supplied by a theoreticiangroup, enabling us relatively easily assigning the observed
levels to the calculated ones. We feel that a time when we
could freely design ﬂuorescent materials according to our
purpose is rapidly approaching.
Acknowledgments
We deeply thank Dr. A. Kato and Prof. S. Iida for the
stimulating discussions and encouraging during the study.
Thanks are also to Prof. P. Dorenbos for his collaborationand helpful discussions. This work is partly supported by
the grant for the High-Technology Research Center Projectfor private universities from Ministry of Education, Sports,
Culture and Technology, Japan.
References[1] T.E. Peters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230.
[2] P.C. Donohue, J.E. Hanlon, J. Electrochem. Soc. 121 (1974) 137.

[3] D.J. Newman, B. Ng (Eds.), Crystal Field Handbook, Cambridge
University Press, Oxford, 2000.
[4] A. Bessière, P. Dorenbos, C.W.E. van Eijk, E. Yamagishi, C. Hidaka,
T. Takizawa, J. Electrochem. Soc. 151 (2004) H254.
[5] K. Tanaka, Y. Inoue, S. Okamoto, K. Kobayashil, K. Takizawa, Jpn.
J. Appl. Phys. 36 (1997) 3517.
[6] S. Iida, T. Matsumoto, N.T. Mamedov, G. An, Y. Maruyama,
A.I. Bairamov, G. Tagiev, O.B. Tagiev, R.B. Dzhabbarov, Jpn.
J. Appl. Phys. 36 (1997) L857.
[7] S. Iida, A. Kato, H. Najafov, C. Hidaka, T. Takizawa, in:Proceedings of International Conference ‘‘Fizika-2005,’’ Baku,
Azerbaijan.
[8] C. Komatsu, T. Takizawa, J. Crystal Growth 210 (2000) 677.
[9] C. Hidaka, T. Takizawa, J. Crystal Growth 237–239 (2002) 2009;
IPAP Book Series 1, Japanese Research Review for Pioneering
Ternary and Multinary Compounds in the 21st Century, 2002,
pp. 68–72.
[10] C. Hidaka, N. Makabe, T. Takizawa, J. Phys. Chem. Solids 64 (2003)
1797.
[11] C. Hidaka, M. Goto, M. Kubo, T. Takizawa, J. Crystal Growth 275(2005) e439.
[12] T. Takizawa, M. Kubo, C. Hidaka, C. Hidaka, T. Takizawa,
J. Crystal Growth 275 (2005) 433.
[13] J. Ronot-Limousin, A. Garcia, C. Fouassier, C. Barthou,
P. Benalloul, J. Benoit, J. Electrochem. Soc. 144 (1997) 687.
[14] C. Hidaka, T. Takizawa, Papers presented at ISPN-2, Kaohsiung,
2005.
[15] K.W.H. Stevens, Proc. Phys. Soc. A65 (1951) 209.
[16] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, J. Chem. Phys.
90 (1989) 3443.
[17] J. Hölsa, M. Lastusaari, M. Maryško, M. Tuika, J. Solid State Chem.
178 (2005) 435.
[18] See the related web page.
[19] S. Nomura, T. Takizawa, S. Endo, in: Proceedings of 14th ICTMC,
Denver, USA, 2004, J. Phys. Chem. Solid, in press.
[20] A. Kato, M. Yamazaki, H. Najafov, K. Iwai, A. Bayramov,
C. Hidaka, T. Takizawa, S. Iida, J. Phys. Chem. Solids 64 (2003) 1511.
[21] A.N. Georgobiani, B.G. Tagiev, O.B. Tagiev, B.M. Izzatov, Inorg.
Mater. 31 (1995) 16–19.

Journal of Physics and Chemistry of Solids 66 (2005) 2090–2093
www.elsevier.com/locate/jpcs

Electronic states of lanthanide in the ternary thiogallate CaGa2S4
Shigetaka Nomura a,*, Takeo Takizawa b, Saburo Endo aa

Department of Electrical Engineering, Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japanb
Department of Physcis, College of Humanities & Sciences, Nihon University 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
The electronic states of lanthanide (Ln) doped CaGa2S4 are investigated by the molecular orbital calculations for a spherical cluster of
LnCa8Ga12S24 using the FORTRAN program DVSCAT on the basis of the Discrete Variational method with Xa potentials (DV-Xa). In view of
the SCF convergence, the Ln-doped lattice should contract to 85–90% of the mother crystal around the Ln atom for the lightweight lanthanides
from Ce to Sm. On the other hand, the lattice contraction is very small for the heavyweight lanthanides, especially for Er, Tm and Yb in contrast to
the generally known lanthanide contraction for Ln3C ions. This is probably attributed to the effective charges of Ln atoms calculated here to beless than C1 for all lanthanides contrary to the chemically accepted value of C3. The energy level scheme of 4f and 5d related molecular orbitals
is proposed for each Ln substituting Ca in CaGa2S4, showing that the optical processes relating to the 5d/4f transition must be complicatedespecially for the lightweight Ln-doped CaGa2S4.
q 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Chalcogenides; A. Optical materials; C. Ab initio calculations; D. Electronic structure

1. Introduction
The ternary thiogallate CaGa2S4 doped with lanthanide (Ln)has recently been noticed as promising for a wavelengthtunable light emitter in the visible region [1–3]. Now, detailed
knowledge on the opt-electronic mechanism is necessary forthe materials design and device engineering. In this work, theelectronic states of Ln-doped CaGa2S4 are investigated by themolecular orbital (MO) calculations for a spherical cluster of
LnCa8Ga12S24 using the FORTRAN program DVSCAT basedon the Discrete Variational method with Xa potentials (DVXa) [4].
2. Calculation
2.1. The DV-Xa method
The DV-Xa is one of the cluster methods based on the firstprinciple and all-electron calculation, suitable for investigating
local electronic states of the constituent atoms. The calculating
system consists of the SCF (self-consistent field) calculation in* Corresponding author. Tel.: C81 3 3260 4272; fax: C81 3 5261 4805.
E-mail address: nomura@rs.kagu.tus.ac.jp (S. Nomura).

0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2005.10.120

reference to the respective electrons occupying the atomic
orbitals for all the atoms in a cluster. The convergence in the
SCF calculation corresponds to the minimum in the totalenergy of the electronic system. An effective calculation can be
performed by the discrete variational method and the usage ofthe Xa potentials in solving the secular equations to determine
the eigen states of the molecular orbitals. In this work, the nonspin and non-relativistic calculations were performed as thefirst step of the electronically complicated system of Ln-doped
CaGa2S4.
2.2. Cluster model
Ternary thiogallate CaGa2S4 belongs to the space group of
Fddd:2, including 224 atoms in an orthorhombic unit cell of
aZ2.0122, bZ2.0090, and cZ1.2133 nm [5]. In this work, aspherical cluster of Ca9Ga12S24 was used as a mother body forthe electronic state calculation. One of the Ca sites is set to thecluster center surrounded by eight S atoms with a distance ofabout 0.3 nm. The centric Ca atom is substituted by a Ln atom
for the calculation of the Ln-doped electronic states. Actually,
the distance from Ln to the surrounding S atoms, d(Ln–S), andnaturally to all atoms in the cluster must have been adjusted inorder to achieve the SCF convergence of the MO calculation.
In other words, a lattice deformation around a doped Ln atom
can be estimated from the electronic state calculation.

S. Nomura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2090–2093
1.050

d (Ln-S) / d (Ca-S)

1.000

0.950

0.900

0.850
less converged
0.800

0.750

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb Dy Ho Er Tm Yb Lu

Lanthanide

Fig. 1. Bond length d(Ln–S) variation normalized by that of the Ca–S bond,d(Ca–S), when the calculation converged.

3. Results and discussion

shows the bond length d(Ln–S) variation normalized by that of
the Ca–S bond, d(Ca–S)Z0.29672 nm, for full Ln members
when the MO calculation converged. For the lightweightlanthanides from Ce to Sm, the lattice contracts to 85–90% of
the mother cluster. The lattice contraction is very small for theheavyweight lanthanides, especially for Er, Tm and Yb.
The best convergence was achieved for Yb without changing
the lattice configuration around the cluster center of
Ca9Ga12S24. Both ends of the lanthanide series, La and Lu
are exceptions whose 4f orbitals are completely empty and full,respectively. These results are different from the generally
called lanthanide contraction for Ln3C ions, where ionic radii
become shorter with increasing the atomic number Z, or thenumber of 4f electrons. The difference comes from the fact that
estimated effective charges for Ln atoms are less than C1, i.e.
LnC instead of Ln3C, as mentioned later.

3.1. Lattice contraction by doping a Ln atom

14
12

Molecular orbital calculations were attempted on
LnCa8Ga12S24 clusters for full members of lanthanide.
The SCF calculation converged only when all atomic positionsin the cluster were rearranged from those in the mother cluster
Ca9Ga12S24 so as to properly adjust the specific distance
between a Ln atom and surrounding eight S atoms. Fig. 1

10

S:3s
S:3p
S:3d

8

S

6
4
2
0
–15

(a) 15

–10

–5

0

5

10

5
4f
5s
5p

4

Ce:4f
Ce:5d

3
10

Ce

5

0
57

58

59

60

61

62

63

64
Z

65

66

67

68

69

70

71

Density of States (DOS)[1/eV]

Number of Electrons

2091

2
1
0
–15

–10

–5

0

5

10

5
4
3

Ga:3d
Ga:4s
Ga:4p

Ga

Number of Electrons(b) 2.0

2
5d
6s
6p
Z*

1.5

1
0
–15

–10

–5

0

5

10

5

1.0

4

Ca:4s
Ca:4p

3

0.5

Ca
2
1

0.0
57

58

59

60

61

62

63

64
Z

65

66

67

68

69

70

71

La

Ce

Pr

Nd

Pm Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

0
–15

Fig. 2. The number of electrons in valence orbitals of (a) 4f, 5 s and 5p, and (b)5d, 6 s, 6p and effective charge Z* for the lanthanide series.

–10

–5

0

5

10

Energy [eV]Fig. 3. Calculated density of states (DOS) of the atomic orbitals of theconstituent elements for the Ce-doped CaGa2S4.

2092

S. Nomura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2090–2093

Table 1
Molecular orbitals relating to the 4f levels of Ce, showing the hybridization percentages for valence atomic orbitals of constituent elements

1.131
1.103
0.924
0.870
0.695
0.616
0.563
0.493
0.412
0.200
0.133
0.061
0.054
0.015
0
K0.100K0.242

Ce

Ga

Ca

S

4f

5d

6s

6p

3d

4s

4p

4s

4p

3s

3p

3d

54
27
48
39
78
41
45
63
53
32
79
2
14
6
3
1
32

3
0
5
4
1
0
0
1
1
0
0
2
0
0
0
2
0

0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0

0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0

0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0

3
8
4
4
1
7
5
1
4
3
2
2
2
0
1
4
5

14
19
11
18
7
13
17
3
9
11
4
4
4
2
3
13
16

0
5
0
1
1
3
6
10
6
21
2
51
47
54
55
40
8

2
8
4
2
1
4
7
9
5
9
2
21
20
21
22
14
4

0
0
0
1
0
0
1
0
0
1
1
1
1
0
0
1
1

15
26
18
15
4
19
9
6
13
12
5
7
9
15
14
13
25

8
7
10
14
7
12
10
6
10
12
6
9
3
1
2
12
10

Sufficient convergence in the SCF calculation has not beenachieved for Eu and Gd. For the two cases, we show the d(Ln–S)/d(Ca–S) ratios which minimize the respective mismatches
in the SCF calculation. The ternary thiogallate EuGa2S4 has an
orthorhombic unit cell of aZ2.0716, bZ2.0404 and cZ
1.2200 nm, belonging to the space group of Fddd:2, similar to
CaGa2S4, [3]. From the crystal data, the bond length d(Eu–S) is
estimated as 0.30094 nm which is slightly shorter than thed(Eu–S)Z0.30488 nm estimated from the SCF calculation.
3.2. Valence electrons of the lanthanide atoms
The valence electrons of each lanthanide atom were
estimated from the MO calculations by the Mulliken’s method,where covalent electrons are evenly shared with the bonding
atoms. Fig. 2a shows the 4f, 5 s and 5p electrons for eachlanthanide. The increment of the atomic number Z simply
increments the number of the 4f electrons from La to Yb, while
the 5 s and 5p orbitals are fully occupied for all lanthanides.
Fig. 2b shows the 5d, 6 s, 6p electrons and effective charge Z*
as well. The number of the 5d electrons is calculated as the
maximum of 1.53 for Ce, and gradually decreases to 0.71 forYb with increasing Z, reflecting the decrease in the effective
charge Z* from 0.88 for Ce to 0.55 for Yb. This result poses a
problem that the generally accepted feature of lanthanide ionsin crystals as being trivalent or divalent is not necessarily true.
The number of electrons occupying the outer 6 s and 6p orbitals
slightly increases with increasing Z. The sum of them is no
more than one for all lanthanides.
3.3. Molecular orbitals and the density of states of atomic
orbitals

be 0 eV here. Most of valence electrons belong to the 3p
orbitals of S atoms. On the other hand, the hybrid orbitals of 3d
of S, 4 s and 4p of both Ga and Ca form the unoccupied states.
The 4f peak of Ce spreads around the HOMO level. Table 1
lists the molecular orbitals highly relating to the 4f orbitals ofCe, showing the hybridization percentage of the constituent
atomic orbitals. Only one occupied level is seen for 4f orbital at
K0.24 eV with the hybridization rate of 32%. Other MO levels
relating to 4f are unoccupied states making strong hybridization mainly with the atomic orbitals of 4p of Ga and 3p and 3dof S. As can be seen in Fig. 3, the 5d atomic orbitals of Ce
widely spreads in the unoccupied energy range, making many
molecular orbitals strongly hybridizing with other unoccupied
components such as 3d of S. On the other hand, there are somelanthanides showing weak hybridization on the 4f atomic
orbitals such as Dy and Er.
3.4. Energy level scheme
Fig. 4 shows the energy level scheme of the molecularorbitals relating to 4f and 5d atomic orbitals of the Ln series
except for Eu and Gd, because the SCF convergence was not
5.0
4.0

5d

3.0

Eg
Energy [eV]

Energy (eV)

2.0
1.0
0.0

4f
–1.0
–2.0

Fig. 3 shows the calculated density of states (DOS) of therespective atomic orbitals for the Ce-doped case, where thediscrete DOS data are profiled with Gaussian type functions.
The HOMO (highest occupied molecular orbital) level is set to

–3.0
Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Fig. 4. Energy level scheme of 4f and 5d related molecular orbitals for thelanthanide series.

S. Nomura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2090–2093

sufficient for the two cases. The highest occupied 4f level isdirectly below the HOMO level or corresponds to the HOMO
level for all lanthanides. An increase in the number of the 4f
electrons of Ln makes the relating molecular orbitals simply
deeper in energy. On the other hand, the uppermost unoccupied
4f levels shown by the gray lines are located up to about 1 eV
above the HOMO level. The unoccupied 5d related levels
disperse in the midgap range for the lightweight Ln group,
while those of the heavyweight group concentrate on theenergy range of about 3.5–4 eV. The optical processes
followed by the 5d/4f transition would be complicatedbecause of the widely expanded 5d levels for the lightweightlanthanides, resulting in multiple and/or broad light emissions.
In view of the expected density of states, the possibility or theefficiency is very small with respect to the visible lightemissions brought by the 5d/4f transitions for the heavyweight lanthanides, from Dy- to Yb-doped cases.
4. Conclusion
The electronic states of lanthanide-doped CaGa2S4 were
investigated from the molecular orbital calculations for2093

spherical LnCa8Ga12S24 clusters using the DV-Xa method.
Substitution of Ln for Ca would induce some lattice contraction
especially for the lightweight lanthanides from the view pointof the SCF convergence in the molecular orbital calculation.
Effective charges of the lanthanide elements when substituting
Ca in CaGa2S4 are estimated to be less than C1 in contrast to
the generally accepted trivalent assignments. An energy level
scheme of the molecular orbitals highly relating to the 4f and5d atomic orbitals for the lanthanide series in CaGa2S4 was
proposed.

References[1] S. Iida, T. Matsumoto, N.T. Mamedov, Y. Maruyama, A.I. Bairamov,
B.G. Tagiev, O.B. Tagiev, R.B. Dzhabbarov, Jpn. J. Appl. Phys. 36 (1997)
857 Lett..
[2] A.N. Georgobiani, B.G. Tagiev, O.B. Tagiev, B.M. Izzatov, R.B. Jabbarov,
Cryst. Res. Technol. 31 (1996) 849.
[3] T.E. Peter, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230–236.
[4] H. Adachi, M. Tsukada, C. Satoko, J. Phys. Soc. Jpn 45 (1978) 875.
[5] B. Eisenmann, M. Jakowski, W. Klee, H. Shäfer, Revue de Chimieminérale 20 (1983) 225.

Journal of Physics and Chemistry of Solids 66 (2005) 2076–2078
www.elsevier.com/locate/jpcs

Optical gain due to the Eu transition in the alloy of Ca1-xEuxGa2S4
A. Kato a,*, S. Iida a,1, M. Yamazaki b, E. Yamagishi c, C. Hidaka c, T. Takizawa caNagaoka University of Technology, Kamitomioka 1603-1, Nagaoka 940-2188, Japanb
Nagaoka National College of Technology, Nagaoka 940-8532, Japanc
College of Humanities and Sciences, Nihon University, Tokyo 156-0045, Japan

Abstract
Room temperature photoluminescence quantum efficiency of the alloy of Ca1KxEuxGa2S4 was measured as a function of x, and was found to benearly unity under excitation at peak wavelength of excitation spectrum (510 nm) in the x range of 0.01%x%0.2. At larger x values, it tends todecrease, but still as high as 30% for stoichiometric compound EuGa2S4. Taking these backgrounds into account, pump-probe experiments weredone with Ca1KxEuxGa2S4 for searching optical gain at xZ0.2. The optical gain of nearly 30 cmK1 was confirmed to exist, though the pumping
induced transient absorption which seems to limit the higher gain was found.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Alloys; A. Optical materials; D. Luminescence; D. Optical properties

1. Introduction
Discussion [1] was given on laser application feasibility ofthe transition 5D1/7F2 in Eu2C[2] for the stoichiometric rareearth compound of EuGa2S4, similarly to the case of
CaGa2S4:Eu [3]. The alloy of these compounds CaGa2S4 andEuGa2S4, Ca1KxEuxGa2S4, show an intense green–yellow Euemission in the whole range of 0%x%1 [4]. This alloy withproper x value, Ca1KxEuxGa2S4, is expected to have an optical
gain larger than EuGa2S4, since the Eu emission in the alloy
with proper x is free from the concentration quenching. The
present study concerns quantum efficiency measurements andan estimation of optical gain against x. The description also
covers the result of pump-probe experiments for searching thegain at xZ0.2.

2. Experimental
Single crystals of Ca1KxEuxGa2S4 (0.01%x%0.4) weregrown by the Bridgeman method [4]. A single crystal of
EuGa2S4 grown by iodine transport method was also used forquantum efficiency standard. In order to measure transient* Corresponding author. Tel.: C81 258 47 9542; fax: C81 258 47 9500.
E-mail address: arikato@vos.nagaokaut.ac.jp (A. Kato).
1
Present address: Shizuoka Institute of Science and Technology, Fukuroi
437-8555, Japan.

0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2005.09.057

absorption under optical pumping, a crystal of xZ0.2 waspolished to be about 23 mm thickness.
Absolute PL quantum efficiency measurements were doneusing an integration semi-sphere as described in our reports[1,3] for two samples (EuGa2S4 and Ca0.99Eu0.01Ga2S4). For
samples with other x values, relative intensities to these two
samples with corrections taking into account the real
absorption at corresponding x were treated as quantum
efficiency values for these samples and plotted as x dependenceof quantum efficiency.
The experimental setup of pump-probe experiments isshown in Fig. 1. An optical parametric oscillator (Spectra
Physics, MOPO-SL) pumped by the third harmonic of a
Q-switched Nd:YAG laser (Spectra Physics, PRO-230-10) wasused as a pump source. Wavelength, pulse duration andmaximum excitation density were 500 nm, 10 ns and 2 MW/
cm2, respectively. This wavelength nearly corresponds to thepeak of the photoluminescence excitation band [1]. The 546
and 577 nm lines of a super high pressure Hg lamp (Ushio,
USH-102B) were used as probe light. These wavelengths lie inthe photoluminescence band.
The pumping light was monitored using a photo-diode byinserting a splitting plate in the pumping pass. Emission fromthe front side of the sample was dispersed through a
monochromator (Ritsu, MC10) and detected by a photomultiplier. Transmitted light of the probe light of the sample
was dispersed by a monochromator (Jobin Ybon, HR-320) in
conjunction with a photomultiplier (Toshiba, PM55). Pumping
light, emission and transmitted light were recorded simultaneously by an oscilloscope (Sony Techtoronix, TDS 3054)

A. Kato et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2076–2078

Attenuator

OPO
Signal (500nm)

2077

Nd:YAG
THG(355 nm)

Trigger

Hg LampBeam
Splitter

Monochromator

PM

FilterAttenuator
Pump

Emission
Monochromator

SamplePD

PM

FilterMonitor
CH2

Probe

CH1
Oscilloscope

CH4

CH3

Fig. 1. Schematic diagram of arrangement of the pump-probe experiment.

TrIðtÞ Z IPuCPr
ðtÞK

Ð Em
ðtÞdt TrIÐ PuCPrEm ðtÞdt IPu ðtÞ;
IPu

(1)

Tr
Tr
ðtÞ and IPu
ðtÞ are temporal variations of thewhere IPuCPr
observed transmitted light with and without the probe light,
EmEm
respectively, and IPuCPr
ðtÞ and IPu
ðtÞ are those of the observed
emission with and without the probe light, respectively.

4000

80

3000

60
2000
40
1000

20
0
0.0

0.2

0.4
0.6
0.8
Eu Alloy Composition x

Maximum Optical Gain (cm–1)

Quantum Efficiency (%)

100

0
1.0

Fig. 2. Eu concentration dependence of the quantum efficiency under
excitation at peak wavelength of the PLE spectrum at room temperature in
Ca1KxEuxGa2S4 system, and expected maximum optical gain according to the
MacCumber’s theory (see the text).

3. Results and discussion
Fig. 2 shows PL quantum efficiency vs. x relation. Actual
measurements were done using the 441.6 nm laser line. The
result shown in Fig. 2 is the efficiency vs. x relation
corresponding the peak of the excitation spectrum at 510 nm[1]. Almost unit quantum efficiency of the luminescence is seenin the range x%0.2 and the efficiency gradually decreased withx to w30% for xZ1 (EuGa2S4) at room temperature. Fig. 2
also shows x dependence of the maximum optical gain
expected from the McCumber’s theory [5] with the use ofquantum efficiency corresponding to the dotted curve. The
maximum optical gain as high as 3000 cmK1, which is larger
than that of EuGa2S4, is expected to be obtained at xZ0.7 andthe value of 1300 cmK1 is expected at xZ0.2 where the pumpprobe measurements were done.
Fig. 3(a) shows temporal variation of net intensity of thetransmitted light I(t). For comparison, temporal variation of theEmemission IPu
ðtÞ is shown in Fig. 3(b). The net intensity I(t)shows an increase under the excitation, indicating the existenceof optical gain. However, I(t) turns to decrease to the level(a)

1.1
Intensity (a.u.)

with and without the probe light. All data were recoded using asingle shot mode, since shot-to-shot fluctuation of the pumplight was too large to be neglected.
The observed transmitted light included a spontaneousemission from the back side of the sample. Therefore, net
intensity of transmitted light I(t) was estimated by making acorrection of this emission contribution with the following
equation.

Transmission@546 nm1.0
0.9

Ca0.8Eu0.2Ga2S4 RT

0.2
(b)

0.1

Emission

0.0
0

2

4
6
Time (µs)

810Fig. 3. Temporal variation of net intensity of the transmitted light I(t) at 546 nm(a) correlated by Eq. (1) in the text and the emission (b) for pump-probe
experiment of Ca1KxEuxGa2S4 with xZ0.2.

A. Kato et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2076–2078

6

60
50
40
30
20
10
0
–10
–20
–30
–40
–50

Emission Intensity (a.u.)

Optical Gain (cm–1)

2078

Ca0.8Eu0.2Ga2S4 RT
546nm
577nm

Fig. 4. Excitation density dependence of optical gain of Ca1KxEuxGa2S4 with
xZ0.2. The value of optical gain was estimated from the increase of thetransmitted light and the thickness of the sample. Closed and open circles show
the optical gain measured by 546 and 577 nm probe light, respectively.

below the initial level, indicating the existence of the pumping
induced transient absorption.
From the increase of I(t) and the thickness of the sample, theoptical gain value can be estimated. The results are shown in
Fig. 4. Closed and open circles show the optical gain measured
at 546 and 577 nm probe light, respectively. Most circles
distribute in the range larger than 0, indicating the existence of
optical gain. Closed circles for 546 nm probe light, which is
close to the peak of the photoluminescence band, distributearound 30 cmK1.
The number of photons per shot across unit area is(2)

where P is the excitation intensity (1 MW/cm2), -u is thephoton energy (2.5 eV (5000 Å)) and t is the pulse duration(10 ns) which is much smaller than the radiative lifetime of Eu
ion (470 ns) [1]. The number of excited Eu ions per unit area isestimated to be
nEu Z

dEu
Z 9:2 !1017 cmK2 ;a(3)

where dEu is the Eu density (1.2!1021 cmK3) for Eu compositionof xZ0.2 and a is an absorption coefficient at wavelength of thepumping light which is assumed to be maximum optical gain gmax
(1300 cmK1) for Eu composition of xZ0.2. The ratio of excited
Eu ions to total Eu ions in the crystal isnph
Z 0:027(4)nEu
and optical gain is estimated to benph
Z 35 cmK1 :g Z gmax
nEu

4
3
2
1
0
0.0

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Excitation Density (MW/cm2)

Pt
Z 2:5 !1016 cmK2 ;
nph Z
Zu

5

(5)

Observed optical gain values in Fig. 4 distribute around
30 cmK1, which corresponds to the optical gain estimated above.
The gain in Fig. 4 does not show further increase with pumping
intensity. This tendency is probably related to the emission
saturation observed in Fig. 5. Even with the gain presently
observed, laser construction is thought to be possible if a highquality crystal of 1 mm size can be obtained.

Ca0.8Eu0.2Ga2S4 RT

0.5
1.0
1.5
Excitation Density (MW/cm2)

2.0

Fig. 5. Excitation density dependence of the emission intensity of
Ca1KxEuxGa2S4 with xZ0.2.

Fig. 5 shows excitation intensity dependence of theemission intensity. The intensity of the emission showssaturation behavior at above 1 MW/cm2. This saturation isprobably related to the pumping induced transient absorption
shown in Fig. 3(a). The origin of this saturation and thetransient absorption is not clear at present. Re-absorption oftrapped carriers can be considered as a candidate, since thisalloy system shows thermoluminescence.
4. Summary
Room temperature photoluminescence quantum efficiencyof the alloy of Ca1KxEuxGa2S4 was measured as a function of x,
and was found to be nearly unity under excitation at peakwavelength of excitation spectrum (510 nm) in the x range of
0.01%x%0.2. At larger x values, it tends to decrease, but still
as high as 30% for stoichiometric compound EuGa2S4.
Pump-probe experiments were done with Ca1KxEuxGa2S4
for searching optical gain at xZ0.2. The transmitted lightshowed an increase at initial stage, indicating an optical gain.
However, it turned to decrease, indicating the existence of thepumping induced transient absorption. The value of observed
gain at excitation density of 1 MW/cm2 was nearly 30 cmK1,
which favorably compares with the theoretically estimatedvalue based on the number of excited Eu ions corresponding to
the excitation density. Even under this gain value, laser
construction is thought to be possible if high quality crystals
with 1 mm size are available.
The observed transient absorption seems to be related withobserved intensity saturation of the emission under highexcitation.
References[1] S. Iida, A. Kato, M. Tanaka, H. Najafov, H. Ikuno, J. Phys. Chem. Solids 64(2003) 1815.
[2] A.N. Georgobiani, B.G. Tagiev, O.B. Tagiev, B.M. Izzatov, R.B. Jabbarov,
Cryst. Res. Technol. 31 (S-2) (1996) 849.
[3] S. Iida, in: T. Matsumoto et al. (Ed.), IPAP Books vol. 1, Institute of Pureand Applied Physics, Tokyo, Japan, 2001, p. 302.
[4] C. Hidaka, E. Yamagishi, T. Takizawa, to be presented in this Conference.
[5] D.E. McCumber, Phys. Rev. 134 (1964) A299.

Journal of Physics and Chemistry of Solids 66 (2005) 2061–2064
www.elsevier.com/locate/jpcs

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals
C. Hidaka *, E. Yamagishi, T. Takizawa
Department of Physics, College of Humanities and Sciences, Nihon University, Sakura-josui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
Six kind CaGa2S4 single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and theirphotoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broadline shapes due to the phonon assisted 4f–5d transitions. On the other hand, those of Pr3C, Tb3C, Er3C or Tm3C doped samples show narrow ones
owing to the 4f–4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed insame materials.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials; B. Crystal growth; D. Luminescence; D. Optical properties

1. Introduction

2. Experimental procedures

Rare earth (RE) elements of the lanthanide series arewell known as light emitting sources when doped intotransparent host materials. The mechanism of light emission
in these materials is rather well understood from bothexperimental and theoretical [1–4], except for some detailed
information such as emitting characteristics concerning
efficiencies, spectral intensities and wavelengths. To
improve the emission efficiency and to control the function
are the central issues to be addressed for the materials
mentioned above.
Alkaline earth thiogallates are long expected as promising hosts for visible light emitters. However, most of theresearches on these materials are carried out using
polycrystalline thin films or polycrystals, so that physicalcharacteristics fundamental for the single crystal have notbeen fully clarified. Photoluminescence spectra of Ce andEu doped crystals gave broad PL lines similar to thosealready described in previous papers [5,6]. Here, only the
results of photoluminescence measured using CaGa2S4
single crystals doped with the respective elements (Pr3C,
Tb3C, Er3C, Tm3C) successfully grown by the melt method
are presented. These emission levels are discussed incomparison to those in the same hosts ever investigated [7].

The host compounds of CaGa2S4 were prepared using theelements of Ca, S and a compound of Ga2S3 weighed to 7 g
in total in argon atmosphere. The detailed procedure is
described elsewhere [8]. Rare earth elements were added at
an amount of 0.5 wt% in a powder form of their sulfides.
The crystal growth was performed in a horizontal Bridgman
furnace at a growth speed of 2.5 mm/h. Four single crystals
doped with different RE elements (Pr3C, Tb3C, Er3C,
Tm3C) were grown. The EDX measurements show thateach of them has a desired concentration of the doped RE
element. The typical crystal size was approximately
6 mmf?!3 mm.
A He–Cd laser with 325 nm was used as a light sourcefor excitation. For photoluminescence (PL) measurements,
two kind spectrometers with a different resolution wereused. Broad spectra were measured by a multi-channel
detector (HAMAMATSU PMA-11) in the wavelength from
200 to 950 nm with resolution less than 2 nm, while narrowspectra by a spectrometer (Nikon G250) with resolution of
0.3 nm together with a photomultiplier (HAMAMATSU
R562). A cut filter was placed in front of a detector for
removing the scattered laser beam. Temperature variation of
the spectra was observed between 10 and 300 K using acryo-cooler.
Absorption spectra were measured with samples of
20 mm thickness using a deuterium or halogen lamp together
with a monochromator (NALUMI RM-23) and a photomultiplier (HAMAMATSU R562) in the 10–300 K temperature range.

* Corresponding author. Fax: C81 3 5317 9772.
E-mail address: komaz@phys.chs.nihon-u.ac.jp (C. Hidaka).

0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2005.09.068

2062

C. Hidaka et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2061–2064

3

3P

0

F2

3P
2

300K

Intensity [arb.unit]

3
3

3P
0

3F
3

1.4

3H
4

F2

3

3P

F2

2

3P
2

P2

3

P1

3P
1
3P
0

3F

4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3F
4
3F
3
3F
2
3
H4

Photon energy [eV]Fig. 1. Photoluminescence spectra of a Pr doped CaGa2S4 single crystal at 300 K. No difference of temperature variation was observed. A proposal assignment of thelevels is shown in the right hand side.

the excitation method (they used 365 nm line of mercury
lamp as a light excitation source).
Fig. 1 shows the spectrum of a Pr doped crystal at 300 K. Its
temperature variation was measured at 10, 100, 200 and 300 K,
but no big difference was observed except a small broadeningof the linewidth and a little enhancing of the line intensity withincrease of temperature. The observed PL lines can be
explained as the transitions between the 4f multiplets of Pr3C
ions [9]. A series of lines near 1.9 eV were identified astransitions between 3P0, 3P1, 3P2 and 3F2. Their energy
differences were determined as 1.89, 1.97 and 2.03 eV,
respectively. Two similar transition series corresponding to
the transitions from (3P0, 3P1, 3P2) to 3F3 and 3F4 were observed

3. Results and discussions
3.1. PhotoluminescenceFig. 1–4 show respective PL spectra of four CaGa2S4
single crystals each doped with one of the rare earth
elements (Pr3C, Tb3C, Er3C, Tm3C), all consisting of sharplines. Using samples prepared by solid state reaction andsynthesis above 1200 8C, Georgobiani et al. alreadymeasured the PL spectra due to the RE elements above[7], which were however entirely different from the present
ones. The main reason for this may be ascribed to thedifference in sample preparations or to the difference in

5D
3

7F

5

5D
4

Intensity [arb.units]

10K

5D
3
7F
3
7F
2

7F
0

7F
6

7F

4

7F
1
5D
4

7F
1
7F
2

7F
0

7F
0
7
7F

7F
3

4

7F
5

F1

7
7F
6

F2
7F
3

7F
4

1.5

2.0

2.5

Photon energy [eV]

3.0

3.5

7

F5

7F
6

Fig. 2. Photoluminescence spectrum of a Tb doped CaGa2S4 single crystal at 10 K. Lines from 1.8 and 2.6 eV are almost not varied and those between 2.6 and 3.3 eV
disappeared above 200 K. A proposal assignment of the levels is shown in the right hand side.

C. Hidaka et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2061–2064

4
4
4

F9/2

4I

S3/2

H11/2

4I
15/2

15/2
2H
9/2

4I
13/2

4I

2063
4G
11/2

15/2
2

4G
11/2

4I

H9/2

13/2

Intensity [arb.unit]

2H
11/2

300K

2

200K

4F
9/2

S3/2

100K
4I
13/2

10K

1.8

2.0
2.2
Photon energy [eV]

2.4

4I
15/2

2.6

Fig. 3. Photoluminescence spectrum of an Er doped CaGa2S4 single crystal. Temperature variation from 10 to 300 K is also presented. There are two types of lines;
appearing and diminishing with increasing temperature. A proposal assignment of levels is shown in the right hand side.

measured Er doped thiogallates at 300 K [11]. Our data at
the same temperature agreed very well with theirs, andthe assignments according to them are shown in Fig. 3. On thebase of these assignments, it can be inferred that among the PL
ground states only the 4I15/2 state may be the origin of thepositive temperature dependence, since the lines assigned from
4
F9/2, 4S3/2 and 2H11/2 to 4I15/2 showed positive ones, while thosefrom 2H9/2 and 4G11/2 to 4I13/2 showed the negative ones. The
4
I15/2 state is considered to spread out, i.e, it may consist of aband having some width. At low temperature, only the lowest
energy part is occupied by electrons, so that the emission
terminates at the higher energy part. As temperature increases,
the empty states spread to the lower energy side in the 4I15/2
band, and then the transition to the lower levels become allowedwith temperature, which account for the appearing the new lines
at the higher energy part at high temperatures in the transitions
from 4F9/2 and 4S3/2 to 4I15/2. On the other hand, the transition
from 2H11/2 to 4I15/2 cannot be explained by this assumption.
4D
2

Intensity [arb.inits]

at 1.67 and 1.42 eV, respectively. The sharp structure at
2.50 eV was also interpreted as the lowest transition from3
P2–3H4 in the transition group between (3P0, 3P1, 3P2) and 3H4.
Fig. 2 shows the PL spectrum of a Tb doped crystal at 10 K.
Sharp lines between 1.8 and 2.6 eV had almost no temperature
variation, but those between 2.6 and 3.3 eV gradually lowered
their intensity and disappeared above 200 K. The broad
spectrum observed in the 1.3–2.0 eV range also disappeared
above 200 K. Referencing the 4f electron configuration data[9], the fine structures between 1.8 and 2.6 eV were assigned tothe transitions from 5D4 to 7Fi (iZ0–6), the transition energies
of which are 1.83, 1.86, 1.90–1.92, 1.99–2.00, 2.10–2.13, 2.25–
2.28, 2.51–2.53 eV, where the hyphenation shows a doublet.
The structures appearing in the 2.6–3.3 eV region were also
assigned to the transitions from 5D3 to 7Fi (iZ0–6), where eachenergy can be obtained by shifting each energy value
mentioned above by 0.71 eV to the higher energy side. These
spectra however diminished at higher temperature than 200 K.
This may be attributed to the difference in characteristics
between the excited states, that is, 5D4 and 5D3, since theelectrons excited at the higher excited state 5D3 may be more
rapidly relaxed to the other lower states as temperature
increases. A broad spectrum (1.3–2.0 eV) only observed at
low temperatures was also observed in the X-ray excited PL
spectra by Bessière et al. who proposed the self trapped excitonmodel for its interpretation [10]. To support their idea much
experiment should be required.
The PL spectra of an Er doped crystal and their temperature
variation were shown in Fig. 3. Yellow colored emission
observed at low temperature changed its color to green as
temperature increased. These spectra differ from those of theother RE doped thiogallates in a point that new lines appearedwith increase of temperature (here after we say it as a ‘positive’temperature dependence), in contrast that the lines at low
temperature diminished or disappeared (we call it as a‘negative’ one) in the other cases. Garcia et al. already

3F

3

4D
2

3F
4

1G
4

1.4

1.6

1.8 2.0 2.2 2.4
Photon energy [eV]

43
6

2.6

2.8

3.0

Fig. 4. Photoluminescence spectrum of a Tm doped CaGa2S4 single crystal at
300 K.

2064

C. Hidaka et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2061–2064

4. Conclusion
Tm doped
host crystal

Absorption coefficient [cm–1]

80

Six kind CaGa2S4 single crystals doped with different
rare earth elements were grown by the horizontal Bridgman
method, and their PL spectra were measured. These spectra
were very different from those ever reported [7]. The PL
spectra of Pr3C, Tb3C, Er3C or Tm3C doped ones presented
sharp spectral shapes originating from the 4f–4f transitions,while those of Ce3C or Eu2C doped crystals were explainedby the 4f–5d transitions in the RE electronic energy levels,
resulting in broad line shapes owing to the phonon relatedtransitions. The assignments of the electronic levels were
performed in comparison to the reported data of RE 4f
multiplets doped in same host materials.

60

40

20

0
2.5

3.0

3.5

4.0

Photon energy [eV]Fig. 5. Absorption spectra of Tm doped CaGa2S4 single crystal. The absorption
spectra of a host crystal are also presented by the dashed line.

There may be some mechanisms that the 2H11/2 state ispopulated only at high temperature. For the upper state of 4I13/
2, such phenomenon might not happen because of its narrowenergy width, resulting in the normal temperature variation.
The photoluminescence spectrum of Tm doped crystal at
300 K was shown in Fig. 4, where temperature variation was
not measured yet. A proposed assignment is shown in Fig. 4,
but for the explanation of 1.5 eV lines (tentatively assigned as
3
H4–3H6), there still exist alternative ones, so that much
experiment is required for the accurate assignment.
3.2. Absorption spectra
Optical absorption was measured for (Ce, Pr, Eu, Tb, Er,Tm) doped crystals. Their spectra were similar to each other,and therefore only a Tm case is shown in Fig. 5. Each RE ion
gave an additional absorption band just below the fundamental
optical band edge of the host crystal.

Acknowledgements
This work is partly supported by the grant for the hightechnology research center project for private universities fromministry of education, sports, science, culture and technology,
Japan.
References[1] T.E. Pters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230–236.
[2] P. Dorenbos, J. Lumin. 104 (2003) 239–260.
[3] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 575–594.
[4] E. Nakazawa, F. Shiga, Jpn. J. Appl. Phys. 42 (2003) 1642.
[5] A. Kato, K. Nadjafov, R. Hayashi, T. Matsumoto-Aoki, M. Yamazaki,
B. Tagiev, R. Dzhabbarov, A. Bayamov, N. Mamedov, S. Iida, J. Appl.
Phys. 39 (Suppl. 39-1) (2000) 440–441.
[6] C. Hidaka, E. Yamagishi, T. Takizawa, Preparation of Ca (1Kx)
EuxGa2S4 crystals and their photoluminescence, absorption andexcitation spectra, J. Phys. Chem. Solids (2005).
[7] A.N. Georgobiani, B.G. Tagiev, O.B. Tagiev, B.M. Izzatov,
R.B. Jabbarov, Cryst. Res. Technol. 31 (1996) 849–852.
[8] C. Hidaka, T. Takizawa, J. Cryst. Growth 237–239 (2002) 2009–2013.
[9] G.H. Dieke, Spectra and Energy Leveis of Rare Earth Ions in Crystals,
Interscience, New York, 1968.
[10] A. Bessière, P. Dorenbos, C.W.E. van Eijk, E. Yamagishi, C. Hidaka,
T. Takizawa, J. Electrochem. Soc. 151 (2004) H254–H260.
[11] A. Garcia, C. Fouassier, P. Doigier, J. Electrochem. Soc. 129 (1982)
2063–2069.

Journal of Physics and Chemistry of Solids 66 (2005) 2058–2060
www.elsevier.com/locate/jpcs

Preparation of Ca(1Kx)EuxGa2S4 crystals and their photoluminescence,
absorption and excitation spectra
Chiharu Hidaka *, Eri Yamagishi, Takeo Takizawa
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
The single crystal of CaGa2S4:Eu is expected as a useful laser material with a high quantum efficiency of light emission. However, as far as ourknowledge is concerned, the systematic study of the mixed compounds of Ca(1Kx)EuxGa2S4 as a function of x has not been reported up to now.
Here, we have first constructed the phase diagram of the CaGa2S4 and EuGa2S4 pseudo binary system, and show that it forms the solid solution.
Then we have grown single crystals of these compounds. The maximum photoluminescence efficiency is achieved at xZ0.25. From the three peakenergies observed in the photoluminescence excitation (PLE) and absorption spectra, the 5d excited states are suggested to consist of three levels
arising from the multiplets of Eu2C ions.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials; B. Crystal growth; D. Luminescence; D. Optical properties

1. Introduction
Rare earth element doped CaGa2S4 compounds are regarded
as host materials for fluorescent devices [1]. Especially, Ce3C
or Eu2C doped compounds exhibit strong emission caused by
the 5d-4f electronic transitions [2,3]. In the case of Ce3C
doping, the photoluminescence (PL) intensity is limited below
a certain value because the Ce3C concentration could not be
increased more than 1 mol% in the melt grown single crystals[4]. On the other hand, in the case of Eu doping, Ca atoms in
CaGa2S4 can be substituted by Eu atoms in any amount andeven 100% substitution by Eu is possible. Since the emission
intensity is expected to increase with the Eu content, EuGa2S4
can be a profitable laser material with a high quantum
efficiency of light emission [3,5]. However, too highconcentration of Eu may cause the so-called concentrationquenching to the photo-emission, so that an optimum Eucontent may be expected in this material. In addition, it has
been known difficult to prepare a EuGa2S4 single crystal by themelt growth using a quartz ampoule, because of the highmelting point of 1215 8C [6]. On the other hand, that of
CaGa2S4 is low enough to grow it in a sealed quartz ampoule.
Thus we have tried to prepare Ca(1Kx)EuxGa2S4 compounds atvarious x values between 0 and 1 expecting to find an* Corresponding author. Fax: C81 3 5317 9772.

0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2005.09.048

appropriate one for the laser action [7], and at the same time tofind the upper limit value of x for the single crystal growthusing quartz ampoules.
In this report, first we have constructed the phase diagram ofthe pseudo-binary system of CaGa2S4 and EuGa2S4 to obtainthe necessary information on the crystal growth of thecompounds above. Then using the powder compounds
prepared in the process above, photo-luminescence (PL) andphoto-luminescence excitation (PLE) are measured to investigate the relation between the efficiency of PL and the Eu
concentration. Further, based on the phase diagram constructed, single crystals of Ca(1Kx)EuxGa2S4 compounds aregrown for xZ0.01, 0.02, 0.10 and 0.20. Optical absorption, PL
and PLE spectra of these crystals are discussed together withthe powder data.
2. Phase diagram and crystal growthFirst, the pseudo-binary phase diagram for Ca(1Kx)EuxGa2S4 was constructed in the composition range from xZ0 to 1
using differential thermal analysis (DTA) and powder X-ray
diffraction (XRD). For DTA measurements, the elements of Ca
and S as well as the compounds of EuS and Ga2S3 were
weighed according to the designed prescription to approximately 0.3 g in total and sealed in a quartz ampoule of
7 mmf!40 mm in vacuum. The inner wall of an ampoule wascoated with carbon by firing acetone to avoid the fracture of an
ampoule due to the chemical reaction between mixed elements
and quartz. DTA measurements were carried out twice; the firstC. Hidaka et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2058–2060
DTA sample
crystalHeating
Cooling

1240

Liq.

1200
1180
1160
1140

Liq. + S.S

25mol%

20mol%

Intensity [arb.units]

Temperature [˚C]

1220

2059

50mol%

2mol%
1mol%

60mol%
100mol%

1120

S.S

1100
0

20

40

60

80

100

1.8

Eu concentration [mol%]

2.0 2.2 2.4 2.6
Photon energy [eV]Fig. 1. Pseudo-binary phase diagram of the CaGa2S4–EuGa2S4. Solid solution(S.S) was found in the whole composition of Eu.

Fig. 2. Photoluminescence spectra of Ca1KxEuxGa2S4 compounds. The
maximum efficiency is observed at 25 Eu mol.%.

was to investigate the chemical reaction process and the second
was to determine the phase transition points in the phase
diagram. Using the products produced during DTA measurements, the lattice parameters of the compounds were
determined by XRD, where a and b axes were shown tocontinuously increase and the c axis to decrease against thecomposition x. The resultant pseudo-binary phase diagram of
the CaGa2S4–EuGa2S4 system is shown in Fig. 1. The solid
solution was found in the whole composition range. Consequently, Ca(1Kx)EuxGa2S4 compounds are shown to beprepared at any value of x, i.e. the Ca ions can be continuously
substituted by the Eu ions in the Ca thiogallate.
Single crystals of Ca(1Kx)EuxGa2S4 were grown at four x
values of 0.01, 0.02, 0.10 and 0.20, using the same elements as
used in DTA, weighed to approximately 5 g in total. A mixtureof Ca, EuS and Ga2S3 was put on a quartz boat coated withcarbon film. Here, the boat and the deficient amount of sulfur
were separately set near the both ends of a long quartz ampoule(13 mm f!280 mm). For sulfurizing the Ca element, the Capart was set to 450 oC while keeping the S part at 300 oC for
24 h, where S was completely absorbed in the Ca part of theboat above, resulting in the CaS compound. Then the mixture
was heated just above the melting point. After rapidly coolingit down, the resultant compound was taken out and ground. It
was again put in a carbon crucible and sealed in a quartz
ampoule in vacuum. The detailed growth procedure is
described elsewhere [4].
As can be seen in Fig. 1, there observed a region, where theliquid and solid phases co-existed. Thus on naturally cooling
down the composition deviation might be expected in a grown
crystal. However, the region was rather narrow with respect tothe composition x, so that the deviation might not be large.
Actually, the composition of grown crystals was checked by an
X-ray Guide Tube (EDX) to find that a desired-composition
part (xZ0.21–0.22) could be cut out from an ingot and wasserved for optical experiments.

the multi-channel detector (HAMAMATSU PMA-11) in the
wavelength from 200 to 950 nm with resolution less than 2 nm.
A cut filter was used for removing the scattered laser beam.
Temperature variation was measured from 10 to 300 K. To
eliminate the difference in size and shape of samples,polycrystals used for PL were powdered and filled into quartz
capillaries with a same size of 3 mm f!20 mm.
Excitation spectra were observed by a monochromator(Nikon G250) with a photomultiplier (MAMAMATSU R562)
at 300 K at a PL peak of 2.2 eV using a 300 W Xenon lampwith a spectrometer (BUNKOKEIKI SM-25) as a light source.
The grating in the latter spectrometer was blazed at 300 nm.
The measurements were carried out using single crystals withcompositions of xZ0.01, 0.02, 0.10 and 0.20 having a size of
6 mm f!3 mm.
Optical absorption was measured using a monochromatorwith a photomultiplier (HAMAMATSU R562) in the
10–300 K temperature range. The sample thickness wasapproximately 10 nm.

4. Photoluminescence, excitation and absorption spectra
Fig. 2 shows PL spectra of the compounds with various Eu
mol.% at 300 K. The spectra were broad and similar to thosepreviously observed [2]. Integrated PL intensities largely
depended on Eu concentration and show a maximum near xZ
0.25 (see Fig. 3). The peak energy stays at a constant value

Intensity [arb.units]

1.2

DTA sample
crystal

1.0
0.8
0.6
0.4
0.2
0.0

3. Experimental procedure for optical measurements
PL measurements were carried out using a He–Cd laser(325 nm, 23 mW) as a light source for excitation together with0

20 40 60 80 100
Eu concentration [mol%]Fig. 3. Integrated intensity of photoluminescence at various concentrations of
Eu. The intensity has the maximum at 25 Eu mol.%.

2060

C. Hidaka et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2058–2060

1.4
Intensity [arb.units]

1.2

PLE
PL
Absorption

1.0
0.8
0.6

Fig. 5 shows excitation and absorption spectra of Ca0.90Eu0.10Ga2S4 (10 Eu mol.%). The difference from that of the
1.0 Eu mol.% sample is that the steep decrease seen in the latter
at 4.0 eV turned to a gradual one. As seen in Fig. 5, the opticalabsorption became stronger at the lower energy side, which
was due to the absorption band around 2.5 eV presumably
enhanced by the increase of the replaced Eu ions.

0.4
0.2

5. Conclusion

0.0
2.0
3.0
4.0
Photon energy [eV]5.0

Fig. 4. Excitation and absorption spectra of Ca0.99Eu0.01Ga2S4 together with aphotoluminescence spectrum.

1.4

Intensity [arb.units]

1.2

PLE
PL
Absorption

1.0
0.8
0.6
0.4
0.2
0.0
2.0
3.0
4.0
Photon energy [eV]5.0

Fig. 5. Excitation and absorption spectra of Ca0.90Eu0.10Ga2S4 together with aphotoluminescence spectrum.

between xZ0 and 0.25, but shifted to the high energy side at
the composition larger than xZ0.25. The energy shift
amounted to 50 meV at xZ1.0 (see Fig. 2). The reduction of
PL intensities at higher concentration above 25 mol% Eu may
be due to the concentration quenching. When all Ca issubstituted by Eu, the distance between Eu atoms is estimatedas approximately 5 Å from the X-ray data. As the distance
between Eu atoms at xZ0.25 is estimated as 7 Å, assuming that
the Eu atoms are homogeneously distributed, the interaction
between the substituted Eu ions may take places at thatconcentration, resulting in the peak shift and concentrationquenching as well.
Fig. 4 shows excitation and absorption spectra of Ca0.99Eu0.01Ga2S4 (1.0 Eu mol.%) together with photoluminescence(the peaks of PL and PLE are normalized to 1.0). The
excitation spectrum has a maximum near the absorption edge at
2.5 eV and two peaks at 3.1 and 3.7 eV which correspond to
those observed in the absorption spectrum, suggesting that theexcited states consist of three levels being ascribed to thesplitting of the 5d multiplets of a Eu ion. The PLE intensity
decreases greatly above 4.0 eV, corresponding to the increasein the optical absorption. This is attributed to the strong lightabsorption just above the fundamental optical band edge of thehost crystal.

We have constructed the pseudo-binary phase diagram of
the CaGa2S4–EuGa2S4 system by DTA and XRD analysis andshown that the system forms the complete solid solution,suggesting that the Ca1KxEuxGa2S4 compounds can be grownat any value of x from 0 to 1. During the investigation above,
more than 10 compounds with different values of x weresynthesized in a same crystallographic phase. Using powdersof these products, PL and PLE spectra were observed. And it
was found that the most efficient compound for PL existed near
xZ0.25.
We have then grown Ca1KxEuxGa2S4 single crystals at fourx values (xZ0.01, 0.02, 0.10 and 0.20), and their PL, PLE andoptical absorption were observed. As x increased, a newabsorption band appeared in the lower energy side below the
intrinsic absorption edge. For small x values, the absorption isdue to the deep band created by Eu doping. However, for larger
value than xZ0.1, it seems that a band-structure related
absorption takes places, which should be confirmed by further
experiments and/or by the theoretical band calculation [8]. PLE
and absorption spectra have three peaks at the same energies of
2.5, 3.1 and 3.7 eV, suggesting that the excited states consist of
three levels ascribed to the splitting of the 5d multiplets of Eu
ions.
Acknowledgements
This work is partly supported by the grant for the hightechnology research center project for private universities fromministry of education, sports, science, culture and technology,
Japan.
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