Physica C 471 (2011) 690–692

Contents lists available at ScienceDirect

Physica C
journal homepage: www.elsevier.com/locate/physc

Relationship between structural phase transition and carrier concentrationin La2 x ySrxRyCuO4 (R = Ce or Tb)
T. Naito ⇑, Y. Ueno, R. Takahashi, H. Fujishiro
Faculty of Engineering, Iwate University, Morioka 020-8551, Japana r t i c l ei n f oArticle history:
Available online 27 May 2011
Keywords:
La2 x ySrxCeyCuO4
La2 x ySrxTbyCuO4
Structural transition
Low temperature X-ray diffraction
Resistivitya b s t r a c t
We have studied the relationship between the crystal structure and the carrier concentration inLa2 x ySrxCeyCuO4 by low-temperature X-ray diffraction method. The analysis for the [1 1 0]t peak of
the tetragonal index conﬁrms that the high-temperature tetragonal phase changes to the low-temperature orthorhombic one in both La1.89Sr0.11CuO4 and La1.88Sr0.11Ce0.01CuO4. We have also examined theeffects of Tb substitution for La-site on the superconductivity and the structure in La2 x ySrxTbyCuO4.
A dip of the critical temperature Tc(x) due to the 1/8 anomaly and a maximum of Tc(x) at the optimum
carrier concentration do not depend on the Tb concentration. This result suggests the possibility that
Tb is introduced as the trivalent ion for x = 0.07–0.18.
Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction
It is widely accepted that a structural phase transition from thehigh-temperature tetragonal (HTT) phase to the low-temperatureorthorhombic (LTO) phase occurs in La2 xAExCuO4 (AE = Sr, Ba) withdecreasing temperature. This HTT–LTO transition causes a bucklingof the CuO2 planes and a tilting of the CuO6 octahedra, and then hides
an intrinsic nature relating with superconductivity such as criticaltemperature Tc [1], spin ﬂuctuation [2] and so on. The temperature
at the structural phase transition Td1 decreases with increasing x,
and ﬁnally reaches to zero at around x = 0.2 [3]. Therefore, the HTT
phase only exists in an entire temperature range for the x P 0.2 region. Dabrowski et al. reported that the relaxation of the orthorhombic distortion of the CuO2 planes with increasing x by observing theincrease of the Cu–O(1)–Cu bond angle, where O(1) is in the CuO2plane [4]. If the distortion of the CuO2 planes depends only on thex value, La2 xSrxCuO4 with the ﬂatter CuO2 planes can be obtainedby the introduction of electrons by substituting tetravalent or higher
valence ion for the La-site. In view of the so-called 1/8 problem,
La2 x yBaxCeyCuO4 and La2 x yBaxTbyCuO4 were studied. As a result,
Ce was tetravalent [5] and Tb took the average valence from 3.1(x = 0.060) to 3.6 (x = 0.26) [6].
We studied the effects of the substitution of tetravalent Ce ionon the superconductivity and the HTT–LTO transition inLa2 x ySrxCeyCuO4 by measuring the electrical resistivity and thethermal dilatation to detect the lattice anomaly [7]. Tc(x) wasshifted to the higher x region by the introduction of Ce, on theother hand, Td1(x) was not affected. This suggests that the struc⇑ Corresponding author. Tel.: +81 19 621 6362; fax: +81 19 621 6362.
E-mail address: tnaito@iwate-u.ac.jp (T. Naito).
0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2011.05.028

tural phase transition is only affected by the Sr concentration.
However, the single-phase of La2 x ySrxCeyCuO4 can be prepared
only up to y = 0.01.
In this paper, we report the HTT–LTO transition inLa2 x ySrxCeyCuO4 which is measured directly by a low-temperature X-ray diffraction method. We also report the effects of the Tbsubstitution on the superconductivity and the HTT–LTO transition.
2. Experimental
Polycrystalline samples of La2 x ySrxRyCuO4 (R = Ce, Tb;
x = 0.07–0.21, y = 0–0.05) were prepared by a standard solid-state
reaction method. Raw powders of La2O3, SrCO3, CeO2, Tb4O7 andCuO were weighted with an appropriate molar ratio and ground.
The mixture was calcined at 900 °C for 24 h in air, and subsequently the calcined compound was pelletized and ﬁred at1150 °C for 72 h in air. As-sintered samples were annealed at800 °C for 24 h in a 1 bar ﬂowing oxygen gas. La2 x ySrxCeyCuO4
for y = 0 and 0.01 and all La2 x ySrxTbyCuO4 samples were conﬁrmed to be the single-phase by a powder X-ray diffraction(XRD) pattern at room temperature. XRD at low temperatures below room temperature was performed using a hand-made cryostat
with a Gifford–McMahon cycle helium refrigerator. Electrical resistivity, q(T), was measured by a four-probe method.
3. Results and discussion
3.1. Effects of Ce substitution
Fig. 1a and b shows the x dependence of Tc and Td1 ofLa2 x ySrxCeyCuO4 for y = 0 and 0.01. The dip of Tc(x) due to the691

T. Naito et al. / Physica C 471 (2011) 690–692

measuring the thermal dilatation using a strain gage. To detect directly the HTT–LTO transition, we performed the X-ray diffraction
at low temperatures.
Fig. 2a and b shows the XRD pattern around the [1 1 0]t peak of
the tetragonal index for La1.89Sr0.11CuO4 at 300 K and 182 K,
respectively. The data were ﬁtted using a Gaussian curve. The[1 1 0]t peak at 300 K and 182 K appears identical around2h = 33.45° within an experimental error. The peak becomes broadwith decreasing temperature; the full width at half maximum(FWHM) is about 0.24° and 0.33° at 300 K and 182 K, respectively.
Since the [1 1 0]t peak of the tetragonal phase equivalent to the[2 0 0]o and [0 2 0]o peaks of the orthorhombic index, the orthorhombicity is evaluated by the separation of the [1 1 0]t peak. The
difference of the peak angle, D(2h), between the [2 0 0]o and[0 2 0]o peaks changes from 0.01° at 300 K to 0.17° at 182 K, whichclearly demonstrates that the crystal structure changes from theHTT phase at 300 K to the LTO one at 182 K. Fig. 2c and d represents the XRD pattern for La1.88Sr0.11Ce0.01CuO4 at 300 K and
220 K, respectively. The shape of XRD pattern is quite similar tothat of non Ce-substituted sample. The intensity of La1.88Sr0.11Ce0.01CuO4 is about half of that of La1.89Sr0.11CuO4. The similar analysis was adopted for both XRD patterns. D(2h) is approximately
0.015° at 300 K and 0.14° at 220 K, suggesting that the HTT–LTOtransition occurs between both temperatures. Td1 could not be exactly determined but the HTT–LTO transition in La1.89Sr0.11CuO4
and La1.88Sr0.11Ce0.01CuO4 was conﬁrmed directly by the low-temperature XRD method. Therefore, we can regard that the thermal
dilatation anomaly reported in the previous paper [7] represents
the HTT–LTO transition.

40
y=020(a)y=0.01cT (K)3010La2-x-ySrxCeyCuO40400

La2-x-ySrxCeyCuO4d1T (K)300
200100(b)y=0y=0.01
Ref.[9]

00.05

0.1

0.15

0.2x0.25

Fig. 1. (a) Sr-concentration x dependence of Tc for La2 x ySrxCeyCuO4 samples withy = 0 and 0.01. (b) Td1(x) for La2 x ySrxCeyCuO4 samples with y = 0 and 0.01. The
broken line shows the reported Td1(x) [9].

1/8 problem shifts from x = 0.115 [8] for y = 0 to x = 0.125 fory = 0.01. The x value at the maximum of Tc also shifts from 0.15(y = 0) to 0.16 (y = 0.01). The change in the x values are consistent
with the Ce concentration. On the other hand, Td1(x) is not affectedby the introduction of Ce. The Td1 values were determined by

800

400

La

Sr

1.89

CuO

0.11

La

T=300 K[110]t

CuO

0.01

T=300 K

4[110]t300[200]oIntensity (cps)Intensity (cps)

Ce

0.11

4600

400[020]o200(a)0200[200]o100[020]o(c)

0400

800

Sr

1.89

CuO

0.11

600

T=182 K

4[110]

La

Intensity (cps)

La

Intensity (cps)

Sr1.88t

400[200]o(b)200[020]
033

33.2

33.4

Sr1.88Ce0.11

CuO0.01

T=220 K

4300

[110]t200[200]o(d)100[020]oo

33.6

2θ (deg.)

33.8

34033

33.2

33.4

33.6

33.8

34

2θ (deg.)

Fig. 2. X-ray diffraction patterns at around [1 1 0]t peak of the tetragonal index at (a) 300 K and (b) 182 K for La1.89Sr0.11CuO4 and at (c) 300 K and 220 K for
La1.88Sr0.11Ce0.01CuO4.

692

T. Naito et al. / Physica C 471 (2011) 690–692

40

0.60.04

y=0.01

0.05

30

Tc (K)

ρ (mΩcm)

0.03

0.4

0.02

0.2

y=0200.01
y=010
La2-x-ySrxTbyCuO4La1.82-ySr0.18TbyCuO40020

4060

80

00.05

1000.17

0.16

0.15

0.14

0.2

0.25

Fig. 5. Sr-concentration x dependence of Tc for La2 x ySrxTbyCuO4 samples withy = 0 and 0.01.

trivalent ion in La2 x ySrxTbyCuO4 for x = 0.07–0.18 and acts as animpurity which suppresses the superconductivity.

Hole concentration x-y0.18
380.15xT(K)
Fig. 3. Temperature dependence of the electrical resistivity of La1.82-ySr0.18TbyCuO4
samples for y = 0–0.05.

0.1

0.13

0.12
4. SummaryLa2-xSrxCuO4Tc (K)363432
La1.82-ySr0.18TbyCuO4

30
28
0

0.01

0.02

0.03

0.04

0.05

0.06

Tb-concentration yFig. 4. Tb-concentration y dependence of Tc of La1.82-ySr0.18TbyCuO4. Upper
horizontal axis represents the expected hole concentration x y. The dotted line
represents Tc(x) of La2 xSrxCuO4.

3.2. Effects of Tb substitutionFig. 3 shows the temperature dependence of the resistivity ofLa1.82 ySr0.18TbyCuO4 for y = 0–0.05. q(T) decreases monotonicallywith decreasing temperature and shows a sharp zero-resistivity
transition for all samples. Fig. 4 shows the Tb-concentration ydependence of Tc of La1.82 ySr0.18TbyCuO4. Upper horizontal-axis
represents the expected hole concentration, x y. Tc(y) decreases
monotonically with increasing y. This suppression of the superconductivity suggests that Tb ion acts as an impurity. If Tb-substitution introduces the electrons, Tc(y) is expected to increase
initially up to y = 0.03 which corresponds to the optimum holeconcentration 0.15, and subsequently to decrease with increasingy, as the dotted line which is Tc(x) of La2 xSrxCuO4. Fig. 5 shows
the Sr-concentration x dependence of Tc of La2 x ySrxTbyCuO4 for
y = 0 and 0.01. Tc(x) for y = 0 is the same as that in Fig. 1. The dip
due to the 1/8 problem appears at around x = 0.115 for y = 0.01
samples, although the dip structure is not so clear. On the otherhand, the maximum of Tc appears at x = 0.15. Tb-substitution
seems not to affect the structure of Tc(x). Bao et al. reported thatthe average valence of Tb changes from 3.1 at x = 0.060 to 3.6 at
x = 0.26 in La2 x yBaxTbyCuO4 [6]. Our results suggest that Tb is

We have studied that the structural phase transition between
the high-temperature tetragonal (HTT) and the low-temperatureorthorhombic (LTO) phases by the temperature-variable X-ray diffraction method in La1.89Sr0.11CuO4 and La1.88Sr0.11Ce0.01CuO4. The
orthorhombicity was evaluated at the [1 1 0]t peak of the tetragonalindex by the peak separation using the [2 0 0]o and [0 2 0]o of theorthorhombic index. We conﬁrmed directly that the HTT phasechanges to the LTO one passing through the structural phase transition temperature reported previously for both samples. This suggests that the thermal dilatation can detect the HTT–LTOtransition.
The effects of Tb-substitution on the superconductivity and thestructural phase transition in La2 x ySrxTbyCuO4 were also studied.
The dip due to the 1/8 problem and the maximum at the optimum
carrier concentration in Tc(x) are not affected by the Tb-substitution. The monotonic decrease in Tc(x = 0.18, y) was observed. These
results suggest that Tb is not introduced as tetravalent or higher
valence ion in La2 x ySrxTbyCuO4 for x = 0.07–0.18.
Acknowledgments
We thank Ms. S. Ohata and Mr. T. Konno for their performing apart of experiments.
References[1] B. Büchner, M. Breuer, A. Freimuth, A.P. Kampf, Phys. Rev. Lett. 73 (1994) 1841.
[2] M. Enoki, M. Fujita, K. Yamada, J. Phys. Chem. Solids 68 (2007) 2111.
[3] R.M. Fleming, B. Batlogg, R.J. Cava, E.A. Rietman, Phys. Rev. B 35 (1987) 7191.
[4] B. Dabrowski, Z. Wang, K. Rogacki, J.D. Jorgensen, R.L. Hitterman, J.L. Wagner,B.A. Hunter, P.G. Radaelli, D.G. Hinks, Phys. Rev. Lett. 76 (1996) 1348.
[5] Y. Koike, T. Kawaguchi, N. Watanabe, T. Noji, Y. Saito, Solid State Commun. 79(1991) 155.
[6] X. Bao, Y. Maeno, F. Nakamura, S. Nishizaki, T. Fujita, in: K. Yamafuji, T.
Morishita (Eds.), Advances in Superconductivity VII, Springer-Verlag, Tokyo,1995, p. 241.
[7] T. Naito, S. Ohata, T. Konno, R. Takahashi, H. Fujishiro, Physica C 470 (2010)
S261.
[8] Y. Koike, A. Kobayashi, T. Kawaguchi, M. Kato, T. Noji, Y. Ono, T. Hikita, Y. Saito,
Solid State Commun. 82 (1992) 889.
[9] B. Keimer, N. Belk, R.J. Birgeneau, A. Cassanho, C.Y. Chen, M. Greven, M.A.
Kastner, A. Aharony, Y. Endoh, R.W. Erwin, G. Shirane, Phys. Rev. B 43 (1992)
14034.