PHYSICAL REVIEW B 80, 174409 共2009兲

Quantum phase transition in (CuCl)La(Nb1−xTax)2O7
A. Kitada,1 Y. Tsujimoto,1 H. Kageyama,1,* Y. Ajiro,1 M. Nishi,2 Y. Narumi,2 K. Kindo,2 M. Ichihara,2 Y. Ueda,2
Y. J. Uemura,3 and K. Yoshimura1
1Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3Department of Physics, Columbia University, New York, New York 10027, USA
共Received 16 February 2009; revised manuscript received 28 July 2009; published 13 November 2009兲

We demonstrate the synthesis and magnetic properties of a quasi-two-dimensional frustrated quantum spin
system 共CuCl兲La共Nb1−xTax兲2O7. We observed persistence of the spin-singlet state in 共CuCl兲LaNb2O7 up tox ⬃ 0.4, accompanied by a slight reduction in the spin gap with increasing x. In spite of unaltered cell parameters and a preserved CuCl plane, 共CuCl兲LaTa2O7 exhibits collinear antiferromagnetic 共CAF兲 order with TN⬃ 7 K as observed in 共CuBr兲LaNb2O7. In the intermediate region 共0.4⬍ x ⬍ 1兲, we observed CAF order witha significantly reduced magnetic moment but with a nearly constant TN, suggesting that the CAF state coexists
with the spin-singlet state in agreement with recent ␮SR results.
DOI: 10.1103/PhysRevB.80.174409

PACS number共s兲: 75.30.Et, 75.30.Kz, 75.45.⫹j, 75.50.Ee

I. INTRODUCTION

Phenomena driven by quantum fluctuations in the vicinityof a quantum critical point in two-dimensional 共2D兲 systems
are one of the most important topics in modern physics. Motivated by the discovery of the 2D S = 1 / 2 spin correlation in
high-Tc superconducting cuprates, 2D quantum spin insulators based on the square lattice and its analogs have been
searched for and extensively investigated. The examples include the J1-J2 lattice 共where J1 and J2 denote the nearest
andnext-nearest
exchange
constants兲suchas
Li2VO共Si, Ge兲O4,1 the checkerboard lattice A2F2Fe2OQ2
共A = Sr and Ba; Q = S and Se兲,2 the 1/5 depleted square lattice
CaV4O9,3 and the Shastry-Sutherland lattice SrCu2共BO3兲2.4
Although those studies have revealed intriguing properties
such as a spin-disordered state and quantized magnetization
plateaus to name only a few,3–5 there are still many open
issues left unsolved and therefore further search for compounds and systematic studies are required for global understanding of quantum fluctuations in 2D systems.
共CuCl兲LaNb2O7 is an S = 1 / 2 square-lattice-based antiferromagnet, where the magnetic CuCl layers are sandwichedby nonmagnetic LaNb2O7 blocks 共Fig. 1兲. This material has aspin-singlet ground state and an excitation gap of ⌬ZF= 2.3 meV. The triplet excitations are nearly Q independent
despite the apparent 2D crystal structure.6 Furthermore,
field-induced magnetic order occurs at a remarkably smaller
magnetic field of Hc1 ⬃ 10 T than that expected from ⌬ZF.7–9
The loss of the tetragonal symmetry and thus the deviationfrom the idealized J1-J2 model is suggested from thenuclear magnetic resonance 共NMR兲 studies.9 In contrast,
共CuBr兲LaNb2O7 exhibits collinear antiferromagnetic 共CAF兲
order at a Néel temperature of TN = 32 K with a propagation
vector q = 共␲ , 0 , ␲兲.10 Common to the two compounds,
there exist competing antiferromagnetic and ferromagnetic
interactions. Recent studies on a solid solution
共CuCl1−yBry兲LaNb2O7 have revealed magnetic order by
5%-Br substitution.11,12 However, it is noteworthy that theBr-for-Cl substitution with different ionic radii 关1.82 Å forBr− and 1.67 Å for Cl− 共Ref. 13兲兴 is subject to direct and1098-0121/2009/80共17兲/174409共5兲

considerable disorder of the CuCl layer itself. Not only theratio of superexchange constants but also chemical disordermight play a significant role in driving the phase transition.
In this paper we investigated the magnetic properties of
共CuCl兲La共Nb1−xTax兲2O7 by means of susceptibility, pulsed
high-field magnetization, and elastic/inelastic neutron
scattering measurements. A crucial advantage of the
Ta-for-Nb substitution over the Br-for-Cl substitution is thatthe magnetic CuCl plane is preserved and that pentavalent
Nb and Ta ions have almost the same radius 共0.64 Å兲.13
The present study has been performed in parallel withthe muon spin relaxation 共␮SR兲 measurements of theand
共CuCl兲La共Nb1−xTax兲2O7
共CuCl1−yBry兲LaNb2O7
systems12 by a research team involving some of the authorsof the present work.
II. EXPERIMENTAL PROCEDURE

The precursor phases RbLa共Nb1−xTax兲2O7 共x = 0, 0.2, 0.3,
0.4, 0.6, 0.8, and 1.0兲 were prepared via a conventional hightemperature route, using stoichiometric amounts of La2O3
共99.99% purity兲, Nb2O5 共99.99%兲, Ta2O5 共99.99%兲, and
25% molar excess of Rb2CO3 共99.9%兲. RbLa共Nb1−xTax兲2O7
was then mixed with a twofold molar excess of ultradry
CuCl2 共99.999%兲 and pressed into pellets in an Ar-filled
glove box 共⬍1 ppm O2 / H2O兲. The ion-exchange reactions
expressed as
RbLa共Nb1−xTax兲2O7 + CuCl2→ 共CuCl兲La共Nb1−xTax兲2O7 + RbCl

共1兲

were carried out in a sealed, evacuated 共⬍10 Torr兲 Pyrex
tube at 320 ° C for seven days.14 The final products were
washed with distilled water to eliminate RbCl and excess
CuCl2, and dried at 120 ° C.
Room temperature x-ray diffraction 共XRD兲 profiles of
共CuCl兲LaNb2O7 and 共CuCl兲LaTa2O7 were indexed into a tetragonal cell with nearly the same lattice constants 共a= 3.879 Å, c = 11.754 Å for Nb, and a = 3.879 Å, c
= 11.748 Å for Ta兲, consistent with those previously

174409-1−3©2009 The American Physical Society

PHYSICAL REVIEW B 80, 174409 共2009兲

KITADA et al.
16(CuCl)La(Nb1-xTax)2O7

14

10
8

-3

χ (10 emu/mol)

12x = 1.0
0.8
0.6
0.4
0.2
0

6
4
2
0
0

10

20

30

40

50

T (K)

III. RESULTS AND DISCUSSIONS

Shown in Fig. 2 are the magnetic susceptibilities ␹ for all
the samples. ␹ of 共CuCl兲LaTa2O7 共x = 1兲 above 50 K wasoffitted to the Curie-Weiss law, ␹ = C / 共T − ␪兲, where C and ␪
represent the Curie constant and the Weiss temperature. The
value of C of 0.410 emu K/共mol Cu兲 suggests the completionof the designed ion-exchange reaction 关Eq. 共1兲兴. The value of
␪ of −1.2 K is about 1/8 of what was obtained for x = 0
共−9.6 K兲.6 This does not simply mean much reduced magnetic interactions because the susceptibility has a broad␹= 11.5 K. This is a characteristic feature of
maximum at Tmax
low-dimensional magnetic materials, indicating that a dominant antiferromagnetic interaction is of same order of the␹does not differ so
magnitude as in x = 0. The observed Tmax
much from that of x = 0 共16.5 K兲. However, unlike ␹ for x= 0 having a sharp drop due to spin-singlet formation, ␹ for␹
, followed
x = 1.0 exhibits only a slight decrease below Tmax
by a Curie tail, presumably due to paramagnetic impurities
and defects. Thus a magnetic ground state is expected for x= 1.0.
0.5

1

0

M/Msatreported.14,15 No trace of impurity phases were found. The
XRD patterns of the 0 ⬍ x ⬍ 1 samples at room temperaturedemonstrated successful synthesis of the solid solution having nearly the same tetragonal cell parameters as those of
共CuCl兲LaNb2O7 and 共CuCl兲LaTa2O7. The quality of the
XRD patterns is as good as that of the end members. Recent
NMR and transmission electron microscopy 共TEM兲 experiments showed that the space group of 共CuCl兲LaNb2O7 is not
P4 / mmm; the C4 symmetry is lost both at the Cu and Cl sites
and the original unit cell is doubled along the a and b axes.
Similar features might be also present in 共CuCl兲LaTa2O7 andthe solid solution.
To check the chemical homogeneity in the solid solution
system, energy-dispersive spectroscopy 共EDS兲 was carriedout at ambient temperature using a JEM2010F system with
an operating voltage of 200 kV at the Institute for Solid State
Physics 共ISSP兲 at the University of Tokyo. The specimen
was finely ground in methanol and then placed on a Cu microgrid mesh for TEM observations. As a typical example,
we show the TEM image of the x = 0.8 sample in Fig. 1共b兲and the corresponding EDS spectrum in Figs. 1共c兲 and 1共d兲,
which demonstrates a uniform distribution of the Nb and Ta
atoms.
Magnetic susceptibilities were measured using the Quantum Design MPMS 共Magnetic Property Measurement System兲 over the temperature range T = 2 – 300 K in a magnetic
field H of 2 T. High-field magnetization measurements up to
57 T were conducted using a pulsed magnet installed at ISSP.
Elastic and inelastic neutron scattering experiments were
performed using the ISSP-PONTA triple-axis spectrometer
共5G兲, installed at the JRR-3 reactor at the Japan Atomic Energy Agency 共JAEA兲, Tokai. Powder samples 共x = 0.3, 0.6,
0.8, and 1.0兲 of about 20 g each were put into aluminum
cylinders. Neutrons with a wavelength of 2.358 Å were obtained from the 002 reflection of pyrolytic graphite 共PG兲, anda horizontal collimation of open-40⬘-sample-80⬘-80⬘ in combination with a PG filter was placed before the sample toeliminate higher-order beam contamination.

FIG. 2. 共Color online兲 Magnetic susceptibilities
共CuCl兲La共Nb1−xTax兲2O7 measured at H = 2 T.

M/MsatFIG. 1. 共Color online兲 共a兲 Schematic view of the crystal structure of 共CuCl兲LaB2O7 共B = Nb, Ta兲. 共b兲 TEM image and 关共c兲 and共d兲兴 the corresponding EDS maps of the x = 0.8 sample, where blue
and red dots represent Nb and Ta, respectively.

0.0

0

0

0.2
0.4

0

5
10
H (T)

10

(CuCl)La(Nb1–xTax)2O7

x = 1.0
0.8
0.6
0.4
0.2
0
fit (x = 0.6)
fit (x = 0.8)
(CuBr)LaNb2O7

15

20

30

40

H (T)

FIG. 3. 共Color online兲 Magnetization curves M of
共CuCl兲La共Nb1−xTax兲2O7 at T = 1.3 K. The normalized magnetization for 共CuBr兲LaNb2O7 共broken line兲 was obtained from the data
in Ref. 10 as its saturation field equals to that of x = 1.0 共⬃23 T兲.
Circles and triangles represent fitted curves to the x = 0.6 and 0.8
data, respectively 共see text for details兲. Inset is an enlarged plot for
x = 0, 0.2 and 0.4, highlighting the critical fields Hc, which wereassigned to the fields of the inflection points in dM / dH. The very
slow increases in magnetization below Hc are due to tiny amount ofdefect and/or unknown magnetic impurity as mentioned in Refs. 7
and 9.

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Intensity(103 counts /~6 min)

(a)

1.7 K
17 K

x = 1.0(d)x = 1.0

22
21

17

(b)

(e)

x = 0.8

x = 0.8

16(c)Backgroundx = 0.6

x = 0.6

(f)

8

4

20

17

18

0

19

0

2

4

6

8

Difference(10 2 cnt. /~12 min)21

Intensity(103 counts /~6 min)Intensity
3(10 counts /~6 min)23

PHYSICAL REVIEW B 80, 174409 共2009兲
Intensity(103 counts /~6 min)Intensity(103 counts /~6 min)QUANTUM PHASE TRANSITION IN 共CuCl兲La共Nb…

10

T (K)

2

FIG. 4. 关共a兲–共c兲兴 Neutron diffraction profiles of x = 1.0, 0.8, and11
0.6 around the 共 2 , 0, 2 兲 magnetic reflection. Open and closed circles
represent the data collected at 1.7 and 17 K, respectively, and solidlines are a guide to the eyes. A broken line in 共c兲 represents the
background. 关共d兲–共e兲兴 Temperature dependence of the intensity of11
the 共 2 , 0, 2 兲 reflection. The lines are a guide to the eyes. 共f兲 Tem1
perature dependence of the difference between intensity of the 共 2 , 0,
1
2 兲 reflection and the background. The lines are a guide to the eyes.

This speculation is supported by the magnetization measurements at 1.3 K 共see Fig. 3兲. While the magnetization
curve for x = 0 remains zero until the field-induced magnetic
order occurs at Hc, apart from a tiny magnetization comingfrom a small amount of impurities/defects,7,9 the magnetization curve for x = 1.0 has a sizable slope even in low fields.

(a)

-1

0.3(Q = 1.0 Å )

(b)

x =0
0.3

400

200

Difference (counts)

600

0

0

0.0

800

1500
-1

x = 0 (Q = 0.8 Å )I(1.7 K) - I(20 K) (counts /~10 min)I(3.3 K) - I(20 K) (counts /~12 min)

500

In order to probe probable magnetic order, we performed
the neutron powder diffraction measurements at zero magnetic field as shown in Fig. 4共a兲. We found a peak at around18° corresponding to the 共 21 , 0, 21 兲 magnetic reflection.
Hence, it is natural to consider that 共CuCl兲LaTa2O7 exhibits
CAF order as in 共CuBr兲LaNb2O7.10 The ordered magnetic
moment was estimated to be 0.69⫾ 0.1␮B, which is comparable to that of 共CuBr兲LaNb2O7 共0.60⫾ 0.11␮B兲. However,
the T dependence of the intensity of this reflection 关Fig. 4共d兲兴
revealed that the transition temperature 7 K is much smaller
than that for 共CuBr兲LaNb2O7 共32 K兲. The magnetization
curve for x = 1.0 becomes very nonlinear compared with thenormalized one for 共CuBr兲LaNb2O7 共Fig. 3兲, suggesting thatthe effect of quantum fluctuations should be considerablystronger in the former material.
We wish to recall here that the cell parameters of
共CuCl兲LaTa2O7 and 共CuCl兲LaNb2O7 are almost the same.
Therefore, if the LaB2O7 perovksite slabs acted simply as
spacers that spatially and magnetically isolated the CuCl layers, the magnetic properties of the two compounds would be
identical. The present result indicates that superexchange interactions through Cu-BO6-BO6-Cu 共B = Nb, Ta兲 play animportant role as well as those through Cu-X-Cu in the magnetic properties. This is compatible with the observations
that the unpaired electron of Cu2+ ion occupies the d共3z2− r2兲 orbital pointing parallel to the c axis9 and that the stability of the 1/3 magnetization plateau in 共CuBr兲A2B3O10
共A = Ca, Sr, Ba, and Pb兲 is tuned by substituting different
atoms in the B site.16 The cases in which d共3z2 − r2兲 orbitalsalign perpendicular to the magnetic layers is also found in
the S = 1 / 2 honeycomb antiferromagnet InCu2/3V1/3O3,
where superexchange interactions via InO6 are expected toinfluence its magnetic properties.17
Once we confirmed that our two isostructural compounds
had different ground states 共i.e., spin-singlet and CAF states兲,we were able to study the magnetic phase diagram in

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

E (meV)

0.5

1.0

1.5

2.0

2.5

0
3.0

-1

Q (Å )

FIG. 5. Results of inelastic neutron scattering measurements for x = 0 共closed circles兲 共Ref. 6兲 and 0.3 共open circles兲. 共a兲 The difference
I共1.7 K兲 − I共20 K兲 for x = 0 and I共3.3 K兲 − I共20 K兲 for x = 0.3. The curve represents a least-squares fit to a Gaussian. 共b兲 Peak intensities of
the transition from the singlet ground state to the one-triplet excited state as a function of Q obtained at E = 2.1 meV for x = 0 and at E
= 2.0 meV for x = 0.3. The solid lines represent the fits according to the equation written in Ref. 6.
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PHYSICAL REVIEW B 80, 174409 共2009兲

KITADA et al.
1.0

(a)

0.8
0.6

S

CAF

0.4

NPD
HFM
µSR

0.2
0.0

30

P

25
20
15

TN (K)∆/kB (K)

INS
NPD
µ SR

(b)

S

10
5
0

0.0

S+CAF

0.2

0.4

x

0.6

CAF

0.8

1.0

FIG. 6. 共Color online兲 共a兲 Antiferromagnetic volume fraction ␣
obtained using neutron powder diffraction 共NPD兲 共closed circles兲,high-field magnetization 共HFM兲 共closed triangles兲, and ␮SR 共Ref.
12兲 共open circles兲. S stands for the spin-singlet state. The solid line
is a guide to the eyes. 共b兲 Magnetic phase diagram of
共CuCl兲La共Nb1−xTax兲2O7 as a function of temperature and concentration. P stands for the paramagnetic state. Closed squares represent the zero-field gap obtained by inelastic neutron scattering
共INS兲. S stands for the spin-singlet state. Closed and open circles
denote the transition temperatures obtained by NPD and ␮SR
共Ref. 12兲.

共CuCl兲La共Nb1−xTax兲2O7. Systematic evolution of the magnetic susceptibilities and the magnetization curves is shown
as a function of x in Figs. 2 and 3, respectively. For example,
the samples with higher x 共from 0, 0.2, 0.4, 0.6, 0.8 to 1.0兲␹
共16.5, 15.4, 14.2, 13.2, 12.5, and 11.5 K,
have lower Tmax
respectively兲. The value of ␪, derived from Curie-Weiss fitting to the susceptibility data, also gradually decreases with x
共␪ = −9.6, −5.6, −3.8, −3.1, −2.5, and −1.2 K兲.
The M-H curves for x = 0.2 and 0.4 trace that of x = 0 in
the low-field region, suggesting the persistence of a spinsinglet ground state. Persistent nature of the spin-singlet
ground state up to 40%-Ta substitution is in marked contrast
to the Cl-Br solid solution where the spin-singlet to CAF
ground state transition was induced by 5%-Br substitution at
most.11,12 When the magnetic field is further increased, we
found an anomaly at Hc = 10.3, 9.1, and 8.0 T for x = 0.0, 0.2,
and 0.4, respectively, suggestive of a field-induced phasetransition. Hc decreases with x. Above Hc, the magnetization
increases in proportion with H and arrives at the saturation
magnetization. For x ⱖ 0.4, magnetization curves have finite
slope even from low-field region, as in the case of x = 1.0.
The energy scan at Q = 1.0 Å−1 for x = 0.3 关Fig. 5共a兲兴shows a singlet-triplet excitation, the peak of which is centered at ⌬ZF = 2.0 meV. This gap energy is much bigger thanthat expected from the high-field magnetization measure-ments; though we have not performed magnetization measurements for x = 0.3, Hc should be between 8.0 T 共x = 0.4兲and 9.1 T 共x = 0.2兲 corresponding, to 0.9–1.0 meV assumingg = 2.0. No reason has yet been given for the source of thediscrepancy of the gap energy derived from neutron andmagnetization measurements but the fact that there is a systematic decrease in ⌬ZF and Hc with x strongly suggests that
it is an intrinsic property. The triplet mode is nearly Q independent, indicating the localized nature of the triplet excitations as observed in x = 0.6 As shown in Fig. 5共b兲, the Q scan
result at E = 2.0 meV for x = 0.3 exhibits rapidly oscillating
behavior similar to x = 0. The fit to the isolated dimer model6
yielded an unreasonably long intradimer distance of R
= 9.5 Å, implying more complex and competing magnetic
interactions. Neutron powder diffraction measurements for
x = 0.6 and x = 0.8 关Figs. 4共b兲 and 4共c兲兴 show a magnetic reflection corresponding to 共 21 , 0, 21 兲 indexed with respect to thechemical unit cell. Hence they should have the same spin
structure as that of x = 1.0. We also found that the orderedtemperature hardly changes 关Figs. 4共e兲 and 4共f兲兴 while theordered moment sizes decrease with decreasingx : 0.50⫾ 0.1␮B 共x = 0.8兲 and 0.27⫾ 0.1␮B 共x = 0.6兲.
Uemura et al.12 recently performed ␮SR measurements of
共CuCl兲La共Nb1−xTax兲2O7 and proposed magnetic phase separation between the static magnetic state and spin-singlet statein the range of 0.4⬍ x ⬍ 1.0. This scenario requires that themagnetization curves for x = 0.6 and 0.8 are expressed by two
terms both in partial volume fractions: the spin-singlet phase
共exemplified by the magnetization curve for x = 0.4兲 and the
CAF phase 共x = 1.0兲,
M共x兲 = ␣ M共1.0兲 + 共1 − ␣兲M共0.4兲.

共2兲

In Eq. 共2兲 ␣ represents the volume fraction of the CAF phaseand 共1 − ␣兲 represents that of the spin-singlet one. As shownin Fig. 3, the fitting curves reproduce the experimental dataquite well. The obtained value of ␣ is 0.35 for x = 0.6 and0.78 for x = 0.8 which is consistent with the ␮SR results
共0.37 for x = 0.6 and 0.80 for x = 0.8兲.12 The decrease in intensity of the magnetic Bragg peaks can be also interpreted
as a gradual decrease in the CAF partial volumes. Assuming
a constant ordered moment in the CAF phase, we estimate
the volume fraction ␣ = 0.39⫾ 0.14 for x = 0.6 and0.72⫾ 0.15 for x = 0.8, again consistent with the ␮SR
results12 and with the magnetization results presented above.
IV. CONCLUSIONS

We have demonstrated that a quantum phase transition
from spin-singlet state to antiferromagnetic order occurs in
共CuCl兲La共Nb1−xTax兲2O7, summarized in the magnetic phase
diagram in Fig. 6共b兲. It is found that 共CuCl兲LaTa2O7 exhibits, despite the closeness of the lattice parameters in the solid
solution, CAF order at TN ⬃ 7 K. This clearly shows that thesubstitution of Ta5+ for Nb5+ in nonmagnetic slabs can affect
the ground state of this quasi-2D magnet. The spin-singlet
ground state in 共CuCl兲LaNb2O7 is fairly robust against Ta
substitution 共0 ⱕ x ⱕ 0.4兲, accompanied by a slight reductionin the spin gap, which is in marked contrast to the drastic
collapse of the spin-singlet state in 共CuCl0.95Br0.05兲LaNb2O7.

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QUANTUM PHASE TRANSITION IN 共CuCl兲La共Nb…

PHYSICAL REVIEW B 80, 174409 共2009兲

In the intermediate region 共0.4⬍ x ⬍ 1.0兲, we observed CAF
order but with a nearly constant TN, likely coexisting withthe spin-singlet state with systematic variation in the volume
fraction 关Fig. 6共a兲兴, in agreement with the recent ␮SR results. This is in stark contrast to the case of
共CuCl1−yBry兲LaNb2O7, where TN increases gradually from 7
K 共y = 0.05兲 to 32 K 共y = 1.0兲.
ACKNOWLEDGMENTS

It is our pleasure to acknowledge fruitful discussions with
K. Totsuka, D. I. Khomskii, J. P. Carlo, and A. A. Aczel. The*Author to whom correspondence should be addressed;
kage@kuchem.kyoto-u.ac.jp
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N. Oba, H. Kageyama, T. Kitano, J. Yasuda, Y. Baba, M. Nishi,

authors would like to thank M. Matsuura for assistance withneutron scattering measurements. This work was supported
by Grants-in-Aid for Science Research in the Priority Areas“Novel States of Matter Induced by Frustration” 共No.
19052004兲 from the Ministry of Education, Culture, Sports,
Science and Technology of Japan, by the Global COE program International Center Science, Kyoto University, Japan,
and by the Japan-U.S. Cooperative Science Program 共Contract No. 14508500001兲. This work was also supported by
Murata Science Foundation. The work at Columbia was supported by U.S. National Science Foundation grant for Materials World Network, under Grants No. DMR-05-02706 andNo. DMR-08-06846.

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