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Physica B 359–361 (2005) 980–982
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Crystal structure and lattice dynamics ofSrCu2ðBO3Þ2 at high pressures
I. Loaa,, F.X. Zhanga, K. Syassena, P. Lemmensa,
W. Crichtonb, H. Kageyamac, Y. Uedad
aMax-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germanyb
European Synchrotron Radiation Facility, F-38043 Grenoble, Francec
Department of Chemistry, Kyoto University, Kyoto 606-8502, JapandInstitute for Solid State Physics, University of Tokyo, Chiba 277-8581, JapanAbstract
We investigate the effect of hydrostatic pressure on the structural and lattice dynamical properties of SrCu2 ðBO3 Þ2 byangle-dispersive synchrotron X-ray powder diffraction and Raman spectroscopy. SrCu2 ðBO3 Þ2 undergoes a subtle
tetragonal-to-monoclinic structural distortion near 5 GPa and a ﬁrst-order structural transition at 15 GPa. The effectsof the structural changes on the magnetic properties of this quasi-two-dimensional quantum antiferromagnet are brieﬂydiscussed.
r 2005 Elsevier B.V. All rights reserved.
PACS: 61.50.Ks; 62.50.+p; 78.30.Hv; 61.10.Nz
Keywords: SrCu2 ðBO3 Þ2 ; High pressure; Structural phase transition

SrCu2 ðBO3 Þ2 is a model system of a frustrated
two-dimensional quantum antiferromagnet and arealisation of the Shastry–Sutherland model [1,2].
The compound is characterised by competing
nearest and next-nearest neighbour in-plane spin
interactions together with a weak coupling between the layers. Various experiments and theoreCorresponding author. Tel.: +49 711 689 1469;

fax: +49 711 689 1010.
E-mail address: i.loa@fkf.mpg.de (I. Loa).

tical investigations have shown that orthogonallycoupled spin-dimer singlets (located on Cu dimers)
form a singlet ground state which is separated
from dispersionless triplet excitations by a spin
gap of D ¼ 34 K [2–4].
Application of high pressures is the means ofchoice to change continuously the magnetic
coupling constants via the variation of bondlengths. Recent magnetic susceptibility measurements indicate a reduction of the spin–gap underhydrostatic pressures up to 0.7 GPa [5]. On the0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.01.284

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I. Loa et al. / Physica B 359– 361 (2005) 980–982

SrCu2(BO3)2Pressures (GPa)T = 300 K

20.6

N2III

16.1

14.6

A' B'Intensity

basis of these results, a quantum phase transition
has been predicted at a critical pressure of
2.5–3.5 GPa [5].
We investigate here the effect of pressure on thecrystal structure and lattice dynamics ofSrCu2 ðBO3 Þ2 under hydrostatic pressure. The ﬁrst
purpose of this work is to explore the stability
range of the ambient-pressure phase of
SrCu2 ðBO3 Þ2 in order to ﬁnd out whether apressure-induced quantum phase transition could
possibly be intercepted by a structural phasetransition. Secondly, we provide high-pressure
structural data as a basis for a detailed analysisof the pressure-induced changes in the magnetic
properties of SrCu2 ðBO3 Þ2 :The phase stability and lattice dynamical
properties of SrCu2 ðBO3 Þ2 were studied by Raman
spectroscopy at pressures up to 21 GPa andambient temperature. The Raman spectra wereexcited at 633 nm and recorded with a single-stage
spectrometer equipped with a holographic notchﬁlter to suppress the laser line (Jobin–YvonLabram). The structural properties were studied
up to 23 GPa and ambient temperature by mono( X-ray powder diffracchromatic (l ¼ 0:3738 A)
tion at the European Synchrotron Radiation
Facility (ESRF Grenoble, beamline ID30). In
both experiments, pressures were generated indiamond anvil high-pressure cells with condensed
nitrogen as the pressure transmitting medium. The
ruby luminescence method [6] was used forpressure determination. The synthesis of theSrCu2 ðBO3 Þ2 material was described elsewhere [7].
The Raman spectra shown in Fig. 1 evidence
two structural transitions at 5–7 and 15 GPa,respectively. The ﬁrst transition manifests itself inthe disappearance of two low-frequency modes
marked A and B and the appearance of two newmodes A0 and B0 at slightly lower frequencies.
None of the other Raman lines exhibits ananomaly near 6 GPa. The observation that only
two out of nine observed Raman modes areaffected by the transition indicates the occurrenceof a rather subtle structural distortion.
As pressure is increased, the Raman peak C near
300 cm1 decreases in intensity and vanishes completely at 15 GPa. The Raman spectra undergo
radical changes between 14.6 and 16.1 GPa with981

II

10.3

6.3
A

4.7

NeN2

B200IC400

600

800

2.21000

Raman Shift (cm–1)
Fig. 1. Raman spectra of SrCu2 ðBO3 Þ2 at pressures up to21 GPa (T ¼ 295 K; lex ¼ 633 nm). The broad band below
200 cm1 originates from the nitrogen pressure medium; thelines marked ‘Ne’ are neon plasma lines.

the number of observed Raman lines increasingfrom 9 to 20. Clearly, there is a sharp structuralphase transition at 15 GPa which is probably ofreconstructive nature. The Raman experiments
were complemented by high-pressure infraredabsorption measurements at the infrared beamlineof the synchrotron ANKA (Karlsruhe). They
conﬁrm the subtle distortion near 5 GPa and theabrupt phase transition at 15 GPa.
X-ray diffraction measurements provide details
on the pressure-induced structural changes inSrCu2 ðBO3 Þ2 : The diffraction diagrams of phase
II between 5 and 15 GPa can be indexed with amonoclinic lattice. The transition near 5 GPa isthus a continuous tetragonal-to-monoclinic distortion (Fig. 2). The structure of the high-pressure
phase above 15 GPa remains to be solved. Bothstructural transitions are fully reversible uponpressure release.
The compressibility of SrCu2 ðBO3 Þ2 is rather
anisotropic with the stacking direction c being
the soft direction. The anisotropy itself changes
with pressure, in particular near the monoclinic

ARTICLE IN PRESS
I. Loa et al. / Physica B 359– 361 (2005) 980–982

982

SrCu2(BO3)2

b/b0

T = 300 K
100a/a00.96

Murnaghan EOS
V0 = 538.2(7) Å3
B0 = 58(2) GPa
B' = 2.9(4)

0.94
0.92

95

c/c0 :

0.88

a/a0

0.90

0.86

c/c0∇16141210860.84

Pc≈4 GPa90002

β = 90° + η(P–Pc )ε

Monoclinic Angle β (deg)

0.98

∇

Normalized Lattice Parameters

1.00

54

106Pc = 4.7 GPa

158101214

Pressure (GPa)

02

468

101214Pressure (GPa)( c ¼ 6:6508ð4Þ A).
( The inset
Fig. 2. Normalised lattice parameters of SrCu2 ðBO3 Þ2 as a function of pressure (a0 ; b0 ¼ 8:9922ð3Þ A;
0displays the ratio of the relative compressions Dc=c0 : Da=a0 along c and a.

distortion, as can be seen from the ratio of therelative compressions Dc=c0 : Da=a0 along c and a(inset of Fig. 2). Quite unusually, the axis
compressibilities of the a and b axes increase
with increasing pressures up to 7 GPa(B0a ¼ d2 a=dP2 o0). This can in part be attributed
to a reduction of the corrugation of the Cu2 O6 B2 layers under pressure, as a detailed analysis of thediffraction diagrams suggests.
Regarding the magnetic properties ofSrCu2 ðBO3 Þ2 ; the preliminary results presented
here let us conclude that it should be possible toreach the quantum critical point at pressures of
2.5–3.5 GPa without the interception of a structural phase transition. The highly anisotropic compression of SrCu2 ðBO3 Þ2 indicates a strengtheningof the interlayer magnetic coupling in comparison
to the intralayer couplings. A detailed analysis of

the intralayer structural changes and their effecton the coupling constants is the subject of ongoingwork.

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