CuSiO3: A quasi-one-dimensional S=1/2 antiferromagnetic chain system

Article abstract of DOI:10.1103/PhysRevB.62.12201

CuSiO₃, isotypic to the spin-Peierls compound CuGeO₃, was discovered recently as a metastable decomposition product of the silicate mineral dioptase, Cu₆Si₆O₁₈·6H₂O. We investigated the physical properties of CuSiO₃ using susceptibility, magnetization, and specific heat measurements on powder samples. The magnetic susceptibility _X(T) is reproduced very well above T=8 K by theoretical calculations for an S=1/2 antiferromagnetic Heisenberg linear chain without frustration (α= 0) and a nearest-neighbor exchange coupling constant of J/k_B=21 K, much weaker than in CuGeO₃. Below 8 K the susceptibility exhibits a substantial drop. This feature is identified as a second-order phase transition at T₀ = 7.9 K by specific heat measurements. The influence of magnetic fields on T₀ is weak, and ac-magnetization measurements give strong evidence for a spin-flop phase at μ₀H_SF?3 T. The origin of the magnetic phase transition at T₀=7.9 K is discussed in the context of long-range antiferromagnetic order (AF) versus spin-Peierls (SP) order. Susceptibility and specific heat results support the AF ordered ground state. Additional temperature dependent ^63.65Cu nuclear quadrupole resonance experiments have been carried out to probe the Cu²⁺ electronic state and the spin dynamics in CuSiO₃.

Full text of DOI:10.1103/PhysRevB.62.12201

PHYSICAL REVIEW B
VOLUME 62, NUMBER 18
1 NOVEMBER 2000-II
1
CuSiO3: A quasi-one-dimensional SÄ 2 antiferromagnetic chain system
M. Baenitz, C. Geibel, M. Dischner, G. Sparn, and F. Steglich
Max-Planck-Institute of Chemical Physics of Solids, D-01187 Dresden, Germany
H. H. Otto and M. Meibohm
¨
TU-Clausthal, Institut fur Mineralogie und Mineralische Rohstoffe, Adolph-Roemer-Straße 2a, D-38678 Clausthal-Zellerfeld, Germany
A. A. Gippius
Faculty of Physics, Moscow State University, 119899 Moscow, Russia
͑Received 17 May 2000͒
CuSiO3, isotypic to the spin-Peierls compound CuGeO3, was discovered recently as a metastable decom-
position product of the silicate mineral dioptase, Cu6Si6O18•6H2O. We investigated the physical properties of
CuSiO3 using susceptibility, magnetization, and specific heat measurements on powder samples. The magnetic
susceptibility ␹(T) is reproduced very well above Tϭ8 K by theoretical calculations for an Sϭ 21 antiferro-
magnetic Heisenberg linear chain without frustration (␣ϭ0) and a nearest-neighbor exchange coupling con-
stant of J/kBϭ21 K, much weaker than in CuGeO3. Below 8 K the susceptibility exhibits a substantial drop.
This feature is identified as a second-order phase transition at T0ϭ7.9 K by specific heat measurements. The
influence of magnetic fields on T0 is weak, and ac-magnetization measurements give strong evidence for a
spin-flop phase at ␮0HSFӍ3 T. The origin of the magnetic phase transition at T0ϭ7.9 K is discussed in the
context of long-range antiferromagnetic order ͑AF͒ versus spin-Peierls ͑SP͒ order. Susceptibility and specific
heat results support the AF ordered ground state. Additional temperature dependent 63,65Cu nuclear quadrupole
resonance experiments have been carried out to probe the Cu2ϩ electronic state and the spin dynamics in
CuSiO3.
ing Cu2ϩ ions via the O2Ϫ ions. The most crucial structural
I. INTRODUCTION
modification is the reduction of the Cu-O(2)-Cu bond angle
The one-dimensional spin system CuGeO3 has attracted
considerable attention in past years, since it was the first ͑and
up to now the only͒ inorganic compound that exhibits a spin-
Peierls transition ͑SP͒.1,2 The simplicity of crystal growth
and the large variety of possible substitutions on the Cu and
Ge sites promoted a huge amount of experimental and theo-
retical studies. Completely new phenomena like the coexist-
ence of the SP state with a long-range antiferromagnetically
ordered state in slightly doped CuGeO3 ͑Si on the Ge site or
Zn on the Cu site͒,3–9 and the strong influence of the frustra-
tion due to the next-nearest-neighbor exchange on the mag-
netic and thermodynamic properties10 were reported. Partial
substitution of Ge by Si in CuGe1ϪxSixO3 has been an im-
portant subject in this field, but despite considerable efforts,
it was not possible to substitute more than 50% Ge by Si
without changing the structure (xр0.1 for single crystals,4–7
xр0.5 for polycrystals8͒. Pure CuSiO3 was considered to be
nonexistent. However, recently Otto et al.11 succeeded in the
synthesis of reasonable amounts of pure isostructural CuSiO3
by using the silicate mineral dioptase Cu6Si6O18•6H2O as a
starting material. Compared to the Ge homologue the lattice
parameters a and c are reduced by about Ϫ3.5% and
Ϫ3.7%, respectively, whereas b increases by about ϩ3.5%.
The unit cell volume of CuSiO3 is reduced (ϷϪ3.8%) in
accordance with the smaller size of Si. This naturally leads to
a modification of bond angles and lengths,11 which should
have a strong influence on the strength of the magnetic in-
teraction governed by the superexchange between neighbor-
from 99° in CuGeO3 to 94° in CuSiO3 which is accompa-
nied by a shrinking of the in-chain Cu-Cu distance. The ob-
vious question is how these structural changes do affect the
ground state properties of this system. Here we present an
investigation of the physical properties of CuSiO3, based on
susceptibility, specific heat, and Cu-nuclear quadrupole reso-
nance experiments.
II. EXPERIMENTAL DETAILS
High quality crystals of dioptase from the type locality
Altyn Tyube ͑Kazakhstan͒ were heated up to 873 K and held
at this temperature for six hours in order to obtain dehy-
drated dioptase ͑black dioptase͒. The black dioptase was de-
composed by a subsequent heat treatment at higher tempera-
tures ͑1050 K͒ and for 20 hours under nitrogen atmosphere.
The finely ground samples of darkish brown color were char-
acterized by x-ray powder diffraction using the Guinier
method. The diffraction pattern reveals a mixture of three
different phases. A quantitative phase analysis reveals that
about 76 wt. % of the mixture consist of the new phase
CuSiO3. The other phases are CuO ͑tenorite, 13.7 wt. %)
and SiO2 ͑amorphous, 10.3 wt %). The orthorhombic unit
cell of the new phase CuSiO3 was refined from Guinier data
giving aϭ4.6357(6) Å, bϭ8.7735(11) Å, and cϭ2.8334͑4͒
Å. The lattice constants of CuSiO3 are in good agreement
with an extrapolation of the results from the diluted system
CuGe1ϪxSixO3 ͑Ref. 8͒ to xϭ1. The crystal structure was
determined to be isostructural to CuGeO3. Details of the
0163-1829/2000/62͑18͒/12201͑5͒/$15.00
PRB 62
12 201
©2000 The American Physical Society

12 202
M. BAENITZ et al.
PRB 62
FIG. 1. Raw data of the magnetic susceptibility of a CuSiO3
sample measured under a field of ␮ Hϭ1 T in a log-T scale. The
FIG. 2. Heat capacity of a CuSiO3 sample and of CuGeO3 ͑data
taken from Ref. 15͒. The dashed line represents the phonon contri-
bution Cphϭ␤T3 (␤ϭ0.32 mJ/mol K4) for CuGeO3.
0
¨
solid curve corresponds to the numerical calculations of Klumper
et al. ͑Ref. 13͒ for Sϭ1/2 1D Heisenberg chains with nearest-
neighbor coupling and without next-nearest-neighbor coupling (J
Ј
ϭ0). The fitting of the numerical results to the experimental ␹(T)
data requires a coupling constant J/kBϭ21 K and an effective mo-
pure CuSiO3 in the sample results in an ␮eff value of 1.79␮
and a g factor of 2.06. These values are very close to those
found for CuGeO3 and expected theoretically for free S
B
ment of
у200 K, ␹(T) is fitted with a Curie-Weiss law ͑dashed line͒ with
effϭ1.56 ␮ (gϭ1.80) and ␪ϭϪ7.2 K. The inset shows d␹/dT
␮effϭ1.60 ␮ (gϭ1.85). At high temperatures T
B
ͱ
ϭ1/2 spins with gϭ2 and ␮ϭg␮ S(Sϩ1)ϭ1.73␮B .
B
␮
B
At lower temperatures the susceptibility exhibits a broad
as a function of temperature.
maximum at Tm,␹ϭ13.5 K, which is a hallmark for low-
dimensional spin systems.12 Above 8 K, ␹(T) could be fitted
synthesis and structure characterization are given in Ref. 11.
Magnetization, specific heat, and NQR measurements were
performed on samples taken from the same batch. The dc-
13
¨
very well with the numerical results of Klumper for S
ϭ1/2 Heisenberg chains. The best fit is obtained with a
nearest-neighbor coupling of J/kBϭ2Tm,␹/1.28212 ϭ21 K
and a slightly enhanced ␮ value of 1.60 ␮ ͑note that we
magnetization measurements at low fields ␮ Hр1 T were
0
carried out using a commercial SQUID magnetometer
eff
B
used the unscaled susceptibility for the fit and that we define
J by the exchange Hamiltonian: HϭJ͚sisiϩ1ϩ␣sisiϩ2). The
quality of the fit suggests that frustration effects, i.e., an an-
͑MPMS, Quantum Design͒. The ac- and dc-magnetization
measurements in higher fields (1 Tр␮ Hр14 T) and the
0
specific heat measurements were performed in a commercial
multipurpose device ͑PPMS, Quantum Design͒. Ac and dc
magnetization are extracted from the induction signal of a
mutual inductance coil arrangement. The specific heat was
determined by standard relaxation technique and an ad-
vanced two-tau model was applied to analyze the thermal
response. The data obtained were corrected by subtracting
the specific heat contributions of the sample holder, ther-
mometers, heaters, and glue. The NQR measurements were
performed on a conventional pulsed spectrometer using the
point-by-point method. The masses of the samples are m
ϭ91.40 mg, 8.72 mg, and 180.20 mg for susceptibility, spe-
cific heat, and NQR measurements, respectively.
tiferromagnetic interaction J between next-nearest neigh-
Ј
bors, are negligible in CuSiO (␣ϭJ /Jϭ0). This is in con-
Ј
3
trast to CuGeO3 where ␣ values between 0.24 and 0.35 are
proposed.10,14 Furthermore, it is evident that the Cu-O͑2͒-Cu
exchange in CuSiO3 is much weaker (J/kBϭ21 K) than in
CuGeO3 (J/kBϷ160 K). Below 8 K, ␹(T) decreases very
rapidly and saturates at the lowest temperatures. The deriva-
tion of the susceptibility d␹/dT shows a peak at T0ϭ7.9 K
which gives clear evidence for a cooperative phase transition
at this temperature ͑see inset of Fig. 1͒. The signature of the
transition in the susceptibility looks more like a long-range
antiferromagnetic order in a polycrystaline sample than a
spin-Peierls transition. For the latter scenario, one expects a
vanishing susceptibility at lowest temperatures ␹(T→0)
ϭ0, which is not observed. The absence of a Curie-like tail
in the susceptibility at the lowest temperatures indicates the
absence of defects and thus points to a high crystalline per-
fection of the CuSiO3 phase.
The presence of a phase transition at T0 is clearly con-
firmed by the specific heat results. The temperature depen-
dence of the total specific heat of three CuSiO3 crystals ͑total
mass of 8.72 mg, 1 mol ϭ 139.6 g͒ below 20 K is shown in
Fig. 2. For a comparison, the data of Liu et al.15 for CuGeO3
are plotted in the same figure. The specific heat of CuSiO3
shows a very clear ␭-shaped, asymmetric anomaly at T0
III. RESULTS
The temperature dependence of the magnetic susceptibil-
ity ␹͑T͒ of the CuSiO3 powder sample in a magnetic field of
1 T is shown in Fig. 1. Below room temperature, ␹(T) in-
creases with decreasing temperature, confirming localized
Cu2ϩ moments. Between 200 K and 300 K the susceptibility
data follow nicely a Curie-Weiss law with a Weiss tempera-
ture of ␪ϭϪ7.2 K, indicating rather weak antiferromagnetic
coupling. An effective moment of ␮effϭ1.56␮ per Cuϩ2
B
ion is determined which corresponds to a g factor of 1.80.
Both impurity phases CuO and SiO2 exhibit a small suscep-
tibility which could be neglected in a first analysis. Scaling
the measured susceptibility with the estimated amount of
ϭ7.9 K with
a
specific heat jump of ⌬CϷ1.50
Ϯ0.05 J/mol K. This is comparable to the ⌬C value of ap-

CuSiO3: A QUASI-ONE-DIMENSIONAL Sϭ 12 . . .
PRB 62
12 203
proximately 1.8 J/mol K found for CuGeO3.15 The total spe-
cific heat obtained is the sum of the lattice term Cph(T) from
the phonons and the magnetic term Cm(T) from the spin
system. The separation of the two contributions is not trivial.
At low temperatures, the phonon contribution should follow
a T3 law, i.e., Cphϭ␤T3. Because of the smaller mass of the
Si ions compared to the Ge ions (mSi /mGeϷ0.39) one can
expect a harder phonon spectra in CuSiO3 and thus a reduced
phonon contribution
␤
3Ͻ␤CuGeO Ϸ0.32 mJ/mol K4
CuSiO
3
͓
͑Refs. 15, 10 and 16͔͒. Thus, the estimate of the phonon
contribution for CuGeO3 ͑see Fig. 2, dashed line͒ yields
therefore an upper limit for the phonon contribution in
CuSiO3. This clearly demonstrates, that below 20 K, the
specific heat of CuSiO3 is dominated by the magnetic con-
tribution. Well below T0 , C(T) follows a T3 power law with
a coefficient ␤m (Ϸ4.5 mJ/mol K4), more than one order of
magnitude larger than that expected for the phonon contribu-
tion, indicating that it has to be related to the magnetic in-
teractions. Such a power law is expected for long-range or-
dered 3D antiferromagnets with weak or absent anisotropy.
The T3 power law at low temperatures is in contrast to the
experimental findings on the spin-Peierls compound CuGeO3
where the opening of a gap in the magnetic excitation spectra
leads to an exponential decrease of the specific heat. There-
fore our specific heat measurements, too, support long-range
antiferromagnetic order rather than a spin-Peierls dimeriza-
tion at T0.
FIG. 3. Temperature dependence of the specific heat ͑left side;
note that the origin of the vertical axis is shifted by 1.6 J/mol K for
the curves with ␮0Hу0T) and the magnetic susceptibility ͑right͒
for different magnetic fields. The inset shows the shift of the tem-
perature Tm,␹ defined by the maximum susceptibility ␹(Tm,␹
)
ϭ␹ in reduced units as a function of applied field and in com-
max
¨
parison with the theoretical prediction of Klumper et al. ͑Ref. 13͒
broadened ͑due to the random orientation of the powder par-
ticles͒ transition at ␮0HSFϷ3T, which looks very similar to
a spin-flop transition. The phase diagram corresponds to that
observed for the AF phase in doped CuGeO3, but it is quite
different from that expected and confirmed for a spin-Peierls
transition. The almost field independent transition tempera-
ture and the presence of a spin-flop-like transition are strong
evidences for an antiferromagnetically ordered ground state
in CuSiO3.
At higher temperatures TϾT0, the specific heat Cm(T) of
a Heisenberg chain without frustration exhibits a maximum
at Tm,cϭ0.75 Tm,␹ ͑Ref. 12͒ ϭ10.1 K with a value of
Cm(Tm,c)Ϸ0.35RϷ2.9 J/mol K, which is independent of
J.12,13,10,17 Scaling this value with the amount of pure
CuSiO3 in the sample results in an expected magnetic con-
tribution of 2.2 J/mol K. The experimental value of the spe-
cific heat at 10 K is only slightly lower. Further measure-
ments and analysis are currently under progress to improve
the estimation of the magnetic specific heat contribution. At
even higher temperatures (TϾJ/kB) Cm(T) becomes very
small and C(T) originates mainly from the phonon contribu-
tion. Therefore the germanate exhibits a larger total specific
heat than the silicate as evidenced from Fig. 2.
We have investigated the nuclear quadrupole resonance
͑NQR͒ of Cu in CuSiO3 to deduce microscopic informations
at nuclear sites. A spectrum at 4.2 K is shown in Fig. 5. The
lines are fitted well by a Gaussian function and the central
63,65
frequencies
␯
and the line widths 63,65⌬␯
were
NQR
NQR
determined at different temperatures ͑see inset of Fig. 5͒.
The NQR signals at 4.2 K have been found at 26.88
Ϯ0.02 MHz for 63Cu and 24.88Ϯ0.02 MHz for 65Cu. The
The influence of magnetic fields up to 14 T on the mag-
netic susceptibility and the specific heat is shown in Fig. 3.
In the susceptibility ␹(T,H) the signature of the transition at
T0ϭ7.9 K is smeared out, the drop in the susceptibility is
reduced and the susceptibility maximum at Tm,␹ is shifted
significantly to lower temperatures with field. This shift of
Tm,␹ , which indicates the suppression of the antiferromag-
netic in-chain correlations with increasing field, is in good
13
¨
agreement with the theoretical calculations of Klumper
͑see inset in Fig. 3͒ for the Sϭ1/2 Heisenberg chain. In the
specific heat C(T) the transition at T0ϭ7.9 K is clearly ob-
servable in fields up to 14 T and the T3 power law at low
temperatures is preserved. The antiferromagnetic order tem-
perature T0 shows only a weak field dependence as indicated
in the magnetic phase diagram plotted in Fig. 4͑a͒. No other
transitions are visible in our C(T,H) measurements. The
temperature dependent ␹(T) measurements at fixed fields
͑Fig. 3͒ and additionally field dependent ac-susceptibilty
measurements at fixed temperatures ͓Fig. 4͑b͔͒ evidences a
FIG. 4. Magnetic phase diagram of CuSiO3 probed by specific
heat and susceptibility measurements ͑left͒. Hc denotes the AF tran-
sition whereas HSF indicates the spin flop transition as evidenced
from ac-susceptibility measurements ͑left side; note that the origin
of the vertical axis is shifted by a constant value for curves at
different temperatures͒.

12 204
M. BAENITZ et al.
PRB 62
IV. DISCUSSION
CuSiO3, isotypic to the spin-Peierls compound CuGeO3
was synthesized from the mineral dioptase and the physical
properties of this new compound were determined by means
of susceptibility, specific heat, and Cu nuclear quadrupole
resonance measurements. The susceptibility of CuSiO3 is in
good agreement with the theoretical results for a quasi-one-
dimensional Sϭ1/2 Heisenberg antiferromagnet with a
nearest-neighbor exchange constant of J/kBϭ21 K and with-
out a significant next-nearest neighbor exchange interaction
(J ϭ0). This is in contrast to the findings for CuGeO
Ј
3
where frustration effects due to next-nearest-neighbor inter-
actions play a crucial role (␣ϭJ /JϷ0.35) and where a
Ј
much higher exchange coupling constant of J/kB
Ӎ160
K
was found. According to the so-called
FIG. 5. 63Cu and 65Cu NQR spectra in CuSiO3 at 4.2 K. The
Goodenough-Kanamori-Anderson23 ͑GKA͒ rules a change
from an antiferromagnetic exchange to a ferromagnetic ex-
change is expected when the bond angle is near 90°. There-
fore the much smaller J value in CuSiO3 could easily be
attributed to the reduction of the Cu-O(2)-Cu bond angle
from 99° in CuGeO3 to 94° in CuSiO3. In contrast, the
disappearance of the frustration is surprising, since the cor-
responding bond angles are fare away from 90°. One pos-
sible explanation is, that the smaller Cu-O͑2͒-Cu bond angle
of 94° leads to a cancellation of the antiferromagnetic and
ferromagnetic exchange contributions and a vanishing
nearest-neighbor exchange J. The remaining next-nearest-
solid curves correspond to the fitting of a Gaussian function to the
experimental data. The inset shows the temperature dependence of
the 63Cu NQR frequency ͑right axis͒ and of the 63Cu NQR line-
widths ͑left axis͒.
ratio of the frequencies of the NQR signals are in good
agreement with that expected from the nuclear quadrupole
moments of Cu (63Q/65Qϭ1.081). Also the ratio of the sig-
nal intensities 63I/65Iϭ2.8 corresponds to that of the natural
abundance of the isotopes 63Cu and 65Cu with 63I/65I
ϭ2.20. Furthermore the presence of the impurity phase CuO
is confirmed nicely by L measurements. At room temperature
63
the 63Cu NQR frequency of CuO is found at
␯
neighbor exchange J would then transform one atomic
Ј
NQR
chain into two independent spin chains. Since the suscepti-
bility ͑and the specific heat͒ is the same as that of a single
spin chain, the present experimental results do not allow to
distinguish between both cases. General calculations for the
edge-sharing cuprates leads to a ferromagnetic exchange
ϭ20.6 MHz, which is in good agreement with the
literature.18 Due to the AF order in CuO at TNϭ230 K ͑Ref.
63
19͒ the
␯
is shifted to much higher frequencies below
NQR
TN (63␯NQRϷ137 MHz for the central line at 4.2 K͒.20
Therefore the observed resonance signals presented in Fig. 5
could be clearly assigned to the Cu NQR lines of the CuSiO3
phase. No other resonance lines are observed in the fre-
quency range of 20–90 MHz indicating the crystallographi-
cally equivalence of the Cu site. Compared to CuGeO3 with
34.23Ϯ0.02 MHz for 63Cu and 31.66Ϯ0.02 MHz for 65Cu
at 4.2 K,21,22 the NQR lines in CuSiO3 are shifted to lower
frequencies. One possible explanation is the effect of the
modified bond lengths and angles on the electric field gradi-
coupling constant J for an bond angle of 94°, wheras J is
Ј
always antiferromagnetic with only a weak dependence on
the angle.24 The ferromagnetic nearest-neighbor exchange
for the 94° configuration has been experimentally found in
Li2CuO2.25 However the arrangement of the chains ͑parallel
chains in Li2CuO2 versus tilted chains in CuSiO3) and the
nature of the side groups ͑Li versus Si͒, which are also ex-
pected to influence J, are different to CuSiO3. Thus an esti-
mate of the J value for CuSiO3 based on an LDA calculation
performed by using the real lattice structure of CuSiO3 does
not support a ferromagnetic nearest-neighbor exchange.26
The large residual susceptibility below TN , the T3 power
law in the magnetic specific heat at low temperatures, the
spin-flop-like transition in a magnetic field as well as the
H-T magnetic phase diagram are very strong evidence for the
antiferromagnetic nature of the transition. The comparatively
large ratio between the ordering temperature TN and the tem-
perature of the maximum in the susceptibility Tm,␹ indicate
that the ratio between interchain-and intrachain exchange
(JЌ /J) is significantly larger in CuSiO3 than in CuGeO3.
This is a natural consequence of the weakness of the intra-
chain exchange. An estimate of the interchain coupling con-
stant for quasi-one-dimensional chains is given by the fol-
18
63,65
ent EFG which affects strongly
␯
.
The EFG at the
NQR
Cu site originates mainly from the ionic charge distribution
of the surrounding ions ͑so called lattice contribution͒ and
the 3d Cu charge distribution ͑valence͒ itself.18 Compared to
CuGeO3 the Cu-O bond lengths are enlarged. In a simple
point charge model for the lattice contribution this could
reduce the EFG and therefore lower the NQR frequencies.
Details of the NQR results and a complex analysis of the
63,65
data will be presented elsewhere. Surprisingly
␯
and
NQR
the line widths 63,65⌬␯
exhibit only a weak temperature
NQR
dependence between 4.2 and 40 K and especially around the
transition at T0ϭ7.9 K, no anomaly is observed ͑see inset
Fig. 5͒. Usually an antiferromagnetic phase transition is as-
sociated with the appearance of strong internal magnetic
fields on the Cu site. This should result in a remarkable
transformation of the pure NQR spectrum at TϾTN to a high
frequency AFMR spectrum for TϽTN which is perturbed by
quadrupole interaction. The origin of this absence of any
signature of the transition is presently not clear.
lowing expression:27 JЌϭTN/1.28 ln(5.8J/TN)ϭ3.7 K.
ͱ
From this we obtain a ratio of JЌ /JϷ0.18. LDA band struc-
ture calculations also reveal a much weaker intrachain cou-
pling J and a larger JЌ /J ratio26 (Ϸ0.14) compared to

CuSiO3: A QUASI-ONE-DIMENSIONAL Sϭ 12 . . .
PRB 62
12 205
Among all inorganic low-dimensional Cu based spin systems
the edge-sharing compound CuSiO3 exhibits the smallest in-
trachain exchange AF-coupling constant of J/kBϭ21 K. The
existence of the two homologeus compounds CuGeO3 and
CuSiO3 with different ground state properties provides an
excellent basis for the application of theoretical models and
methods of low-dimensional physics.
CuGeO3, which is in agreement with our experimental re-
sults. However, they also suggests that the frustration should
still be significant in CuSiO3. The weaker intrachain cou-
pling J and the much larger JЌ /J ratio obviously favor the
occurrence of an AF transition instead of a SP transition. The
presented investigations do not give any information about
the strength of the magnetoelastic coupling in CuSiO3. This
coupling is a prerequisite for a spin-Peierls transition.
In conclusion, it is shown that CuSiO3 is a quasi-one-
dimensional Sϭ1/2 Heisenberg chain system which under-
goes a transition to long range antiferromagnetic order at
TNϭ7.9 K. There is no direct evidence for spin-Peierls tran-
sition in this new compound. Field dependent susceptibility
measurements reveal a spin-flop phase at ␮0HSFϷ3 T.
ACKNOWLEDGMENTS
We acknowledge fruitful discussions with H. Rosner and
S.-L. Drechsler. A. A. Gippius thanks INTAS ͑Grant No.
99-01-55͒ for partial support of the work.
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