The new perovskite PbVO 3 was synthesized under high-temperature and high-pressure conditions. Its crystal structure (a ) 3.80005(6) Å, c ) 4.6703(1) Å, Z ) 1, S.G. P4mm) contains isolated layers of corner-shared VO 5 pyramids, which are formed instead of octahedra due to a strong tetragonal distortion (c/a ) 1.23). The lead atom is shifted out of the center of the unit cell toward one of two [VO 2 ]-layers due to the influence of the lone pair. This new perovskite exhibits a semiconductor-like F(T) dependence down to 2 K. This behavior can be qualitatively explained by taking into account strong electron correlations in electronic structure calculations.
Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu2Te2O5X2, X=Cl and Br. While the phase transition observed in the Cl system at To=18.2 K is consistent with 3D AF ordering, the phase transition at To=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets. 75.40.Gb, 75.40.Cx, 75.10.Jm, Reduced dimensionality of a quantum spin system in combination with frustration leads in many cases to unconventional and interesting ground states or magnetic phase diagrams. Prominent examples are the frustrated and dimerized spin-1/2 chain, represented by the lowtemperature phase of CuGeO 3 , or the two-dimensional Shastry-Sutherland lattice with orthogonally arranged spin dimers and a frustrating inter-dimer coupling, realized in SrCu 2 (BO 3 ) 2 . These systems show a spin liquid ground state with a singlet-triplet gap. Frustration is evident in the latter system as dispersionless elementary triplets and multi-particle bound states of triplet and singlet character [1][2][3].Spin triangles and tetrahedra that are strongly coupled into Kagomé or pyrochlore structures are at the origin of another important class of frustrated spin systems [4,5]. Although the consequences of the classical ground-state degeneracy for the quantum case have not been fully elucidated theoretically, there are good reasons to believe that such models do not possess magnetic long-range order (LRO) but low-lying singlets [6,7]. Scenarios leading to the development of LRO within this non-magnetic manifold have been put forward [8,9].The limit of weakly-coupled tetrahedra with spin S=1/2 has been studied in some detail in 3D and 1D, and the physics is expected to be very interesting [10][11][12][13]. However, they have not been investigated experimentally so far due to the lack of appropriate materials. The recently found spin system Cu 2 Te 2 O 5 X 2 , with X=Cl, Br, contains tetrahedral clusters of Cu 2+ with S = 1/2 in a distorted square planar CuO 3 X-coordination [14]. These tetrahedra align to tubes or chains along the [001] direction, as they are separated along [100] and [010] direction by different Te-O coordinations (see Fig. 1a). Substituting Br for Cl in this system widens up the unit cell and increases its volume from 367 to 391Å 3 , by 7%, while bond angles or anisotropies do not change essentially. Therefore, this system allows in a unique way to study the interplay of frustration and coupling in a tetrahedra quantum spin system. Preliminary measurements of the magnetic susceptibility χ(T) of both compounds showed a maximum at T χmax = 23 K and 30 K for X=Cl, Br and a strong reduction at low temperatures, typical for a spin gap system. Assuming that the compounds consist of weakly-coupled units of 4 spins with couplings J 1 and J 2 (see Fig. 1b), the best fit of the susceptibility -quite a good one actually -was obtained for J 1 = J 2 = 38.5 K ...
The static and dynamic properties of V 4+ spins ͑S =1/ 2͒ in the frustrated square-lattice compound Pb 2 ͑VO͒͑PO 4 ͒ 2 were investigated by means of magnetic susceptibility and 31 P nuclear magnetic resonance ͑NMR͒ shift ͑K͒ and 31 P nuclear spin-lattice relaxation rate 1 / T 1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below T N Ӎ 3.65 K. NMR spectra above T N show two distinct lines corresponding to two inequivalent P sites present in the crystal structure. The observed asymmetry in hyperfine coupling constant for the in-plane ͑P1͒ P site directly points toward a distortion in the square lattice at the microscopic level, consistent with the monoclinic crystal structure. The nearest-and next-nearestneighbor exchange couplings were estimated by fitting K versus temperature T by a high-temperature series expansion for the spin susceptibility of the frustrated square lattice to be J 1 / k B = ͑−5.4Ϯ 0.5͒ K ͑ferromag-netic͒ and J 2 / k B = ͑9.3Ϯ 0.6͒ K ͑antiferromagnetic͒, respectively. 1 / ͑T 1 T͒ is almost T independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of 1 / ͑T 1 T͒ which arises from a growth of antiferromagnetic spin correlations above T N . Below T N and for the field applied along the c axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing T which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the ab plane. This type of magnetic ordering is consistent with expectation from the J 2 / J 1 Ӎ −1.72 ratio. The critical exponent  = 0.25Ϯ 0.02 estimated from the temperature dependence of the sublattice magnetization as measured by 31 P NMR at 11.13 MHz is close to the value ͑0.231͒ predicted for the two-dimensional XY model.
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