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.

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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.