We report on the magnetic properties of an insulating cubic compound Cu3TeO6 studied by ac and dc susceptibility, torque magnetometry and neutron powder diffraction. A novel three-dimensional magnetic lattice composed of almost planar regular hexagons of Cu2+ S = 1/2 spins is present in Cu3TeO6. The magnetic susceptibility in the paramagnetic state obeys the Curie–Weiss law in the 200–330 K regime with ΘCW = −148 K and at TN = 61 K system undergoes an antiferromagnetic phase transition. Above TN the susceptibility is isotropic. Below TN a large anisotropy develops in fields H≥500 Oe. Torque measurements reveal the presence of antiferromagnetic domains below TN. In a rather low magnetic field ( Oe) switching of domains is observed. The dynamics related to movement of domain walls is very slow at low temperatures (of the order of 102 s) and interferes with all torque measurements. The presence of domains is a consequence of the symmetry of the underlying magnetic lattice. Neutron powder diffraction reveals that antiferromagnetic long-range order is associated with the wavevector . The dominant component of the magnetic moment is along one of the space diagonals of the cubic unit cell, but it is not possible to resolve whether the structure is collinear or canted.

Static electrical and magnetic properties of single crystal BaVS3 were measured over the structural (TS = 240K), metal-insulator (TMI = 69K), and suspected orbital ordering (TX = 30K) transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at TX signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal field quasi-doublet. The disordered insulator at TX < T < TMI is described as a classical liquid of non-magnetic pairs.Spatial ordering of the occupancy of degenerate electronic orbitals plays important role in the diverse magnetic phenomena of transition metal compounds [1]. To cite a well-known example: the interplay of magnetic and orbital long range ordering, and strong coupling to the lattice, account for the metal-insulator transitions of the V 2 O 3 system [2,3]. In contrast, the metal-insulator transition of the S = 1/2, 3d 1 electron system BaVS 3 is not associated either with magnetic long range order, or with any detectable static spin pairing. As an alternative, the possibility of an orbitally ordered ground state was discussed [4], while other proposals emphasized the quasione-dimensional character of the material [5][6][7]. The crystal structure is certainly suggestive of a linear chain compound since along the c axis, the intrachain V-V distance is only 2.81Å, while in the a-b plane the interchain separation is 6.73Å [8,9]. It is thus somewhat surprising that our present studies show that electrically BaVS 3 is nearly isotropic. This means that BaVS 3 provides one of the few realizations of a Mott transition within the non-magnetic phase of a three-dimensional system. Since this case (or rather its D → ∞ counterpart) is much studied theoretically, but scarcely investigated experimentally, a good understanding of BaVS 3 should be valuable for strong correlation physics in general.BaVS 3 has a metal-insulator transition at T MI = 69K, accompanied by a sharp spike in the magnetic susceptibility [5,10]. The high temperature phase is a strongly correlated metal with mean free path in the order of the lattice constant. There is no sign of a sharp Fermiedge in the UPS/XPS spectra [6] and instead of a Pauli-susceptibility it exhibits Curie-Weiss like behavior. Though the magnetic susceptibility is similar to that of an antiferromagnet [10,11], no long-range magnetic order develops at the transition [9,12]. The transition is clearly seen in the thermal expansion anomaly [5], and in the specific heat [7]. The d-electron entropy right above T MI is estimated as ∼ 0.8R ln 2, and it seems that a considerable fraction of the electronic degrees of freedom is frozen even at room temperature [7]. It appears that the 69K transition is not symmetry breaking [13]: it is a pure Mott transition which does not involve either magnetic order or any static displacement of the atoms.Hints of long range order were found well below T MI , at T X = 30K, in rece...

We report the pressure dependence of the three phase transitions in TTF-TCNQ as determined by resistivity measurements. There is only one transition above 15 kbar, and we find a 4-kbar regime centered at 20 kbar where the transition temperature peaks (the distortion temperature T d = 71 K at 20 kbar), and the transition is first order. We suggest that there is a commensurate x3 superlattice in this pressure regime resulting from an increase in charge transfer to §.Previous high-pressure conductivity measurements of TTF-TCNQ 1 " 3 were performed before structural information on the sequence of three phase transitions now observed became available, although some of the first evidence for more than one transition in this compound came from these experiments. 1 ' 2 In the present work we have followed all three transitions in the pressure range up to 33 kbar by measurements of the stackingaxis resistivity,. Our results are in broad agreement with the previous investigations, although we have been able to assemble a more detailed phase diagram.X-ray and neutron diffraction experiments on the phase transitions at 53, 49, and 38 K are reviewed by Comes. 4 EPR, 5 C 13 NMR, 6 and isotopeeffect 7 experiments indicate that the TCNQ stacks distort at 53 K, and that the lower transitions are associated with the TTF stacks. The presence of 4k F scattering has been taken as evidence for the importance of Coulomb correlations. 8 " 10 These are more important on the TTF stacks, 5 since the TTF bandwidth is appreciably smaller than that of the TCNQ. n Both normal and N 15 -substituted crystals were used, the latter because they have been found previously to show particularly sharp phase transitions. 7 Standard low-frequency ac resistivity and high-pressure techniques were employed, 12 using He gas as the pressure transmitting medium up to 8 kbar, and either isopentane or a mixture of isopentane and isoamyl alcohol above.