NMR and EPR measurements in V0 2 under uniaxial stress in the [110]^ direction lead to a phase diagram entirely similar to that obtained in V 1 _ x Cr 3C 0 2 alloys. Two intermediate phases M 2 and T are observed, and, as established for V 1 . 3C Cr x 0 2 , correspond to linear Heisenberg chains of spin i (M 2 phase) on one V sublattice. These chains undergo a progressive dimerization in the T phase as the temperature is lowered.V0 2 undergoes a metal-insulator transition at 340 K from the high-temperature rutile phase (R) to a monoclinic phase (M 2 ) in which all the vanadium atoms are bonded to form well-defined pairs. 1 " 4 Recently it was established that the addition of minute amounts of Cr (^0.1 at.%) stabilized a new insulating monoclinic phase {M 2 ) in which one-half the V atoms are in pairs while the other half are in equispaced chains of paramagnetic V 4+ ions. 5 " 7 In this Letter we report the stabilization of the M 2 phase in pure V0 2 under the application of modest uniaxial stresses. Using both NMR and EPR as probes of the V ions we find that a uniaxial stress can progressively depair one half of the V sites leading to a firstorder transition to the M 2 phase in which these sites act as paramagnetic V 4+ ions.The simplest way to understand these phases is to view the rutile phase as two interpenetrating sublattices of V chains parallel to the [001]^ axis, each V atom being surrounded by an oxygen octahedron whose axis points in the [ 110] ^ and [ 110] ^ directions on the A and B sublattices, respectively. The two V sublattices are equivalent by symmetry in R and M 19 but are differentiated in M 2 . In the M 2 phase, the V atoms of the A sublattice are strongly paired along the [001]^ axis, while the V atoms of the B sublattice form zigzag chains along the same direction. The localization of the one d conduction electron per V site of the zigzag chains, due to electron-electron correlations, has been established by both NMR 6 and EPR 7 measurements. These results were analyzed in terms of linear Heisenberg chains of spin i which do not interact with each other. A transitional triclinic phase (T) is found to occur between the M x and M 2 phases. 8 ' 6 The T phase corresponds to a progressive dimerization of the magnetic chains on the B sublattice and a progressive tilting of the V pairs of sublattice A, leading to the two equivalent V sublattices 6 in the M x phase. These new insulating phases, which are stabilized by other impurities (Al 9 and Fe 10 ) have been interpreted as alternative phases of pure V0 2 . 6 The exact mechanism by which a particular group of impurities can stabilize T and M 2 and break the symmetry between sublattices A and B is unknown. This break in symmetry can can be done more directly by applying a unaxial stress in the [ 110]^ direction in pure V0 2 . On the A sublattice the oxygen octahedra point in the direction of the stress, and the oxygen-oxygen apical distance should decrease. This shortening should reduce the tilting of the V-V pairs on the A sublattice and induce a...

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The nuclear magnetic resonance (NMR) of V" has been studied in the series of compounds V, "Cr"02 between 100 and 350 K. Three insulating phases are clearly distinguished. In the low-temperature M, phase only one V site is seen with a positive Knight shift and electric-field gradient identical to the insulating phase of pure VO, . At temperatures just below the metal-insulator transition a second phase M, is stable in which two sites are resolved. One site has a small positive Knight-shift characteristic of a paired V'+ site while the other has a negative Knight shift showing a localized V'+ site. These two sites are identified as the V atoms on the paired chains and the equispaced chains in C2/m structure of Marezio et al. At intermediate temperatures a transitional phase T is stable in which two sites can be resolved by their electric-field gradients. The two sites are progressively differentiated by increasing temperature and Cr concentration and are interpreted as arising from two sets of inequivalent paired chains, one of which is depairing with increasing temperature and Cr concentration. These results are inconsistent with the monoclinic symmetry and a disordered bond model proposed previously for the transitional phase. Triclinic splittings were observed recently by Villeneuve et al. and additional crystallographic evidence supporting this result is presented. The magnetic properties are interpreted as a set of noninteracting linear Heisenberg chains and the M, -T transition as a dimerization of the linear Heisenberg chain. The results demonstrate a breakdown of the band model in the insulating phases.

The concept of hidden Fermi surface nesting was introduced to explain the general observation that certain low-dimensional metals with several partially filled bands exhibit charge density wave (CDW) instabilities, although their individual Fermi surfaces do not reveal the observed nesting vectors. This concept was explored by considering the Fermi surfaces of the purple bronze AMo(6)O(17) (A = sodium or potassium) and then observing the CDW spatial fluctuations expected from its hidden nesting on the basis of diffuse x-ray scattering experiments. The concept of hidden Fermi surface nesting is essential for understanding the electronic instabilities of low-dimensional metals.

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