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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