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...

The coefficient of thermal expansion has been determined in the temperature range 25–400 °C for InP, the ternary alloy Ga0.47In0.53As, and the quaternary alloy Ga0.26In0.74As0.40P0.60 grown on (100) InP by liquid-phase epitaxy. This parameter is (4.56±0.10) ×10−6/°C for InP, (5.42±0.10) ×10−6/°C for Ga0.26In0.74As0.60P0.40, and (5.66±0.10) ×10−6/°C for Ga0.47In0.53As.

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Substantial solid-solution strengthening of GaAs by In acting as InAs4 units has recently been predicted for an intermediate-temperature plateau region. This strengthening could account, in part, for the reduction of dislocation density in GaAs single crystals grown from the melt. Hardness measurements at high temperatures up to 900 °C have been carried out on (100) GaAs, Gllo. 9975 lIlo.0025 As, and Gao.99 II1o.o! As wafers, all of which contain small amounts of boron. Results show a significant strengthening effect in In-doped GaAs. A nominally temperature-independent flow-stress region is observed for all three alloys. The In-doped GaAs shows a higher plateau stress level with increasing In content. The results are consistent with the solid-solution strengthening model. The magnitude of the solid-solution hardening is sufficient to expl.ain the reduction in dislocation density with In addition.

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