ELASTIC ANOMALIES IN VANADIUM-CHROMIUM SESQUIOXIDES (CONSTANT, ULTRASONICS, METAL-INSULATOR TRANSITION, LATTICE DYNAMICS)

The transit times of 30MHz ultrasonic waves have been measured in (V(,1-x)Cr(,x))(,2)O(,3) single crystals with nominal values of x = 0.015, 0.03, and 0.06. Hydrostatic pressures up to about 4 kbar and temperatures between 200 and 650 K were used. The results are used to determine the pressure and temperature dependences of the elastic constants, C(,(mu)(nu)). Two of the elastic constants, C(,11) and C(,12), have unusual negative pressure dependences resulting in the (lamda)(,4) elastic eigenvalue decreasing strongly with increasing pressure. This indicates that pressure tends to make the lattice unstable toward a stretching deformation which alters the c/a ratio without changing the crystal symmetry and is consistent with the structural change known to occur at the pressure-induced, paramagnetic insulator-to-metal transition. For (V(,0.97)Cr(,0.03))(,2)O(,3), the Gruneisen anharmonicity parameter deduced from our elastic data does not agree with that determined from thermal data in contrast to the excellent agreement between the two quantities for pure V(,2)O(,3). In all our (V(,1-x)Cr(,x))(,2)O(,3) crystals the C(,44) shear elastic constant softens drastically with decreasing temperature. This, along with other elastic behavior, indicates a smooth change from rhombohedral to almost hexagonal elastic symmetry before a nearly second-order transition to the low-temperature monoclinic phase. Some of the elastic constants of (V(,1-x)Cr(,x))(,2)O(,3) go through minima as a function of temperature near where the high-temperature electrical transition occurs in V(,2)O(,3). The minima are found to be due to the transfer of electrons accompanying the ultrasonic strain - induced change in the separation of 3d-subbands whose separation depends on temperature also. Values are deduced for the deformation potentials describing the strain dependence of the separation between the 3d-subbands. We have made the first lattice dynamical calculations on V(,2)O(,3), (V(,0.985)Cr(,0.015))(,2)O(,3), and Al(,2)O(,3) in the long-wavelength limit using a rigid-ion model employing an effective ionic charge and various short-range interactions. The calculations accounted for the experimental values of all but one of the elastic constants and of the Raman mode frequencies at room temperature. The effective ionic charge of each (V(,1-x)Cr(,x))(,2)O(,3) crystal was found to be considerably larger than that of Al(,2)O(,3). This is in accordance with the ionicity of V(,2)O(,3), estimated from the dielectric theory of Levine, being larger than that of Al(,2)O(,3). It was also found that the short-range force constants of (V(,1-x)Cr(,x))(,2)O(,3) are quite different from those of Al(,2)O(,3) and that most of the model parameters for semiconducting (V(,0.985)Cr(,0.015))(,2)O(,3) have values in between those of metallic V(,2)O(,3) and insulating Al(,2)O(,3). The effect of alloying a small amount of Cr(,2)O(,3) into V(,2)O(,3) on the interatomic force-field-parameters is discussed also