ELASTICITY OF BAFCL SINGLE-CRYSTAL UNDER HYDROSTATIC-PRESSURE

The sound velocities for longitudinal and transverse waves have been m easured in single crystalline BaFCl at room temperature using ultrason ic pulse echo and Brillouin scattering techniques. The complete set of elastic constants is deduced and lead to the bulk moduli values of Ba FCl at ambiant conditions (B-0 = 44 GPa, B-perpendicular to = 133 GPa, B-parallel to = 131 GPa) which are compared with those obtained by a shell model. Moreover, using the ultrasonic technique under pressure, the pressure derivatives of the second order elastic constants at 298 K have been determined up to 0.3 GPa. All moduli increase linearly wit h pressure in this pressure range, allowing to determine directly and separately the first derivative of the bulk modulus B-0' = 5.8. These data are used to calculate a Murnaghan equation of state. A detailed c omparison is given between our results with those recently obtained by X-ray diffraction on powder or calculated using the local density app roximation method. Finally, the anisotropy of BaFCl under pressure is discussed.