Acoustic vibrational modes softening in gadolinium metaphosphate glasses

The acoustic properties of Gd sup 3+ metaphosphate glasses have been determined from measurements of the effects of temperature, hydrostatic pressure and uniaxial stress on ultrasonic wave velocity. The ultrasonic wave velocities for these glasses increase monotonically down to about 120K, in accord with the usual effects of vibrational anharmonicity. At lower temperatures the elastic stiffnesses become more strongly dependent upon temperature and attenuation shows a broad peak, behaviour associated with a thermally activated relaxation process of two-level systems. The hydrostatic pressure derivatives (∂ C sup S sub IJ / ∂ P) sub p=0 of the elastic stiffnesses C sup S sub IJ and also (∂ C sup S sub IJ / ∂ P) sub p=0 of the bulk modulus B sup S are small and can be either positive or negative . Measurements of the effect of uniaxial stress on ultrasonic wave velocities have been used to determine the temperature dependences of the third order elastic stiffnesses C sub IJK of (Gd sub 2 O sub 3) sub 0.229 (P sub 2 O sub 5) sub 0.771 glass between 293 K and 77 K; C sub 111, C sub 113, C sub 155 and C sub 456 are negative while C sub 112 and C sub 114 are positive. The longitudinal and shear acoustic mode Gruneisen parameters are small and negative : application of pressure softens the long wavelength acoustic phonon mode frequencies. The mode softening is enhanced as the temperature is reduced