Elastic and non-linear acoustic properties and thermal expansion of ceriummetaphosphate glasses

To test predictions of the soft potential model (SPM) for the thermal and a coustic properties of glasses, the thermal expansion and the ultrasonic wav e velocity and attenuation have been measured in cerium metaphosphate glass es with compositions in the vicinity of (Ce2O3)(0.25) (P2O5)(0.75). The ult rasonic wave velocities have been determined as fimctions of temperature an d hydrostatic pressure, the results provide the temperature dependences of the adiabatic elastic stiffnesses C-11 and C-44 and related elastic propert ies, and the hydrostatic-pressure derivatives (partial derivativeC(11)/part ial derivativeP)(P=0) and (partial derivativeC(44)/partial derivativeP)(P=0 ) of the elastic stiffnesses and (partial derivativeB(S)/partial derivative P)(P=0) of the bulk modulus. The longitudinal ultrasonic wave velocities in crease under pressure. The hydrostatic pressure derivative (partial derivat iveB(S)/partial derivativeP)(P=0) of the bulk modulus B-S is positive: when compressed, the cerium metaphosphate glasses show a normal volume elastic response. However, the pressure derivative (partial derivativeC(44)/partial derivativeP)(P=0) of the shear modulus is negative but small, indicating w eak softening of shear modes under pressure. The shear wave ultrasonic atte nuation is characterised by a broad peak; the calculated relaxation paramet ers are consistent with phonon-assisted relaxation of two-level systems. Th e results found for C-IJ and (partial derivativeC(IJ)/partial derivativeP)( P=0) are used to determine the long-wavelength acoustic-mode Gruneisen para meters, which quantify the vibrational anharmonicity and are needed to obta in the acoustic mode contribution to thermal expansion. The temperature dep endence of the shear wave ultrasound velocity, after subtraction of the rel axation and anharmonic contributions. follows a linear law as predicted by the SPM for relaxation of soft harmonic oscillators. At low temperatures th e excess low-energy vibrational states provide a negative contribution to t hermal expansion, which can be understood on the basis of the SPM. (C) 2001 Elsevier Science B.V. All rights reserved.