ELASTIC AND NONLINEAR ACOUSTIC PROPERTIES AND THERMAL-EXPANSION OF RARE-EARTH METAPHOSPHATE GLASSES

The thermal expansions and the ultrasonic wave velocities and attenuat ions of (Sm2O3)(0.224)(P2O5)(0.776) and (La2O3)(0.222)(P2O5)(0.778) an d mixed (La2O3)(x)(Sm2O3)(y)(P2O5)(0.75) (where x + y = 0.25) metaphos phate glasses have been measured as functions of temperature. The chan ge in the ultrasonic wave velocity induced by application of hydrostat ic pressure up to 0.16 GPa has also been measured at selected temperat ures between room temperature and 375 K. The experimental results prov ide the temperature dependences of the adiabatic elastic stiffnesses C -11 and C-44 and related elastic properties and provide the hydrostati c-pressure derivatives (partial derivative C-11/partial derivative P)( P=0) and (partial derivative C-44/partial derivative P)(P=0) of the el astic stiffnesses and (partial derivative B-S/partial derivative P)(P= 0) of the bulk modulus. The results obtained for C-IJ and (partial der ivative C-IJ/partial derivative P)(P=0) are used to determine the long -wavelength acoustic-mode Gruneisen parameters; these quantify the vib rational anharmonicity, which is essential information for developing the acoustic mode contribution to the thermal expansion of the glasses . Hence the ultrasonic velocity measurements in metaphosphate glasses modified with Sm3+ and La3+ ions enable separation of these acoustic-m ode contributions from those due to the excess modes. Using the soft-p otential model, it has been shown that, at low temperatures, the exces s low-energy vibrational states provide negative contributions to the thermal expansion and to the nonlinear acoustic properties. For the mi xed metaphosphate glasses of the type (La2O3)(x)(Sm2O3)(y)(P2O5)(0.75) , the acoustic-mode softening induced by Sm3+ is negated by the additi on of La3+: as a result the acoustic-mode Gruneisen parameters and the thermal expansion are extremely small.