The elastic bahaviour and vibrational anharmonicity of molybdenum phosphate glasses

To examine the effects of vibrational anharmonicity on the long-wavelength phonon dynamics of a series of phosphate glasses, hydrostatic pressure and temperature dependences of ultrasonic wave velocities Ave been measured in molybdenum phosphate glasses (MoO3 x (P2O5)1-x over the composition range 35 to 76 mol % MoO3. Marked discontinuities occur in the variations of elastic constants with composition, indicating distinct differences in the nature of the structure and bonding in the glasses as a function of composition. The pressure derivatives of the elastic constants are found to be positive and the temperature derivatives negative. Both the longitudinal (γL) and shear (γS) mode Grüneisen parameters are positive, showing that application of hydrostatic pressure produces an increase in the long-wavelength acoustic phonon mode frequencies. The temperature dependences of both longitudinal and shear acoustic phonon velocities are found to be markedly anomalous in that they continue to increase substantially as the temperature is reduced below about 100 K. The low-temperature elastic constant data are compatible with the interaction of the phonons with two-level systems, and provide direct evidence for such systems in phosphate glasses