Elastic behaviour under pressure of superionic glasses

To establish the effects of hydrostatic pressure upon the elastic behaviour of fast ionic glasses, measurements have been made of the changes induced by pressure on the velocities of ultrasonic waves propagated in glasses belonging to the systems (Ag2O)y(B2O3)1-y and (AgI)x[(Ag2O)y(B2O3)1-y]1−x. The pressure derivative (∂Bs/∂P)p=0, of the adiabatic bulk modulus Bs is found to be positive. However, the shear modulus (∂C44/∂P)p=0 is small but negative, indicating slight softening of shear modes under pressure. In the binary glass series (Ag2O)y(B2O3)1-y, increase in Ag2O content results in decreases in the pressure gradients of the natural velocity for both shear and longitudinal waves. In contrast, an increase in AgI in a ternary glass of fixed Ag2O to B2O2 ratio (y = 0.33) produces an increase in these pressure gradients. The implication that the silver ions introduced via Ag2O and AgI occupy different sites is in accord with the microdomain model for the structure of the ternary glasses. The vibrational anharmonicity of the long-wavelength acoustic phonons in these glasses is discussed on the basis of the Grüneisen parameter approach.