Vibrational anharmonicity of vitreous samarium phosphate

When subjected to hydrostatic pressure, samarium phosphate glasses become easier to squeeze, an extraordinary effect attributable to fluctuating valence of the samarium ions. To examine how the vibrational anharmonicity varies with temperature in these glasses, the hydrostatic pressure derivatives of the ultrasonic wave velocities have been measured for a 20 mol% Sm 2O 3-P 2O 5 glass between 250 and 450 K and the thermal expansion from 85 to 275 K. Below room temperature, the pressure derivatives ( ∂C 11 S / ∂P) T, P=0 and ( ∂B S / ∂P) T, P=0 of the longitudinal elastic stiffness tensor component C 11 S and the bulk modulus B S become much more negative: longitudinal acoustic mode softening becomes more pronounced with decreasing temperature. This can be attributed to enhancement of the proportion of the 3+ valence state of intermediate valence (2+ to 3+) samarium ions with increasing pressure and/or decreasing temperature. The thermal expansion is small and below 110 K becomes negative; at low temperatures the contributions from the long wavelength acoustic modes, which the measured negative hydrostatic pressure derivatives of the elastic constants show have negative Grüneisen parameters, dominate due to the increased relative proportion of phonons in these low energy, small wavevector states.