A Ultrasonic Study of the Elastic Properties and Acoustic Mode Vibrational Anharmonicity in Glasses and Crystals.

Available from UMI in association with The British Library. Measurements of ultrasonic wave velocities and their hydrostatic pressure and temperature dependences have enabled determination of linear and nonlinear elastic properties of a number of glasses and single crystals. In each material studied, some form of softening of acoustic modes occurs either with pressure or temperature or both. To quantify this softening, the zone centre acoustic phonon mode vibrational anharmonicities are expressed in terms of the mode Gruneisen parameters gamma_ {i}(=-d lnomega _{i}/d ln V). i. Softening the shear modes under pressure is found in (Ag_2O)_ {y}(B_2O _3)_{1-y} and (AgI) _{x} ((Ag_2O) _{y}(B_2 O_3)_{1-y}] _{1-x} superionic glasses: dC _{44}/dP and gamma s are negative. Differences between dC _{rm IJ}/dP in the binary and ternary glasses implies that silver ions introduced via Ag_2O and AgI occupy different microscopic sites. ii. In (Sm_2O _3)_{x}(P_2 O_5)_{1-x} glasses, an unusual compositional dependence of the elastic behaviour is correlated with the microscopic structure changes. As pressure is increased, these glasses become easier to compress: their longitudinal and shear modes soften with pressure whilst a reduction of temperature causes slight mode softening. A relaxation process of two-level systems affects the elastic contents at low pressure. iii. The pressure dependences of the elastic constants of the cubic Ce_3S _4 crystal have been measured. Although this material has positive temperature derivatives of C _{11} and C^ ' (=(C_{11} - C_{12})/2): evidence of a phase transition at low temperatures, application of pressure does not result in mode softening. iv. In the two piezoelectric crystals, rhombohedral AlPO_4 and tetragonal Li _2B_4O_7 , application of pressure does induce shear mode softening: dC_{66}/dP and the respective gamma_{rm i} are negative. For AlPO_4 , this is consistent with a positive temperature dependence of C_{66} associated with interaction between the transverse-acoustic and the optic phonons.