Effects of alkali and alkaline-earth cations on the high-pressure sound velocities of aluminosilicate glasses

The sound velocities (compressional wave velocity [V-P] and shear wave velocity [V-S]) of four types of aluminosilicate glasses (Mg3Al2Si6O18 (MAS), Ca3Al2Si6O18 (CAS), Na3AlSi3O9 (NAS), and K3AlSi3O9 (KAS)) are measured using the ultrasonic technique at high pressures of up to 7.8 GPa. The V-P and V-S of MAS glass decrease up to a pressure of 2 GPa and subsequently increase with increasing pressure. The pressure dependence of the CAS glass velocities changes; V-P remains almost constant when P <= 2 GPa and subsequently increases above 2 GPa. The minimum V-S can be observed at approximately 2 GPa, which is similar to that in the case of the MAS glass. The sound velocities of the NAS and KAS glasses monotonically increase with pressure. The increments in the V-P and V-S of the KAS glass show less sensitivity when compared with that observed in the case of the NAS glass within the pressure range of our experiments. The differences in the properties of the modifying cations in the glasses, such as size (Mg-[5](2+) < Ca-[similar to 6-7](2+) approximate to Na-[similar to 6-7](+) < K-[similar to 9-11](+)) and field strength (ratio of the charge to the square radius), can be considered responsible for each sound velocity trend. The effects of the cation field strength on the structure and elasticity of the aluminosilicate glasses could govern the pressure-induced change in sound velocities. The results indicate that the type and amount of cation control the elastic behavior of silicate glass under high pressure.