Effect of alkali cations on the compressibility of MAlSi3O8 glasses (M=Na, K, Rb, Cs) in the pressure range up to 6.0 GPa

The refractive indices of RbAlSi3O8 and CsAlSi3O8 glasses in the range of pressures up to 6.0 GPa were measured using a polarization-interference microscope and a diamond anvil apparatus. The changes in the relative density which characterize the compressibility of the glasses were estimated in the pressure range under investigation from the measured refractive indices within the framework of the theory of photoelasticity. The results were compared with previous data obtained for Na2O center dot Al2O3 center dot 6SiO(2) and K2O center dot Al2O3 center dot 6SiO(2) glasses. Although the molar contents of Al2O3 and M2O are identical in the MAlSi3O8 glasses (M=Na, K, Rb, Cs), the isothermal initial bulk modulus (K-t,K-0) of these glasses decreases in the order Na+, K+, Rb+, and practically does not change when Rb+ is replaced by Cs+. This change in K-t,K-0 indicates an increase in the degree of polymerization of the structural network of the glasses in the order Na+, K+, Rb+. The degree of polymerization reaches a maximum for the glasses containing Rb+ or Cs+. The increase in K'(t) for glasses in the series of alkali metal cations can be explained by the fact that the voids are occupied by cations K+, Rb+, Cs+, which have a larger orbital radius than Na+, and more strongly inhibit compression. For glasses containing Rb+ and Cs+, a more densely packed structure containing three-membered rings of tetrahedra confers additional resistance to compression.