Single-crystal elastic properties of (Cs,Na)AlSi2O6⋅H2O pollucite: A zeolite with potential use for long-term storage of Cs radioisotopes

Abstract

The single-crystal and aggregate elastic properties of the zeolite pollucite (Cs,Na)AlSi2O6⋅H2O, a potential host for Cs radionucleides in geological repositories, have been determined by Brillouin scattering spectroscopy at ambient conditions. The three nonzero individual elastic constants of cubic pollucite are: C11=105.0(1.3) GPa, C44=27.0(3) GPa, and C12=25.7(6) GPa. The Voigt–Reuss–Hill average of the aggregate bulk, shear modulus, Young's modulus and Poisson’s ratio are KS=52.2(1.0) GPa, G=31.5(7) GPa, E=78.6(1.0) GPa, and ν=0.248(4), respectively. The bulk modulus of pollucite is 12.7% lower than that of the all-Na isotypic mineral analcime NaAlSi2O6⋅H2O whereas the shear moduli G are identical within mutual uncertainties. The higher compressibility of pollucite results from the weaker Cs–O bonds compared to Na–O bonds, suggesting strong control of the nature and configuration of the extraframework content on the behavior of the structure. The elastic properties of pollucite reported here will help in the prediction of its behavior as a geological barrier and in the modeling of the short-term and long-term safety of the Cs-repositories.