Brillouin scattering study on the single-crystal elastic properties of natrolite and analcime zeolites

The Brillouin light-scattering technique was used to investigate the single-crystal elastic properties of two aluminosilicate zeolites, natrolite (NAT) and analcime (ANA), at ambient conditions. An inversion of the acoustic velocity data results in the full set of elastic stiffness moduli (C-ij's) for both materials. From the single-crystal moduli the aggregate adiabatic bulk moduli (K-s), shear moduli (G), and Poisson's ratios (nu) were found to be K-s=48.5(1.0) GPa, G=31.6(1.0) GPa, and nu=0.232(5) for NAT, and K-s=59.8(1.2) GPa, G=32.1(1.0) GPa, and nu=0.272(5) for ANA (Voigt-Reuss-Hill averages). The bulk and shear moduli of both zeolites are relatively low compared with those of densely packed aluminosilicates, reflecting an open framework structure of (Al,SiO4) tetrahedra which is easily deformed by bending the Si-O-Al angles. As expected for a less dense crystal, NAT is softer and more compressible than ANA. An evaluation of the directional Young's moduli shows that the compressibility of NAT is nearly uniform along the [100] and [010] axes, while [001] is stiffer, in agreement with previous compression studies. We do not find experimental evidence of negative Poisson's ratios for NAT zeolites as predicted by recent theoretical calculations. (c) 2005 American Institute of Physics.