Direct, static measurement of single-crystal Young’s moduli of the zeolite natrolite: Comparison with dynamic studies and simulations

Young’s moduli of zeolite single crystals (natrolite, Na2(Al2Si3O10) · 2H2O, NAT-type) have been determined directly using static three-point bend, compression and indentation experiments for the first time. The values compare well with those previously determined from dynamic studies (Brillouin scattering and ultrasound) of the same zeolite. A strong anisotropy in elasticity is detected, in particular a larger Young’s modulus along the crystal z-axis compared to those along the x and y axes, which reflects the anisotropic, fibrous structure of natrolite. The idealised, siliceous natrolite structure has previously been predicted to possess on-axis negative Poisson’s ratios (auxetic behaviour), but the measured elastic constants are not consistent with this hypothesis. We use new simulation results via a molecular mechanics approach to show that the presence of extra-framework cations and water molecules in the true aluminosilicate zeolite framework removes the on-axis auxetic behaviour expected for the hypothetical siliceous framework and then discuss our new calculated elastic constants in relation to the experimental values.