THE SINGLE-CRYSTAL ELASTIC-MODULI OF NEIGHBORITE

The adiabatic elastic moduli of a single crystal of Neighborite (NaMgF3 perovskite) have been measured at ambient conditions using Brillouin spectroscopy. The adiabatic aggregate (Voight-Reuss-Hill) bulk modulus is K = 75.6 GPa, and shear modulus is mu = 46.7 GPa. The experimental results show the ratio of linear compressibilities beta(bBAR)/beta(aBAR) = 0.80 for neighborite. These ratios reflect the different amounts of tilting freedom of the octahedral framework along each lattice axis of the perovskite structure. It is understood that the elastic compliance S(ij) of the crystal can directly sense the behavior of the octahedral tilting in the structural distortion of NaMgF3 perovskite. The octahedral tilting angles are considered to be the order parameters of the ferroelastic phase transition in the perovskite structure. Single crystal elasticity data provide a basis for understanding the role of octahedral tilting in the ferroelasticity of perovskite. Together with high pressure compressional data, one can thus elucidate the relationship between crystal structure and physical properties of perovskite. A detailed assessment indicates that the dominant compression mechanism for NaMgF3 perovskite is shortening of the octahedral [Mg-F] bond, which is also true for orthorhombically distorted MgSiO3 perovskite.