Thermoelastic properties of olivine-type sinhalite and chrysoberyl

Similar to the mullite-types, the structures of olivine-type materials are dominated by parallel chains of edge-sharing MO6 octahedra. The large number of crystal species belonging to these groups makes it possible to systematically study the influence of different cations and of different types of linkage of the octahedral chains on their properties. Therefore, we determined full sets of the coefficients of thermal expansion and elastic stiffnesses of orthorhombic sinhalite, MgAlBO4, and chrysoberyl, Al2BeO4, in the temperature range from 100 K to 1273 K and, respectively, from 100 K to 1473 K, employing a combination of dilatometry and resonant ultrasound spectroscopy. At ambient conditions the elastic behavior of sinhalite is more anisotropic than the one of chrysoberyl [1]. Both crystal species follow the general trend of increasing bulk modulus with increasing oxygen packing density as observed in many oxides with hcp-like packing of oxygen atoms. With bulk moduli of 178 GPa and 233 GPa sinhalite and chrysoberyl belong to the stiffest olivine-type materials known so far. While in chrysoberyl the softening of the elastic stiffnesses at high temperatures can be well explained by lattice vibrations, certain stiffness coefficients of sinhalite soften at an increasing rate above about 600 K. The anomalous behavior is probably related to the increasing instability of the tetrahedral BO4 groups that eventually leads to decomposition of sinhalite at about 1400 K. [1] Wang H., Gupta M.C., Simmons G.: J. Geophys. Res. 80 (1975) 3761-3764.