ISOTHERMAL EQUATION OF STATE AND COMPRESSIONAL BEHAVIOR OF TETRAGONAL EDINGTONITE

Abstract

The high-pressure (HP) structural evolution of a natural tetragonal edingtonite from Ice River, Canada, was investigated up to 5.1 GPa using in situ single-crystal X-ray diffraction and a diamondanvil cell. The isothermal equation of state was determined. The values of V0, KT0, and K′ refined with a third-order Birch-Murnaghan equation of state (BM-EoS) are V0 = 601.6(3) Å3, KT0 = 59(2) GPa, and K′ = 3.4(8). Under high-pressure conditions the main deformation mechanisms can be described by rotation/kinking of “rigid units,” represented by the 4 = 1 secondary building unit (SBU), due to the tetrahedra tilting. The angle between the SBUs (ϕ) increased from 17.15(8)° at 0.0001 GPa to 20.03(9)° at 4.61 GPa. The bulk structural compression results from the combination of the “soft” behavior of the not fully occupied channels [KT0 = 19(1) GPa for [100]-channels; KT0 = 21(1) GPa for [110]-channels] and of the rigid behavior of the tetrahedral framework. The extra-framework cations do not increase in coordination number within the pressure range investigated. The barium occupancy factors for the Ba1 and Ba2 sites change with increasing pressure. For P > 2.3 GPa the Ba2 site is completely empty, and only the position Ba1 is occupied.