Effect of hydrostatic pressure up to 6 GPa on the crystal structures of ammonium and sodium hexafluorosilicates, (NH4)2SiF6and Na2SiF6; a phase transition in (NH4)2SiF6at 0.2–0.3 GPa

Abstract

Anin situX-ray powder diffraction study has shown the cubic modification of (NH4)2SiF6(cryptohalite, space groupFm¯3m) at 0.2–0.3 GPa to transform irreversibly into the trigonal bararite, space groupP¯3m1. The anisotropy of lattice strain in the high-pressure phase of (NH4)2SiF6was measured up to 6 GPa and compared with that in the trigonal Na2SiF6. The pressure-induced changes in the packing of (SiF6)2–anions were analyzed, the changes in the IR-spectra of (SiF6)2–ions induced by increasing pressure and on decompression were measured and compared for trigonal (NH4)2SiF6and Na2SiF6.