Effect of hydrostatic pressure up to 6 GPa on the crystal of ammonium and sodium hexafluorosilicates, (NH4)(2)SiF6 Na2SiF6; a phase transition in (NH4)(2)SiF6 at 0.2-0.3 GPa

An in situ X-ray powder diffraction study has shown the cubic modification of (NH4)(2)SiF6 (cryptohalite, space group Fm (3) over barm) at 0.2-0.3 GPa to transform irreversibly into the trigonal bararite, space group P (3) over bar m1. The anisotropy of lattice strain in the high-pressure phase of (NH4)(2)SiF6 was 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)(2)SiF6 and Na2SiF6.