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

✍ Scribed by Boldyreva, Elena V.; Shakhtshneider, Tatyana P.; Sowa, Heidrun; Ahsbahs, Hans

• Book ID: 115462138
• Publisher: Oldenbourg Wissenschaftsverlag
• Year: 2007
• Tongue: English
• Weight: 461 KB
• Volume: 222
• Category: Article
• ISSN: 2194-4946
• DOI: 10.1524/zkri.2007.222.1.23

✦ Synopsis

An in situ X-ray powder diffraction study has shown the cubic modification of (NH~4~)~2~SiF~6~ (cryptohalite, space group Fm¯3__m__) at 0.2–0.3 GPa to transform irreversibly into the trigonal bararite, space group P¯3__m__1. The anisotropy of lattice strain in the high-pressure phase of (NH~4~)~2~SiF~6~ was measured up to 6 GPa and compared with that in the trigonal Na~2~SiF~6~. The pressure-induced changes in the packing of (SiF~6~)^2–^ anions were analyzed, the changes in the IR-spectra of (SiF~6~)^2–^ ions induced by increasing pressure and on decompression were measured and compared for trigonal (NH~4~)~2~SiF~6~ and Na~2~SiF~6~.