A Raman spectroscopic and energy-dispersive x-ray diffraction study of the high-pressure phase transitions of sodium hydrogen fluoride

Under ambient conditions, NaHF₂ adopts a rhombohedral structure. Two high-pressure phase transitions have been noted previously, which the authors have located at 5 kbar and 41 kbar. They have studied all three phases by Raman spectroscopy and energy-dispersive X-ray diffraction. The symmetric (FHF) stretching vibration is at a significantly higher frequency in phase I (631 cm^-1 at ambient) than in phase II (623 cm^-1 and 612 cm^-1 at 10.6 kbar). The phase I structure is also incompressible down the triad axis, along which the (FHF) groups are oriented. The phase II X-ray data were fitted by a monoclinically distorted marcasite-like cell. This structure is easily derived from that of phase I by a shear, with accompanying rotation of the (FHF) groups into two distinct orientations consistent with the observed splitting of the Raman modes. A further 5% volume discontinuity occurs at the II-III transition. The diffraction data for phase III index on a tetragonal cell whose dimensions suggest a variant of the structure adopted by KHF₂ at ambient pressure.