X-Ray and Thermal Analysis of Quenched Ammonium Fluoride II and III: Three New Phases

Abstract

Ammonium fluoride II and III have been recovered at 100°K and atmospheric pressure and examined by x-ray powder diffraction and by thermal analysis. The structure of phase III is closely related to that of cesium chloride. Phase II when heated from 100°K at atmospheric pressure transforms to a new phase, designated V, at about 120°K and V in turn transforms to I at about 230°K. Phase III under the same conditions transforms at about 120°K to a new phase, designated VI, and this in turn transforms to another new phase designated VII at about 200°K. On further heating to 230°K, phase VII transforms to I. Phases V and VII are probably stacking variants of the wurtzite-type structure of ammonium fluoride I. Phases II and VI have powder patterns too complicated to analyze unambiguously on the data available.

Variation of the Raman frequency of a soft mode with the pressure (20 oC) for the phase transitions in NH4F

“…As the pressure increases to 11.5 kbar at 25 o C, II-III transition occurs in NH₄F, which was also obtained in earlier experiments [7]. It was shown that at 100 K and 1 bar, there were three possible phases at low temperatures [8]. From the measurements of the Raman spectra at 20 o C up to 20 kbar, two new phases of V and VI have been observed at 15 and 143 kbar, respectively [5].…”

Energies of the phases of ice at low temperature and pressure relative to ice Ih

“…Stacking variants are known in the closely related system ammonium fluoride (28), phase I of which is isomorphous with ice Ih, but have never been reported in the phases of ice. Stacking faults are known to occur in ice Ih (29) but seem not to have been identified in the numbers needed to e x~l a i n differences in energy of transformation of as much as 40 the difference in the energies of transformation of ice Ic to Ih.…”

Behavior of the AB-type compounds at high pressures and high temperatures

“…This phase could be indexed on the basis of a wurtzite-like structure. A study of quenched phase III revealed two new metastable phases NH4F(VI) and N~ F(VII), which appeared at approximately 115 K and 200 K, respectively, upon heating from 77 K to room temperature [134]. Neither of these phases could be unambiguously identified.…”