Variation of the Raman frequency of a soft mode with the pressure (20 oC) for the phase transitions in NH4F
Abstract
The Raman frequency of a soft mode (238 cm-1)
is analyzed as a function of pressure at 20 oC for NH4F
using the experimental data from the literature. This analysis is performed for
the pressure dependence of the Raman frequency shifts (1/n)(¶n/¶P)T of the
soft mode close to the I - III, III - V and V – VI transitions in NH4F.
The frequency shifts increase as the pressure increases toward the phase
transitions at T = 20 oC (293 K) in this ammonium structure. From
the frequency shifts of the soft mode studied, the pressure dependence of the
isothermal compressibility is predicted through the mode Grüneisen parameter. Our
calculated isothermal compressibility can be compared with the experimental
measurements.
Keywords
References
- [1] Raghurama G, Narayan R. The structures of the ammonium halides. J Phys Chem Solids 1983; 44: 633-638.
- [2] Levy HA, Peterson SW. Neutron diffraction study of the crystal structure of ammonium chloride. Phys Rev 1952; 86: 766.
- [3] Stevenson R. Phase transitions in the ammonium halides. J Chem Phys 1961; 34: 1757.
- [4] Pistorius CWFT. Melting curves and phase transitions of the ammonium halides to 40 kbar. J Chem Phys 1969; 50: 1436.
- [5] Zou G, Zhao Y, Cui O, Jin Z. Raman spectra and phase transitions of NH4F at high pressure. Solid State Physics Under Pressure. In: Mimonura S (Ed.) Terra Scientific Publishing Company, 1985. pp. 191.
- [6] Salihoğlu S, Yurtseven H, Enginer Y. P-T phase diagram for NH4F. Solid State Sciences 2002; 4: 529-534.
- [7] Swenson CA, Tedeschi JF. Phase transitions in ammonium fluoride. J Chem Phys 1964; 40: 1141.
- [8] Nabar MA, Calvert LD, Whalley E. J Chem Phys 1989; 91: 1353.
- [9] Wyckoff RGW. Crystal Structure. Interscience Publishers Inc., New York, 2nd ed. Vol. 1, 1963.
- [10] Pistorius CWFT. Progress in Solid State Chemistry vol 11, 1976 ed. McCaldin JO, Somorja G. New York: Permagon. pp. 11.