Low‐temperature Raman spectra of polycrystalline NH4F and ND4F

Abstract

AbstractRaman spectra of polycrystalline ammonium fluoride and fully deuterated ammonium fluoride were recorded at various temperatures between 80 and 300 K. The spectra were obtained using a cold stage attached to a Raman microscope. At low temperatures the Raman lines were sharper and shifted to higher wavenumbers compared with their room temperature counterparts. However, no indications of any crystalline phase change were observed at low temperatures. A strong line at 2876 cm−1 was assigned to the infrared inactive symmetric NH stretching mode ν1(A1). A very strong line at lower wavenumbers (2818 cm−1) was assigned to the antisymmetric stretching mode ν3(F2). Observed deuterium isotope shifts were in agreement with those calculated using the Redlich–Teller isotope product rule. The degenerate bending modes ν2(E) and ν4(F2) were observed in the Raman spectra of NH4F and ND4F for the first time. The ν2 fundamental was split into doublets in the low‐temperature spectra, as predicted from a factor group analysis based on the known crystal structure. Three bands due to translational lattice modes were observed between 400 and 200 cm−1 in the spectra of both isotopic molecules. Several bands in both spectra are assigned to overtones and combinations involving an ammonium ion libration, but no bands directly attributable to ammonium ion librations were observed. Copyright © 2001 John Wiley & Sons, Ltd.