Dynamics of Sodium and Potassium Bifluoride: Infrared, Raman, and Neutron Studies

Abstract

The crystal and molecular dynamics of NaHF2 and KHF2 have been studied by infrared and Raman spectroscopy and by neutron inelastic scattering. Infrared absorption spectra have been measured in the region of the bending (v2) and asymmetric stretching (v3) vibrations of NaHF2 and KHF2 containing 2%, 11%, and ≈ 80% DF2−. The spectra show a rather striking reduction in the widths of the DF2− absorption peaks as well as shifts in peak frequencies as the percent DF2− is lowered toward a ``defect'' concentration. These results indicate that the considerable width of the infrared absorption bands assigned to the v2 and v3 modes in pure NaHF2 and KHF2 is associated with the coupling of near-neighbor HF2− oscillators having similar frequencies or energy states. The Raman spectrum for NaHF2 shows peaks at 630.5 and 145 cm−1 which can be unambiguously assigned, respectively, to the HF2− symmetric stretching mode (A1g) and the Eg librational lattice mode. The KHF2 Raman spectrum is similar to a previous result, with some differences in the frequencies of the lattice-mode peaks. The combined neutron and far-infrared spectra on NaHF2 provide information on the acoustic and optical translational lattice modes. An approximate vibrational frequency distribution is derived from the neutron spectrum and compared with previous spectroscopic and theoretical results on NaN3. A detailed assignment of the internal and lattice modes for both sodium and potassium bifluoride is presented.