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

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 $\text{≈ 80\%\hspace{0.17em}}{\mathrm{DF}}_2^{-}.$ The spectra show a rather striking reduction in the widths of the ${\mathrm{DF}}_2^{-}$ absorption peaks as well as shifts in peak frequencies as the percent ${\mathrm{DF}}_2^{-}$ 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 ${\mathrm{HF}}_2^{-}$ 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 ${\mathrm{HF}}_2^{-}$ symmetric stretching mode (A_1g) and the E_g 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.