Polarized Raman scattering study of terminal stretching modes inK0.30MoO3

Abstract

Polarized Raman spectra of blue bronze ${\mathrm{K}}_{0.30}{\mathrm{MoO}}_{3}$ are reinvestigated focusing particularly on a frequency range from 900 to 1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, where remarkably sharp Raman bands, ``spikes,'' appear. This Brief Report proves that previously reported results on the spikes of blue bronze include serious faults with respect to (1) the selection rule, (2) the number, and (3) the origin of the bands: (1) The spikes are Raman active both above and below the Peierls transition temperature ${(T}_{p}),$ in contrast to previous studies, where the spikes were Raman inactive above ${T}_{p}.$ (2) Four spikes are observed with $\mathbf{E}\ensuremath{\perp}\mathbf{b}$ polarization, while in all the previous studies, the number of the spikes was reported to be three. (3) The origin of the spikes should be attributed to terminal Mo-O stretching vibrations rather than to electronic transitions previously proposed. In addition, vibrational mode assignment of the spikes based on the terminal Mo-O stretching vibration model is successfully made. This suggests that an analysis of terminal stretching Mo-O vibrations with Raman spectroscopy can be a useful probe for blue bronze.