Vibrational spectra of transition metal hexafluoride crystals. I. Orthorhombic MoF6, WF6, and UF6 neat crystals

Neat crystal Raman spectra of orthorhombic MoF₆, WF₆, and UF₆ are presented and discussed. This work lays the foundation for presentation of two-particle and total band structures in the two following papers. The neat crystal spectra are interpreted in terms of an internal-external mode separation and a conventional factor group analysis. In general, these data lead to four essential conclusions concerning hexafluoride crystals: (a) Fermi resonance plays an important role in producing the observed energy and intensity differences in this series; (b) k = 0 exciton structures are remarkably similar in the three crystals; (c) similarities and differences between these solids can be directly correlated with molecular parameters such as dipole derivatives, polarizabilities, etc.; and (d) phonons (external modes) can be neatly separated into rotational and translational normal modes.