Optical Phonons and Crystalline Symmetry of InSe

A survey of the Raman and infra red data for the layered structure InSe shows that there is no agreement upon its crystallographic structure. The polytypes, β, ε, and γ are equally invoked by the various groups. Some experiments on samples from different laboratories are repeated. Infrared transmission at different temperatures and Raman spectra away from resonance are obtained. It is concluded on the basis of the optical data that InSe crystallizes only in the β‐polytype. All previous optical measurements can be explained with the β‐polytype if one takes into account the resonance Raman effect. It is inferred from the Raman and infra red data that the interlayer interaction cannot be exclusively of the van der Waals type and that long range Coulomb forces contribute to the lattice dynamics of InSe.