Infra-red effective charges for amorphous, monoclinic and trigonal Se

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Abstract
Model calculations are presented which identify the dynamic effective charge mechanisms responsible for infrared (i.r.) absorption via vibrational modes in amorphous (a-Se), monoclinic (mono-Se) and trigonal (trig-Se) selenium. The calculations support previous studies which showed that the vibrational properties of amorphous and monoclinic Se were similar, and dominated by intra-molecular effects, and in contrast that the vibrational properties of trigonal Se were qualitatively different due to stronger inter-molecular interactions. The i.r. absorption spectrum of the Se, molecule (the molecular constituent of mono-Se) is fitted using short-range valence force field parameters and dynamic i.r. effective charges. A new dynamic effective charge mechanism is introduced, to account for i.r. absorption in the bond-bending bands, which is generally applicable for atoms having non-bonding electrons. The same force constants and i.r. erective charges give an excellent quantitative description of the vibrational properties of a-Se, subject to constraints on the nature of the dihedral angles assumed for the polymeric constituent. The same parameters are used to calculate the vibrational density of states and i.r. spectrum for an isolated helical chain, but it has been found that this structure does not give a qualitatively correct description of the i.r. properties of the intra-chain modes of trig-Se. We demonstrate that the i.r. properties of trig-Se can be adequately described by (a) using a force constant model that includes both inter-and intrachain forces, and (b) introducing a dynamic effective charge mechanism for charge displacement between intra- and inter-chain bonds.