Lattice dynamics of trigonal selenium. I. Phonon spectra

Phonon dispersion curves of trigonal selenium were measured as 300K. Determination of acoustic branches was performed for the directions ( zeta 00), ( zeta zeta 0) and (00 zeta ), where good agreement with ultrasonic data was found. All six optic modes could be measured in the directions ( zeta 00), ( zeta zeta 0), (00 zeta ), ( 1 / 3 1 / 3 zeta ) and ( zeta zeta 1 / 2 ). The low symmetry of Se required close interaction of theory and experiment. Several successively more elaborate models were developed which are dealt with in paper II. A valence force field (VFF) model which assumes partly covalent interchain bonding yielded good agreement in the acoustic and lower optic regime. The upper optic modes showed an important dispersion indicating the existence of long-range forces. They were treated within the framework of the shell model.