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Zone-center optical phonons have been measured using infrared reflectivity and Raman scattering for single-crystal, orthombic GeS. Using Kramers-Kronig analysis and oscillator fits of the reflectivity data, we have determined the infrared-active TO and associated LO phonon frequencies, and dielectric constants for all three principal polarizations. Group-theoretical analysis of the three-dimensional space group of GeS (D₂₇^16) predicts seven infrared-active phonons. Only six of these were observed. Raman measurements yielded an additional 12 zone-center phonons (in agreement with theoretical predictions). Despite the fact that GeS possesses a center of inversion symmetry, many of the infrared and Raman modes are found to be nearly degenerate. This is taken as evidence for the importance of the diperiodic layer symmetry (DG32, no inversion center) in determining the vibrational properties of GeS. The ratios of various intralyer to interlayer force constants are estimated and found to lie in the range 12: 1 to 30: 1.
Wiley et al. (Mon,) studied this question.