In molecular solids, vibrational frequencies can be profoundly affected (shifted by > 50%) by relatively modest hydrostatic pressures. We have carried out an extensive Raman-scattering investigation of the influence of pressure and temperature on a large number of phonons in two inversely related chalcogenide molecular crystals, ${\mathrm{As}}_4$${\mathrm{S}}_4$ and ${\mathrm{S}}_4$${\mathrm{N}}_4$. Our results for ${\mathrm{As}}_4$${\mathrm{S}}_4$ in particular, with over 30 phonons tracked under pressure, provide the most complete picture thus far available for the effect of compression on the vibrational spectrum of any solid. These detailed data for the two chalcogenides have allowed us to test and extend a vibrational scaling law for molecular solids. The logarithmic pressure derivative $\left(\frac{1}{\omega }\right)\left(\frac{d\omega }{\mathrm{dP}}\right)$ is very large and varies little from mode to mode within the external-mode regime, then drops rapidly with increasing $\omega$ within the internal-mode regime. Raman spectra measured at low temperatures, combined with those observed at high pressures, have allowed us to discern many intermolecular-interaction effects (Davydov splittings, low-lying lattice lines) and to locate most of the lines predicted for these crystals by a weak-coupling picture. Also, knowledge of both the temperature and the pressure dependences has enabled us to separate the phonon-excitation ("explicit") and the volume-driven ("implicit") contributions to $\frac{d\omega }{\mathrm{dT}}$. ${\mathrm{As}}_4$${\mathrm{S}}_4$ exhibits a regular behavior: for external modes, $\frac{d\omega }{\mathrm{dT}}$ is dominated by the implicit effect; for internal modes, it is dominated by the explicit effect. ${\mathrm{S}}_4$${\mathrm{N}}_4$, anomalous in several respects, exhibits an erratic spectral variation of the explicit-implicit mix.

• Received 24 April 1978

DOI:https://doi.org/10.1103/PhysRevB.18.5775