Temperature and pressure dependence of the Raman spectrum of crystallineP4S3

The Raman spectrum of ${\mathrm{P}}_4$ ${\mathrm{S}}_3$ has been investigated as a function of temperature (10 to 320 K) at ambient pressure and as a function of hydrostatic pressure to 86 kbars at room temperature. External and internal mode frequencies have been identified by studying the spectrum in solution and by observing pressure-induced changes in the spectrum. The external modes, which are believed to be due to the librational motion of the molecules in the crystal, disappear completely at the crystalline-plastic phase transition near 314 K, leaving only a wing to the Rayleigh scattered component. From this result it is concluded that the ${\mathrm{P}}_4$ ${\mathrm{S}}_3$ molecules gain rotational freedom in the high-temperature plastic $\beta$ phase. The external mode frequencies are particularly strongly pressure dependent whereas the internal mode frequencies are affected to a lesser extent by pressure. The low-temperature spectrum reveals crystal-field and Davydov splitting because of line narrowing, while pressure enhances these splittings because of stronger intermolecular interaction resulting from compression. The external mode spectrum is richer at high pressure due to increased resolution resulting from a selective pressure effect on the external mode frequencies. The $\frac{\mathrm{dv}}{\mathrm{dP}}$ data are analyzed in terms of the scaling law for the Grüneisen parameter proposed by Zallen. The volume and the phonon excitation contributions to $\frac{\mathrm{dv}}{\mathrm{dT}}$ are analyzed.