Temperature dependence of the Raman spectrum of tetraphosphorus triselenide

Abstract

The Raman spectrum of P 4Se 3 has been measured as a function of temperature in the range 20–370 K, at ambient pressure. The internal and external first-order Raman-active phonons vary smoothly in wave number over the temperature interval 20–350 K, but the crystalline-plastic phase change at 355 K. results in the loss of the external mode manifold from the 370 K Raman spectrum, leaving only a wing to the Raleigh-scattered component. The temperature coefficients of most external phonons are larger than those of the energetically well-separated internal phonons. The lowest wave number rigid layer phonon displays soft behaviour (d ṽ/d T positive) whereas the remaining members of the external mode manifold have negative temperature coefficients which increase in magnitude approximately as the phonon wave number increases. Some of the internal mode Davydov components show near-zero or positive temperature coefficients, a behaviour similar to that observed for other chalcogenide semiconductors.