Raman spectroscopic study of BeCl2 in the crystalline, glassy and liquid states and of molten BeCl2-CsCl mixtures

Raman spectra of solid and liquid beryllium chloride and of binary liquid B eCl2-CsCl mixtures have been measured. Systematic investigation of the Rama n spectra upon heating the crystalline forms of BeCl2 revealed two differen t solid phase transitions at 350 and 405 degrees C. A glass phase was also obtained by vapour transport with a crystallisation temperature at similar to 250 degrees C. The Raman spectra of the molten and glassy BeCl2 show ban ds at common frequencies that are correlated to certain vibrational modes o f two solid allotropic phases. Temperature dependent measurements of the Ra man spectra of a series of BeCl2-CsCl melt compositions suggest that: (a) a t X-BeCl2 less than or equal to 0.5 the liquid structure is predominated by tetrahedral and trigonal beryllium species in equilibrium, BeCl42- reversi ble arrow BeCl3- + Cl-; and (b) at X-BeCl2 > 0.5 two types of polynuclear b eryllium ionic species are formed, one consisting of a "chain'' of edge-bri dged BeCl4 tetrahedra and the other having a "cluster'' like structure of v ertex-bridged BeCl4 tetrahedra. The spectra of pure BeCl2 liquid and glass are interpreted in terms of similar neutral "chain'' and "cluster'' structu res having as ending units trigonally coordinated beryllium atoms. On going from the glass to the melt and/or with increasing temperature the "cluster '' structures unfold to form "chains''. Similarities also exist between the structures of molten BeCl2 and of glassy SiSe2.