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

Abstract

Raman spectra of solid and liquid beryllium chloride and of binary liquid BeCl₂–CsCl mixtures have been measured. Systematic investigation of the Raman spectra upon heating the crystalline forms of BeCl₂ revealed two different solid phase transitions at 350 and 405 °C. A glass phase was also obtained by vapour transport with a crystallisation temperature at ∽250 °C. The Raman spectra of the molten and glassy BeCl₂ show bands at common frequencies that are correlated to certain vibrational modes of two solid allotropic phases. Temperature dependent measurements of the Raman spectra of a series of BeCl₂–CsCl melt compositions suggest that: (a) at X_BeCl2⩽0.5 the liquid structure is predominated by tetrahedral and trigonal beryllium species in equilibrium, BeCl₄²⁻⇌BeCl₃⁻+Cl⁻; and (b) at X_BeCl2>0.5 two types of polynuclear beryllium ionic species are formed, one consisting of a ‘‘chain’’ of edge-bridged BeCl₄ tetrahedra and the other having a ‘‘cluster’’ like structure of vertex-bridged BeCl₄ tetrahedra. The spectra of pure BeCl₂ liquid and glass are interpreted in terms of similar neutral ‘‘chain ’’ and ‘‘cluster ’’ structures 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.