Glass-forming ability of TeO₂ and temperature induced changes on the structure of the glassy, supercooled, and molten states

Polarized (VV) and depolarized (VH) Raman spectra are obtained for glassy, supercooled, and molten TeO₂ at temperatures up to 1000 K in order to elucidate the temperature evolution of the pertinent structural and vibrational properties. The intrinsic tendency of the system for crystallization is avoided by means of a newly applied protocol, thereby enabling the recording of Raman spectra of pure TeO₂ on going from the molten to the supercooled liquid and to the room temperature glass states. Following an appropriate fitting procedure, the revealed bands are assigned to specific modes of structural polymorphs. A weak polarised band at ∼880 cm^-1 is assigned to Te=O terminal stretching in agreement with the literature ab initio molecular orbital calculations. Subtle changes to the relative band intensities within the 550-900 cm^-1 stretching region are caused by temperature increase. The network-like structure of the glass/melt is composed by TeO₄ trigonal bipyramid and TeO₃ trigonal pyramid units. With increasing temperature, TeO₄ units convert to TeO₃ units with a concurrent increase in the number of Te=O sites resulting from cleavages within the network structure. The fraction of such terminal oxygen atoms has been directly estimated from the spectroscopic data. The relative populations of the basic building blocks and the average number of O atoms around Te have been estimated for a wide temperature range directly from the Raman spectra, implying a gradual transformation of TeO_4/2 to TeO_2/2(= O) trigonal pyramid units. The results are discussed in the context of the current phenomenological and theoretical status of the field.