Melting and premelting of silicates: Raman spectroscopy and X-ray diffraction of LiSiO and NaSiO.

The isostructural lithium (LiSiO) and sodium (NaSiO) metasilicates have been investigated from room temperature up to the melting point by single-crystal Raman spectroscopy and energy-dispersive X-ray powder diffraction. The unit-cell parameters and Raman frequencies of LiSiO vary regularly with temperature up to the melting point, which is consistent with the lack of premelting effects in calorimetric measurements. In contrast, NaSiO undergoes a transition at about 850 K from orthorhombic Cmc 2 symmetry, to a lower symmetry (possibly Pmc 2), and shows near 1200 K changes in the Raman spectra that correlate well with the premelting effects as determined from calorimetry observations. In both compounds, a high alkali mobility likely sets in several hundreds of degrees below the melting point. Premelting in NaSiO is associated with extensive deformation of the silicate chains as evidenced near the melting point by similarities in the Raman spectra of the crystalline and liquid phases. [ABSTRACT FROM AUTHOR]