Coordination of Ti in crystalline and glassy fresnoites: A high-resolution XANES spectroscopy study at the Ti K-edge

The coordination environment of Ti in a variety of Ti-oxide model compounds and five fresnoites (crystalline and glassy Ba2TiOSi2O7, Sr2TiOS2O7 and Ba2TiOGe2O7) has been measured using high resolution X-ray absorption near edge structure (XANES) spectroscopy at the Ti K-edge at ambient temperature and pressure. Five-coordinated Ti, Ti-[5], is the dominant Ti species (approximate to 60% of the total Ti atoms) in the two glassy fresnoites studied. However, Ti-[4] and Ti-[6] were also detected (similar to 20% of the total Ti atoms each). Both glassy fresnoites (silicate and germanate) show the same amounts of Ti-[4], Ti-[5] and Ti-[6]. Ti-[5] is most likely present as square planar, titanyl-bearin_g moieties. Based on thermal expansion models around Ti-[5] in crystalline fresnoite, some breakage in the square planar units (and the formation of Ti-[4] and Ti-[6] by disproportionation) may be responsible for the presence of these three Ti-coordinations in the melt phase (and, consequently, in the glassy modifications).