Abstract
The thermal transformation of hydrothermally prepared Cs–Nb–W–O mixed oxides has been studied in the 25–1000 °C temperature range. Structural evolution of the prepared solids was monitored by in situ synchrotron X-ray diffraction, absorption spectroscopy techniques, conventional powder X-ray diffraction, thermogravimetric analysis, infrared and Raman spectroscopy, and high resolution transmission electron microscopy (HRTEM and STEM). The as-grown solids show the hexagonal tungsten bronze (HTB) structure, which can be stabilized above 900 °C depending on the chemical composition. From this temperature, solids are selectively transformed into the orthorhombic Cs0.5(Nb2.5W2.5)O14 structure-type. The same results are obtained for Cs0.5(Nb2.5W2.5–xMx)O3, with M = V or Mo (x = 0–1.25). A mechanism for the quantitative transformation at high temperature is proposed in terms of the relative occupancy of the hexagonal channels in the HTB-type phase as well as the Nb/(W + V) ratio.
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