The Self-Organized Transformation from Hexagonal to Orthorhombic Bronze of Cs-Nb-W-O Mixed Oxides Prepared Hydrothermally

[EN] The thermal transformation of hydrothermally prepared Cs-Nb-W-O mixed oxides has been studied in the 25-1000 degrees 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 degrees C depending on the chemical composition. From this temperature, solids are selectively transformed into the orthorhombic Cs-0.5(Nb2.5W2.5)O-14 structure-type. The same results are obtained for Cs-0.5(Nb2.5W2.5-xMx)O-3, 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.