Development of tungsten oxide hydrate phases during precipitation, room temperature ripening and hydrothermal treatment

Morphology and structure transformation of tungstic acid hydrate, H2WO4·nH2O precipitates, have been studied in aqueous phase with various levels of residual [Na+]s at room temperature and at 120 °C in autoclave. Spindle-shaped morphology is characteristic for the washing product of the freshly prepared precipitates. Residual sodium is a persistent contaminant in H2WO4·H2O. Sodium ion content in the solid phase has been found to control reaction route at hydrothermal treatment of tungstic acid hydrates. Morphological changes against [Na+]s content at 120 °C are significant. Rectangular crystallites of WO3·H2O, hexagonal platelets of WO3·1/3H2O or their mixture can develop from the precursor spindle-shaped H2WO4·H2O particles with various [Na+]s at 120 °C. It is noteworthy that particles that ripened at room temperature display faces and axes seemingly corresponding to those of the crystallites developed in the hydrothermal process.