PREPARATION OF YVO4 POWDER FROM THE Y2O3+V2O5+H2O SYSTEM BY A HYDROLYZED COLLOID REACTION (HCR) TECHNIQUE

Prior to the formation of YVO4 in the Y2O3 + V2O4 + H2O system, two intermediate, partially hydrophobic, complex colloidal mixtures with metastable characteristics can be produced at room temperature and atmospheric pressure. The ball-milled system, having both hydrophobic and hydrophilic species, transforms into the stable yttrium orthovanadate phase due to intensive hydrolysis. At room temperature an orange mixture (possessing dispersed Y2O3 and 4Y(2)O(3-p)(OH)(p)(p+). 2VO(3)(-), Y2O3-p(OH)(p)(p+). 6VO(3)(-). XH(2)O-like heteroaggregations) formed by 20 h mixing at pH ca. 4.0 transforms slowly, another red-brown heavily flocculated colloidal mixture (with dispersed Y2O3 and Y2O3-p(OH)(p)(p+). V10O286-. yH(2)O-like aggregation) formed by 70 h mixing at pH ca. 4.5 transforms rapidly into YVO4 in water. During additional mixing of highly diluted red-brown mixtures this transformation can be completed at room temperature. At elevated temperatures (50-95 degrees C) the orange mixture precipitates into a red-brown decavanadate-type precipitatium which subsequently can also rapidly hydrolyse into an orthovanadate phase in the diluted aqueous systems. Both vanadium excess meta- and decavanadate-type aggregations exhibit amorphous character by X-ray diffraction. The semi-hydrophobic colloidal structure can modify the dissociation mechanism, which prevents the system from returning to the starting oxides, and gives a new HCR technique for YVO4 preparation with a simple hydrolysis process at low temperatures and atmospheric pressure.