GROWTH-STUDIES OF YVO4 CRYSTALS-(II) - CHANGES IN Y-V-O-STOICHIOMETRY

Incongruent vanadium oxide vaporization of yttrium orthovanadate (YVO4) melt generates changes in both oxygen and yttrium-vanadium (Y-V) stoichiometry. Slightly modified YVO4 or YVO4 + Y8V2O17 phases are formed from continuously changing melt systems depending on their starting compositions. The occurrence of Y8V2O17 in yttrium orthovanadate crystal fibers grown by laser healed pedestal growth (LHPG) technique can be simply eliminated by utilization of suitable V2O5-excess starting compositions. In contrast, the oxygen-deficient YVO4-x black phase is an inherently oxygen deficient phase with limited solid solution in the YVO4-YVO3 subsystem of Y2O3-V2O5-V2O3 complex ternary phase diagram close to the congruent YVO4 composition (49.3 mol% V2O5 - 50.7 mol% Y2O3). The greater oxygen deficiency YVO4-x spe cimens have smaller lattice parameters as determined from detailed XRD data. Ceramic feed rod and fiber crystal grown from the feed rod with slight yttrium oxide excess starting composition were characterized by electron microprobe analysis. Changes in both Y-V and oxygen stoichiometry were observed along the fiber, due to double limited solid solution system of three dimensional ternary phase diagram. Consequently, both the presence of Y8V2O17 phase and change of Y-V stoichiometry over oxygen deficiency cause difficulties for YVO4 crystal growth from yttrium oxide excess melt.