Hydrothermal synthesis and phase stability of new zircon- and scheelite-type ZrGeO4

Single-phase zircon- and sheelite-type ZrGeO4 were selectively synthesized from the reaction of a ZrOCl2 solution and GeO2 under mild hydrothermal conditions at 120-240 degreesC by PH control of the solution via homogeneous generation of a hydroxide ion through the decomposition of urea. New phase zircon-type ZrGeO4 octahedral-like particles that were assemblies of small crystals (10-80 nm in size) were directly formed under acid conditions, and the particle size was controlled (0.4-1.5 mum in size) by the absence or presence of urea as well as hydrothermal treatment temperature. Scheelite-type ZrGeO4 particles (50-100 nm in size) that were a coagulation of nanometer-sized crystals were formed under moderate neutral to acid conditions (6 greater than or equal to PH > 2) in the presence of urea. The zircon-type ZrGeO4 was stable below 1160 degreesC in air and transformed to the scheelite-type structure at 1180 degreesC. The dissociation of ZrGeO4 to tetragonal ZrO2 and GeO2 occurred by heat treatment under conditions above 1350 degreesC for 5 h or 1450 degreesC for 1 h.