Hydrothermal Synthesis and Phase Stability of New Zircon- and Scheelite-Type ZrGeO4

Single-phase zircon- and sheelite-type ZrGeO 4 were selectively synthesized from the reaction of a ZrOCl 2 solution and GeO 2 under mild hydrothermal conditions at 120−240 °C by pH control of the solution via homogeneous generation of a hydroxide ion through the decomposition of urea. New phase zircon-type ZrGeO 4 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 μm in size) by the absence or presence of urea as well as hydrothermal treatment temperature. Scheelite-type ZrGeO 4 particles (50−100 nm in size) that were a coagulation of nanometer-sized crystals were formed under moderate neutral to acid conditions (6 ≥ pH > 2) in the presence of urea . The zircon-type ZrGeO 4 was stable below 1160 °C in air and transformed to the scheelite-type structure at 1180 °C. The dissociation of ZrGeO 4 to tetragonal ZrO 2 and GeO 2 occurred by heat treatment under conditions above 1350 °C for 5 h or 1450 °C for 1 h.