Studies of New Order-Disorder Structural Transitions in Ln2M2O7 (Ln = Lu, Gd; M = Ti)

Phase formation processes at T = 350–1000°C in Lu(Gd)-Ti-O samples obtained by co-precipitation method are studied. Thermal processing of freeze dried co-precipitation products results in formation of Lu2Ti2O7 with fluorite structure at 650°C. The phase transition of fluorite to pyrochlore occurs at 750–800°C. At T > 800°C Lu2Ti2O7 possesses the structure of the disordered pyrochlore with antistructural defects. Further heating leads to the ordering of pyrochlore at T > 900°C. This process is accompanied by the appearance of a maximum at the temperature dependence of dielectric permeability of Lu2Ti2O7. The grain size of Lu2Ti2O7 ceramics annealed at 950°C was found to be 13–15 nm. The electric conductivity of this ceramics at 200°C (10−5 S/cm) is 5 orders higher than the values usually observed for Lu2Ti2O7 fabricated by a common solid state synthesis method. In Gd-Ti-O system only pyrochlore crystallization process at 740–900°C was observed.