Complex Rare-Earth Tantalates with Pyrochlore-Like Structure: Synthesis, Structure, and Thermal Properties

The existence of a wide range of pyrochlore-like (PL) Ln 2 FeTaO 7 compounds (Ln = Pr–Yb, including Y) with a hexagonal structure (space group R ) was shown. Two new low-temperature methods for the synthesis of Ln 2 FeTaO 7 were developed to reduce the production temperature and time, namely, coprecipitation followed by annealing and synthesis in molten salts NaCl/KCl. The synthesis of PL phases was found to involve the formation of a nanocrystalline metastable fluorite phase, the fluorite irreversibly transforming to a PL phase as the crystallization time or temperature increases. No other transitions in Ln 2 FeTaO 7 were recorded by DSC up to the melting point of 1320–1450°C (depending on the composition of the Ln 2 FeTaO 7 compound). The isobaric heat capacity C p ( T ) was measured and the thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) were calculated in the temperature range 5–1300 K for Y 2 FeTaO 7 by the way of example. In the low-temperature region of 2–25 K, an abnormal behavior of Cp(T), apparently related to the Y 2 FeTaO 7 magnetic transition, was detected.