Bismuth niobates doping by lutetium and dysprosium: Synthesis, formation enthalpies, heat capacity

Abstract

Bismuth niobates doped by lutetium and dysprosium (Bi3Nb0.2Lu0.8O6.2, Bi3Nb0.2Dy0.8O6.2) were synthesized by solid state reaction. Bi3Nb0.2Lu0.8O6.2 was prepared for the first time. Compound Bi3Nb0.2R0.8O6.2 is one of the perspective materials for oxygen ceramic generators, photocatalytic processes, etc. For the first time, the standard formation enthalpies for compounds Bi3Nb0.2Lu0.8O6.2, Bi3Nb0.2Dy0.8O6.2 were studied by reaction calorimetry, and lattice enthalpies were calculated. It was found that standard formation enthalpies and lattices enthalpies for compounds Bi3Nb0.2R0.8O6.2 (R ​= ​Dy, Lu) were increasing in absolute value with decreasing radius of rare-earth element. To predict thermodynamic characteristics of uninvestigated compounds of Bi3Nb0.2R0.8O6.2 with other rare-earth elements, formulas were derived that related lattice enthalpies and standard formation enthalpies with radii of rare-earth elements at low doping. It was shown that dependences were linear. The heat capacity for Bi3Nb0.2Dy0.8O6.2 was measured by DSC calorimetry in the temperature range of 167–674 ​K. It was found that compound Bi3Nb0.2Dy0.8O6.2 had phase transition at temperatures about 500 ​K. It was shown that Bi3Nb0.2Lu0.8O6.2 was most thermodynamically stable and preferred for application.