Phase stability and thermodynamic properties of Nb-bearing compounds in the CaO-Nb2O5-TiO2 system

This study investigates the phase stability and isobaric heat capacities of some compounds in the CaO-Nb 2 O 5 -TiO 2 slag system derived from the Bayan Obo ore deposit. Four compounds in the CaO-Nb 2 O 5 system and three compounds in the Nb 2 O 5 -TiO 2 system were synthesized by solid-state sintering, viz CaNb 2 O 6 , Ca 2 Nb 2 O 7 , Ca 3 Nb 2 O 8 , Ca 4 Nb 2 O 9 , Nb 2 TiO 7 , Nb 10 Ti 4 O 29 and Nb 24 TiO 62' . In addition, we also tried to synthesize the potential ternary compounds C 8 N 7 T 6 , C 6 NT 3 , C 3 NT 3 , and “X” reported in literature. The phase synthesis results were characterized using a combined approach of SEM-EDS, XRD, and Rietveld refinement; then the isobaric heat capacities of the successfully synthesized compounds were determined over the temperature range from 373 K to temperatures below their melting points, from which the corresponding temperature-dependent polynomial expressions were established. Meanwhile, the standard molar enthalpies of formation and standard entropies of the synthesized compounds at 298 K were calculated using the two-parameter model. The current results can provide necessary experimental data for thermodynamic assessment and database development of the CaO-Nb 2 O 5 -TiO 2 system.