Enthalpy of formation of selected mixed oxides in a CaO–SrO–Bi2O3–Nb2O5 system

The heats of drop-solution in 3Na2O + 4MoO3 melt at 973 K and 1073 K for calcium and strontium carbonates, Bi2O3, Nb2O5 and several stoichiometric mixed oxides in CaO–Nb2O5, SrO–Nb2O5 and Bi2O3–Nb2O5 systems were measured using a Setaram Multi HTC-96 calorimeter. The values of enthalpy of formation from constituent binary oxides at 298 K, ΔoxH, were derived for the mixed oxides under investigation: ΔoxH(CaNb2O6) = −132.0 ± 23.8 kJ mol−1, ΔoxH(Ca2Nb2O7) = −208.0 ± 31.9 kJ mol−1, ΔoxH(SrNb2O6) = −167.9 ± 19.1 kJ mol−1, ΔoxH(Sr2Nb2O7) = −289.2 ± 37.5 kJ mol−1 and ΔoxH(BiNbO4) = −41.9 ± 11.1 kJ mol−1. Additionally, the values ΔoxH for other mixed oxides with different stoichiometries were estimated on the basis of these experimental results.

Research highlights► Heats of drop-solution in 3Na2O + 4MoO3 melt. ► Enthalpy of formation for CaNb2O6, Ca2Nb2O7, SrNb2O6, Sr2Nb2O7 and BiNbO4. ► Estimation of the enthalpy of formation for calcium, strontium and bismuth niobates.