Thermodynamic Regularities in Perovskite and K2NiF4 Compounds

It has been found that the enthalpy of formation of perovskite compounds, Δ_fH° (ABO₃, B = transition metais), from binary oxides can be well characterized in terms the tolerance factor, t≡(r_A+ r_o)√2 (r_B+ r_o), where r_A and r_B are the radii of A-site ions with 12-coordination and B-site ions with 6-coordination, respectively, and Δ_fH°=?168 + 270(1 ? t) kJ·mol^?1 for A^IB^VO₃, Δ_fH°=?125 + 1000(1 ? t) kJ·mol^?1 for A^IIB^IVO₃, and Δ_fH°=? 90 + 720(1 ? t) kJ·mol^?1 for A^IIIB^IIIO₃. Although the thermodynamic data of K₂NiF₄ compounds are not extensive, a similar regularity can be found when use is made of the radii of A-site ions with 9-coordination for the K₂NiF₄ compounds. These correlations will be quite useful in predicting.