Energetics of La_1-xA_xCrO_3-δ perovskites (A = Ca or Sr)

A series of perovskites with the general formula La_1-xA _xCrO_3-δ (A = Ca or Sr) have been synthesized in the solid solution range 0.0<x≤0.3 and 0.0≤δ≤0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B₂O₃ at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction 2Cr_Cr^x + V _O^.. + 0.5O₂(g) = 2Cr_Cr^. + O_O^x is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from -620±260 to -280±80 kJ/mol O₂ when x = 0.1-0.3 for Ca-doped samples, and from -440±150 to -290±50 kJ/mol O₂ for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of V_O^..) for the reaction A _A^x + O_O^x = A′_La + 0.5V_O^.. + 0.5O2(A = Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO ₃ is more stable than its Ca-doped counterpart, which is consistent with basicity arguments.