STANDARD MOLAR ENTHALPIES OF FORMATION OF PRO2 AND SRPRO3 - THE UNUSUAL THERMODYNAMIC STABILITY OF APRO(3) (A=SR, BA)

The standard molar enthalpies of formation Delta(f)H(m)(o) of the fluorite oxide PrO2 and the perovskite oxide SrPrO3 have been determined by solution calorimetry. A combination of appropriate thermodynamic cycles leads to the values Delta(f)H(m)(o)(PrO2, 298.15 K) = -(959.8 +/- 4.1) kJ . mol(-1) and Delta(f)H(m)(o)(SrPrO3, 298.15 K) = -(1588.4 +/- 4.1) kJ . mol(-1). Unusual stability is found for SrPrO3 in terms of the enthalpy of reaction of the binary oxides to form the ternary oxide: Delta(f)H(m)(o){SrO(cr) + PrO2(cr) = SrPrO3(cr)} = -39 kJ . mol(-1). This stability of SrPrO3 is consistent with earlier thermochemical work on BaPrO3. The Pr members of the series of perovskite oxides AMO(3)(A = Sr, Ba; M = transition elements: Ce, Pr, Tb, actinides) are substantially more stable than expected from the trend established by the other members of the series, in which the enthalpy of reaction of the binary oxides to form the ternary oxide becomes less negative as the perovskite becomes more distorted. Electronic stabilization of the perovskite (PrO3)(2-) framework in these 4f(1) compounds is proposed as the origin of the additional thermodynamic stability of the perovskite oxides of Pr4+.