A thermochemical study of the phase reaction TbO_1.5+x + ( 3 28 - x 2) O₂ → ( 1 7)Tb₇O₁₂

The heat of reaction and equilibrium pressure for the oxidation reaction TbO_1.5+x + ( 3 28 - x 2) O₂ → ( 1 7)Tb₇O₁₂ have been measured by means of a Tian-Calvet-type calorimeter and a thermal balance. The results of the weight measurement show a reproducible hysteresis loop. The heat of reaction has been measured along the oxidation branch of the hysteresis loop. The partial molar enthalpy indicates four distinct compositional regions. First, the TbO_1.5+x region which can be described in terms of a point defect model and strong interaction between neighboring excess oxygen atoms. Second, the region between TbO_1.54 and TbO_1.61 is interpreted as the intrinsic hysteresis region and discussed in terms of the regular solution model. Third, the region between TbO_1.61 and TbO_1.70 is recognized as a pseudophase region. Fourth, a region of ι phase exists in which the partial thermodynamic quantities can be compared with those of CeO_1.714, PrO_1.714, and TbO_1.714. The partial molar enthalpy was also measured for a scanning loop which is also interpreted in terms of the four regimes.