Thermodynamic modeling of phase transformations and defects: From cobalt to doped cobaltate perovskites

The goal of this dissertation is to develop a thermodynamic model capable of predicting the stability of La1-xSrxCoO 3-delta at arbitrary temperatures, strontium contents, oxygen partial pressures, and gas compositions. Information regarding the defect chemistry responsible for the oxygen nonstoichiometry can also be predicted from the model.;The thermodynamic model is constructed with the CALculation of PHAse Diagrams (CALPHAD) approach, where parameterized Gibbs energy functions are fitted to experimentally measured and theoretically predicted thermochemical and phase equilibria data. Complex, multi-component systems can be efficiently and accurately described with this approach by modeling a system's constituent subsystems, where data may be more plentiful, and then extrapolating the Gibbs energy functions of the subsystems into the desired higher-order compositional space, adding more parameters as needed. Thermochemical data for these neighboring phases have not been measured, so first-principles calculations, based on density functional theory, are employed to predict their Gibbs energy functions. The model predictions of the combined La1-xSrxCoO 3-delta are discussed, following an extensive statistical assessment of the published and new experimental oxygen nonstoichiometry data for the perovskite.;Since the Gibbs energy function for several complex cobaltates will be predicted for the CALPHAD modeling of the perovskite, the capability of first-principles methods to predict the vibrational contribution to the free energy for cobaltates is examined for a case where the predictions can be compared to experiments, Co3O4 spinel. A scheme to efficiently predict the vibrational contribution to the free energy is developed, utilizing the Debye-Gruneisen Model and harmonic phonon calculations with the supercell approach. It is found that this scheme can predict the heat capacity and entropy of Co 3O4 with sufficient accuracy for CALPHAD modeling, with error on the order of 2--3 kJ/mol-atom in the Gibbs energy at around 1000 K. The heat capacity, room temperature entropy, and enthalpy of formation of Sr2Co2O5 brownmillerite and Sr6Co 5O15 are then predicted, for use in the CALPHAD modeling of SrCoO3-delta. However, due to errors in the prediction of the brownmillerite entropy and enthalpy of formation, the stability from for the two phases is incorrect, with brownmillerite more stable at all temperatures. This error is corrected by treating the enthalpy of formation and entropy as model parameters in CALPHAD with the experimental phase equilibria data.;Following the results of the CALPHAD modeling of the LaCoO3-delta and SrCoO3-delta perovskites, several predictions are made. For instance, charge disproportionation of Co+3 in LaCoO3-delta is on the of 40% in air. Similarly, the presence of Co+2, Co+3, and Co+4 is predicted at around 1200 K. However, the combination of the LaCoO3-delta and SrCoO3-delta models, assuming ideal mixing between La+3 and Sr+2 ions, predicts oxygen nonstoichiometry that agrees well at lanthanum-rich compositions but gives poor agreement at strontium-rich compositions, particularly at low temperatures. It is speculated that this is due to defect-defect interactions and the formation of complex defect associates for strontium contents near SrCoO3-delta, suggested by the results of the statistical analysis of oxygen nonstoichiometry data. Several attempts are made to improve the agreement of the CALPHAD model with the experimental oxygen nonstoichiometry data by including interaction parameters to describe regular and sub-regular interactions between La +3 and Sr+2. Although better agreement is achieved, a limited set of parameters capable of agreement across the entire composition space of La1-xSrxCoO3-delta was not found. More data concerning the nature of defects at strontium-rich compositions is requested. (Abstract shortened by UMI.)...