The high-temperature phase chemistry and thermochemistry of the lead magnesium niobium titanium oxide system

The phase equilibrium diagrams for the PbO-MgO-Nb₂O₅ -TiO₂ system were experimentally evaluated in the vicinity of 1000°C. In addition, the equilibrium vapor pressure of lead oxide was measured as a function of temperature and composition for most of the phase compatibility relationships. Together, this information was combined to map out the thermochemistry of the perovskite and pyrochlore solid solutions, and the compounds in the surrounding compositional space. The focus of this investigation has been to provide a thermodynamic description of the stability of the perovskite phase as it pertains to high-temperature synthesis and compositional control.; The ternary diagram for the PbO-MgO-Nb₂O₅ system was previously reported at 1000°C. Here, the three remaining ternary diagrams, PbO-MgO-TiO₂, PbO-Nb₂O₅-TiO₂, and MgO-Nb₂O₅-TiO₂ are presented at 1000°C. The individual equilibrium compatibilities were assessed via more than one reaction path. Typically, samples were combinations of the constituent oxides and/or previously synthesized ternary compounds pelletized and welded shut in platinum capsules. Equilibration was a particularly slow process in the MgO-Nb₂O₅-TiO₂ system and, therefore, samples were analyzed at higher temperatures, up to 1450°C, and then extrapolated to 1000°C. Samples prepared that did not contain lead oxide were annealed in air. Upon finishing the four ternary diagrams, the complete quaternary phase equilibrium was evaluated at 1000°C. Details surrounding the perovskite and extended pyrochlore solid solutions were revealed.; Lead oxide vapor pressures were measured by Knudsen cell thermogravimetric analysis (KC-TGA). Due to the multiple lead oxide species present, and discrepancies in the thermodynamic descriptions of the molecular distributions, a calibration procedure was devised from which the lead oxide activity was deduced directly from the measured weight-loss rates. From the activities, the Gibbs energy of formation was calculated for the perovskites. Finally, the combined information is presented in stability diagrams for the three lead-oxide-containing ternary diagrams.