Ca4Nb2O9–CaTiO3: Phase Equilibria and Microstructures

CaTiO3–Ca4Nb2O9 is a quasi-binary section, and all the phases participating in equilibrium are solid solutions of the binary end-members. The phases are perovskite-based ABO3-type, with a common stoichiometry Ca[CaxNbyTiz]O3. The differences between the lower-temperature phases is in the type of ordering between Ca/Nb/Ti ions occupying the B-site, as well as in the type of octahedral tilting. The proposed phase diagram has the following single-phase fields: (1) cubic disordered C (Pm3m); (2) a series of disordered/tilted CaTiO3-based polymorphs T, O1, and O2 (Pnma) with different combinations of tilting; (3) disordered/tilted O′2 (Pnma), same as O2 but lower in Ti; (4) ordered/tilted HTP2 (P21/c) with 1:1 ordered (111)c planes; (5) ordered/tilted LT1/6 (P21/c) with 1:2 ordered (111)c planes. The single-phase fields are connected with the following reactions of the phases: (1) high-temperature monotectoid C→C′+O′2; (2) miscibility gap C→C′+C″; (3) eutectoid HTP2→LT1/6+O2; (4) a series of peritectoid reactions between O′2 and CaTiO3 polymorphs; (5) an ordering transition O′2→HTP2. In addition, a metastable transition to the triclinic LT1/4 phase (with 1:3 ordered (111)c planes) occurs for a wide range of compositions (0