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 a re perovskite-based ABO,type, with a common stoichiometry Ca[CaxNbyTiz]O-3. The differences between the low-er-temperature phases is in the type of or dering between Ca/Nb/Ti ions occupying the B-site, as well as in the type o f octahedral tilting. The proposed phase diagram has the following single-p hase fields: (1) cubic disordered C (Pm (3) over barm); (2) a series of dis ordered/tilted CaTiO3-based polymorphs T, O-1, and O-2 (Pnma) with differen t combinations of tilting; (3) disordered/tilted O-2' (Pnma), same as O-2 b ut lower in Ti; (4) ordered/tilted HTP2 (P2(1)/c) with 1:1 ordered (111)(c) planes; (5) ordered/tilted LT1/6 (P2(1)/c) with 1:2 ordered (111)(c) plane s. The single-phase fields are connected with the following reactions of th e phases: (1) high-temperature monotectoid C --> C ' + O-2'; (2) miscibilit y gap C --> C ' + C "; (3) eutectoid HTP2 --> LT1/6 + O-2; (4) a series of peritectoid reactions between O-2' and CaTiO3 polymorphs; (5) an ordering t ransition O-2' --> HTP2. In addition, a metastable transition to the tricli nic LT,, phase (with 1:3 ordered (111)(c) planes) occurs for a wide range o f compositions (0 < x < 0.6).