Phase relations in the BaTi03-Ti02 system were studied at temperatures above 1300°C in air. Quenching experiments were performed with high-purity reagents, and a new equilibrium phase diagram was constructed. Results include redetermination of the liquidus boundaries, the eutectic temperature, the melting or decomposition temperatures of the stable compounds in the system, the cubic-hexagonal transition in BaTiO,, and the solid solubility of TiOz in BaTi03. [

No abstract

Rietveld refinements using time-of-flight neutron diffraction data are reported for two forms of Mg2TiO4 (Mr=160-51), MgTiO3 (Mr=120.20) and two samples of MgTi205 (Mr = 200" 10). The compounds were synthesized at 1673K and subsequently annealed and quenched from other temperatures. All data were collected at room temperature on a 10 m powder diffractometer at a nominal scattering angle of 150 ° 20. Mg2TiO4 annealed at 973 K has the spinel structure, space group Fd3m, with a= 8.4376(5),~, V=600.71(ll)A 3, Z=8. Mg2TiO4 prepared at 773 K is tetragonal, space group P4122, a = 5.9748 (5), c = 8.414 (7) A, V= 300.37 (7) A 3, Z = 4. Final weighted-profile R values are 0.0376 and 0-0327 for the cubic and tetragonal spinels, respectively. In the cubic form, Mg and Ti are disordered in a single octahedral site to form a nearly perfect inverse spinel, although there may be considerable short-range order. The tetragonal structure is a slight distortion of the cubic one, with two inequivalent octahedral sites over which the Mg and Ti are highly, but completely, ordered. MgTiO3 annealed_at 1073 K has the ilmenite structure, space group R3, a = 5.05478 (26), c = 13.8992 (7)/~, V = 307.56 (4) A 3, Z = 6. The final Rwp is 0.0257. Mg and Ti are completely ordered between two octahedral sites, and probably remain so at temperatures up to Mg-Ti distribution in both samples is disordered, with the 1773 K sample substantially more disordered. The lattice parameters of MgTi205 are sensitive to the degree of disorder. These results have been combined with thermochemical data obtained on the same specimens to derive an understanding of the effects of order-disorder on the phase-stability relations of these compounds.

Solution calorimetry of MgAI~04-Als,304 solid solutions was performed in a molten 2Pb0 * B203 solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the a-y transition in AI2O3 and on the MgA1204-Al~304 (Mg0-AI2O3) subsolidus phase relations to estimate the partial molar entropy of mixing of y-Alsi304 in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values.

No abstract