The crystal chemistry of complex perovskite dielectric oxides is reviewed, with an emphasis on structures derived from ordering of the cations on the octahedral B-sites. New classes of perovskites, designed to exhibit 1:2 or 1:3 B-site order for application as low-dielectric-loss microwave ceramics, are identified, and their synthesis, structure, and properties are described. Through the use of B-site chemistries based on Li, Nb, Ta, Ti, and W, members of four new families with 1:2 order, A(βI 1/3βII 2/3)O3, and three new families with 1:3 order, A(βI 1/4βII 3/4)O3, were successfully prepared. The formation and stability of the new and previously prepared ordered perovskites are rationalized through the use of familiar crystal chemical tools such as cation size and charge difference, bond valence, tolerance factor, and new concepts related to the local charge imbalance on the A- and B-sublattices. These tools can be successfully applied to develop stability field maps for each structure and to predict other new ordered systems.

Oxide melt solution calorimetric measurements were made to determine the enthalpy of formation of cubic-yttria-stabilized zirconia (c-YSZ) with respect to the oxides m-ZrO2 and C-type YO1.5. The enthalpy of formation can be fit either by a quadratic equation or by two straight line segments about the minimum near x = 0.40. The quadratic fit gives a strongly negative interaction parameter, Ω=-93.7 ± 12.0 kJ/mol, but does not imply regular solution behavior because of extensive short-range order. In this fit, the enthalpy of transition of m-ZrO2 to c-ZrO2, 9.7 ± 1.1 kJ/mol, is in reasonable agreement with earlier estimates and that of C-type to cubic fluorite YO1.5, 24.3 ±14.4 kJ/mol, is consistent with an essentially random distribution of oxide ions and anion vacancies in the high-temperature fluorite phase. The two straight-line segments give 6.1 ± 0.6 kJ/mol and 5.5 ± 2.5 kJ/mol for these transitions, respectively. The latter value would imply strong short-range order in cubic fluorite YO1.5. Clearly more complex solution thermodynamic descriptions need to be developed. The enthalpy of transition from the disordered c-YSZ phase to the ordered δ-phase at 25 °C was also measured and was 0.4 ± 1.6 kJ/mol. No energetic difference between the disordered c-YSZ phase and the ordered δ-phase underscores the importance of short-range order in c-YSZ. Enthalpy data were combined with Gibbs free energy data to calculate entropies of mixing. Using the quadratic fit, negative excess entropy of mixing in the cubic solid solution, relative to a system with maximum randomness on cation and anion sublattices, was found and was another indication of extensive short-range order in c-YSZ.

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