The perovskite Ba 3 ZnTa 2 O 9 (BZT) is the parent of a family of high dielectric constant, low-loss oxides currently applied in mobile telecommunications base stations. The preparation of BZT microwave dielectric resonators with optimal dielectric properties requires careful attention to thermal treatment to control the formation of domains in which the Ta and Zn cations order on the octahedral sites of the perovskite structure. In this paper we use powder X-ray and neutron diffraction together with X-ray fluorescence and electron microscopy to demonstrate the strong correlation between the size of the cation-ordered domains, the extent of the ordering within them, and their composition, particularly the ZnO content. Phase separation into two trigonal BZT phases, which differ in the extent of cation order and the size of the ordered domains, is a common feature of all the synthetic procedures. A zincdeficient impurity phase of nominal stoichiometry Ba 8 ZnTa 6 O 24 , involving face-as well as corner-sharing octahedra, is identified in strongly zinc-deficient samples. A mechanism in which a fully ordered BZT phase grows at the expense of a slightly Zn-deficient and partially ordered phase is proposed.

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A new molten salt synthesis route to the mixed-valence sodium titanate NaTi 2 O 4 has been discovered. Reduction of Na 8 Ti 5 O 14 by Ti metal powder in a 3:1 molar mixture of NaCl:KCl at 770°C produced crystals of NaTi 2 O 4 . Use of the molten salt flux lowered the synthesis temperature of this compound by over 400°C. Time-resolved in situ high-temperature X-ray powder diffraction was used to probe the kinetics and mechanism of the reaction. Energydispersive X-ray diffraction (EDXRD) data revealed that the reaction is rapid; the phase begins to form in 30 min at 770°C, and product formation is essentially complete within 2 h. Crystalline solids are present in the molten salt flux at all times during the course of the reaction, indicating that the mechanism most likely involves reactions occurring at the surfaces of the solid particles, mediated by the molten salt flux. Possible key intermediates identified through EDXRD and quenching studies are Ti 3 O, Na 2 Ti 6 O 13 , and Na 0.54 TiO 2 . This new molten salt synthesis route offers a facile way to reproducibly prepare large samples of this mixed-valence compound for further study.

Neutron spectroscopic measurements of the magnetic excitations in PrO2 reveal (1) sharp peaks characteristic of transitions between levels of the 4f(1) configuration of Pr4+ split by the cubic crystal field, and (2) broad bands of scattering centered near 30 and 160 meV. We present a simple model based on a vibronic Hamiltonian that accounts for these contrasting features of the data. The analysis shows that 90%+/-10% of the Pr ions have a localized 4f(1) configuration and provides strong evidence for a dynamic Jahn-Teller effect in the gamma(8) electronic ground state.