D.D. Khalyavin

Synthesis and dielectric properties of tungsten-based complex perovskites

Ba 2 MeWO 6 (Me = Mg, Ni, Zn) double perovskites were prepared by the conventional solid-state reaction in a wide temperature range. Single-phase ceramics were obtained only at low temperatures approximately 1200 °C, whereas a small amount of second phases existed in the samples sintered at higher temperatures. All the compounds are characterized by the cubic perovskite structure (space group Fm3m) with a complete NaCl type ordering between B-site ions. Anomalous temperature variation of the dielectric loss tangent found in the Ba 2 NiWO 6 perovskite is supposed to be connected with a dielectric relaxation due to electronic hopping within thermally activated Ni 3 + -6W ( 6 - 1 / 6 ) + /W 5 + -6Ni ( 2 + 1 / 6 ) + clusters. Dielectric measurements showed that the other two perovskites-Ba 2 ZnWO 6 and Ba 2 MgWO 6 -exhibit a positive value of the temperature coefficient of permittivity. Such temperature variation is assumed to be caused by a considerable influence of the second polar mode involving B-site ion vibrations on the low-frequency dielectric properties.

Crystal structure of Sr2MgWO6 and Ba2SrWO6 determined by powder X-ray diffraction

The crystal structures of Sr 2 MgWO 6 and Ba 2 SrWO 6 compounds, studied by powder X-ray diffraction, were found to be distorted perovskites with a complete NaCl type ordering between Mg 2+ /W 6+ and Sr 2+ /W 6+ ions, respectively. The unit cells are characterized by the tetragonal (space group I4/m; a 0 a 0 a − ) symmetry, with the parameters a=5.5849(10) A and c=7.9455(10) A, for Sr 2 MgWO 6 and triclinic (space group F-1; a − b − b − ) symmetry, with the parameters a=8.5409(10) A, b=c=8.5860(10) A, α=89.35(1)° and β=γ=90.31(1)°, for Ba 2 SrWO 6 .

Structural and Dielectric Characteristics of Ba2MeWO6 (Me=Mg, Ni, Zn) Double Perovskites

Ba2MeWO6 (Me=Mg, Ni, Zn) double perovskites were prepared by the conventional reaction method. The crystal structure of these compounds is characterized by the cubic symmetry (space group Fm3m) with a complete NaCl type ordering between Me and W ions. In all cases, dense (~98 %) and homogeneous ceramics were obtained. An investigation of the dielectric characteristics was carried out at radio and microwave frequencies. Comparative analysis with dielectric properties reported early for another complex barium-based perovskites has been done. It was concluded that the charge distribution between B-site ions has a considerable influence on the temperature behaviour of the permittivity.

Microstructure Characterization and Dielectric Properties of La4Mg3W3O18 Layered Oxide

The microstructure development and the dielectric properties of La4Mg3W3O18 layered oxide were investigated. A strong anisotropic grain growth is revealed in ceramics sintered above 1400oC. The anisotropy of the grain reflects the anisotropic character of the crystal structure. Dielectric measurements yield a relative dielectric permittivity, ε, and a temperature coefficient of the dielectric permittivity, TC ε, to be 21 and 95 ppm K-1, respectively. The positive value of TC ε is assumed to be caused by the octahedral tilting by analogy with the behaviour of other complex ordered perovskites.

Structure and Dielectric Behavior of the (1−x)La(Mg1/2Ti1/2)O3 xBa(Mg1/2W1/2)O3 Microwave Ceramics

The sequence of structure transformations, P2 1 /n − I2/m − Fmm, involving oxygen octahedral tilting has been revealed in the ceramics of the whole-range B-site ordered perovskite system (1−x)La(Mg 1/2 Ti 1/2 )O 3 −xBa(Mg 1/2 W 1/2 )O 3 . Microwave dielectric characteristics of the ceramics have been evaluated as a function of their composition. The observed structure sequence and structure-dependent dielectric behavior are discussed in respect to an order of the transition between anti-phase tilted and untilted configurations as well as a likely clustering in the system.