Oleg Ovchar

The effect of partial isovalent substitution in the A-sublattice on MW properties of materials based on Ba6–xLn8+2x/3Ti18O54 solid solutions

Abstract Investigations of materials based on the three systems: (Ba 1– y Pb y ) 6– x La 8+2 x /3 Ti 18 O 54 , (Ba 1– y Pb y ) 6– x Nd 8+2 x /3 Ti 18 O 54 , and (Ba 1– y Ca y ) 6– x Ln 8+2 x /3 Ti 18 O 54 revealed that a partial isovalent substitution in cation sublattices allows the control of the electrophysical parameters of microwave dielectrics. The investigations were carried out over a wide range of x values. Data on the relationship between the crystal-cell parameters and the microwave dielectric properties ( e , Q , and t f ) have been obtained and related to the size of the isovalent ion residing at crystallographic sites in the A-sublattice. Based on the data obtained, a non-random distribution of Ca 2+ and Pb 2+ ions on different A-sites as a function of their concentration in different systems was concluded. Various temperature-stable microwave dielectrics, based on the above three systems, have been synthesized and shown to exhibit excellent MW properties.

Abnormal behavior of the dielectric parameters of Ba6−xLn8+2x/3Ti18O54 (Ln=La–Gd) solid solutions

The temperature dependence of the dielectric parameters e and tan δ for the entire x range of barium lanthanide titanate solid solutions (BLT ss) using the general formula Ba6−xLn8+2x/3Ti18O54 (Ln=La, Nd, Sm, Gd) was measured over wide frequency and temperature ranges. The results show the presence of diffuse regions of e(T) and tan δ(T) anomalies, which are located below (in the La- and Nd-containing solid solutions) or above (in the Sm- and Gd-containing solid solutions) room temperature. However, no evidence of a structural transition responsible for these anomalies could be determined by differential scanning calorimetry and x-ray diffraction analysis. To examine the nature of the abnormal temperature behavior the dielectric parameters in the systems studied were considered in terms of a simple oscillator model. Within the framework of this model correlations among the average size of the A-site ions, tilting of the oxygen octahedra, and the lattice strain are discussed, and the effect of crystallogra...

Anomalies in the temperature dependence of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54

Measurements of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54 solid solutions revealed the existence of a broad maximum in the temperature dependence of the permittivity as well as the dielectric loss. Submillimeter-wave frequency measurements confirmed the intrinsic origin of the observed anomaly, which could possibly be ascribed to phase transition with unknown nature in the Ba6−xSm8+2x/3Ti18O54 solid solution. The temperature of the anomaly, which is in the range of 90–120 °C, depends on composition of Ba6−xSm8+2x/3Ti18O54 and decreases with an increase in x.

Application of positron annihilation and Raman spectroscopies to the study of perovskite type materials

Defect properties of perovskite type materials, Ba3B′Nb2O9 (where B′=Mg, Zn, or Co), with near-stoichiometric compositions were studied by positron annihilation and Raman spectroscopies. Theoretical simulations of stoichiometric perovskites revealed a dependence of the positron bulk lifetime on the degree of ordering. In Ba3MgNb2O9 (BMN) the positron bulk lifetime for a completely disordered structure is 195 ps versus 237 ps for a completely ordered one. The predicted bulk lifetimes for Ba3ZnNb2O9 (BZN) and Ba3CoNb2O9 (BCN), with Pm3¯m symmetries are 193 ps and 194 ps, respectively. It was found that deviation from stoichiometry results in the appearance of secondary Ba- and Nb-rich phases, which according to theoretical simulations have bulk lifetimes much longer than that of the host material. Positron lifetime spectroscopy was used to monitor changes in the concentration of these second phases. The difference between predicted defect lifetimes and the bulk values for the studied perovskites was less th...

Low-Loss Microwave Dielectrics Based on the Columbites A2+Nb2O6 and Perovskites : The Effect of Compositional Variation

Due to their high quality factor in the microwave (MW) range the columbites A2+Nb2O6 and perovskites can find a variety of applications in the MW engineering. However, the attainment of low-microwave-loss structure of the above compounds is rather complicated task, and their MW properties may vary significantly due to the structural inhomogeneity. In this work the effect of slight compositional non-stoichiometry on the microstructure and MW dielectric properties of the columbites A2+Nb2O6 and perovskites has been studied and will be discussed. The effects of the quality of starting reagents as well as the synthesis conditions were also considered. The non-linear dependence of the Q-factor magnitude on the chemical composition has been found and associated with the evolution of the microstructure.