Study On Relationships Between Composition, Structure And Dielectric Properties Of Perovskite-Like Compounds

In order to explore novel high performance materials with perovskite-like structure, series of A_nB_n-1O_3n-type B-site deficient hexagonal perovskites and A₆B₁₀O₃₀-type filled tungsten bronzes are systematically investigated in this dissertation. According to characteristics of the crystal structure and the cations in A and B sites (radii, valence and polarization), a series of perovskite-like new compounds and their density ceramics are designed, synthesized and prepared for the first time. The relationship between the crystal structure and dielectric properties, and the effect of composition on structure and properties of these materials are extensively investigated. The possibility in microwave application is discussed.New dielectric ceramics Ba-nTi_n-5Nb₄O_3n(n=6, 7, 8) with cation-deficient hexagonal perovskite-related structures were prepared in Ba₅Nb₄O₁₅-BaTiO₃ system by the conventional solid-state reaction route. The phases and structures of the ceramics were characterized. Ba₆TiNb₄O₁₈ formed "shift" structure and Ba₈Ti₃Nb₄O₂₄ formed "twinned" structure, while Ba₇Ti₂Nb₄O₂₁ decomposed to Ba₆TiNb₄O₁₈ and Ba₈Ti₃Nb₄O₂₄ phases at high temperature. These ceramics show high dielectric constant (ε_r) of in the range of 45.1～48.6, high quality factors with Q·f values in the range 13179～22882, and the temperature coefficient of resonant frequency Ï„_f in the range +59～+175ppm℃^-1. The values of ε_r and Ï„_f for the ceramics gradually increased and Q·f values decreased with increasing number of n.Three new compounds Ba_6-nLa_nTi_1+nNb_4-nO₁₈ (n=1, 2, 3) were synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, they are identified as the A₆B₅O₁₈ type cation-deficient perovskite with space group R(3|ˉ)m, and the structures can be described as a fully ordered stacking of blocks of five octahedral layers with corner shared octahedral blocks separated by one vacant octahedron layer. Their ceramics exhibit high ε_r in the range 48.3～57.3, high quality factors (Q·f) in the range 18456 ～ 24865 GHz and Ï„_f in the range +138 to +37.3 ppm℃^-1. The ε_r and Ï„_f of these ceramics gradually decreased and Q·f values increased with the increase of n or contents of Ti and La. The ceramics...