This work is mainly concerned about the crystal structures of B-site ordered complex perovskites and the correlation between the crystal structures and dielectric properties which were varied with composition in complex perovskite solid solution. According to subjects, this work could be divided into three parts.
Part A.
The Crystal Structures of Complex Perovskites.
The room temperature crystal structures of B-site ordered complex perovskite $SrYb_1/2Nb_1/2O_3$, $SrMg_1/2W_1/2O_3$ and $CaMg_1/2W_1/2O_3$ were investigated by using transmission electron microscope, X-ray and high-resolution neutron powder diffraction method. As in the most case of non-ferroelectric simple perovskites, the main mechanism of structural deviation from ideal cubic perovskite is oxygen octahedra tilting and the octahedra tilting nature depends on the volume difference of A cation and $BO_6$ octahedra. However, in contrast to simple perovskites, it was found that the oxygen octahedra are distorted. The main reason of existence of oxygen octahedra distortion is the nonequivalent volume of two different octahedra caused by the site ordering. This volume difference inevitably inhibits rigid rotation of $B'O_6$ and $B"O_6$ network. The crystal structures of $SrYb_1/2Nb_1/2O_3$ and $CaMg_1/2W_1/2O_3$ are monoclinic $(P2_1/n)$ with $a^-a^-a^+$ tilting system and that of $SrMg_1/2W_1/2O_3$ is monoclinic (I2/m) with $a^-a^-a^o$ tilting system according to Glazer notation, respectively. The monoclinic structures are accompanied by triclinic distortion of proto type ordered perovskite cells. During the structure verifications of $SrYb_1/2Nb_1/2O_3$ and $CaMg_1/2W_1/2O_3$, it was found that the oxygen atoms lying in the in-phase tilting axis make effort to leveling in (001) plane made by Sr cations. The simple geometric and energy consideration of tendency of being same plane is also discussed.
Based on the crystal structure investigations of $SrYb_1/2Nb_1/2O_3$ and $SrMg_1/2W_1/2O_3$, the possible reason of different lattice distortion behavior during antiferroelectric phase transitions in typical antiferroelectric complex perovskites $PbYb_1/2Nb_1/2O_3$ and $PbMg_1/2W_1/2O_3$ is considered. In $PbYb_1/2Nb_1/2O_3$, there is suitable space to shift of Pb ion without any inhibition. However, in $PbMg_1/2W_1/2O_3$, the oxygen octahedra must be elongated to produce the proper space of Pb ion shift. Therefore, the unit cell volume of $PbYb_1/2Nb_1/2O_3$ decrease and that of $PbMg_1/2W_1/2O_3$ increase during antiferroelectric phase transition, respectively.
Part B.
The Correlation between Crystal Structures and Microwave Dielectric Properties in Complex Perovskite Solid Solutions.
The prime objective of this part is to find the correlation between crystal structure and microwave dielectric properties in B-site ordered complex perovskites. In order to study about this, structural and dielectric properties of two complex perovskite solid solutions of $xBaYb_1/2Nb_1/2O_3 - (1-x)SrYb_1/2Nb_1/2O_3$ and $(1-x) CaMg_1/2W_1/2O_3 - xCaTiO_3$ are investigated by using X-ray powder diffraction, transmission electron microscope, low frequency dielectric measurements, and microwave dielectric measurements.
In the case of $xBaYb_1/2Nb_1/2O_3 - (1-x)SrYb_1/2Nb_1/2O_3$ solid solution, The numbers of oxygen octahedra tilting axes and the tilting angles are continuously relaxed from $a^-a^-a^+$ (0≤x≤0.5) to $a^oa^oa^o$ ($BaYb_1/2Nb_1/2O_3$) via $a^-a^-a^o$ (0.5≤x≤0.8). Refined atomic positions indicate that the average bonding length of B-site cations and oxygen is not varied with composition. However, the average bonding length of A-site cation and oxygen is continuously increase with increasing Ba contents. This fact suggests that structural environments around A-site are more intensely changed than that of B-site with composition. In addition to this the monoclinic angle is markedly decrease for 0
