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Ca[(Mg1/3Ta2/3)1-xTix]O3 (hereafter denoted as CMTT) and (Ca1-0.39xNd0.26x) [(Mg1/3Nb2/3)1-xTix]O3 (hereafter denoted as CNMNT) complex perovskite ceramics were studied by Raman spectroscopy combined with Rietveld analysis. The crystal structures changed from monoclinic structure with P21/c space group to orthorhombic structure with Pbnm space group for both systems. The main structure of the two systems was orthorhombic and the characteristic modes for the orthorhombic structure with space group Pbnm could be found in all the compositions. Raman modes at around 408 cm-1 could only be found for compositions with 1:2 long range cation ordering while modes at around 800 cm-1 could be found for all the compositions in both systems. They were concluded to be correlated with the 1:2 long range cation ordering and different short range cation ordering, respectively. Two Ag modes at around 800 cm-1 represented for 1:2 and 1:1 short range cation ordering were observed for CMTT ceramics with x=0.1–0.6 while only 1:2 short range Ag mode was observed in all compositions for CNMNT ceramics. The weak A-O hybridization was considered to be the reason for the absence of the 1:1 short range cation ordering. The short range cation ordering was suggested to be appreciably beneficial to the Qf value. [ABSTRACT FROM AUTHOR]

Dense (1− x)Ca(Mg1/3Ta2/3)O3/ xCaTiO3 ceramics (0.1≤ x≤0.9) were prepared by a solid-state reaction process. The crystal structures and microstructures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Single-phase solid solutions were obtained in the entire composition range. Long-range 1:2 ordering of B-site cations and oxygen octahedra tilting lead to the monoclinic symmetry with space group P21/ c for x=0.1. For x above 0.1, the long-range ordering was destroyed and the crystal structure became the orthorhombic with space group Pbnm. The microwave dielectric properties showed a strong dependence on the composition and microstructure. The dielectric constant and temperature coefficient of resonant frequency increased nonlinearly as the CaTiO3 content increased while the Qf values decreased approximately linearly. Good combination of microwave dielectric properties was obtained at x=0.45, where ℇr=45.1, Qf=34 800 GHz, and τf=17.4 ppm/°C. [ABSTRACT FROM AUTHOR]

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilt, and it may provide a promising way to develop new room-temperature single-phase multiferroic materials with a strong magnetoelectric effect. Though ferroelectric polarizations are confirmed by the electric hysteresis loops at room temperature, the ferroelectric-paraelectric phase transition has not been well understood. In the present work, the crystal structures, dielectric properties, and phase transition behaviors of Ruddlesden-Popper Sr3–xCaxSn2O7 ceramics are thoroughly determined. Ferroelectricity is confirmed through the polarization-electric field hysteresis loops at room temperature, and the Curie temperature is identified by the variable-temperature differential scanning calorimetric analysis and dielectric characterization. The polarizations and Curie temperatures of the present ceramics increase with increasing the calcium content. An improper ferroelectric transition is experimentally evidenced by a small dielectric constant and a step variation of dielectric constant around the Curie temperature, and the thermal hysteresis of dielectric constant confirms the first order transition nature. Moreover, the fitting result of Landau phase transition theory also suggests the first-order improper ferroelectric transition nature. The variable-temperature Raman result implies a tetragonal paraelectric phase above TC. The present work confidently confirms the first-order hybrid improper ferroelectric transitions in the present ceramics, and ferroelectric properties can be readily tuned by substituting the cations at the A-site. [ABSTRACT FROM AUTHOR]