Defect chemistry and dielectric properties of Yb3+:CaTiO3 perovskite

Abstract

The defect chemistry of Yb3+:CaTiO3 solid solutions has been investigated both theoretically and experimentally. Three different incorporation mechanisms with similar solution energy were predicted for Yb3+ by atomistic simulation: (i) Ca site substitution with Ca vacancy compensation; (ii) Ti site substitution with O vacancy compensation; (iii) simultaneous substitution at both Ca and Ti sites with self-compensation. X-ray diffraction and scanning electron microscopy results strongly support the possibility to realize the above defect chemistries in CaTiO3 by changing the Ca∕Ti ratio to force Yb3+ on the Ca site (Ca∕Ti<1), on Ti site (Ca∕Ti>1), or on both sites (Ca∕Ti=1) according to the calculations. The temperature dependence of the relative dielectric constant (102–105Hz) of ceramics corresponding to predominant Yb substitution either at the Ca site or the Ti site is qualitatively similar to that of undoped CaTiO3. The Curie-Weiss temperature is shifted to more negative values in comparison to CaTiO3, suggesting that the compositions Ca1−3∕2xYbxTiO3 and CaYbxTi1−xO3 are further driven away from the ferroelectric instability. In contrast, the dielectric properties (102–105Hz) of ceramics corresponding to Ca1−x∕2YbxTi1−x∕2O3 are radically different. The relative dielectric constant is increased of about one order of magnitude (2200 at 30K), is almost independent of temperature, with a maximum variation of 20% in range of 20–300K, and shows frequency dispersion above 150K. The loss tangent at 20–300K is <5% for frequencies ⩾1kHz. The possible mechanism for the observed dielectric behavior is discussed.