Defect chemistry and dielectric properties of Yb3+: CaTi O3 perovskite

The defect chemistry of Yb3+:CaTi O3 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 CaTi O3 by changing the CaTi ratio to force Yb3+ on the Ca site (CaTi<1), on Ti site (CaTi>1), or on both sites (CaTi=1) according to the calculations. The temperature dependence of the relative dielectric constant (102 - 105 Hz) of ceramics corresponding to predominant Yb substitution either at the Ca site or the Ti site is qualitatively similar to that of undoped CaTi O3. The Curie-Weiss temperature is shifted to more negative values in comparison to CaTi O3, suggesting that the compositions Ca1-32x Ybx Ti O3 and Ca Ybx Ti1-x O3 are further driven away from the ferroelectric instability. In contrast, the dielectric properties (102 - 105 Hz) of ceramics corresponding to Ca1-x2 Ybx Ti1-x2 O3 are radically different. The relative dielectric constant is increased of about one order of magnitude (2200 at 30 K), is almost independent of temperature, with a maximum variation of 20% in range of 20-300 K, and shows frequency dispersion above 150 K. The loss tangent at 20-300 K is <5% for frequencies 1 kHz. The possible mechanism for the observed dielectric behavior is discussed.