Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd4 − xMxAl2O9 − x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd4 − xCaxAl2O9 − x/2 and Gd4 − xSrxAl2O9 − x/2 show the monoclinic cuspidine structure with space group of P21/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd4Al2O9, GdAlO3 and SrGd2Al2O7 appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10− 4 S/cm for Gd3.95Ca0.05Al2O8.98. In comparison, the conductivity of pure Gd4Al2O9 was σ = 1.73 × 10− 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd4Al2O9 at 1000 °C was 7.4 × 10− 6 K− 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.