Titanite-type solid solutions (Ca,Pb)TiGeO₅: A competition between triclinic and antiferroelectric distortions

Abstract

Titanite-type oxides, ABMO₅, have been recently recognized as a novel vein of antiferroelectric (AFE) materials due to their unique crystal structure including an antiparallel arrangement of polar chains. This study presents the results of synthesis, structure analysis, and dielectric measurement of titanite-type solid solutions (Ca,Pb)TiGeO₅. With the Pb composition increasing, structural symmetry at room temperature changes from monoclinic to triclinic. We identified the crystal structure of PbTiGeO₅ by Rietveld refinement similar to that of triclinic titanite-type CaZrGeO₅. In addition, the AFE phase transition temperature is decreased by Pb substitution. These indicate the competition between triclinic and AFE distortions, in other words, lone-pair electrons on Pb²⁺ ions and the second-order Jahn–Teller effect on the Ti⁴⁺ ions.