Crystal Structures and Phase Transitions in A-Site Deficient Perovskites Ln_1/3TaO₃

The synthesis and structures of the perovskites Ln 1/3 TaO 3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P 4/ m m m /La → C m m m /Ce−Gd → P m m a /Tb, Dy → P m c 2 1 /Ho, Er. Although apparently tetragonal in P 4/ mmm, electron diffraction patterns of Tm 1/3 TaO 3 reveal this has a complex incommensurate structure. Likewise Gd 1/3 TaO 3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd 1/3 TaO 3 is thought to be due to the proximity to the first-order Cmmm − Pmma transition. Variable temperature studies show both Tb 1/3 TaO 3 and Dy 1/3 TaO 3 undergo a first-order Cmmm − Pmma transition upon heating.