Tilting structures ininverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= Si, Ge, Sn, Pb)

Abstract

Single-crystal X-ray diffraction experiments were performed for a series of inverseperovskites, M ₃ TtO ( M= Ca, Sr, Ba, Eu; Tt= tetrel element: Si, Ge, Sn, Pb) in the temperature range 500–50 K. For Tt= Sn, Pb, they crystallize as an `ideal' perovskite-type structure (Pm\bar 3m, cP5); however, all of them show distinct anisotropies of the displacement ellipsoids of the Matoms at room temperature. This behavior vanishes on cooling for M= Ca, Sr, Eu, and the structures can be regarded as `ideal' cubic perovskites at 50 K. The anisotropies of the displacement ellipsoids are much more enhanced in the case of the Ba compounds. Finally, their structures undergo a phase transition at ∼ 150 K. They change from cubic to orthorhombic ( Ibmm, oI20) upon cooling, with slightly tilted OBa ₆octahedra, and bonding angles O—Ba—O ≃ 174° (100 K). For the larger Ba ²⁺cations, the structural changes are in agreement with smaller tolerance factors ( t) as defined by Goldschmidt. Similar structural behavior is observed for Ca ₃ TtO. Smaller Tt ⁴⁻anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca ₃SiO and Ca ₃GeO with cubic structures at 500 K, change into orthorhombic ( Ibmm) at room temperature. Whereby, Ca ₃SiO is the only representative within the M ₃ TtO family where three polymorphs can be found within the temperature range 500–50 K: Pm\bar 3m– Ibmm– Pbnm. They show tiny differences in the tilting of the OCa ₆octahedra, expressed by O—Ca—O bond angles of 180° (500 K), ∼ 174° (295 K) and 170° (100 K). For larger M(Sr, Eu, Ba), together with smaller Tt(Si, Ge) atoms, pronounced tilting of the O M ₆octahedra, and bonding angles of O— M—O ≃ 160° (295 K) are observed. They crystallize in the anti-GdFeO ₃type of structure ( Pbnm, oP20), and no phase transitions occur between 500 and 50 K. The observed phase transitions are all accompanied by multiple twinning, in terms of pseudo-merohedry or reticular pseudo-merohedry.