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 ofinverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= tetrel element: Si, Ge, Sn, Pb) in the temperature range 500–50 K. ForTt= 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 theMatoms at room temperature. This behavior vanishes on cooling forM= 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 OBa6octahedra, and bonding angles O—Ba—O ≃ 174° (100 K). For the larger Ba2+cations, the structural changes are in agreement with smaller tolerance factors (t) as defined by Goldschmidt. Similar structural behavior is observed for Ca3TtO. SmallerTt4−anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca3SiO and Ca3GeO with cubic structures at 500 K, change into orthorhombic (Ibmm) at room temperature. Whereby, Ca3SiO is the only representative within theM3TtO 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 OCa6octahedra, expressed by O—Ca—O bond angles of 180° (500 K), ∼ 174° (295 K) and 170° (100 K). For largerM(Sr, Eu, Ba), together with smallerTt(Si, Ge) atoms, pronounced tilting of the OM6octahedra, and bonding angles of O—M—O ≃ 160° (295 K) are observed. They crystallize in theanti-GdFeO3type 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.