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Tilting structures in inverse perovskites, M 3 Tt O ( M = Ca, Sr, Ba, Eu; Tt = Si, Ge, Sn, Pb)
Author(s) -
Nuss Jürgen,
Mühle Claus,
Hayama Kyouhei,
Abdolazimi Vahideh,
Takagi Hidenori
Publication year - 2015
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520615006150
Subject(s) - orthorhombic crystal system , octahedron , crystallography , perovskite (structure) , atmospheric temperature range , crystal structure , anisotropy , materials science , chemistry , physics , thermodynamics , quantum mechanics
Single‐crystal X‐ray diffraction experiments were performed for a series of inverse perovskites, M 3 Tt O ( 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 (, cP 5); however, all of them show distinct anisotropies of the displacement ellipsoids of the M atoms 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 , oI 20) upon cooling, with slightly tilted OBa 6 octahedra, 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 Tt O. Smaller Tt 4− anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca 3 SiO and Ca 3 GeO with cubic structures at 500 K, change into orthorhombic ( Ibmm ) at room temperature. Whereby, Ca 3 SiO is the only representative within the M 3 Tt O family where three polymorphs can be found within the temperature range 500–50 K: – Ibmm – Pbnm . They show tiny differences in the tilting of the OCa 6 octahedra, 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 6 octahedra, and bonding angles of O— M —O ≃ 160° (295 K) are observed. They crystallize in the anti ‐GdFeO 3 type of structure ( Pbnm , oP 20), 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.