Ordering of tetrahedral chains in the Sr2MnGaO5 brownmillerite

Abstract Tetrahedral chain ordering in the Sr 2 MnGaO 5 structure is studied using electron diffraction (ED) and high-resolution electron microscopy. The ED patterns show the presence of satellite reflections, which indicate a commensurately modulated structure with a =5.4056(8) A, b =16.171(3) A, c =5.5592(7) A, q =1/2 c * , superspace group Imma (00 γ )s00. The superstructure arises due to ordering of the two types of symmetry related tetrahedral chains (L and R) according to a …LRLR… sequence, where L and R chains alternate along the c -axis within the same (GaO) layer. Numerous defects at different structural levels were observed, comprising interleaving L and R chains, violation of the …LRLR… chain sequence within one layer, different stacking modes of the …LRLR… ordered layers with subsequent alternation of blocks of different width along the b -axis of the brownmillerite subcell and island fragmentation of the modulated superstructure. By in situ heating ED experiments it is found that the long-range ordering of the tetrahedral chains is stable up to 665°C and is completely suppressed at 905°C.