Structural principles of cation ordering and octahedral tilting in A-site ordered double perovskites: ferroelectric CaMnTi 2 O 6 as a model system.

We have used quantum chemical methods to study the structural principles and energetics of A-site ordered AA'B 2 O 6 double perovskites. 33 combinations of A-site ordering and Glazer tilting have been systematically studied for the ferroelectric CaMnTi 2 O 6 model system. The used approach was able to predict the correct combination of A-site ordering and tilting of octahedra in comparison to the experimentally known CaMnTi 2 O 6 . The energy differences between the various combinations of A-site ordering and tilt systems show a large variation of tens of kJ mol -1 per formula unit, which suggests that the methodology used here can be used as a starting point for making reliable predictions on the structures of yet unknown A-site ordered double perovskites. The energy differences due to A-site ordering and octahedral tilting were larger compared to the energy difference arising from ferroelectric distortion in CaMnTi 2 O 6 . The energy differences between various hypothetical double perovskite structures could be explained by studying their structural characteristics in detail. The relative energies are closely correlated with the Mn-O distances and Mn coordination in the studied structures.