Centrosymmetric to non-centrosymmetric transition in the Ca2-xMnxTi2O6 double perovskite system studied through structural analysis and dielectric properties.

We have used high-pressure synthesis to synthesize samples of Ca2-xMnxTi2O6 double perovskite, where x varies between 0.2 and 1. The synthesized materials were structurally characterized with powder X-ray diffraction (XRD). Rietveld refinement of the XRD patterns was used to study the change from CaTiO3 (x = 0) to the composition CaMnTi2O6 (x = 1) where half of the Ca(II) ions are replaced by smaller Mn(II) ions. We analyzed the peak shapes in the XRD patterns, as well as lattice parameters, and it appears that smooth symmetry change from the centrosymmetric space group Pbnm to the non-centrosymmetric space group P42mc occurs between x = 0.3 and x = 0.5. We also confirmed the centrosymmetric to non-centrosymmetric transition by characterizing the dielectric properties of the materials with ferroelectric measurements.

Investigation on the predictive power of tolerance factor τ for A-site double perovskite oxides.

We have used a recently introduced new tolerance factor τ to create a stability map of all possible A-site double perovskite titanates AA'Ti2O6 and niobates AA'Nb2O6. The predictive power of τ is relatively good based on comparisons with available experimental data for A-site double perovskites. We carried out quantum chemical calculations on two hypothetical double perovskite compositions CsScTi2O6 and YRbTi2O6, where τ predicts high probability for their existence. In both cases, we found limits in the predictive power of the new tolerance factor for ion combinations on the A and A' site which are very different in size. A difference in oxidation state may decrease the accuracy, as well. Overall, the A-site double perovskite stability mapping provides a starting point for the discovery of novel A-site double perovskites.

Structural principles of cation ordering and octahedral tilting in A-site ordered double perovskites: ferroelectric CaMnTi2O6 as a model system.

We have used quantum chemical methods to study the structural principles and energetics of A-site ordered AA'B2O6 double perovskites. 33 combinations of A-site ordering and Glazer tilting have been systematically studied for the ferroelectric CaMnTi2O6 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 CaMnTi2O6. 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 CaMnTi2O6. 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.