LaSrVMoO6: A case study for A-site covalency-driven local cationic order in double perovskites

An unusual atomic scale chemical fluctuation in LaSrVMoO6, in terms of narrow patches of La, V and Sr,Mo-rich phases, has been probed in detail to understand the origin of such a chemical state. Exhaustive tuning of the equilibrium synthesis parameters showed that the extent of phase separation can never be melted down below a unit cell dimension making it impossible to achieve the conventional B-site ordered structure, which establishes that the observed inhomogeneous patchlike structure with minimum dimension of few angstroms is a reality in LaSrVMoO6. Therefore another type of local chemical order, hitherto unknown in double perovskites, gets introduced here. X-ray diffraction, electron microscopy elemental mapping, magnetic, and various spectroscopic studies have been carried out on samples, synthesized under different conditions. These experimental results in conjunction with ab initio electronic structure calculation revealed that it is the energy stability, gained by typical La-O covalency as in LaVO3, that leads to the preferential La, V and Sr,Mo ionic proximity, and the consequent patchy structure.