Cationic order versus La-O covalency in La A (Ca,Ba)VMoO6 double perovskites

We have investigated the structural and physical properties of double perovskite (DP) La A VMoO₆ (A =Ca^{2 +} , Ba^{2 +}; abbreviated as LCVMO and LBVMO from now on) compounds, proposed to be possible half-metallic antiferromagnets (HMAFMs). Here we show that within La A VMoO₆ double perovskite structure, La-O covalency competes against B -site as well A -site cationic order and this competition critically influences their physical properties. Evidently, the presence of Ca^{2 +} or Ba^{2 +} at the A site along with La^{3 +} would offer a tool to modify the A -site ordering and consequently influence the La-O covalency as well. Our experimental results reveal that LCVMO lies at the extreme end of this family and accommodates large scale phase separation in terms of La, V, and Ca, Mo-rich phases as a result of dominant La-O covalency. On the other hand, LBVMO is more correctly described as a layered A -site ordered and nearly complete B -site disordered double perovskite where cationic order dominates the La-O covalency. The general trend of our experimental findings is in agreement with the ab initio electronic structure calculations, carried out on realistic structures based on local coordination obtained from extended x-ray-absorption fine-structure study.