LaSrVMoO$_6$: A case study for $A$-site covalency-driven local cationic order in double perovskites

TL;DR

This study investigates the atomic-scale chemical fluctuations in LaSrVMoO$_6$, revealing that La-O covalency drives local cationic order and phase separation, which cannot be eliminated through synthesis adjustments.

Contribution

It introduces a new type of local chemical order in double perovskites driven by A-site covalency, supported by experimental and ab-initio calculations.

Findings

Phase separation persists at the unit cell scale despite synthesis tuning.

La-O covalency favors La,V and Sr,Mo ionic proximity, causing patchy structures.

A novel covalency-driven local order in double perovskites is identified.

Abstract

An unusual atomic scale chemical fluctuation in LaSrVMoO$_6$, 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 an unit cell dimension making it impossible to achieve the conventional $B$-site ordered structure, which establishes that the observed `inhomogeneous' patch-like structure with minimum dimension of few angstroms is a reality in LaSrVMoO$_6$. 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 {\it ab-initio} electronic structure calculation revealed that it is the energy stability, gained by typical La-O covalency as in LaVO$_3$, that leads to the preferential La,V and Sr,Mo ionic proximity, and the consequent patchy structure.