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

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 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 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 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.

Figure 1

(a)–(f) X-ray powder diffraction from the six samples at 300 K showing the observed data (open circles), calculated pattern (continuous line), and difference pattern. Insets show contributions from different crystallographic phases.

Figure 2

(a) Elemental mapping from LSVMO-1 (upper panel) and (b) LSVMO-2 (lower panel) samples. The total area under investigation has been divided into 12 equal area cells, shown by the overlaid lines. Integrated intensities of La, Sr, V, and Mo from each of the small cells are plotted as histograms below the corresponding panel after topographic correction. Numbers of the cells are marked in the top left La subpanel to correlate intensity bars to the corresponding cell.

Figure 3

(a)–(f) ZFC (lower curve) and FC (upper curve) magnetization vs temperature data collected from six samples.

Figure 4

(Left) Mo $K$-edge $k^2$-weighted experimental XAFS data (points) $k\chi \left(k^2\right)$, and best fit (full line) of LSVMO-2, LSVMO-3, and LSVMO-6 samples (shifted for clarity). The right panel shows the modulus of the Fourier transform of experimental $k^2\chi \left(k\right)$ (points) and best fit (full lines).

Figure 5

The height normalized O 1$s$ (a), Sr 3$d$ (b), and Mo 3$d$ (c) core level and valence band (d) spectra of LSVMO-1 (open circle), 2 (up triangle), and 3 (open rhombus).

Figure 6

(a)–(f) Different possible structural models of LaSrVMoO${}_6$. The corresponding relative stabilization energies ($\Delta E$) are also shown in the figure.

Figure 7

(a) and (b) Charge density plots for structure (d) [see Fig. 6] in two different lattice planes, (c) and (d) charge density plots for structure (c) in two different lattice planes. (e) and (f) show off-site COHPs and integrated COHPs (ICOHP) per bond for the nearest neighbors Mo-O, V-O, and La-O for structures (d) and (c), respectively. All energies are measured with respect to the Fermi energy.