  Double perovskites extend the design space for new materials, and they often
host phenomena that don't exist in their parent perovskite compounds. Here, we
present a detailed first principles study of the correlated double perovskite
Sr$_2$VNbO$_6$, where inter-cationic charge transfer and strength of electronic
correlations depend strongly on the cation order. By using Density Functional
Theory + Embedded Dynamical Mean Field Theory, we show that this compound has a
completely different electronic structure than either of its parent compounds
despite V and Nb being from the same group in the periodic table. We explain
how the electronic correlations' effect on the crystal structural parameters
determines on which side of the Hund's metal-Mott insulator transition the
material is. Our results demonstrate the emergence of Hund's metallic behavior
in a double perovskite that has $d^1$ parents, and underlines the importance of
electronic correlation effects on the crystal structure.
