  We study the effect of substrate induced strain on the structural, transport,
optical and electronic properties of Sr$_2$CoNbO$_6$ double perovskite thin
films. The reciprocal space mapping, $\phi$-scan and high-resolution
$\theta$-2$\theta$ scans of x-ray diffraction patterns suggest the epitaxial
nature and high-quality of the films deposited on various single crystal
ceramic substrates. A systematic enhancement in the dc electronic conductivity
is observed with increase in the compressive strain, while a sharp reduction in
case of tensile strain, which are further supported by change in the activation
energy and density of states near the Fermi level. The optical band gap
extracted from two distinct absorption bands, observed in the visible-near
infrared spectroscopy show a non-monotonic behavior in case of compressive
strain while significant enhancement with tensile strain. Unlike the bulk
Sr$_2$CoNbO$_6$ (Co$^{3+}$ and Nb$^{5+}$), we observe different valence states
of Co namely 2+, 3+ and 4+, and tetravalent Nb (4$d^1$) in the x-ray
photoemission spectroscopy measurements. Moreover, a reduction in the average
oxygen valency with the compressive strain due to enhancement in the covalent
character of Co/Nb--O bond is evident. Interestingly, we observe sharp Raman
active modes in these thin films, which indicates a significant enhancement in
structural ordering as compared to the bulk.
