Structural, transport, optical, and electronic properties of Sr₂CoNbO₆thin films

We study the effect of substrate induced strain on the structural, transport, optical, and electronic properties of Sr 2CoNbO 6 double perovskite thin films. The reciprocal space mapping, φ-scan, and high-resolution θ-2 θ 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 an increase in the compressive strain while there is a sharp reduction in the case of tensile strain, which is further supported by a change in the activation energy and the density of states near the Fermi level. The optical bandgap extracted from two distinct absorption bands, observed in the visible-near infrared spectroscopy, shows a non-monotonic behavior in the case of compressive strain while there is significant enhancement with tensile strain. Unlike the bulk Sr 2CoNbO 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.