Structural, transport, optical and electronic properties of Sr$_2$CoNbO$_6$ thin films

TL;DR

This study investigates how substrate-induced strain affects the structural, electronic, optical, and transport properties of Sr$_2$CoNbO$_6$ thin films, revealing strain-dependent conductivity, valence states, and enhanced structural order.

Contribution

It provides new insights into strain effects on double perovskite thin films, including changes in valence states, optical band gaps, and structural ordering, which were not previously characterized in detail.

Findings

Conductivity increases with compressive strain, decreases with tensile strain.

Optical band gap shows non-monotonic behavior under compressive strain and increases with tensile strain.

Presence of multiple Co valence states and tetravalent Nb in thin films.

Abstract

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.