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We use temperature dependent neutron powder diffraction (NPD) to investigatethe structural changes and magnetic interactions in double perovskiteSr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0.4 and 0.6). A structural phase transitionfrom tetragonal ($I4/m$) to monoclinic ($P2_1/n$) is observed between $x=$ 0.4and 0.6 samples. Interestingly, temperature evolution of the unit cellparameters follow the Gr\"uneisen approximation, and the analysis suggest theisotropic thermal expansion in case of the $x=$ 0.4 sample, whereas the $x=$0.6 sample shows the anisotropy where the thermal expansion along the $c$-axissignificantly deviates from the Gr\"uneisen function. We observe the $z$-outJahn-Teller distortion in the CoO$_6$ and consequently $z$-in distortion in theadjacent NbO$_6$ octahedra. With increase in the La substitution, a decrease inthe degree of octahedral distortion is evident from the significant reductionin the local distortion parameter $\Delta$ around the B-site atoms.

Observation of anisotropic thermal expansion and Jahn-Teller effect in double perovskites Sr$_{2-x}$La$_x$CoNbO$_6$ using neutron diffraction