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

Abstract

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr${}_{2-x}$La${}_x$CoNbO${}_6$ ($x=$ 0.4 and 0.6). A structural phase transition from tetragonal ($I4/m$) to monoclinic ($P{2}_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic 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 $c$-axis significantly deviates from the Grüneisen function. We observe the $z$-out Jahn-Teller distortion in the CoO${}_6$ and consequently $z$-in distortion in the adjacent NbO${}_6$ octahedra. With increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter $\Delta$ around the B-site atoms.

submitted