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

TL;DR

This study investigates structural and magnetic changes in La-doped Sr2CoNbO6 double perovskites using neutron diffraction, revealing anisotropic thermal expansion, a phase transition, and Jahn-Teller distortions affected by La substitution.

Contribution

It provides new insights into the anisotropic thermal expansion and Jahn-Teller effects in Sr2-xLaxCoNbO6, highlighting the impact of La doping on structural distortions and phase transitions.

Findings

Observation of a tetragonal to monoclinic phase transition.

Identification of anisotropic thermal expansion in La-doped samples.

Reduction of octahedral distortion with increased La substitution.

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 ($P2_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Gr\"uneisen 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 Gr\"uneisen 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.