Observation of Anisotropic Thermal Expansion and the Jahn-Teller Effect in Double Perovskites Sr 2- x La x CoNbO 6 Using Neutron Diffraction.

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 ( I 4/ m ) to monoclinic ( P 2 1 / n ) is observed between x = 0.4 and 0.6 samples. Interestingly, the temperature evolution of the unit cell parameters follows the Grüneisen approximation, and the analysis suggests an isotropic thermal expansion in the 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 an increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter Δ around the B-site atoms.