Observation of Anisotropic Thermal Expansion and the Jahn–Teller Effect in Double Perovskites Sr2–xLaxCoNbO6 Using Neutron Diffraction

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr_2-xLa_xCoNbO₆ (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P2₁/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 CoO₆ and consequently z-in distortion in the adjacent NbO₆ 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.