Observation of Anisotropic Thermal Expansion and the Jahn–Teller Effect in Double Perovskites Sr_2–xLa_xCoNbO₆ 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 (P2 1 /n) is observed between x = 0.4 and 0.6 samples. Interestingly, the temperature evolution of the unit cell parameters follows the Gruneisen 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 Gruneisen 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.

Seeing the Unseen: The Structural Influence of the Lone Pair Electrons in PbWO4

“…Clearly, there is a relationship between deformation of the polyhedra and the ionic radii on both the A and B sites. As V 5+ , W 6+ , and Re 7+ are all n d 0 cations, they are additionally likely to have an electronic contribution, such as susceptibility to SOJT distortions as observed in perovskite and double perovskite oxides. − The importance of this is evident in the Bi B O 4 ( B 5+ = V, As) compounds, where the VO 4 (d 0 ) tetrahedra are more distorted than the related AsO 4 (d 10 ) tetrahedra, in line with the larger V 5+ ionic radii (0.355 vs 0.335 Å). , …”