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 Sr2−xLaxCoNbO6 (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P 21/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 CoO6 and consequently z-in distortion in the adjacent NbO6 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 ∆ around the B-site atoms.

Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures

“…Note that the position of the white line not solely depends on the valence state but also on the local coordination environment around the absorbing atom [51]. A structural transition from the tetragonal to the monoclinic phase was reported in these samples for x 0.6 from our previous x-ray and neutron powder diffraction measurements [9,59] . Thus, a cumulative effect of both valence shift and structural transformation results in the minimal shift in the feature D for the x 0.6 samples.…”

“…This is found to be interesting as the NbO 6 octahedra accommodates the chemical pressure exerted by the substitution of smaller La 3+ ions at the Sr 2+ site [67] up to the x = 0.4 sample. However, with further increase in x, the NbO 6 octahedra rotates around x and y-axis with the same magnitude, leading to the monoclinic distortion having b − b − c + tilt [9,29,59], which results in the abrupt decrease in the distortion of NbO 6 octahedra for the x 0.6 samples. This is also manifested by the sudden enhancement in the B-site ordering (refer Table I and discussion below) and hence antiferromagnetic interactions in x 0.6 samples through Co-O-Nb-O-Co 180 0 superexchange path [9].…”

“…However, a small inconsistency is present in case of the x = 1 sample possibly due to decrease in the quality of the signal because of the lower Sr concentration. In I4/m symmetry, two different oxygen positions lead to three sets of Sr-O bond distances with four bonds in each group, which further split into twelve different bonds in P 2 1 /n symmetry [59]. However, we take three Sr-O scattering paths with the same DW factor to fit the Sr K-edge EXAFS spectra in 1.4-3.5 Å range for all the samples, as shown in Figs.…”