Lattice dynamics and structural correlations in antiferromagnet SrLaCoNbO${}_6$ double perovskite

Abstract

We report a detailed temperature-dependent Raman spectroscopy and Co $K$-edge extended x-ray absorption fine structure (EXAFS) study of the insulating antiferromagnetic double perovskite SrLaCoNbO${}_6$. Interestingly, pronounced changes in the phonon frequencies, linewidths, and intensities are observed around 60~K, well above the long-range antiferromagnetic ordering temperature, $T_{\mathrm{N}}\approx 15$~K. These anomalies indicate a modification of the lattice dynamics that may involve local structural distortions, short-range magnetic correlations, or an interplay between them. Additional changes in both high- and low-energy Raman modes are detected near 160--180~K via variations in phonon intensities and linewidths, together with changes in the Fano asymmetry of selected low-energy modes. The asymmetric line shapes of these modes are well described phenomenologically by Fano profiles and indicate coupling of the discrete phonons to an underlying continuum, whose possible microscopic origins are discussed. Moreover, the EXAFS analysis reveals correlated changes in bond distances and Debye--Waller factors around the Co ions near 60~K and 160~K, indicating subtle local structural distortions at temperatures corresponding to the Raman anomalies. The combined Raman and EXAFS results therefore demonstrate a close correlation between anomalous lattice dynamics and local structural evolution in SrLaCoNbO${}_6$.

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