Disentangling octahedral distortion and symmetry breaking in the ordered double perovskite SrLaCoNbO${}_6$

Abstract

We report a comprehensive high temperature structural investigation of the B-site ordered double perovskite SrLaCoNbO${}_6$ using combined Raman spectroscopy and x-ray diffraction (XRD) over the range 25--900~${}^{\circ }$C. Two distinct temperature-driven anomalies are identified at $T_{\mathrm{OD}}=300\pm 25{}^{\circ }$C and $T_{\mathrm{PT}}=750\pm 25{}^{\circ }$C through pronounced renormalizations in the phonon frequency and linewidth, accompanied by the evolution and suppression of characteristic XRD reflections. The low-temperature anomaly at $T_{\mathrm{OD}}$ is associated with an isostructural octahedral distortion within the monoclinic $P{2}_1/n$ phase. At $T_{\mathrm{OD}}$, the radial and angular distortion parameters of the CoO${}_6$ and NbO${}_6$ octahedra decrease to near-zero values before reversing upon further heating due to an inversion of the in-plane Co--O and Nb--O bond-length hierarchy. The high-temperature anomaly at $T_{\mathrm{PT}}$ corresponds to a symmetry-breaking structural phase transition from $P{2}_1/n$ to the monoclinic $I2/m$ phase, confirmed by group-theoretical analysis and Rietveld refinement. These results demonstrate that integrating temperature-dependent Raman spectroscopy with laboratory-based XRD, complemented by quantitative octahedral distortion analysis, provides a robust approach to disentangle isostructural octahedral distortions from symmetry-breaking phase transitions in complex double perovskite oxides without recourse to synchrotron or neutron diffraction facilities.