A remarkable hardening (∼ 30 cm −1 ) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba 2 FeReO 6 and Sr 2 CrReO 6 double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the anti-symmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of local 3d and 5d occupations and spin magnitudes.

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Temperature dependent normal modes and lattice thermal expansion of the Sr 2 B'UO 6 (B' = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron Xray diffraction, respectively. Monoclinic crystal structures with space group P2 1 /n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ~1 cm-1 upon cooling below T* ~ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr 2 B'UO 6 (B' = Ni, Co).

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