Unusual electron–phonon coupling in K5Li2SmF10

Unusually intense vibronic sidebands appear in the 4.2 K absorption spectra corresponding to transitions to sextet states 6FJ (J=1/2–9/2) of Sm3+ in K5Li2SmF10 single crystal. R value defined as the ratio of integrated intensity of vibronic sideband and the integrated intensity of no-phonon lines has been estimated to be as high as 2.64 for the 6H5/2–6F7/2 transition. Contribution of vibronics to transitions to quartet states is negligible. Instead, the electron–phonon coupling induces strong thermal line broadening as evidenced by temperature dependence of line widths associated with the 4G5/2–6H5/2 transition, consistent with the electron–phonon coupling parameter α∼500 cm−1.