Latent symmetry versus accidental degeneracy effects in the vibrational spectra of dopant chromate anions in M 2Cr xS 1− xO 4 solid solutions (M∈{K, Rb, Cs})

Abstract

Fourier transform IR and Raman spectra of chromate anions isomorphously isolated in potassium, rubidium and cesium sulfate matrices were recorded at room and low temperature (∼100 K). On the basis of the detected second-order vibrational transitions involving the dopant species ν 3 mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Despite the fact that, rigorously speaking, the appearance of the spectra of dopant chromate anions may be explained in terms of a C s site symmetry, the ν 3 stretching region resembles an approximate ( A 1⊕ E) splitting (characteristic for a local C 3 v symmetry). Although such findings may be rationalized in terms of the latent symmetry concept, we propose an alternative explanation, based on the concept of vibrational mode mixing (a ‘Fermi-like’ resonance) of the CrO 4 2− ν 1 mode with the ν 3 a site-group component. We also derive a quantitative model based on the degenerate case stationary perturbation theory that allows an estimation of the relative importance of the latent symmetry versus vibrational mode mixing effects. In the present case, we show that the Fermi-like resonance is predominantly responsible for the observed spectral features.