Vibrational anharmonicity and vibrational Stark effect of sulfate ions trapped in potassium, rubidium and cesium chromate

Abstract

Room and low temperature (≈100 K) FT-IR and Raman spectra of the sulfate doped K 2CrO 4, Rb 2CrO 4 and Cs 2CrO 4 were recorded. The positions of the ν 1, ν 3 and ν 4 fundamental mode components of the dopant anions were measured. Nine (out of possible ten) second-order stretch-stretch vibrational transitions of the dopant anions were detected. On the basis of these data, the anharmonicity constants and the corresponding harmonic eigenvalues were calculated for several vibrational transitions using second-order perturbation theory expressions. The anharmonicity of the studied second-order transitions of the type ν 1+ ν 3i increases in the order (SO 4/K 2CrO 4)<(SO 4/Rb 2CrO 4)<(SO 4/Cs 2CrO 4), while for those of the type ν 3j+ ν 3i it basically follows the trend: (SO 4/K 2CrO 4)>(SO 4/Rb 2CrO 4)<(SO 4/Cs 2CrO 4). The measured relative Stark splittings of the ν 3 and ν 4 mode components of the dopant SO 4 2− anions, as well as the average X 13i/ X 3i3j values decrease in the order (SO 4/K 2CrO 4)>(SO 4/Rb 2CrO 4)>(SO 4/Cs 2CrO 4). In all cases, the splitting is larger for ν 3 than for ν 4 modes, indicating a smaller angular than the bond length distortion. The theory of vibrational Stark effect suggests that the observed frequency shifts of the ν 1, ν 3 and ν 4 mode components may be attributed to the increase of the field strength at the doped anion site going from K 2CrO 4 to Cs 2CrO 4. The Stark splitting of these modes, on the other hand, implies that the internal crystalline field vector is almost parallel to the (hypothetical) C 2 axis of the slightly distorted dopant tetrahedral anions.