A theoretical interpretation of the abnormal 5D0→7F4 intensity based on the Eu3+ local coordination in the Na9[EuW10O36]·14H2O polyoxometalate

The Na9[EuW10O36]·14H2O polyoxometalate studied here has shown a peculiar 4f-4f emission spectrum that has two main differences in comparison to data reported previously in the literature: a 5D0→7F0 line with much smaller intensity and an abnormally high intensity of the 5D0→7F4 transition. These results have been theoretically interpreted in terms of the Ωλ intensity parameters and their dependence on the nature and local symmetry of the chemical environment around the Eu3+ ion, leading to the conclusion that the chemical environment is highly polarizable with a local symmetry corresponding to a slightly distorted D4d coordination geometry. Further evidences corroborating this interpretation were obtained by progressively dissolving the polyoxometalate in water, inducing a progressive accommodation of the coordination polyhedron towards lower symmetries.