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

The Na₉EuW₁₀O₃₆]·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 Ω_λ 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 D_4d 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.