Optical spectroscopy of Pr3+ in the weberite, NaGdSb2O7: High covalence of Pr3+O2− bonding

The spectroscopic properties of the Pr3+ ion in NaGdSb2O7 are discussed in this paper. The host crystallizes in the orthorhombic weberite crystal structure with the general formula, A2B2O7. The Pr3+ luminescence is dominated by emission transitions emanating from the 1D2 state. Transient fluorescent decay of the Pr3+ luminescence is indicative of random distribution of Na+ and Gd3+ ions in NaGdSb2O7. The remarkable feature in the spectroscopic properties of the Pr3+ ion is the very low energy of the interconfiguration Pr3+ 4f2→4f15d1 excitation transition (3.63 eV; 342 nm). The large red shift of the Pr3+ 4f15d1 configuration is connected with significant covalent contribution to the crystal field and the high covalence of the Pr3+O2− bonding is tentatively assigned to chemical covalent bonding within the A3B tetrahedral moieties which surround the O8h2− anion in the weberite structure. A comparative study of the optical properties of Pr3+ in NaGdSb2O7 and La2Hf2O7, which crystallizes with the pyrochlore structure, is also presented.