The influence of nd0 transition metal cations on the Eu3+ asymmetry ratio R = I(5D0-7F2)/I(5D0-7F1) and crystal field splitting of 7F1 manifold in pyrochlore and zircon compounds

This paper examines the influence of transition metal cations with empty nd(0) cores on the Eu3+ asymmetry ratio R = I(5(D0)-7(F2))/I(5(D0)-7(F1)) and the magnitude of crystal field splitting of the F-7(1) manifold (Delta E; F-7(1)). Two families of compounds that crystallize with the pyrochlore and zircon structures are considered. The pyrochlores under consideration have the general formula La2M2O7 (M = Hf, Zr, Sn). Comparison of the Eu3+ optical properties indicate that R increases in order La2Sn2O7 < La2M2O7 (M = Hf, Zr) while (Delta E; F-7(1)) varies as La2M2O7 (M = Hf, Zr) < La2Sn2O7. The strength of M-O covalent bonding is responsible for these trends. The M-cations in La2M2O7 (M = Hf, Zr) have empty nd(0) orbitals that participate strongly in covalent bonding with oxide anions, while the covalent contribution to the Sn4+-O2- bonding is weak due to filled Sn4+ 4d(10) core. Mulliken charge calculations of the cations and anions in isostructural pyrochlore compounds are interpreted to indicate the presence of a stronger low-symmetry crystal field component that increases the asymmetry ratio in compounds with cations having empty nd(o) cores. This stronger covalent bonding is also responsible for decreasing (Delta E; F-7(1)). The second family of compounds under consideration crystallize in the zircon structure with the general formula YZO(4) (Z = P, V). Comparison of Mulliken charges of cations and anions in these compounds indicates strong covalent bonding in the vanadate because it contains V5+ transition metal cation with empty 3d(0) orbital. It leads to R(YPO4) < R(YVO4) and [YVO4: (Delta E; F-7(1))] < [YPO4: (Delta E; F-7(1))]. It is concluded that in isostructural family of compounds, the covalent bonding within the crystal framework determines R and (Delta E; F-7(1)).

Automated summary

This study investigates the influence of transition metal cations with empty nd(0) cores on the optical properties of Eu3+ and the crystal field splitting. The results suggest that the strength of covalent bonding between the cations and oxide anions is responsible for the variations in properties. In isostructural compounds, covalent bonding within the crystal framework determines the asymmetry ratio and crystal field splitting.