The dependence of 10 Dq crystal field parameter for Mn4+ (3d3 configuration) and the magnitude of 7F1 level splitting for Eu3+ (4f6 configuration) on pyrochlore compositions

Abstract

The excitation spectra of the six-coordinated Mn4+ ion in the pyrochlores Ln2Sn2O7 (Ln=Lu3+, Y3+, and Gd3+) have been determined to examine the variation in the crystal field splitting parameter (10 Dq) on the Mn4+–O48f2- distance. The results obtained are entirely consistent with the expectation of the electrostatic point charge model since 10 Dq progressively increase with decreasing Mn4+–O48f2- distance. A comparative study pertaining to the behavior of the Eu3+ (4f6) and Mn4+ (3d3) ions in the pyrochlores, Ln2(Ti,Sn)2O7 (Ln=Lu3+, Y3+, and Gd3+) is presented. The results show that the crystal-field strength that are experienced by the Mn4+ and Eu3+ ions in the Ln2Sn2O7 (Ln=Lu3+, Y3+, and Gd3+) family of materials, are anti-correlated. Thus, for the Mn4+ ion with 3d3 configuration, the crystal-field strength increases with decreasing ionic radii of the Ln3+ cation, while for the Eu3+ ion with 4f6 configuration, an increase with increasing ionic radii of the Ln3+ cation is observed. However, when the Sn4+ ion of the Y2Sn2O7 lattice are replaced by the Ti4+ ion, the crystal-field strength is reduced to the extent that the exposed Mn4+ 3d3 electrons and the screened Eu3+4f6 electrons start to behave similarly. These results are explained by the variations in the covalent mixing between the ligand (O48f2-) and the activator (Mn4+ and Eu3+) cations, which is driven by the competition for the O48f2- electron density by the Ln3+, Sn4+, and Ti4+ ions of the Ln2(Ti,Sn)2O7 lattice.