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

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 Mn ⁴⁺-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⁺ (4f ⁶) and Mn⁴⁺ (3d³) ions in the pyrochlores, Ln₂(Ti,Sn)₂O₇ (Ln = Lu³⁺, Y ³⁺, and Gd³⁺) is presented. The results show that the crystal-field strength that are experienced by the Mn⁴⁺ and Eu ³⁺ 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 4f ⁶ 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 Ti ⁴⁺ ion, the crystal-field strength is reduced to the extent that the exposed Mn⁴⁺ 3d³ electrons and the screened Eu ³⁺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 Ln ³⁺, Sn⁴⁺, and Ti⁴⁺ ions of the Ln ₂(Ti,Sn)₂O₇ lattice.