The dependence of 10 Dq crystal field parameter for Mn~(4+) (3d~3 configuration) and the magnitude of ~7F_1 level splitting for Eu~(3+) (4f~6 configuration) on pyrochlore compositions

摘要

The excitation spectra of the six-coordinated Mn~(4+) ion in the pyrochlores Ln_2Sn_2O_7 (Ln = Lu, Y , and Gd~(3+)) have been determined to examine the variation in the crystal field splitting parameter (10 Dq) on the Mn~4 -O_(48f)~(2-) distance. The results obtained are entirely consistent with the expectation of the electrostatic point charge model since 10 Dq progressively increase with decreasing Mn~4 - O_(48f)~(2-)distance. A comparative study pertaining to the behavior of the Eu~(3+) (4f~6) and Mn~(4+) (3d~3) ions in the pyrochlores, Ln_2(Ti,Sn)_2O_7 (Ln = Lu~(3+), Y~(3+), and Gd~(3+)) is presented. The results show that the crystal-field strength that are experienced by the Mn~(4+) and Eu~(3+) ions in the Ln_2Sn_2O_7 (Ln = Lu~(3+), Y~(3+), and Gd~(3+)) family of materials, are anti-correlated. Thus, for the Mn~(4+) ion with 3d~3 configuration, the crystal-field strength increases with decreasing ionic radii of the Ln~(3+) cation, while for the Eu~(3+) ion with 4f~6 configuration, an increase with increasing ionic radii of the Ln~(3+) cation is observed. However, when the Sn~(4+) ion of the Y_2Sn_2O_7 lattice are replaced by the Ti~(4+) ion, the crystal-field strength is reduced to the extent that the exposed Mn~(4+) 3d~3 electrons and the screened Eu~(3+) 4f~6 electrons start to behave similarly. These results are explained by the variations in the covalent mixing between the ligand (O_(48f)~(2-)) and the activator (Mn~(4+) and Eu~(3+)) cations, which is driven by the competition for the O_(48f)~(2-) electron density by the Ln~(3+), Sn~(4+), and Ti~(4+) ions of the Ln_2(Ti,Sn)_2O_7 lattice.