抄録
The local structures around Ga3+ in the tetrahedral sites at the compositions x=1.0 and 1.6 and around Fe3+ in the octahedral sites at the compositions x=3.8 and 4.6 in the Y3Fe5−xGaxO12 garnet structure were investigated by EXAFS method in order to evaluate the short-range correlation of vibration for each backscattering atom situated in the first-, second- and third-nearest neighbor positions with respect to Ga3+ and Fe3+. The Ga3+ atoms in the solid solution occupy only the tetrahedral site in the composition range 0.0<x≤1.6 while the Fe3+ atoms occupy only the octahedral site in the range 3.8≤x<5.0. The distances of Ga-O for the tetrahedral sites in Y3Fe2(Fe2.0Ga1.0)O12 (1.855(6)Å) and Y3Fe2(Fe1.4Ga1.6)O12 (1.843(6)Å) are in agreement with that in the Ga end-member and empirical distance calculated from the effective ionic radii. The distances of Fe-O for the octahedral sites in Y3(Fe1.2Ga0.8)Ga3O12 (2.026(5)Å) and Y3(Fe0.4Ga1.6)Ga3O12 (2.035(5)Å) are in agreement with the empirical distance but are slightly longer than that in the Fe end-member. Because no anharmonic contribution to the Debye-Waller factor appeares at room temperature, we evaluated the harmonic effective pair potential V(u)=1⁄2αu2 for Ga-O and Fe-O bonds from the Debye-Waller factors. The derived potential values, α, for Ga-O and Fe-O bonds were 18.0(9)×10−12 and 9.9(9)×10−12erg/Å2, respectively. The values of the EXAFS Debye-Waller factors for Y with respect to Ga are small due to the short-range correlation of vibration. The largely correlated displacement between the Ga atom and second-nearest Y atoms should be attributed to the cation-cation interaction or the repulsive force between the cations across the shared edges. It is concluded that the cation-cation interaction in garnet structures, especially between the tetra- and dodecahedral sites, plays an important role in the stability of the crystal structure.
