Local structure and spin state of Co 4+ ions in the perovskite-type SrCo 1− xMn xO 3 solid-solution

Abstract

The local structure of the perovskite-type SrCo 1− x Mn x O 3 solid-solution was investigated in order to study the effect of replacement of Co 4+ ions with Mn 4+ ions on the spin state of Co 4+ ion. The actual distances from Co 4+ or Mn 4+ ions to the first nearest-neighbors in the solid-solution, R[CoO] and R[MnO], are determined by EXAFS method. R[MnO] decreases with increase of Co 4+ ion content in the range 0.33 ≤ x ≤ 1.0 even though the lattice constant increases with Co 4+ ion content. On the other hand, R[CoO] increases with Mn 4+ ion content with a break at x = 0.33. A lever rule is well satisfied: (1− x) · R[CoO] + x · R[MnO] = R[(Co 1− x Mn x )O], where R[(Co 1− x Mn x )O] is the mean (Co,Mn)O distance by X-ray diffraction. These results indicate that the O 2− ion moves toward the Mn 4+ ion and away from the Co 4+ ion in the CoOMn combination. The strength of the ligand field for the Co 4+ ion becomes weak gradually with increase of Mn 4+ ion content because the O 2− ions around Co 4+ ions are more strongly attracted to Mn 4+ ions. Little change in R[MnO] in the range 0.0 ≤ x < 0.33 can be interpreted by forming little MnOMn combinations (i.e., the CoOMn and CoOCo combinations preferentially exist). It is proposed that the strength of the ligand field decreases sufficiently, when four of six CoO bonds in CoO 6 octahedra are lengthened at the composition of x = 0.33, so that the spin state of Co 4+ ion changes from low to high.