LOCAL-STRUCTURE AND SPIN STATE OF CO-4+ IONS IN THE PEROVSKITE-TYPE SRCO1-XMNXO3 SOLID-SOLUTION

The local structure of the perovskite-type SrCo1-xMnxO3 solid-solution was investigated in order to study the effect of replacement of Co4+ ions with Mn4+ ions on the spin state of Co4+ ion. The actual distances from Co4+ or Mn4+ ions to the first nearest-neighbors in the solid-solution, R[CoO] and R[MnO], are determined by EXAFS method. R[MnO] decreases with increase of Co4+ ion content in the range 0.33 ≤ x ≤ 1.0 even though the lattice constant increases with Co4+ ion content. On the other hand, R[CoO] increases with Mn4+ ion content with a break at x = 0.33. A lever rule is well satisfied: (1-x) · R[CoO] + x · R[MnO] = R[(Co1-xMnx)O], where R[(Co1-xMnx)O] is the mean (Co,Mn)O distance by X-ray diffraction. These results indicate that the O2- ion moves toward the Mn4+ ion and away from the Co4+ ion in the CoOMn combination. The strength of the ligand field for the Co4+ ion becomes weak gradually with increase of Mn4+ ion content because the O2- ions around Co4+ ions are more strongly attracted to Mn4+ ions. Little change in R[MnO] in the range 0.0 ≤ x < 0.33 can be interpreted by forming little MnOMn combinations (i.e., the CoOMn and CoOCo combinations preferentially exist). It is proposed that the strength of the ligand field decreases sufficiently, when four of six CoO bonds in CoO6 octahedra are lengthened at the composition of x = 0.33, so that the spin state of Co4+ ion changes from low to high. © 1990.