Disappearance of peculiarly large atomic displacement in the high-temperature phase of magnetite with substitution of Fe2+ by Ni2+ and Mn2.

Single crystal specimens of Fe2.992O4 (magnetite), Fe3-x-yMnx□yO4 (x ≤ 0.980) and Fe3-x-yNix□yO4 (x ≤ 0.513, □ denotes point defects) are prepared using a floating-zone technique, and changes in their structural parameter values with x are examined at room temperature. Preferences of Mn, Ni and point defects at A, B and B sites, respectively, are confirmed by single-crystal X-ray diffraction experiments, while preference of Mn at the A site is not perfectly accomplished in Fe2.010Mn0.980O4. Mean-square displacements of atoms along the 3 axis in [111] are peculiarly large at the B site in Fe2.992O4 and the displacement decreased smoothly in both series, to nearly half in Fe2.00Mn0.980O4, with increasing amount of heteroatom. On the other hand, displacements normal to the direction show only slight convexity with x due to coexistence of the heteroatom. As a result anisotropy in displacements (dominant in [111] at x = 0) is inverted at x = 0.3 in FeMn series and would expectedly be inverted at x ≃ 0.55 in FeNi series. In spite of different locations of heteroatoms and slightly different inversion points on anisotropy, changes in mean-square displacements at the B site in [111] in these series are found on similar lines with changing amounts of heteroatom. In other words, the amount of this displacement is a function of the amount of Fe2+, or remnant electron from the itinerant-electron point-of-view, on the B-site substructure in these compounds. This characteristic lattice mode on the B-site substructure could be interpreted as an average of local distortion of the substructure due to `trimeron' [Senn et al. (2012). Nature, 481, 173-176] and some other modes such as distortion in 〈001〉 [Siratori & Kino (1980). J. Magn. Magn. Mat. 20, 87-90], which moves in the high-temperature structure.