Structure-property relationship in the ordered-perovskite-related oxide ${\mathrm{Sr}}_{3.12}{\mathrm{Er}}_{0.88}{\mathrm{Co}}_4{\mathrm{O}}_{10.5}$

Synchrotron x-ray-diffraction patterns were measured and analyzed for a polycrystalline sample of the room-temperature ferromagnet ${\mathrm{Sr}}_{3.12}{\mathrm{Er}}_{0.88}{\mathrm{Co}}_4{\mathrm{O}}_{10.5}$ from $300\text{to}650\mathrm{K}$, from which two structural phase transitions were found to occur successively. The higher-temperature transition at $509\mathrm{K}$ is driven by ordering of the oxygen vacancies, which is closely related to the metallic state at high temperatures. The lower-temperature transition at $360\mathrm{K}$ is of first order, at which the ferromagnetic state suddenly appears exhibiting a jump in magnetization and resistivity. Based on the refined structure, possible spin and orbital models for the magnetic order are proposed.

Figure 1

(Color online) Observed (+), calculated (solid line), and differential Rietveld refinement x-ray-diffraction profiles of ${\mathrm{Sr}}_{3.12}{\mathrm{Er}}_{0.88}{\mathrm{Co}}_4{\mathrm{O}}_{10.5}$ at (a) $650\mathrm{K}$, (b) $460\mathrm{K}$, and (c) $320\mathrm{K}$. The low angular region $(3.8°⩽2\theta ⩽13.0°)$ was enlarged and shown as the insets. The tick marks indicate the calculated peak positions. The crystal structure at $650\mathrm{K}$ is shown in the inset.

Figure 2

(Color online) Crystal structures of the $\mathrm{Co}{\mathrm{O}}_6$ layer (upper) and the $\mathrm{Co}{\mathrm{O}}_{4.25}$ layer (lower) at (a) $650\mathrm{K}$, (b) $460\mathrm{K}$, and (c) $320\mathrm{K}$, viewed along the $c$ axis. The light red (light gray) spheres in the $\mathrm{Co}{\mathrm{O}}_{4.25}$ layers denote oxygen sites with an occupancy of 25% for (a) and an occupancy of 50% for (b) and (c).

Figure 3

Temperature dependence of (a) monoclinic angle, (b) axis lengths, (c) inverse molar magnetic susceptibility, and (d) resistivity of ${\mathrm{Sr}}_{3.12}{\mathrm{Er}}_{0.88}{\mathrm{Co}}_4{\mathrm{O}}_{10.5}$. The axis lengths are given for $2a_0$ ($a_0$ is the lattice parameter for the simple cubic perovskite cell). The inset shows development of the ($1∕2$ $1∕2$ 0) superstructure reflection in the HTT phase.

Figure 4

(Color online) (a) Averaged Co-O bond lengths for the octahedral and the tetrahedral sites of ${\mathrm{Sr}}_{3.12}{\mathrm{Er}}_{0.88}{\mathrm{Co}}_4{\mathrm{O}}_{10.5}$. Averaged Co-O bond lengths of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ are taken from Ref. 19. (b) Averaged Co-O bond lengths for ${\mathrm{Co}}_{\mathrm{oct}}^{\mathrm{aniso}}$ (left panel) and that for ${\mathrm{Co}}_{\mathrm{oct}}^{\mathrm{iso}}$ (right panel). Typical errors are $\sim 0.02\mathrm{\AA }$. The Co-O bond lengths in the shaded area are not displayed because of poor reliability of the refinements. (c) Possible spin and orbital configurations at ${\mathrm{Co}}_{\mathrm{oct}}^{\mathrm{aniso}}$ and ${\mathrm{Co}}_{\mathrm{oct}}^{\mathrm{iso}}$ in the LTM phase.