Charge and spin order in the perovskite $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$: Charge disproportionation behavior of randomly arranged $\mathrm{F}{\mathrm{e}}^{4+}$

$B$-site-disordered perovskite $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ with unusually high valence $\mathrm{F}{\mathrm{e}}^{4+}$ was synthesized using a high-pressure technique. $\mathrm{F}{\mathrm{e}}^{4+}$ randomly distributed at half of the $B$ sites shows charge disproportionation to $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$. The spins of $\mathrm{F}{\mathrm{e}}^{3+},\mathrm{F}{\mathrm{e}}^{5+}$, and $\mathrm{M}{\mathrm{n}}^{4+}$ order below 90 K. Analysis of low-temperature neutron powder-diffraction data revealed a $G$-type antiferromagnetic structure—where all the nearest-neighboring spins of $\mathrm{F}{\mathrm{e}}^{3+},\mathrm{F}{\mathrm{e}}^{5+}$, and $\mathrm{M}{\mathrm{n}}^{4+}$ couple antiparallel—and the small ordered moment of $0.58{\mu }_{\mathrm{B}}$ reveals local charge ordering that gives rise to predominant $\mathrm{F}{\mathrm{e}}^{3+}$-O-$\mathrm{F}{\mathrm{e}}^{5+}$ antiferromagnetic arrangements. Despite the identical chemical compositions of $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ and $\mathrm{C}{\mathrm{a}}_2\mathrm{FeMn}{\mathrm{O}}_6$, the magnetic structure of the present $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ is very different from the noncollinear one of layered $B$-site-ordered $\mathrm{C}{\mathrm{a}}_2\mathrm{FeMn}{\mathrm{O}}_6$.

Figure 1

Neutron powder-diffraction patterns and refinement results for $B$-site-disordered perovskite $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ (a) at room temperature and (b) at 2 K. Corresponding patterns and results for the layered double perovskite $\mathrm{C}{\mathrm{a}}_2\mathrm{FeMn}{\mathrm{O}}_6$ (c) at room temperature and (d) at 2 K are reproduced from Ref. [14]. The dots, black lines, blue lines, and vertical marks represent observed patterns, calculated patterns, differences between observed and calculated intensities, and Bragg refraction points, respectively. Marked peaks are typical magnetic reflections.

Figure 2

Mössbauer spectra of $B$-site-disordered $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ measured (a) at room temperature and (b) at 3 K. The circles are observed data, and the spectra are fitted with (a) a singlet component and (b) two sextets corresponding to $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$ components.

Figure 3

Magnetic properties of $B$-site-disordered $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$ (red) together with those of the layered $B$-site-ordered double perovskite $\mathrm{C}{\mathrm{a}}_2\mathrm{FeMn}{\mathrm{O}}_6$ (blue). (a) Temperature dependence of magnetic susceptibility χ. (b) Magnetic-field dependence of the magnetic moment.

Figure 4

Idealized model for the spin and charge ordering in $B$-site-disordered $\mathrm{CaF}{\mathrm{e}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_3$. Red, blue, and white arrows represent magnetic spins of $\mathrm{F}{\mathrm{e}}^{3+},\mathrm{F}{\mathrm{e}}^{5+}$, and $\mathrm{M}{\mathrm{n}}^{4+}$, respectively. $G$-type antiferromagnetism over local $\mathrm{F}{\mathrm{e}}^{3+}$-O-$\mathrm{F}{\mathrm{e}}^{5+}$ arrangements give rise to the observed ferrimagnetism.