Structure, electronic and magnetic characterization, and calculated electronic structures of two oxyhalide hexagonal perovskites

We report the crystal structures, initial magnetic and charge transport characterization, and calculated electronic structures of the hexagonal oxyhalide perovskites, ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ and ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$. The experimental information is obtained through the study of single crystals. Face-sharing $\mathrm{Ru}{\mathrm{O}}_6$ octahedra form ${\mathrm{Ru}}_2{\mathrm{O}}_9$ dimers in a layered triangular geometry in these materials; minor amounts of off-magnetic-site structural disorder are present in our crystals. Both are magnetically isotropic, with 2.5 μB/mol-Ru, Curie-Weiss theta −185 K, and 2.9 μB/mol-Ru, Curie-Weiss theta −168 K, respectively. Broad features in the magnetic susceptibility and heat capacity associated with magnetic ordering (at 35 and 37 K, respectively) are observed. The charge transport band gaps are 0.03 eV for the oxychloride and 0.006 eV for the oxybromide. There is no gap at the Fermi level for either of these compounds in density-functional theory electronic structure calculations.

Figure 1

(a) Crystal structure of ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{X}_2$ ($X=\mathrm{Cl}$, Br) from $z=1/4$ to $z=3/4$. (The disordered Ba4 is shown in orange and the disordered Cl9/Br9 is shown in blue). (b), (c) Two ${\mathrm{Ru}}_2{\mathrm{O}}_9$ dimers (two face-sharing $\mathrm{Ru}{\mathrm{O}}_6$ octahedra) corner sharing to each other. The Ru–O bond lengths are labeled and the crystals are displayed.

Figure 2

(a) Temperature-dependent magnetic susceptibility and (b) its inverse in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ at the applied field of 1 T. Insets: Field-dependent magnetization of ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ at 2 and 300 K with the applied field parallel and perpendicular to the $c$ axis, respectively.

Figure 3

(a) Temperature-dependent magnetic susceptibility and (b) its inverse in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$ at the applied field of 1 T. Inset: Field-dependent magnetization of ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$ at 2 and 300 K with the applied field parallel and perpendicular to the $c$ axis, respectively.

Figure 4

(a) Normalization and comparison of the temperature-dependent susceptibilities of these two materials. (The dashed line represents ideal Curie-Weiss behavior.) (b) Heat capacity data measured from 1.8–80 K under zero applied field. A broad hump at around 35 K in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ and 37 K in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$ indicates a short-range magnetic ordering or low-dimensional ordering, previously observed in the magnetic susceptibility. Inset: Low-temperature fittings result to the γ term of zero in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$ and $10\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$.

Figure 5

Fitting of the resistivity data to the Arrhenius equation results to the band gap of (a) 0.03 eV in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ and (b) 0.006 eV in ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$. The insets show the raw resistance data.

Figure 6

(a), (b) Density of states curves and (c), (d) Band structures of ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ and ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$, respectively, plotted from the DFT calculations.