Magnetic structure determination of Ca${}_3$LiOsO${}_6$ using neutron and x-ray scattering

We present a neutron and x-ray scattering investigation of Ca${}_3$LiOsO${}_6$, a 5d material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions. This contrasts with investigations of extended superexchange interactions that have been largely limited to low-dimensional 3d systems involving both superexchange and extended superexchange. Despite the apparent one-dimensional nature and triangular units of magnetic osmium ions in Ca${}_3$LiOsO${}_6$, the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.

Figure 1

Ca${}_3$LiOsO${}_6$ forms a hexagonal crystal structure with space group $R\overline{3}c$. Magnetic Os${}^{5+}$ ions (blue spheres) are surrounded by six oxygen anions (red spheres). These octahedra are separated along the $c$ axis by Li${}^{+}$ ions. Ca${}^{2+}$ ions (gray spheres) provide the charge balance.

Figure 2

Neutron powder diffraction results at 150 and (inset) 4 K with $\lambda =1.54$ $\AA$ measured on HB-2A at HFIR. The crystal-structure symmetry remains unchanged between high and low temperature, however, an additional magnetic reflection indicative of magnetic order is observed at low temperature, indicated by the asterisk.

Figure 3

NPD data with $\lambda =2.41$ $\AA$ at 4 K modeled with two possible magnetic structures from the representational analysis, labeled (a) $\Gamma \left(1\right)$ and (b) $\Gamma \left(5\right)$, following the numbering scheme of Kovalev. The refinement gave (a) $R_p$ 5.10, $R_{wp}=$ 7.21, $R_{exp}$ $=$ 2.82, ${\chi }^2=$ 6.54 and (b) $R_p=$ 4.98, $R_{wp}=$ 7.02, $R_{exp}=$ 2.83, ${\chi }^2$ $=$ 6.17. The corresponding magnetic spin structure is shown for (c) $\Gamma \left(1\right)$ and (d) $\Gamma \left(5\right)$. The schematic shows 180${}^{\circ }$ between spins, however, additional angles are possible in the basal plane as described in the text.

Figure 4

MRXS energy dependence of the (009) reflection. There is a large enhancement observed in the $\sigma$-$\pi$ scattering and a suppression of scattering in the $\sigma$-$\sigma$ channel, as expected for a purely magnetic reflection. A large resonant enhancement was observed at both $L2$ and $L3$ edges.

Figure 5

Integrated intensity of the scattering at purely magnetic reflections with (a) powder neutron scattering and (b) single-crystal MRXS. The determined antiferromagnetic transition ordering temperature $T_N$ is shown to agree for both techniques.

Figure 6

Schematic of magnetic structure of Ca${}_3$LiOsO${}_6$. Blue spheres are Os ions with associated magnetic spin shown as an arrow. Smaller red spheres are oxygen ions. (a) Magnetic extended superexchange pathways Os-O-O-Os: $J_1$ (black), $J_2$ (red), and $J_3$ (blue). $J_1$ and $J_2$ form AFM exchange interactions, whereas $J_3$ forms FM bonds. (b)–(d) The separate magnetic interaction pathways are shown with their respective distances and angles. The angle between spins in the schematic is 180${}^{\circ }$. See the text for discussion of other angles, however, we note that altering the angle between spins does not effect our discussion of the different exchange interactions.