The crystal and magnetic structures of polycrystalline samples of the cobalt oxyhalides ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_2{X}_2$, $X=\mathrm{Cl}$ and Br, have been studied using variable temperature neutron diffraction. The materials are isostructural with the ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type square lattice Heisenberg antiferromagnet ${\mathrm{Sr}}_2\mathrm{Cu}{\mathrm{O}}_2{\mathrm{Cl}}_2$ and contain two-dimensional $\mathrm{Co}{\mathrm{O}}_2$ sheets separated along $z$ by double rocksalt $\mathrm{Sr}X$ layers. Both materials exhibit magnetic Bragg scattering indicative of long range antiferromagnetic order at $T⩽20\mathrm{K}$. The thermal evolution of magnetic reflections has allowed a $T_N=215\left(10\right)\mathrm{K}$ to be determined for ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_2{\mathrm{Cl}}_2$. No evidence of a lowering of crystal symmetry at the onset of magnetic order within ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_2{\mathrm{Cl}}_2$ is found and the crystal structure of both materials in their antiferromagnetically ordered states is well described with $I4∕mmm$ symmetry. The coexistence of domains with the ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$-type and ${\mathrm{La}}_2\mathrm{Ni}{\mathrm{O}}_4$-type spin structures is proposed to account for the observed magnetic scattering. The ordered moments of ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_2{\mathrm{Cl}}_2$ and ${\mathrm{Sr}}_2\mathrm{Co}{\mathrm{O}}_2{\mathrm{Br}}_2$ obtained at $20\mathrm{K}$ and $2\mathrm{K}$, respectively, are $3.22\left(4\right){\mu }_B$ and $3.52\left(7\right){\mu }_B$ indicating the presence of a significant orbital contribution in these high spin $(S=3∕2)$ cobalt (II) systems.

• Received 7 January 2004

DOI:https://doi.org/10.1103/PhysRevB.70.144406