We report the results of the neutron diffraction experiments on the spin-chain compounds ${\text{Ca}}_3{\text{Co}}_{2-x}{\text{Fe}}_x{\text{O}}_6$ ($x=0.2$ and 0.4) crystallizing in the rhombohedral structure (space group $R\overline{3}c$) in the temperature range of 1.5–100 K. Additional magnetic Bragg peaks in the low-temperature neutron powder diffraction patterns mainly indicate an antiferromagnetic ordering for these iron-substituted compounds. The observed magnetic reflections can be indexed in the nuclear lattice with a propagation vector $K=\left(0,0,1\right)$ which indicates that the centering translations have been lost in the magnetic structure. The Rietveld refinement of the neutron diffraction patterns for both the compounds confirms that the ${\Gamma }_2$ irreducible representation of the little group $G^k$ classifies the magnetic structure which corresponds with ferromagnetic planes perpendicular to the $b$ axis, where the magnetic moment is aligned along the $c$ axis, coupled antiferromagnetically along the $b$ direction. We demonstrate that two different magnetic structures, (i) amplitude modulated structure with a propagation vector $K=\left(0,0,1\right)$ and (ii) partially disordered antiferromagnet structure, are able to fit the same neutron diffraction pattern because the Fourier coefficients for each solution only differs in a global phase factor that cannot be determined by the experiment. The temperature-independent intensity of the (110) Bragg peak and the fact that the determined Fourier coefficients assure that the magnetic structure has net zero magnetic moment in the lattice confirm the absence of any phase transition to a ferrimagnetic state.

• Received 3 February 2009

DOI:https://doi.org/10.1103/PhysRevB.79.184428