Dilution of a polar magnet: Structure and magnetism of Zn-substituted Co2Mo3O8

We report crystal structure, magnetization, and specific heat measurements on single crystals of the hexagonal polar magnet ${\mathrm{Co}}_{2\text{\ensuremath{-}}x}{\mathrm{Zn}}_x{\mathrm{Mo}}₃{\mathrm{O}}₈$ magnetically diluted by replacing Co by Zn. In contrast to the transformation from the antiferromagnetic to a ferrimagnetic state observed in the isostructural ${\mathrm{Fe}}₂{\mathrm{Mo}}₃{\mathrm{O}}₈$ upon small Zn doping, a robust antiferromagnetic behavior is preserved in Zn-doped ${\mathrm{Co}}₂{\mathrm{Mo}}₃{\mathrm{O}}₈$ up to $x=0.55$. The N\'eel temperature decreases from ${T}_{\mathrm{N}}=40$ K at $x=0$ to 23 K at $x=0.55$, thus extrapolating to $x=1.27$ (36% filling) as the percolation threshold typical for a three-dimensional, highly coordinated network. This indicates strong magnetic couplings beyond the honeycomb planes in ${\mathrm{Co}}₂{\mathrm{Mo}}₃{\mathrm{O}}₈$. A sharp peak in the specific heat and a clear cusp in the susceptibility associated with the onset of magnetic order is observed up to $x=0.55$, whereas at $x=0.66$ these features are broadened due to increased disorder. Interestingly, the in-plane lattice parameter, the Curie-Weiss temperature, and the magnetic entropy vary with $x$ in a concerted but nonmonotonic manner. These observations can be traced back to the observed site-selective Zn substitution. We found that in the low-doping regime ($x<0.2$) ${\mathrm{Zn}}²⁺$ ions primarily occupy the octahedrally coordinated sites, although they have a clear preference for occupying the tetrahedrally coordinated sites at higher doping levels. Due to the multiple interlayer exchange paths, dependent on the coordination of the ${\mathrm{Co}}²⁺$ ions, this behavior is reflected in the nonmonotonic variation of the Curie-Weiss temperature and magnetic entropy with substitution.