We studied the structural and magnetic properties of the double perovskite oxides containing mercury, $A_2{\mathrm{MnTeO}}_6$ ($A$ = Hg, ${\mathrm{Hg}}_{0.8}{\mathrm{Ca}}_{0.2}$, and Ca). ${\mathrm{Hg}}_2{\mathrm{MnTeO}}_6$ and (${\mathrm{Hg}}_{0.8}{\mathrm{Ca}}_{0.2}{)}_2{\mathrm{MnTeO}}_6$ crystallize in a rhombohedral phase with $R\overline{3}$ space group, while ${\mathrm{Ca}}_2{\mathrm{MnTeO}}_6$ crystallizes in a monoclinic phase with $P{2}_1/n$ space group. The magnetic susceptibility, specific heat, and neutron diffraction measurements show that, upon cooling, the commensurate antiferromagnetic (AFM) order occurs at 10 K, 8 K, and 11 K for $A$ = Hg, ${\mathrm{Hg}}_{0.8}{\mathrm{Ca}}_{0.2}$, and Ca systems, respectively. The first-principles calculations agree well with the experimental results, showing that the type-I AFM spin arrangement, i.e., magnetic moments aligned ferromagnetically within the planes and antiferromagnetically between adjacent planes, has the lowest total energy compared to other types of long-range order. For all three systems, the magnetic entropy associated with the AFM order amounts to less than 80% of the theoretical value for the high-spin state of ${\mathrm{Mn}}^{2+}$, indicating magnetic frustration across the series. Additionally, an incommensurate order at 5 K was identified by neutron diffraction for ${\mathrm{Hg}}_2{\mathrm{MnTeO}}_6$, which can be described by helicoidal spin order. This observation makes ${\mathrm{Hg}}_2{\mathrm{MnTeO}}_6$ one of the most complex magnetic systems among double perovskite oxides.

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• Magnetic order
• Specific heat
• Antiferromagnets
• Manganites
• Perovskites
• GGA
• Neutron diffraction