Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH₂)₃]Mn(NCS)₃

We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH$_2$)$_3$]Mn(NCS)$_3$ which crystallises in the unusual low-symmetry trigonal space group $P\overline{3}$. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature ($T_\mathrm{N,mag} = 18.1(6)\,$)K) and can be described by nearest neighbour antiferromagnetic interactions $J=-11.07(4)\,$K. A method for growing neutron-diffraction sized single crystals (\textgreater10 mm$^3$) is demonstrated. Low temperature neutron single crystal diffraction shows that the compound adopts the collinear antiferromagnetic structure with $T_\mathrm{N,neut}= 18.94(7)\,$K, magnetic space group $P \bar{3}'$. Low temperature second-harmonic generation (SHG) measurements provide no evidence of breaking of the centre of symmetry.