Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH 2 ) 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 3̄. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature T 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 (>10 mm 3 ) is demonstrated. Low temperature neutron single crystal diffraction shows that the compound adopts the collinear antiferromagnetic structure with T N,neut = 18.94(7) K, magnetic space group P 3̄'. Low temperature second-harmonic generation (SHG) measurements provide no evidence of breaking of the centre of symmetry.