Anisotropy of exchange interactions in honeycomb ladder compound ReCl5

The Re5+(5 d2) compounds possess large spin-orbital interaction which urges for large anisotropy, non – collinear structures and other phenomena. Here we present ReCl5 composed by separate [Re2Cl10] units formed by edge – shared chlorine octahedra. It demonstrates the formation of antiferromagnetically ordered state in two steps at TN1 = 35.5 K and TN2 = 13.2 K seen in dc-, ac - magnetic susceptibility and in specific heat. At 4K it can be transformed to the state with spontaneous magnetic moment by relatively weak magnetic field μ0H = 0.5 T via metamagnetic phase transition. Ab initio calculations give anisotropic ferromagnetic exchange interactions J1 and J2 within and between rhenium pairs forming the zig-zag chains along the a – axis. Pairs of zig-zag chains are coupled by ferromagnetic interaction J3 along the c – axis into magnetic honeycomb ladders. The ladders are coupled by significantly weaker interaction J4.