Peculiar crystal structure and long-range order of spin-1/2 ladders in MoOBr₃

Tetragonal crystal structure of molybdenum oxytribromide, MoOBr3, consists of non-planar Mo2Br6 units joined by non-equidistant Mo-O-Mo bonds into infinite ladders, which are connected to each other by the van der Waals forces. Within unit cell, the apical Mo atoms in any given ladder are shifted along the c axis either above (+z) or below (-z) the basal ab plane of the Br atoms. Magnetic susceptibility taken at various frequencies z(ac)(T) and magnetic fields x(dc)(T) as well as specific heat C-p(T) data evidence the formation of a long-range antiferromagnetic order at TN = 34 +/- 1 K with largely reduced effective magnetic moments mu eff = 1.3 +/- 0.1 mu B. First principles GGA+U calculations point to the predominance of intra-ladder ferromagnetic exchange interaction on both rungs J(rung) = - 10 meV and legs J(leg) = - 1 meV of the ladder, while much weaker inter-ladder antiferromagnetic exchange interaction J(inter) = 0.2 meV couples the ladders. The anisotropic part of the exchange K = 0.6 meV in combination with J(inter) allows estimating the spin-flop transition field mu H-0(spin-flop) similar to 6 T in good correspondence with the magnetization M(H) data.