Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: distortion-induced lifting of geometrical frustration

The crystal structure and the magnetism of BaMn${}_2$O${}_3$ have been studied by thermodynamic and by diffraction techniques using large single crystals and powders. BaMn${}_2$O${}_3$ is a realization of a $S=5\mathrm{/}2$ spin ladder as the magnetic interaction is dominant along 180${}^{\circ }$ Mn-O-Mn bonds forming the legs and the rungs of a ladder. The temperature dependence of the magnetic susceptibility exhibits well-defined maxima for all directions proving the low-dimensional magnetic character in BaMn${}_2$O${}_3$. The susceptibility and powder neutron diffraction data, however, show that BaMn${}_2$O${}_3$ exhibits a transition to antiferromagnetic order at 184 K, in spite of a full frustration of the nearest-neighbor inter-ladder coupling in the orthorhombic high-temperature phase. This frustration is lifted by a remarkably strong monoclinic distortion which accompanies the magnetic transition.