Magnetic properties of paramelaconite (Cu4O3): A pyrochlore lattice with S=1/2

We have studied the magnetic properties of a single crystal of the mineral paramelaconite, Cu₄O₃, showing a tetragonally distorted (space group I4₁/amd) magnetic pyrochlore sublattice of S=1/2. The magnetic susceptibility shows a drop around 40 K that could be interpreted as an antiferromagnetic transition or the appearance of a nonmagnetic state at lower temperatures. Neutron diffraction measurements at low temperature show unambiguously the transition to an ordered state of propagation vector k=(1/2,1/2,1/2) with respect to the reciprocal basis of the conventional body centered cell. Referred to the pseudocubic Fd3m setting it is k_C=(0,1,1/2). To the best of our knowledge, this is an unprecedented magnetic ordering in pyrochlore lattices. We argue that the observed magnetic ordering cannot be explained within models limited to isotropic superexchange interactions.