This proposal is part of a project in which we explore the unusual physics of 5d transition metal oxides. Using a special high pressure technique, we have synthesised first samples of lead calcium osmate (Pb2CaOsO6). We found that Pb2CaOsO6 undergoes a metal-insulator transition at 80K, which for this material cannot be understood in the usual Mott-Hubbard scenario. It is more likely a rare realisation of a Slater transition, where the electronic ground state is renormalised by magnetic ordering.We have previously performed a Pb2CaOsO6 powder diffraction experiment on WISH. We were able to determine the propagation vector of the antiferromagnetic order, but the dataset did not allow a complete determination of the magnetic structure. To conclude our study, we here propose a magnetic neutron diffraction experiment of our newly synthesised Pb2CaOsO6 single crystal on WISH.