Magnetism, structural distortion and metal-insulator transition in two novel double perovskite osmate

We are interested in a family of double perovskites which have remarkable properties. One member of this family, Pb2CaOsO6, transitions from behaving like a metal to being insulating upon cooling below 80 K. At that same temperature, the Os magnetic moments in Pb2CaOsO6 order antiferromagnetically. We believe the magnetic order induces the metal-insulator transition. We have found that Pb2CaOsO6 is structurally distorted compared to most other double perovskites. This distortion can relieve magnetic frustration and thus allow the moments to order. The sister ocmpound, Pb22ZnOsO6, remains a metal down to 1.5 K and does not order magnetically. In order to understand the behaviour of these two materials we want to use neutron diffraction to determine if Pb22ZnOsO6 is structurally distorted. The experiment will also reveal any short range magnetic correlations at low temperature.