Structure of the new La3Ni2O7 superconductor under high hydrostatic pressure

The very recent report of superconductivity in the bilayer Ruddlesden-Popper (RP) compound La3Ni2O7 has attracted a great deal of attention and raised hopes that this could herald an entirely new family of high-Tc oxides with phenomenology that is unrelated to the cuprates or other known materials. Consequently, there has been a flurry of reports, mostly concerned with theory and modelling, but with relatively little experimental support beyond the initial report (later confirmed by a second group). There is therefore an urgent need to understand the structure/properties relations in this material. In La3Ni2O7, superconductivity appears under applied pressures between 14 GPa and 43.5 GPa with a maximum of ~80 K, and is apparently linked to a structural phase transition at ~10 GPa. However, the crystallographic symmetry of the high-pressure phase is controversial, with conflicting reports that may be related to oxygen non-stoichiometry. Here, we propose to carry out a crystallographic study of La3Ni2O7-x as a function of temperature and pressure. We will study two samples with well-characterised oxygen contents, hence establishing anchor points in the T-P-x phase diagram that will guide future experiments and theory.