High-pressure synthesis and structural properties of NaMn7O12, a model system for studying charge, spin and orbital orderings in mixed valence manganese oxides

In the last years manganites with perovskite structure are attracting a great interest for the complexity of their electronic structures and the richness of phenomena such as metal-insulator transitions, magneto-resistance effects and charge/orbital/spin-orderings. Within this family of materials, NaMn7O12 can be considered a model system in the study of ordering phenomena. NaMn7O12 has a double-perovskite structure and crystallizes in the cubic Im3 space group, with a = 7.312 Å. Its formula can be written as (NaMn3+3)(Mn3+2Mn4+2)O12 to better visualize the perovskite-like structure and the Mn valence distribution between different crystallographic sites. This feature allows the direct observation of the charge/orbital/spin orderings by crystallographic data. In the half-doped manganites the random occupation of the dopant gives rise to structural inhomogeneities and consequently the Mn3+ and Mn4+ ordering cannot be easily detected by a diffraction technique. NaMn7O12 is a metastable material, synthesized under HP/HT conditions in a multi-anvil apparatus. We present a study on the synthesis of NaMn7O12 as bulk and single crystal, and the structural refinement carried out by powder X-ray and neutron diffraction and by single crystal X-ray diffraction. Measurements of resistivity under pressure by using a DAC, clearly show an insulator to metal transition at about 18 GPa.