Structure resolution of the new phase Ba19Cr12O48

After nearly 30 years of research on the superconducting cuprates, the discovery of high T c superconductivity in related iron based pnictides in 2008 1 has stimulated the search of new superconducting compounds. In particular, phases presenting 2D squared planes of 3d transition elements with antiferromagnetic interactions, like the famous cuprate YBa 2 Cu 3 O 7‐ d , are of great interest. In that sense, the n=1 (Sr 2 CrO 4 (Sr214)) and n=2 (Sr 3 Cr 2 O 7 (Sr327)) members of the Sr n+1 Cr n O 3n+1 Ruddlesden‐Popper (RP) series, synthesized by J.A. Kafalas and J.M. Longo under high pressure ‐ high temperature for the first time in 1972 are interesting 2 . Only recently this Sr‐Cr‐O system has been revisited by Baikie et al. and E. Castillo‐Martinez & M.A. Alario‐Franco 3,4 . To explore new Cr‐based systems where superconductivity could potentially be induced by changing doping and Cr‐Cr interactions, we have synthesized new chromates by replacing Sr with other alkaline earth elements. The replacement of Sr by a smaller Ca alkaline earth has given rise to a new Ca‐based chromate Ca 3 Cr 2 O 7 (Ca327) RP phase synthesized at 4 GPa and 1000°C 5 . When a bigger alkaline earth element, such as Ba, was used, a new unknown phase was isolated instead of the traditional RP‐type phase in this conditions of synthesis; the low pressure Ba 2 CrO 4 orthorhombic phase was first obtained from the solid state reaction of BaCO 3 and Cr 2 O 3 at 1000°C under Ar flow, then this Ba‐based precursor was treated at 1000°C under 6 GPa for 30 minutes. Since it was impossible to solve ab initio the crystallographic structure of this new phase from powder X‐ray diffraction, electron diffraction (ED) appeared to be the most suitable method. First of all the selected area electron diffraction highlighted a big cubic cell with the cell parameter a = 13.6 Å, as illustrated by the two [110] and [111] zone axis electron diffraction patterns on figure 1. The extinction conditions are consistent with a body‐centered lattice. Then an ED tomography was performed manually by executing a rotation of the sample holder from +50° to ‐50° and recording an ED pattern each 1° step, with the application of a 1° precession. The reflections picking, 3D reciprocal space reconstruction, cell determination and reflection intensities extraction were realized with the software PETS 6 , and the structure model was obtained using charge flipping on JANA2006 7 . All the cations were properly determined as well as most of the oxygens, and the remaining oxygens were identified by fast Fourier transform difference. Then the structure was successfully refined from powder X‐ray diffraction data using Rietveld method. On the final structure presented on figure 2, all Cr are situated in face‐sharing octahedra.