Phase coexistence in the charge ordering transition in CaMn7O12

The structural phase transition in CaMn7O12 has been investigated by using high-resolution synchrotron and neutron powder diffraction. Both measurements show a phase coexistence phenomenon: between 409 and 448 K two different crystallographic phases coexist in the material. The first one is trigonal and it has a charge ordering (CO) of the Mn3+ and Mn4+ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from zero at 400 K up to 100% about 460 K. Both phases have domains of at least 150 nm at each temperature in the PS region. A percolation scenario assuming a growth of the volume of the highly conducting CD regions at the expense of the volume of the insulating CO matrix is discussed and it is found to be in agreement with literature data of the CaMn7O12 resistivity.