New structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x}$)O$_{10-x}$ complex oxides and it implications in the mullite family of materials*

Complex oxides with the mullite crystal structure belong to the most important phase in both traditional (porcelains and alumino silicate refractories) and advanced ceramics (heat exchangers, shock resistant composites, optical devices). New complex oxides in the mullite family Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ were synthesized and characterized. Using synchrotron x-ray diffraction we demonstrate a new structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ induced by pressure. We contrast it with the structural stability for mullite senso stricto Al$_{4+2x}$Si$_{2-2x}$O$_{10-x}$ where we did not observe any phase transition.