High pressure structural investigation on alluaudites Na2Fe3(PO4)3-Na2FeMn2(PO4)3 system

Figure 1 The crystal structure of alluaudites Na2M3(PO4)3 (M3=Fe3, Fe2Mn and FeMn2) projected along the c-axis. Alluaudites adopt a flexible framework plus strong covalent P-O bond, which contribute to excellent structural stability up to ~10 GPa. Mn2+ instead of Fe can be described as Na++2Fe2+→Mn2++Fe3++□ where □ represents a lattice vacancy, and it is equivalent to applying negative chemical pressure to the host. The substitution causes a more compressible b-axis, lattice expansion, structural compressibility and intensifies the core/electron-electron interactions of Fe.221