Redox Reactions in Strontium Iron Phosphates: Synthesis, Structures, and Characterization of Sr9Fe(PO4)7 and Sr9FeD(PO4)7

Physical and chemical properties of Sr9Fe(PO4)7 and Sr9FeD(PO4)7 were investigated by Mossbauer, infrared, and Raman spectroscopy, magnetization and dielectric measurements, differential scanning calorimetry, and thermal analysis. Sr9Fe(PO4)7 undergoes an antiferroelectric-paraelectric (AFE-PE) phase transition at Tc = 740 K. Structure parameters of the AFE phase at 293 K were refined from time-of-flight (TOF) neutron powder diffraction data (space group C2/c; Z = 4; a = 14.4971(2) A, b = 10.6005(13) A, c = 17.9632(3) A, and β = 112.5053(9)°), and those of the PE phase at 923 K from synchrotron X-ray powder diffraction data (space group R3m; Z = 3; a = 10.70473(13) A and c = 19.8605(2) A). Parts of Sr atoms and PO4 tetrahedra are highly disordered in the PE phase. Sr9FeD(PO4)7 was prepared by treating Sr9Fe(PO4)7 with D2 at 820 K. The incorporation of D atoms above Tc kept the structure of the high-temperature modification of Sr9Fe(PO4)7. Therefore, Sr9FeD(PO4)7 is isotypic with the PE phase of Sr9Fe(PO4...