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 Mössbauer, 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) Å, b = 10.6005(13) Å, c = 17.9632(3) Å, and β = 112.5053(9)°), and those of the PE phase at 923 K from synchrotron X-ray powder diffraction data (space group R3̄m; Z = 3; a = 10.70473(13) Å and c = 19.8605(2) Å). 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)7. Rietveld refinements from TOF neutron diffraction data and synchrotron XRD data of Sr9FeD(PO4)7 (at 293 K) on the basis of space group R3̄m gave lattice parameters a = 10.67744(13) Å and c = 19.5810(2) Å, making it possible to locate D atoms at two positions. Oxidation of Sr9FeD(PO4)7 in air above 673 K regenerated Sr9Fe(PO4)7. When Sr9FeD(PO4)7 was heated above 940 K in the absence of oxygen and when Sr9Fe(PO4)7 was treated by 5% H2 + 95% N2 above 1100 K, they decomposed to Sr2P2O7 and Sr9.333Fe1.167(PO4)7.