STABILITY, STRUCTURE REFINEMENT, AND MAGNETIC-PROPERTIES OF BETA-FE2(PO4)O

We previously reported the existence and properties of a low-temperature modification of metastable β-Fe2(PO4)O. Its structure was proposed by analogy with NiCr(PO4)O, but the magnetic measurements were hampered by traces of Fe3O4. We have now obtained a purer sample and a single crystal, allowing precise structure refinement, detailed magnetic characterization, and an investigation of the temperature stability range. The single crystal X-ray study confirms the structure as previously proposed: tetragonal (Z = 4), SG I41 amd, with a single iron site in face-sharing octahedra, and isolated PO4 tetrahedra; the reliability factor is R = 0.0345 (Rw = 0.0363). The magnetic susceptibility has been measured from 4 to 850 K. The magnetization at zero applied field is around 0.01 emu/g at 300 and 88 K, and 0.04 emu/g at 3.5 K. The χ = f(T) curve displays several unusual features: above TN (408 K) the Curie constant continuously decreases as a consequence of short-range magnetic order; below 100 K the susceptibility displays a small second maximum at 12 K. The magnetic structure has been investigated by low temperature powder neutron diffraction methods. At 17 K the Fe (II/III) spins are ordered ferromagnetically within the chains of face-sharing FeO6 octahedra, with the spin direction along (001). The chains are ordered antiferromagnetically with respect to each other. The phase stability has been investigated by high temperature X-ray diffraction. The nonreversible β → α transition takes place at ∼800°C and is completed at ∼840°C; however, the β phase stability is slightly lower in the presence of FeCl2 traces or under high pressure. © 1990.