  Fe3PO4O3 forms a non-centrosymmetric lattice structure (space group R3m)
comprising triangular motifs of Fe^{3+} coupled by strong antiferromagnetic
interactions (|\Theta_{CW}| > 900 K). Neutron diffraction from polycrystalline
samples shows that strong frustration eventually gives way to an ordered
helical incommensurate structure below T_N = 163 K, with the helical axis in
the hexagonal ab plane and a modulation length of ~ 86 \AA. The magnetic
structure consists of an unusual needle-like correlation volume that extends
past 900 \AA along the hexagonal c-axis but is limited to ~ 70 \AA in the ab
plane, despite the three-dimensional nature of the magnetic sublattice
topology. The small in-plane correlation length, which persists to at least T =
T_N/40, indicates a robust blocking of long-range order of the helical magnetic
structure, and therefore stable domain walls, or other defect spin textures,
must be abundant in Fe3PO7. Temperature dependent neutron powder diffraction
reveals small negative thermal expansion below T_N. No change in lattice
symmetry is observed on cooling through T_N, as revealed by high resolution
synchrotron X-ray diffraction. The previously reported reduced moment of the
Fe^{3+} ions (S=5/2), \mu ~ 4.2 \mu_B, is confirmed here through magnetization
studies of a magnetically diluted solid solution series of compounds,
Fe3-xGaxPO4O3, and is consistent with the refined magnetic moment from neutron
diffraction 4.14(2) \mu_B. We attribute the reduced moment to a modified spin
density distribution arising from ligand charge transfer in this insulating
oxide.
