Ian D. Watts

POLARIZED NEUTRON DIFFRACTION STUDY OF THE EXTENDED HONEYCOMB MOLECULAR NETWORK D20-P(C6D5)4MNFE(C2O4)3

The MII and MIII magnetic ions in the extended molecular network P(C6D5)4MnFe(C2O4)3 form a two-dimensional honeycomb magnetic lattice. The Mn2+ and Fe3+ ions alternate in the extended network which is formed by the oxalate (C2O4) ligands. These hexagonal layers are separated and charge compensated by large [P(C6D5)4]+ ions, positioned in between the honeycomb layers. P(C6D5)4MnFe(C2O4)3 orders magnetically at TN=27(1) K. A full neutron spin polarization study of the neutron scattering cross section has been carried out which allows the unambigious separation of the magnetic cross section from the total diffraction process. The magnetic structure can be described with the magnetic Shubnikov group R3c. The magnetic moments are antiferromagnetically aligned along the c axis while the Mn2+ and Fe3+ ions form an antiferromagnetic alignment on the honeycomb lattice.

Muon spin relaxation studies of magnetic ordering in the molecular-based ferrimagnets PPh4MnIIFeIII(C2O4)3 and (n-C4H9)4NFeIIFeIII(C2O4)3

Muon spin relaxation measurements are reported on two molecular-based ferrimagnets, PPh4MnIIFeIII(C2O4)3 and (n-C4H9)4NFeIIFeIII(C2O4)3, at temperatures from 8 to 100 K and applied fields up to 2.5 T using pulsed (ISIS) and continuous (PSI) muon sources. In zero field, the initial asymmetry in the muon depolarization falls sharply, and the muon relaxation rate diverges, in the vicinity of the transition to long-range order (Tc) measured by bulk susceptibility. The onset of both effects takes place significantly about Tc, indicating low-dimensional short-range fluctuations in these larger materials. No coherent muon precession is observed in either compound, with pulsed or continuous muons.

Synthesis, structure and magnetic properties of organic-intercalated bimetallic molecular-based ferrimagnets (n-CnH2n+1)PPh3MIIFeIII(C2O4)3, (MII= Mn, Fe; n= 3–7)

Bimetallic tris-oxalato-salts (n-CnH2n+1)PPh3MIIFeIII(C2O4)3 (n = 3–7, MII = Mn, Fe) were prepared and the structures investigated by powder X-ray diffraction in order to study the evolution of the structure and magnetic properties as a function of alkyl chain length. The compounds all have the same two-dimensional honeycomb structure of MII and FeIII bridged by oxalate, with the organic cations lying between the metal–oxalate layers, whose separation ranges from 9.48 A (n = 3) to 11.10 A (n = 7) for the FeII salts and 9.37 to 10.81 A for MnII. The compounds all behave as ferrimagnets, with magnetic parameters similar to the corresponding AMIIFeIII(C2O4)3 with A = NR4+, PPh4+ and Tcs almost insensitive to interlayer separation. The MnII salts exhibit uncompensated magnetisation below Tc and the FeII ones show Neel type N ferrimagnetism, with negative magnetisation at low temperature, the magnitude of which is influenced by the preparation conditions, due to vacancies in the FeII sublattice.

Site dilution of two-dimensional honeycomb molecular ferrimagnets AFeIIFeIII(C2O4)3 (A = (n-C4H9)N, P(C6H5)4)

The effect of site dilution on the bulk magnetic properties of two-dimensional honeycomb ferrimagnets AFeIIFeIII(C2O4)3 (A = (n-C4H9)4N, P(C6H5)4) has been studied by substituting the FeII site with Zn and the FeIII site with Ga. In the (n-C4H9)4N compounds the critical temperature Tc falls to zero at a mole fraction of diamagnetic dopant (Zn or Ga) between 0.6 and 0.65. In the P(C6H5)4 compounds only 50% doping of FeIII with Ga is needed to suppress long-range order, probably because FeII vacancies are already present in the lattice. Monte Carlo calculations predict the quantitative variation of Tc with dilution, but experimentally the dopant has a smaller effect than predicted, possibly because the dopant ions are not randomly distributed.

Structure of iron and manganese ions substituted in the framework of nanoporous AlPO-5 material

The structure of iron and managanese ions substituted in the framework of nanoporous AlPO-5 is determined by ex situ and in situ X-ray absorption spectroscopy. Fe K-edge XANES and EXAFS studies clearly indicate that iron ions are present as Fe(III) in octahedral coordination in the assynthesised material and tetrahedral coordination in the calcined material in both pure FeAlPO-5 and FeMnalPO-5. XANES and EXAFS results also indicate that reaction with hydrogen peroxide causes the removal of Fe(III) ions from the framework. Mn K-edge XANES and EXAFS of FeMnAlPO-5 samples indicate that Mn(II) ions are present in the framework, tetrahedrally coordinated, in the as-synthesised material but upon calcination it is found that the Mn(II) ions are removed from the framework, suggesting a different synthesis strategy is necessary to stabilise the Mn(II) ions in the framework simultaneously with Fe(III) ions.