Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C4H9)4N FeIIFeIII(C2O4)3 by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye–Waller factor has a discontinuity at 40K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.

Bimetallic tris-oxalato-salts (n-CnH2n+1)(PPh3MFeIII)-Fe-II(C2O4)(3) (n = 3-7, M-II = 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 M-II and Fe-III bridged by oxalate, with the organic cations lying between the metal-oxalate layers, whose separation ranges from 9.48 Angstrom (n = 3) to 11.10 Angstrom (n = 7) for the Fe-II salts and 9.37 to 10.81 Angstrom for Mn-II. The compounds ail behave as ferrimagnets, with magnetic parameters similar to the corresponding AM(II)Fe(III)(C2O4)(3) with A = NR4+, PPh4+ and T(c)s almost insensitive to interlayer separation. The Mn-II salts exhibit uncompensated magnetisation below T-c and the Fe-II 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 Fe-II sublattice.

Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh4) [(FeFeIII)-Fe-II(ox)(3)] and (NBu4) [(FeFeIII)-Fe-II(ox)(3)] by neutron polarization analysis and Mossbauer spectroscopy. Polarized neutron diffraction profiles obtained between 2 and 50 K on (d(20)-PPh4) [(FeFeIII)-Fe-II(ox)(3)] show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca. 0.8 Angstrom(-1) is attributed to two-dimensional short-range magnetic correlations, which are described by a Warren function. The correlation length is ca. 50 A between 2 and 30 K and then decreases to ca. 20 Angstrom at 50 K. The Mossbauer spectra of (PPh4) [(FeFeIII)-Fe-II(ox)(3)] and (NBu4) [(FeFeIII)-Fe-II(ox)(3)] have been measured between 1.9 and 293 K and 1.9 and 315 K, respectively, and are very similar. The paramagnetic spectra exhibit both high-spin Fe-II and Fe-III doublets with relative areas which indicate a 5% and 2% excess, respectively, of Fe-III. The coexistence in (PPh4) [(FeFeIII)-Fe-II(ox)(3)] between 10 and 30 K of broad sextets and doublets in the Mossbauer spectra and the paramagnetic scattering observed in the polarized neutron measurements indicate the coexistence of spin-correlated and spin-uncorrelated regions in the layers of this compound. The polarized neutron scattering profiles and the Mossbauer spectra yield the magnetic exchange correlation length and lifetime, respectively, and the combined results are best understood in terms of layers composed of random frozen, but exchange correlated domains of ca. 50 Angstrom diameter at the lowest temperatures, of spin-correlated domains and spin-uncorrelated regions at intermediate temperatures, and of largely spin-uncorrelated regions above the Neel temperature as determined from magnetometry. The similarity of the Mossbauer spectra of (PPh4) [(FeFeIII)-Fe-II(ox)(3)] and (NBu4) [(FeFeIII)-Fe-II(ox)(3)] leads to the conclusion that similar magnetic exchange correlations are present in the latter compound.

Royal Institution of Great Britain, London W1S 4BS, United Kingdom Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom NWO-EW, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom IRI, TU-Delft, Mekelweg 15, 2629 JB Delft, Netherlands Institut für Anorganische Chemie und Analytische Chemie, J. Gutenberg-Universita ̈t Mainz, Staudinger Weg 9, D-55099 Mainz, Germany Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65409-0010 Institut de Physique, B5, Universite ́ de Liège, B-4000 Sart-Tilman, Belgium ~Received 27 November 2001; revised manuscript received 2 July 2002; published 9 September 2002 !

The high symmetry and resulting electronic degeneracy of the C(60)(3)(-) anion is viewed as the key molecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerene systems. The experimental evaluation of this hypothesis requires the synthesis of face-centered cubic (fcc) trivalent fulleride anion salts derived from higher fullerenes such as C(70), which have thus far proved elusive with only stable A(1)C(70), A(4)C(70), and A(6)C(70) phases known. In this paper, we report the synthesis of fcc A(3)C(70) phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are strongly dependent on the cooling protocol due to the existence of metastable partially or completely orientationally disordered phases. EPR data indicate that the phases are metallic but not superconducting. The densities of states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recent observations that four electrons per C(70) anion are required for superconductivity. Size-matching on both the octahedral and tetrahedral sites is required for A(3)C(70) stability - K(2)CsC(70) is only stable at elevated temperature and Na(2)C(70) is unstable, the composition corresponding to C(70) and a sodium-rich trigonal phase.

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.

The effect of site dilution on the bulk magnetic properties of two-dimensional honeycomb ferrimagnets AFe(II)Fe(III)(C2O4)(3) (A = (n-C4H9)(4)N, P(C6H5)(4)) has been studied by substituting the Fe-II site with Zn and the Fe-III site with Ga. In the (n-C4H9)(4)N compounds the critical temperature T-c 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 Fe-III with Ga is needed to suppress long-range order, probably because Fe-II vacancies are already present in the lattice. Monte Carlo calculations predict the quantitative variation of T-c with dilution, but experimentally the dopant has a smaller effect than predicted, possibly because the dopant ions are not randomly distributed.

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.

The crystal structure of AMIIMIII(oxalate)3 class of materials, with M being a transition metal ion, shows an alternating order of the MII and MIII magnetic ions on a honeycomb lattice. In (P(C6D5)4)FeFe(ox)3 no long-range magnetic order has been observed but instead a diffuse magnetic component is present below 35K. XYZ neutron spin polarisation analysis has been used to separate the magnetic scattering from the nuclear-coherent and spin-incoherent contribution. The magnetic elastic as well as the quasi-elastic magnetic scattering profile can be obtained unambigiously. In (P(C6D5)4)FeFe(ox)3 no long-range magnetic order exists, due to the random anisotropy effects introduced by the different electronic configurations of the Fe2+ and Fe3+ ions. A comparison with the long-range magnetic ordering observed in (P(C6D5)4)MnFe(ox)3 is made.