Interchain Constants Research Articles

Fluorine-bridged chain compounds as models in magnetochemistry: structures and magnetism of (imidH) 2[Fe 2oxF 6] and [Fe 2oxF 4(DMSO) 4

The crystal structure of a new Fe(III) dimer obtained by fluorination of FeCl 3 with Me 3SnF in the presence of oxalic acid and crystallization from dimethyl sulfoxide (DMSO) has been determined: [Fe 2oxF 4(DMSO) 4] 1 , Space group P2 1/ n, Z=2, a=7.259(2), b=11.409(3), c=13.374(2) Å, β=97.26(1), R=4.47%. It shows a tetradentate bridging oxalato, equatorial cis fluorine and axial trans DMSO ligands. From an aqueous solution of FeF 3·3H 2O, oxalic acid, and imidazolium fluoride, crystals of (imidH) 2[Fe 2oxF 6] 2 could be precipitated. Space group Pban, Z=2, a=9.143(2), b=20.837(6), c=3.890(1) Å, R=2.51%. The structure shows anionic chains formed by bridging trans fluorine ligands connected, like in the dimer above, by oxalate anions to form a double chain. The magnetic properties were determined on powders by SQUID measurements. Mössbauer experiments were performed on the S=5/2 Fe 3+ double chain of (imidH) 2[Fe 2oxF 6], where the anisotropy is of dipolar origin and is therefore very weak. The striking result is the characteristic aspect of two magnetic split spectra below T N and additionally, the coexistence of a temperature dependent paramagnetic component (doublet) and the magnetic split spectra in the temperature range T N=14.5 and T H=40 K. The origin of that unusual behaviour is attributed to nonlinear excitations (magnetic solitons) in systems with small local anisotropy. Consequently, the domain wall width should be large. The subspectrum of the doublet was identified with intraband solitons. The relative intensity of the fast relaxing component increases with increasing temperature as ∝exp(− E s/ kT). From the slope of fractional intensity of the broadened doublet as a function of inverse temperature, the activation energy E s/ k=40(1) K was derived. Considering the results of magnetic susceptibility measurements of the intra-chain exchange constant J/ k=−17.3(2) K, we found an anisotropy constant of D/ k=0.15(2) K in agreement with our magnetic study. Additionally, parameters as the inter-chain constants J′/ k=0.08(2) K and J″/ k=0.60(5) K were calculated. Speculatively, from the experimental data of the magnetically split spectra the ratio (domain length/domain wall width) ≈ 2 was estimated at T N, in agreement with the theoretical expectations. Finally, the results obtained for the double chain of (imidH) 2[Fe 2oxF 6] with weak anisotropy are compared with previously reported Mössbauer experiments on 57 Fe -doped powder of (ND 4) 2MnF 5, inelastic neutron scattering, and magnetic susceptibility measurements on single crystals of (ND 4) 2MnF 5 with strong anisotropy.

Phase diagram of the Blume-Emery-Griffiths model on the simple cubic lattice calculated by the linear chain approximation

The linear chain approximation is used to study the temperature dependence of the order parameters and the phase diagrams of the Blume–Emery–Griffiths model on the simple cubic lattice with dipole–dipole, quadrupole–quadrupole coupling strengths and a crystal-field interaction. The problem is approached introducing first a trial one-dimensional Hamiltonian whose free energy can be calculated exactly by the transfer matrix method. Then using the Bogoliubov variational principle, the free energy of the model is determined. It is assumed that the dipolar and quadrupolar intrachain coupling constants are much stronger than the corresponding interchain constants and confined the attention to the case of nearest-neighbor interactions. The phase transitions are examined and the phase diagrams are obtained for several values of the coupling strengths in the three different planes. A comparison with other approximate techniques is also made.