Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

Magnetic properties of ultrafine clusters of Fe₅HO₈·₄H₂O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R _cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R _cr = 2 α β η/(1 - T _cc/T ₀), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T _cc = 3/2 Nκ _B T _o ² ηβ ² (N is the number of iron atoms, κ _B is the Boltzmann constant, T _o is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R _cr ～ 1.5–2.0 nm and T _JMT ～ 4.2–6 K, respectively. Satisfactory agreement between the value R _cr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.     

Numerical integration of overlap and ligand contributions to the electric field gradient

The total electric field gradient (EFG) tensor V _pq is calculated by numerical integration of threedimensional integrals. Each of them is solved a) by integrating over one dimension analytically and b) by integrating over the remaining two dimensions on the basis of a Gauss-type integration rule. The use of 100 abscissas in the twodimensional numerical integration scheme yields satisfactory accuracy which was checked by evaluating overlap integrals; an increase to 400 abscissas does not increase the result drastically. Calculating quadrupole splittings E _Q from numerically integrated electric field gradient tensors V _pq we observe that depending a) on the amount of covalency and b) on the amount of deviation from octahedral or tetrahedral symmetry, involved in a molecular system, overlap and ligand contributions to V _pq play an important role. Especially for the sandwich compound ferrocene, Fe(C₅H₅)₂, we find a significant difference between E _Q ^{num. int.} which follows from the numerical integration method, and E _Q ^conventional which is derived from effective charges.Supported by Deutsche Forschungsgemeinschaft     

Calculation of the electronic energy differences of spin crossover complexes

Density functional calculations using different functionals and basis sets have been carried out to calculate the electronic energy difference between the high- and low-spin isomers of spin crossover complexes with a transition metal center. The reparameterized B3LYP* method is confirmed to be most suitable for the calculation of electronic energy differences between isomers with different spin states. If only changes of the electronic energy difference upon modifications of the complex are considered all employed density functional methods show a similar performance. Basis sets with effective core potentials seem to perform as well as large all electron basis sets. Calculations using the polarizable continuum model have been performed to investigate the effect of solvents. In addition the effect of hydrogen bonding between the spin crossover complex and a water molecule is studied.     

Comparison of the dynamics of the magnetic order above Tf in (Au,Fe) and Y(Fe,Al)2

⁵⁷Fe Mössbauer spectra in external fields up to 13.5 T are recorded for two typical cluster glasses (Au, Fe) and Y(Fe, Al)₂ to investigate the spin dynamics above the freezing temperature. The spectra are analyzed with a model in which magnetic clusters are assumed to form and decay stochastically.     

Modulations of exchange coupling in oxoferrylporphyrin cation radical complexes detected by Mössbauer and EPR measurements

Oxoferrylporphyrin cation radical complexes were generated using the prophyrin dianions: tetrakis 2,6-dichlorophenyl (TDCPP) and tetrakis 2,4,6-trimethoxyphenyl (TTMPP). Spin coupling between ferryl iron (S=1) and porphyrin radical S′=1/2), ligand field interaction and hyperfine parameters of iron were studied by Mössbauer and EPR measurements and corresponding spin Hamiltonian analyses. Samples of [Fe^IV=0 TDCPP], which had to be prepared in CH₂Cl₂, were “vacuum dried” in order to obtain Mössbauer spectra.     

Synthesis and characterization of metal-centered, six-membered, mixed-valent, heterometallic wheels of iron, manganese, and indium

Heptanuclear metal-centered, six-membered, mixed-valent, heterometallic wheels 1-3 of iron, manganese, and indium were prepared in a one-pot reaction from N-benzyldiethanolamine (H2L(1)), cesium carbonate, [PPh4]2[MnCl4], and FeCl3 or InCl3. All three complexes were characterized by the combination of elemental analysis, FAB mass spectroscopy, X-ray diffraction and cyclic voltammetry and in the case of 1 additionally by M?ssbauer spectroscopy. In 1, four Mn(II) ions in the periphery are arranged in pairs alternating with one Fe(III) ion each, with an Fe(III) ion located in the center. In 2, three Mn(II) ions alternate with three In(III) ions, whereas in 3, four In(III) ions are arranged in pairs and alternate with one Mn(II) ion each. In 2 and 3 an Mn(II) ion is encapsulated in the center.     

Application of the FAKE molecular-orbital method to diatomic molecules XY (X,Y = H,F, Cl,Br, I)

Theoretical Chemistry Accounts - The FAKE (fast, accurate kinetic energy) method of semiempirical molecular orbital calculation is applied to diatomic molecules XY (X, Y= H, F, Cl, Br, I). The...     

Spin-spin relaxation in heteropolynuclear clusters containing iron(III) and copper(II) ions

The magnetic coupling between Fe(III) and Cu(II) and the relaxation behavior of the spin centered at the iron have been studied in di- and trinuclear clusters. The experimental spectra have been analyzed in terms of a simple cross-relaxation model. In the strongly coupled trimer the relaxation rates are determined by a Boltzmann factor and a single field- and temperature-independent scaling parameter¹ w ₀, while the weakly coupled dimer exhibits obviously a more complex behavior.     

Mössbauer spectroscopy,intracluster atomic mobility and thermodynamics of ultrafine oxide clusters

Summary Dynamic properties of ultrafine clusters of γ-Fe₂O₃ (ferric oxide, FO) were studied by M?ssbauer spectroscopy and by thermodynamic analysis. The data obtained for FO clusters allowed the conclusion that dynamic properties of clusters as well as the decrease of melting point and the appearance of a gap between freezing and melting points depend on intracluster atomic mobility. Intracluster atomic mobility in FO clusters was shown to increase by the action of surfactants which decrease intercluster interactions. The increase in intracluster atomic mobility was suggested to proceed via formation of a solid-liquid state. Paper presented at ICAME-95, Rimini, 10–16 September 1995.