Powder and single-crystal samples of the dimer compound Cs3Cr28r9 have been studied by inelastic neutron scattering. At 1.6 K, the singlet-to-triplet dimer excitation exhibits pronounced energy dispersion as a result of weak interdimer interactions. The results are interpreted within the random-phase approximation with use of a Heisenberg Hamiltonian and considering only nearestneighbor interactions. The agreement between theory and experiment is excellent. Three exchange parameters have been obtained at 1.6 K: J = -1.03 meV (intradimer), J~= -0.054 meV (interdimer, intrasublattice), and J, = -0.039 meV (interdimer, intersublattice). The excitation shows softmode behavior. However, an ordering does not occur above 1.6 K.
The magnetic properties of nickelocene, undiluted and doped into ruthenocene and ferrocene, have been determined by susceptibility measurements on powder samples and crystallites oriented in high magnetic fields, respectively, between 4.4 and about 200 K. The results reveal that there is a predominantly ferromagnetic coupling of a nickelocene molecule to its nearest neighbors in an undiluted sample, whereas the interaction with triplet-state molecules at greater distances in diluted samples seems to be antiferromagnetic. By introduction of two molecular fields, describing the intermolecular interactions and an intramolecular zero-field splitting, the magnetic susceptibility of nickelocene, undiluted and doped into isostructural diamagnetic hosts, can be interpreted by the same spin-Hamiltonian parameters: D0 = 33.6 ± 0.3 cm'1, = 0.63 cm'1, and Ts = 0.89 cm'1. Furthermore the singlet-triplet separation in polycrystalline Ni(C5D5)2 has been determined by inelastic neutron scattering (INS) spectroscopy. The observed magnetic transition at 31.6 ± 1.0 cm'1 in the INS spectra does not correspond directly to the zero-field splitting of the isolated molecule, D0, but shows some influence of the intermolecular coupling and the deuteriated ligands.
Single-crystal EPR experiments have been performed on orthorhombic [(NH3)5Ru(pz)Ru(NH3)5]Cl5'5H20 at 3 K. Two resonance lines appear in the crystallographic ab plane, one each from the two structurally equivalent binuclear ions with different orientation. The resonances are described by g tensors that have their principal axes collinear to the molecular axes. Both gx components are parallel to the crystallographic c axis, which is perpendicular to the plane of the pyrazine rings. The angle between the Z axes (Ru-Ru line) of the two magnetically inequivalent complexes is 86 (1)°i n agreement with X-ray diffraction results (85.5°). The components of the g tensor are gx = 1.346 (3), gY -2.799 (3), and gz = 2.487 (3). No Ru hyperfine structure could be observed owing to the large line width in the undiluted single crystal. The EPR spectrum was simulated by diagonalization of the 6 X 6 perturbation matrix using a one-center basis set in the hole formalism with the parameters D/\ -2.317 (20), E/\ = 0.745 (20), and K = 0.981 (5). Both an axial and a rhombic component of the ligand field must be considered to reproduce the g tensor anisotropy. The orbital reduction factor k within the t2g5 configuration is estimated to be 0.79. The unpaired electron is predominantly in an orbital perpendicular to the plane of the pyrazine ring. Delocalization of the odd electron over the two ruthenium ions via the tr* system of the pyrazine ring is therefore possible.
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