The mixed-ligand complexes [Co(III)(tpy)(Cat-N-SQ)]Y and [Ni(II)(tpy)(Cat-N-BQ)]PF(6) (tpy = 2,2':6',2' '-terpyridine; Cat-N-BQ, Cat-N-SQ = mononegative and radical dinegative Schiff base diquinone ligand; Y = PF(6), BPh(4)) were prepared. Structural and spectroscopic data support the different charge distribution of the two compounds. The temperature-dependent electronic and spectral properties of solutions containing the [Co(III)(tpy)(Cat-N-SQ)](+) suggest that this compound undergoes a thermally driven valence tautomeric interconversion to [Co(II)(tpy)(Cat-N-BQ)](+) complex, the metal ion being in high-spin configuration. The comparison of the electrochemical properties of the cobalt and nickel derivatives supports the observed behavior. The same interconversion process was found to occur also in the solid state with a significant higher T(c) value than in solution. It was found that the previously reported [Co(III)(Cat-N-BQ)(Cat-N-SQ)] shows a similar behavior. The large difference between the interconversion T(c) in the solid state and in solution is suggested to come from the entropy changes associated with the modifications of vibronic interactions.
In this work, we report on x-ray diffraction, electron spin resonance, and magnetic susceptibility measurements performed on-Na 1.286 V 2 O 5. The x-ray data clearly reveal the existence, around 100 K, of a structural second-order phase transition, stabilizing a superstructure associated with charge ordering. The complete superstructure is determined from x-ray scattering data measured at 15 and 40 K. The low-temperature structure remains centrosymmetric with space group P2/c but with a doubled b lattice parameter. An analysis using superspace symmetry shows that one of the vanadium atoms and its apical oxygen, namely the V7 and O7 atoms, are the most displaced. Two sites for V7 atoms are identified below the transition instead of one with the formal valence of V 4.5ϩ. We show that these two sites are occupied by V 4ϩ and V 5ϩ , which corresponds to a charge ordering. We suggest that this transition is driven by electronic repulsions. The magnetic measurements give additional evidence for this transition and for the opening of a spin gap at low temperature. This is discussed in conjunction with superstructure formation.
The structural, optical and magnetic properties of nanostructured ZnO films
co-doped with cobalt and aluminium have been studied. The nanocrystalline
films, with cluster sizes in range 50 - 100 nm, were deposited by pulsed laser
ablation in a mixed atmosphere of oxygen and helium. The nanocrystallites have
the wurtzite structure and are highly oriented with the c-axis perpendicular to
the substrate. Both optical and electron spin resonance (ESR) spectroscopy
results show the substitutional incorporation of Co$^{2+}$ ions on the Zn site
inside the ZnO nanoclusters. The temperature dependence of the ESR spectra
follows Curie law corresponding to a paramagnetic material
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