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Register a new userMagnetism and structure of LixCoO2 and comparison to NaxCoO2
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The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0 are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and 1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling is observed, increasing in strength as more Co4+ is introduced. At an x value of about 0.65, the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. A comparison of the structures of the NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter. Further, the CoO6 octahedra in LixCoO2 are less distorted than those in NaxCoO2. We postulate that these structural differences strongly influence the physical properties in the two systems.
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We report here the synthesis of single-phase bulk samples of CoO2, the x = 0 end member of the AxCoO2 systems (A = Li, Na), from a pristine LiCoO2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetic susceptibility of the CoO2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO2 is a Pauli-paramagnetic metal with itinerant electrons.
We prove the direct link between low temperature magnetism and high temperature sodium ordering in NaxCoO2 using the example of a heretofore unreported magnetic transition at 8 K which involves a weak ferromagnetic moment. The 8 K feature is characterized in detail and its dependence on a diffusive sodium rearrangement around 200 K is demonstrated. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered sodium Coulomb potential on the CoO2 physics is evidenced opening new ways to experimentally revisit the NaxCoO2 phase diagram.
Based on first-principles calculations, the evolution of the electronic and magnetic properties of transition metal dihalides MX$_2$ (M= V, Mn, Fe, Co, Ni; X = Cl, Br, I) is analyzed from the bulk to the monolayer limit. A variety of magnetic ground states is obtained as a result of the competition between direct exchange and superexchange. The results predict that FeX$_2$, NiX$_2$, CoCl$_2$ and CoBr$_2$ monolayers are ferromagnetic insulators with sizable magnetocrystalline anisotropies. This makes them ideal candidates for robust ferromagnetism at the single layer level. Our results also highlight the importance of spin-orbit coupling to obtain the correct ground state.
The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed and compared to experimental results from Co3O4 single crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer of Co3O4 easily forms that leads to the observation of an intense phonon around 700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved crystals from optical floating zone ovens do not show such effects and have a high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96, 167204 (2006)]. We discuss the relation of structural dimensionality, electronic correlations and the high frequency phonon cut-off of the thermolectric cobaltite.
The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measurements have revealed Curie-Weiss behavior with an effective magnetic moment of 7.79 muB per europium atom in agreement with the theoretical value of 7.94 muB for Eu2+. A drop in the magnetic susceptibility at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe Mossbauer spectroscopic measurements have confirmed a beginning cooperative magnetic phenomenon by showing significantly broadened spectra at 4.8 K compared to those at 78 K.
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