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تسجيل مستخدم جديدSynthesis and properties of CoO2, the x = 0 end member of the LixCoO2 and NaxCoO2 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.
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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, increa
sing 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.
The strength and effect of Coulomb correlations in the (superconducting when hydrated) x~1/3 and ``enhanced x~2/3 regimes of Na(x)CoO2 are evaluated using the correlated band theory LDA+U method. Our results, neglecting quantum fluctuations, are: (1)
allowing only ferromagnetic order, there is a critical U_c = 3 eV, above which charge disproportionation occurs for both x=1/3 and x=2/3, (2) allowing antiferromagnetic order at x=1/3, U_c drops to 1 eV for disproportionation, (3) disproportionation and gap opening occur simultaneously, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below U_c ferromagnetism is favored. Comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for x~0.7, but negligible enhancement for x~0.3. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for x>0.5 while there no magnetic behavior or local moments reported for x<0.5. We suggest that the phase diagram is characterized by a crossover from effective single-band character with U >> W for x>0.5 into a three-band regime for x<0.5, where U --> U_eff <= U/sqrt(3) ~ W and correlation effects are substantially reduced.
We report temperature and field dependent measurements of the magnetic susceptibility, specific heat and resistivity of sintered YbB2 pellets, prepared via two distinct reaction routes, utilizing different temperatures, pressures and sintering times.
Sample behavior is affected by the preparation procedure, as a consequence of different secondary phases, most of which were identified via x-ray diffraction. These experiments show that YbB2 is a metal with the Yb atoms in or very close to their 3+ state. YbB2 appears to order anti-ferromagnetically at TN ~ 5.6 K, which can be considered a relatively high ordering temperature for an ytterbium-based intermetallic compound.
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.
Powder Na$_{x}$CoO$_{2}$ ($0.70leq xleq 0.84$) samples were synthesized and characterized carefully by X-ray diffraction analysis, inductive-coupled plasma atomic emission spectroscopy, and redox titration. It was proved that $gamma$-Na$_{x}$CoO$_{2}
$ is formed only in the narrow range of $0.70leq xleq 0.78$. Nevertheless, the magnetic properties depend strongly on $x$. We found, for the first time, two characteristic features in the magnetic susceptibility of Na$_{0.78}$CoO$_{2}$, a sharp peak at $T_{p}=16$ K and an anomaly at $T_{k}=9$ K, as well as the transition at $T_{c}=22$ K and the broad maximum at $T_{m}=50$ K which had already been reported. A type of weak ferromagnetic transition seems to occur at $T_{k}$. The transition at $T_{c}$, which is believed to be caused by spin density wave formation, was observed clearly for $xgeq 0.74$ with constant $T_{c}$ and $T_{p}$ independent of $x$. On the other hand, ferromagnetic moment varies systematically depending on $x$. These facts suggest the occurrence of a phase separation at the microscopic level, such as the separation into Na-rich and Na-poor domains due to the segregation of Na ions. The magnetic phase diagram and transition mechanism proposed previously should be reconsidered.
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