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Register a new userDual electronic states in thermoelectric cobalt oxide
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We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hunds coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties.
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From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30 % of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.
We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$. A large negative magnetothermopower is found to scale with both magnetic field and temperature revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S$_0approx$ 60 $mu$V K$^{-1}$ equal to the high temperature asymptotic value of the spin 1/2 entropy. Low temperature specific heat measurements allow us to determine an enhanced electronic part with $gammaapprox$ 50 mJ (mol K$^{2}$)$^{-1}$ attesting of strong correlations. Thereby, a critical comparison between [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$, other cobaltites as well as other materials reveals a universal behavior of the thermopower low temperature slope as a function of $gamma$ testifying thus a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one.
Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >= 0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35) exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50. This behavior is in sharp contrast to the results of x <= 0.40 for which the S293K value is small and independent of x (+10 uV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.
By means of neutron scattering we have determined new branches of magnetic excitations in orbitally active CoO (TN=290 K) up to 15 THz and for temperatures from 6 K to 450 K. Data were taken in the (111) direction in six single-crystal zones. From the dependence on temperature and Q we have identified several branches of magnetic excitation. We describe a model for the coupled orbital and spin states of Co2+ subject to a crystal field and tetragonal distortion.
Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN~290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hunds rule model the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer.
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