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Doping is considered to be the main method for improving the thermoelectric performance of layered sodium cobaltate (Na$_{1-x}$CoO$_2$). However, in the vast majority of past reports, the equilibrium location of the dopant in the Na$_{1-x}$CoO$_2$s complex layered lattice has not been confidently identified. Consequently, a universal strategy for choosing a suitable dopant for enhancing Na$_{1-x}$CoO$_2$s figure of merit is yet to be established. Here, by examining the formation energy of Gd and Yb dopants in Na$_{0.75}$CoO$_2$ and Na$_{0.50}$CoO$_2$, we demonstrate that in an oxygen poor environment, Gd and Yb dopants reside in the Na layer while in an oxygen rich environment these dopants replace a Co in CoO$_2$ layer. When at Na layer, Gd and Yb dopants reduce the carrier concentration via electron-hole recombination, simultaneously increasing the Seebeck coefficient ($S$) and reducing electric conductivity ($sigma$). Na site doping, however, improves the thermoelectric power factor (PF) only in Na$_{0.50}$CoO$_2$. When replacing a Co, these dopants reduce $S$ and PF. The results demonstrate how thermoelectric performance critically depends on the synthesis environment that must be fine-tuned for achieving any thermoelectric enhancement.
Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of Na$_{x}$CoO$_2$ have predominantly $a_{1g}$ symmetry with significant O $2p$ character. A large transfer of spect
Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water
In this work, we investigated the behaviour of Sb dopants in $Na_{x}CoO_{2}$ for Na concentrations of $x = 0.75, 0.875$ and $1.00$ by density functional theory. We chose $Na_{x}CoO_{2}$ with higher Na concentration of $x > 0.75$ because it has excess
Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved thermoelectric properties upon the reduction of their dimensionality. Here we found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most stable phase among several
Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted TiNiSn-based ingots. This promising conversion efficiency is due to a low lattice thermal conductivity, which is attributed to excess nickel in the half-Heusler structure.