Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca₂CoO_3.34]_0.614[CoO₂], also known as Ca₃Co₄O₉, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µV·K^-1, resistivity of ρ= 15 mΩ·cm and thermal conductivity of κ= 9.8 mW·K^-1·cm^-1. The resulting dimensionless figure of merit becomes Z T ₃₀₀ = 3.5×10^-2, which is comparable to the value reported for a polycrystalline sample of NaCo₂O₄, indicating that the title compound is a potential candidate for a thermoelectric material.

We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a superspace group of C m (0 1- p 0) using powder X-ray and neutron diffraction data. The compound consists of two interpenetrating subsystems of a CoO 2 sheet and a distorted triple-layered NaCl-type Ca 2 CoO 3 block, being incommensurate parallel to the b -axis. The CoO 2 sheet is composed of a CdI 2 -type triangular Co lattice with sharing an edge of the CoO 6 octahedra. In the Ca 2 CoO 3 subsystem, “<”-shaped buckled arrangement of the atoms is found, due to the ordered arrangement of the Ca and Co atoms parallel to the c -axis. Both the subsystems have common a -, c -axes and β-angle with a =4.8339(3) Å, c =10.8436(7) Å and β=98.14(1)°. The b -axis lengths are refined to be b 1 =2.8238(2) Å for the [CoO 2 ] subsystem and b 2 =4.5582(2) Å for the [Ca 2 CoO 3 ] subsystem. An undulating arrangement of the O atom site...

Zintl compound n-type Mg₃(Sb,Bi)₂ was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.

The crystal structure of a polycrystalline sample of higher manganese silicide (HMS) has been determined by means of the $\left(3+1\right)$-dimensional superspace group approach. The structural parameters were refined with a superspace group of $I{4}_1/amd\left(00\gamma \right)00ss$ using powder neutron-diffraction data collected at 295 K. The compound belongs to a composite crystal family consisting of [Mn] and [Si] subsystems, with an irrational $c$-axis ratio (misfit parameter) of $\gamma =c_{\text{Mn}}/c_{\text{Si}}\sim 1.74$. Significant in-plane rotational modulation was revealed in the “chimney”-[Si] subsystem, while positional modulation in the “ladder”-[Mn] subsystem was only realized along $c_{\text{Mn}}$. The electronic structure of the sample was calculated on the basis of a commensurate approximation of the modulated structure using the full potential linearized augmented plane-wave method. The obtained band gap of $E_g\sim 0.6\text{eV}$ agreed well with the experimentally observed one. It appears that the band gap and density of states of the HMS samples depend on the positional modulation of the Si atoms. The various controversial formulas (for example, ${\text{Mn}}_4{\text{Si}}_7$, ${\text{Mn}}_{11}{\text{Si}}_{19}$, and so on) of the HMS phases reported thus far can be regarded as commensurate cases of a series of incommensurate ${\text{MnSi}}_{\gamma }$ phases in which the $\gamma$ value ranges from $\sim 1.70$ to 1.75.