Preparation and Thermoelectric Properties of a Chimney-Ladder (Mn1-xFex)Si\gamma ($\gamma\sim 1.7$) Solid Solution

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A partially Fe-substituted solid solution of a higher manganese silicide, (Mn1-xFex)Si\gamma, has been prepared by means of arc-melting and a subsequent annealing process. According to the linear relationship between $x$ and lattice parameters, the solid solution can be prepared up to $x = 0.35$. The compounds consist of two tetragonal subsystems of [Mn1-xFex] and [Si], with an irrational $c$-axis ratio $\gamma = c_{\text{Mn}}/c_{\text{Si}} \sim 1.7$. With increasing $x$ from 0 to 0.35, equivalent to increase electron carriers, the $a$-axis and $c_{\text{Mn}}$-axis lengths gradually decrease, while the $c_{\text{Si}}$-axis length linearly increases. On the basis of a valence electron counting rule, we expected that a transition from p-type to n-type would occur at $x = 0.23 \pm 0.05$. This transition is experimentally confirmed at $x \sim 0.28$; the n-type samples are prepared at $0.28 \leq x \leq 0.35$. The electrical conductivity at room temperature exhibits symmetrical curves versus $x$ for both p- and n-type samples. However, the solubility limit of Fe appears to suppress any further increase in carrier concentration, resulting in poor electrical conductivity for n-type samples. As a consequence, the maximum dimensionless figure-of-merit $ZT = 0.071$ at 644 K is obtained for a sample with $x = 0.30$.
2011-03-25