Preparation and Thermoelectric Properties of a Chimney-Ladder (Mn1-xFex)Siγ (γ ∼ 1.7) Solid Solution

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(Mn)/c(Si) similar to 1.7. With increasing x from 0 to 0.35, equivalent to increase electron carriers, the a-axis and c(Mn)-axis lengths gradually decrease, while the c(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 +/- 0.05. This transition is experimentally confirmed at x similar to 0.28; the n-type samples are prepared at 0.28 <= x <= 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. (C) 2011 The Japan Society of Applied Physics