Enhanced thermoelectric performance of a chimney-ladder (Mn _1-xCr _x )Si _γ (γ∼ 1:7) solid solution

A partially Cr-substituted solid solution of a higher manganese silicide, (Mn _1-x Cr _x )Si _γ, has been prepared by an arc-melting method and a subsequent annealing process. The compound consists of two tetragonal subsystems of [Mn _1-x Cr _x ] and [Si] with an irrational c-axis ratio, γ = c _Mn/c _Si. According to Vegard's rule, the solubility limit of Cr is determined at x = 0:20. With increasing Cr content x from 0 to 0.20, equivalent to increasing the hole carrier concentration, the a- and cMn-axis lengths linearly increase, while the cSi-axis length gradually decreases. As expected from a valence electron counting rule, the electrical conductivity of (Mn _1-x Cr _x )Si _γ increases with increasing x from 334 S·cm ^-1 (x = 0) to 727 S·cm ^-1 (x = 0.20) at room temperature. The Cr-substituted samples undergo a microscale domain separation; the sample with a nominal composition of x = 0.20 consists of Cr-rich (x ∼ 0:3) and Mn-rich (x ∼ 0:1) dendritic domains with a typical size of ∼100 μm. Despite the substantial increase in electrical conductivity, the total thermal conductivity decreases with increasing x owing to a marked reduction in lattice contribution. As a consequence, the dimensionless figure-of-merit ZT increases from 0.28 (x = 0) at 800 K to 0.45 (x = 0.20) at 900 K.