Crystal Structure and Thermoelectric Properties of Chimney-Ladder Higher Manganese Silicides

Crystal structure, electronic structure and thermoelectric (TE) properties of Nowotny chimney-ladder (NCL) higher manganese silicides (HMSs) are reviewed. According to a (3+1)-dimensional superspace approach, HMSs are revealed to belong to incommensurate composite crystals, wherein [Mn] and [Si] subsystems possess an irrational c-axis ratio γ=cMn/cSi. The γ parameter, also represents a stoichiometry of the HMSs as MnSiγ, dictates the electronic structure, directly relating to a valence electron counting number, VEC. The p-type TE performance can be enhanced by a partial substitution of V, Cr, etc., while n-type HMSs can be obtained by heavily substituting Mn with Fe, etc., to realize VEC>14. A unique thermal expansion has been discovered at T~800 K, which would deteriorate TE performance and hence to be avoided from a practical application point of view.