Crystal Structure and Thermoelectric Properties of Incommensurate Higher Manganese Silicides
Y. Miyazaki, Y. Kikuchi, H. Hamada, K. Hayashi
Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579 Japan
miya@crystal.apph.tohoku.ac.jp
Higher Manganese Silicide (HMS) consists of two tetragonal subsystems of [Mn] and [Si] with an irrational c-axis ratio γ = cMn/cSi, forming an incommensurate Nowotny chimney-ladder (NCL) structure as shown in Fig. 1. The [Mn] subsystem has less displacive modulation and 3d electrons of Mn are responsible for the electronic conduction of this compound. In contrast, significant displacive modulation of Si in the [Si] subsystem effectively blocks the propagation of phonons and hence to reduce lattice thermal conductivity. A good thermoelectric (TE) material must have a high Seebeck coefficient and electrical conductivity as well as a low thermal conductivity and HMS is thus regarded as a potential TE material to be used at a mid-temperature range. Using γ, the structure formula of HMSs is represented as MnSiγ (γ~ 1.73). [1] To comprehend the electronic structure of such a complicated composite crystal and its solid solutions, we adopted the concept of valence electron counts, VEC. Based on the VEC concept, we have successfully prepared V-, Cr- and Fe-substituted HMS solid solutions (Mn1-xMx)Siγ (M = V, Cr and Fe).
Polycrystalline samples were prepared in an arc-melting furnace under an Ar atmosphere. A Rietveld refinement was performed for the X-ray diffraction (XRD) data using the superspace group of I41/amd(00γ)00ss by means of a JANA2006 software package. [2] A VEC value was determined by the valence electron numbers Z of Mn, V, Cr, Fe and Si and refined γ values as VEC = ZMn (1-x) + ZM x + ZSi γ. Samples for TE measurements were densified by the spark plasma sintering (SPS) at 1223 K. The Seebeck coefficient and electrical conductivity were simultaneously measured above 300 K. The thermal conductivity was measured using a standard laser flash thermal constant analyser.
Solubility limit was confirmed to be x = 0.10, 0.20 and 0.35 for M = V, Cr and Fe, respectively. Due to the irrational nature, the VEC value of the parent compound (13.93) is slightly less than 14, responsible to realise good p-type performance. Temperature dependence of the Seebeck coefficient S and electrical conductivity σ of the Cr-system exhibited metallic behaviour up to around 900 K. However, the increase in σ was relatively larger than the decrease in S. As a result, a substantial increase in TE power factor S2σ was achieved; the x = 0.04 sample exhibited the maximum power factor of S2σ = 2.01 mW/K2m compared with that of Cr-free sample of 1.48 mW/K2m at 760 K.
Fig. 1 Incommensurate crystal structure of MnSiγ (a). The unit cell of the [Mn] subsystem (b) and that of the [Si] subsystem.