First-principles and experimental studies of impurity doping into Mg2Si
Introduction
The intermetallic compounds of Mg2X (X = Si, Ge, and Sn) and their solid solutions have been considered as candidates for high-performance thermoelectric materials [1], [2]. For thermoelectric materials, a large Seebeck coefficient S, small electrical resistivity ρ, and small thermal conductivity κ, are required. These parameters determine the so-called thermoelectric figure of merit Z = S2/ρκ. Low lattice thermal conductivity and high carrier mobility are desirable for the improvement of the figure of merit. Vining [1] pointed out that the value of the factor A′of Mg2X (3.7–14) is larger than that of SiGe (1.2–2.6) and β-FeSi2 (0.05–0.8), where A′ = (T/300)(m∗/me)3/2μ/κph, where m∗ is the carrier effective mass, μ is the mobility in cm2/Vs, and κph is the lattice thermal conductivity in mW/cmK. Therefore, an Mg2X system will achieve a higher ZT with further development. However, thus far, there have been few systematic studies of Mg2X compounds and their solid solutions from the perspective of thermoelectric energy conversion.
Impurity doping will drastically affect the thermoelectric properties of the Mg2X compounds and their solid solutions, and it is essential to obtain good dopants for Mg2Si. Some experimental works have been attempted in order to dope impurities into Mg2X to control its semiconducting properties. The conduction types are p-type, produced by doping with Ag and Cu, and n-type, produced by doping with Sb, Al, P, and Bi [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. However, the first-principles calculations of the impurities in Mg2Si have not been investigated systematically. To our knowledge, few theoretical data have been reported on the geometrical and electronic structure of impurity-doped Mg2Si.
In this paper, we report a systematic study of the impurities in Mg2Si based on the first-principles calculations using the density-functional-pseudopotential method, by employing a supercell approach with 48 atoms per cell. We consider the following 12 elements as the impurity species: B, Al, Ga, and In from group IIIb; N, P, As, Sb, and Bi from group Vb; and Cu, Ag, and Au from group Ib. We also present the experimental data of impurity-doped Mg2Si fabricated by spark plasma sintering (SPS), characterized by Hall effect measurements at 300 K.
Section snippets
Experimental and details of the calculations
The following high-purity powders were used as starting materials: Mg (>99.9%), Si (>99.999%), B (>99%), SiB6 (>98%), MgB2 (>99%), Al (>99.9%), Mg3N2 (>99%), Mg3P2 (>99%), Sb (>99.9%), Bi (>99.9%), Cu (>99.9%), Ag (>99.9%), and Au (>99.9%). Constituent Mg, Si, and impurity (X) powders were ground together and then heated at 993–1053 K for 5 min at 20–30 MPa in a graphite die (15 mm in diameter) in vacuum (<4 Pa) by the SPS method at a heating rate of 30–50 K/min. The density of the annealed samples
Results and discussion
Table 1 lists the transport properties of impurity-doped Mg2Si at 300 K in comparison with those of nondoped Mg2Si. RH of nondoped and B-, Al-, N-, P-, Sb-, Bi-, Cu-, and Au-doped Mg2Si is negative, indicating that the conductivity is mainly due to electrons. However, RH of Ag-doped Mg2Si is positive, indicating that the conductivity is mainly due to holes. The carrier concentration strongly depends on the types of dopants and their solubility. The Hall mobility (μH = RH/ρ) of impurity-doped Mg2Si
Conclusions
The formation energy, structural relaxation, and Mulliken charge of impurities in Mg2Si are systematically investigated using first-principles calculations based on the density functional theory. Among the elements in groups Ib, IIIb, and Vb, As, P, Sb, Bi, Al, and N are suggested as n-type dopants, whereas Ga is suggested as a p-type dopant. For In, Ag, Cu, and Au, the conduction type depends on the atomic chemical potentials of Mg and Si. The formation energies of As, P, Sb, and Bi in the
Acknowledgement
This research was partially supported by the Ministry of Education, Sports, and Culture, Grant-in-Aid for Young Scientists (B), No. 18760514, 2007.
References (24)
- et al.
J Phys Chem Solids
(1962) - et al.
Physica B
(2005) - et al.
Intermetallics
(2007) J Cryst Growth
(1974)- et al.
- et al.
Sov Phys Solid State
(1962) - et al.
Mater Trans JIM
(1992) - et al.
Mater Trans JIM
(1992) - et al.
Phys Rev B
(2006)
Phys Rev
Phys Rev
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