First-principles and experimental studies of impurity doping into Mg2Si

doi:10.1016/j.intermet.2007.12.001

Intermetallics

Volume 16, Issue 3, March 2008, Pages 418-423

https://doi.org/10.1016/j.intermet.2007.12.001 Get rights and content

Abstract

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 Si-site substitution are negative at Mg- and/or Si-rich limits, suggesting that these impurities have good solubility. The transport properties of impurity-doped Mg₂Si, fabricated by spark plasma sintering, have been characterized by Hall effect measurements at 300 K. The carrier type of impurity-doped Mg₂Si is influenced by the types of dopants and the substitution site.

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^∗/m_e)^3/2μ/κ_ph, where m^∗ is the carrier effective mass, μ is the mobility in cm²/Vs, and κ_ph 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. R_H 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, R_H 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 (μ_H = R_H/ρ) of impurity-doped Mg₂Si

Conclusions

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)

M.W. Heller et al.
J Phys Chem Solids
(1962)
J. Tani et al.
Physica B
(2005)
J. Tani et al.
Intermetallics
(2007)
H.F. Shanks
J Cryst Growth
(1974)
C.B. Vining
V.K. Zaitsev et al.
E.N. Nikitin et al.
Sov Phys Solid State
(1962)
Y. Noda et al.
Mater Trans JIM
(1992)
Y. Noda et al.
Mater Trans JIM
(1992)
V.K. Zaitsev et al.
Phys Rev B
(2006)

R.G. Morris et al.

Phys Rev

(1958)

R.D. Redin et al.

Phys Rev

(1958)

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First-principles and experimental studies of impurity doping into Mg2Si

Abstract

Introduction

Section snippets

Experimental and details of the calculations

Results and discussion

Conclusions

Acknowledgement

J Phys Chem Solids

Physica B

Intermetallics

J Cryst Growth

Sov Phys Solid State

Mater Trans JIM

Mater Trans JIM

Phys Rev B

Phys Rev

Phys Rev

First-principles and experimental studies of impurity doping into Mg₂Si