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X-ray diffraction studies of Mg2Si and
Ag-doped Mg2Si under pressure
Y.Mori , Y.Kaihara, K.Takarabe : Okayama University of Science
The reuse of waste heat energy is expected as one of the solutions of
the environmental issues. It is necessary for the recovery of the waste
heat energy to investigate the high-performance thermoelectric device
which consists of N-type and P-type semiconductors. Moreover it is
important for the industry and the environment to research the
ecological friendly semiconductor which has abundant deposit on the
earth and is harmless for the health.
Mg
Si
Reserves 32,000 267,700
/ppm
Bi
Te
Pb
Co
Sb
0.06
5(ppb)
8
29
0.2
Mg
Mg
Mg
Mg
a = 0.6340 nm
S.G. = Fm-3m
2
Price
100
45
245
4500
49
1750000
215
Harmless
O
O
X
X
X
X
X
S:Seebeck coefficient
The synthesis is difficult, because the boiling point of Mg
ρ:Electrical conductivity （1363K） is close to the melting point of Mg Si（1358K）.
2
κ:Thermal conductivity
ST
Thermoelectric
ZT 
properties

Purpose
Calculation
• To find the synthetic condition for Ag-doped Mg2Si by hightemperature and high-pressure XRD (Multi Press)
• To calculate the suitable dopant element for P-type
• To synthesize the thermoelectric material (Piston Cylinder) and
evaluate the thermal property
(2)
Method: CASTEP
Fine Powder of Sample
(3)
Fig.5 SEM image of
mixture of Mg and
Si powders.
LDA-GGA
Table 1. Summary of excess energies to be supplied for interstitial occupancy
into one of 4b sites or substitutional occupancy of a Mg site or a Si site of
Mg8Si4 by X (X = Cu, Ag, or Au) to proceed
Interstitial Mg substitution Si substitution
Cu
Experiment
(1)
Fig.2 The synthetic condition of Mg2Si.
Fig.1 Performance of several thermoelectric materials
Fig.3 Crystal structure of
Mg2Si. (anti CaF2-type: )
Mg8Si4X1
Mg7X1Si16
Mg8Si3X1
+0.863
p-type
+1.107
+1.059
Au
+0.034
+0.349
Ag
+0.722
p-type
(4 sites)
+0.733
p-type
(8 sites)
-0.515
n-type
+0.721
n-type
(4 sites)
+0635
+0.773
+0.815
Ag@
2GPa
Fig. 4 Photographs of the starting materials for the XRD study of
Ag-doped Mg 2Si under high-temperature and high-pressure.
The left two material are Mg(1) with particles 150 mm in
diameter, and Si(2) powder and the right one is pure Mg2Si(3)
synthesized by Union Material Co., Ltd.
The average diameters of the particles of Si were (A)~150mm,
(B) ~40mm, (C)~20mm, (D) ~3mm.
Fig. 12 Electronic densities of states of (1)undoped
Mg54Si27 (2)Mg64Si32Cu1 (3)Mg53Si27Ag1
(4)Mg54Si26Au1. Arrows in the Figures show
intrinsic energy band gaps. Fermi energies are
aligned with 0eV.
XRD @ High Temp.
(1)
Fig.6 Particle size distribution of Si powder (D).
300K
973K
873K
773K
673K
573K
300K
P=1.0 GPa
Mg+Si
XRD under HP
300K
973K
873K
773K
673K
573K
300K
P=1.0 GPa
Mg2Si
(2)
422
500
600
700
800
(2)
511,333
422
400
220
311
220
200
111
331
420
300K
873K
823K
723K
673K
523K
300K
Si(400)
400
100
P=1.0 GPa
Mg2Si+Ag
Si(311)
200
80
Si(220)
(1)
400
Si(331)
300K
873K
823K
723K
673K
523K
300K
P=1.0 GPa
Mg+Si+Ag
60
MgO(220)
Electrical conductivity / Sm
600
40
100
MgO(111)
80
-1
60
Si(400)
Mg(112)
Mg(021)
Mg(112)
Mg(013)
Si(311)
Mg(110)
Si(220)
Mg(012)
40
800
Si(111)
1000
Mg(002)
Mg(011)
Si(111)
Thermal properties
Mg(010)
111
220
Fig.7 High-pressure equipment and sample assembly.
331
420
220
400
311
200
BeamLine: PF-AR-NE5C
Pressure technique: Multi Press (MAX80)
511,333
• XRD under pressure by synchrotron radiation
source at Photon Factory in Tsukuba
900
Temperature / K
Fig. 8 Photograph of an electric furnace and automatic
control system.
Fig.10 Electrical conductivity of the standard
sample of Mg2Si was measured.
0
-100
40
60
This work was supported by MEXT KAKENHI(C) Grant Number 11013342, and has been performed under the
approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G668, 2012G566).
80
Fig. 14 EDX diffraction patterns of
Ag-doped Mg2Si under hightemperature at 1 GPa. The starting
materials of the left (1) and the
right (2) are Mg, Si, Ag and Mg2Si,
Ag , respectively. In the left figure,
Mg2Si was synthesized at 523 K
and Mg peaks disappeared and
broad peak appeared at 873 K.
There temperatures are lower than
the case of undoped sample. Ag
peaks disappeared 823 K.
However, in the right figure, Ag
peaks remains still 873 K. The
result means that in the case of
Ag-doping , the Mg and Si
powders are better than Mg2Si
powder as the starting material.
Ag(311)
511,333
422
Ag(220)
331
420
400
220
Ag(200)
311
220
Ag(111)
111
Mg(013)
200
60
100
Energy2=6.0 deg. / keV
MgO
Mg Si
2
673 K
RT
8h
Intensity(arb.unit)
6h
3h
1h
30
40
50
60
Energy2=6.0 deg / KeV
Si
Mg
Ag
Mg Si
2
Ag
Mg
Mg
RT
Mg Si
2
• In the case of mixture of Ag, Mg, Si powders, Mg2Si was synthesized at 523 K,
and Ag peaks disappeared at 823 K and MgO or SiO2 peaks did not appeared.
40
100
Energy2=6.0 deg. / keV
Summary
• The first principle calculation expects that Ag is suitable doping element for the
P-type conduction of Mg2Si and the pressure is effective for the Ag-doping.
80
Mg
Fig.11 Seebeck coefficients of the standard sample
and the synthesized Mg2Si under pressure(red).
Si(311)
Mg(110)
Temperature / K
Si
900
Si(220)
Mg(012)
800
Ag(200)
700
Mg(002)
Mg(011)
Ag(111)
600
Mg(010)
500
Si(111)
-450
Si(400)
Mg(112)
Mg(021)
Mg(112)
Si(331)
Ag(311)
-400
400
511,333
Ag(220)
-350
422
420 331
220
220
-300
400
311
-250
-500
Fig. 9 Photograph of the sampling plate for the thermoelectric
properties. The properties of the small sample such as 5mm
in diameter were measured by using this plate.
200
-150
111
Seebeck coefficient / mVK
-1
-50
-200
Fig. 13 EDX diffraction patterns of
Mg2Si under high-temperature at 1
GPa. The starting material of left
figure (1) is Mg and Si powders.
Mg2Si was synthesized at 573 K
which temperature is very low
than the melting point of Mg (923
K). Mg peaks disappeared and a
broad peak appeared at 973 K.
The quenched sample (top of the
(1)) does not include Mg and Si.
In the right figure (2), the starting
material is powdered pure Mg2Si
synthesized by Union Material Co.,
Ltd.
Some peaks of MgO appeared
with increasing temperature.
70
80
Fig. 15 EDX diffraction patterns of Ag-doped Mg2Si at
673 K, 1 GPa for various synthesis-time. Diffraction
peaks of Mg decreased with increasing synthesis-time,
their peaks almost disappeared after 8hrs.