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Intermetallics 19 (2011) 908e912

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Intermetallics

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Crystal structure and thermoelectric properties of b-MoSi2Takahiro Yamada*, Hisanori YamaneInstitute of Multidisciplinary Research for Advanced Material, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan

a r t i c l e i n f o

Article history:Received 23 November 2010Accepted 3 February 2011Available online 1 March 2011

Keywords:A. Molybdenum silicidesB. CrystallographyB. Thermoelectric propertiesC. Melting

* Corresponding author. Tel./fax: þ81 22 217 5813.E-mail address: yamataka@tagen.tohoku.ac.jp (T. Y

0966-9795/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.intermet.2011.02.003

a b s t r a c t

A powder sample of a metastable phase b-MoSi2 having the C40-type crystal structure was prepared byheating Mo sheets with a NaeSi melt at 858 K for 12 h to determine details of the crystal structure. Thelattice constants (a ¼ 4.6016(3) Å and c ¼ 6.5700(3) Å) and Si atom coordinate (y ¼ 0.1658(2)) of b-MoSi2were determined by Rietveld analysis of the X-ray powder diffraction. The thermoelectric propertieswere refined for a bulk sample prepared by sintering the b-MoSi2 powder at 773 K and 600 MPa.The electrical resistivity of the sintered b-MoSi2 sample with a relative density of 65% of the theoreticalone was 2.5 mU cm at 300 K, and slightly increased with increasing temperature from 300 to 725 K. TheSeebeck coefficients changed from þ60 to þ89 mV/K in the temperature range from 330 to 725 K. Themaximum thermoelectric power factor was 2.2 � 10�6 W cm�1 K�2 at 725 K.

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1. Introduction

Since MoSi2 has attractive properties such as a high meltingpoint, low electrical resistance, superior high-temperature stabilityand strength, and low density, it has already been used as a heatingelement and high-temperature protective coating, and has beenstudied for application not only as high-temperature structuralmaterial, but also as gate electrodes, interconnects, and diffusionbarriers in micro-electronic devices [1e6].

There are two known polymorphs of MoSi2, the a-MoSi2 stablephase and the b-MoSi2 metastable phase [3,7]. Single crystals ofa-MoSi2 have been grown from themelt [8e10], and polycrystallinebulks and powders have been prepared by arc melting [11],combustion synthesis reaction [12e14], high-temperature reaction[15,16], and mechanical alloying [17e21]. b-MoSi2 powders havebeen prepared as mixtures with a-MoSi2 or an amorphous phase bymechanical alloying [17e21] and by plasma spray processing[22,23]. The b-MoSi2 in the mixtures has been found to transformirreversibly to a-MoSi2 by annealing at 973e1273 K [17e23]. Poly-crystalline thin films of b-MoSi2 have been produced by annealingthinMo/Si structures fabricated on the substrates of polycrystalline,single crystal, and hydrogenated amorphous silicon [24e29], and byion implantation for Mo film deposited on a polycrystalline Sisubstrate [30]. The transformation from b- to a-MoSi2 has beenobserved in the films above 775�1273 K [26e29].

The crystal structure of a-MoSi2 is the tetragonal body-centeredC11bwith space group I4/mmm and that of b-MoSi2 is the hexagonal

amada).

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close-packed C40 with P6222 [30e37]. Both structures consist ofMo-Si2 layers, inwhichMo atoms are surrounded by 6 Si atoms. Thestacking sequences of a-MoSi2 and b-MoSi2 are ABAB and ABCABC,respectively. The crystal structure of a-MoSi2 has already beendetermined by X-ray diffraction (XRD) [31e33]. To our knowledge,the structure of b-MoSi2, including the atomic positions, has not yetbeen experimentally studied. Optimization of the crystal structureof b-MoSi2 was performed by ab-initio calculation [34,35].

The electronic transport properties of a-MoSi2 have been studiedfor single crystals and polycrystalline thin films [38,39]. Theirresistivitiesmeasured at room temperaturewere 15e150 mU cm andincreased with temperature. The Seebeck coefficients of a-MoSi2measured for a bulk sample and a polycrystalline thin filmwere þ8and 0 � 2.0 mV/K, respectively, although the measurementtemperature was not clear in the literature [40].

There have been three studies on the electrical resistivity ofb-MoSi2 thin films [40e42]. Krontiras et al. [41] measured theresistivity of a b-MoSi2 film and reported a negative temperaturecoefficient of the resistivity, while de Vries et al. [42] reporteda positive one, although the room-temperature resistivities repor-ted in both reports were close to ca. 400 mU cm. Dvorina et al.reported a positive one; however, the absolute value of the resis-tivity was not presented in the literature [40].

A group VI metal silicide, CrSi2, having the C40-type structure isa p-type degenerate semiconductor with a band gap of about0.35 eV [43,44]. There have been many studies on the thermo-electric properties of CrSi2 because of its large Seebeck coefficient(ca. þ125e200 mV/K at 550e700 K) [45]. However, the Seebeckcoefficient of b-MoSi2, crystallizing in the C40-type structure, hasnot been reported, although the energy gap of b-MoSi2 was shownto be 0.035 and 0.07 eV by theoretical calculations [7,34,35].

T. Yamada, H. Yamane / Intermetallics 19 (2011) 908e912 909

In our previous study, b-MoSi2 powder was prepared by heatingMo powder in a NaeSi melt at 873 K [46]. In the present study, b-MoSi2 powder with a low content of oxygen was prepared usinga Mo sheet as a Mo source. The crystal structure of b-MoSi2 wasdetermined by powder XRD. The thermoelectric properties weremeasured for the bulk sample prepared by pulse-current pressuresintering of the b-MoSi2 powder.

Fig. 1. Observed (dots) and calculated (line) powder X-ray diffraction patterns ofa-MoSi2 (a) and b-MoSi2 (b). The difference, observed minus calculated, is shown in thelower field. Vertical bars indicate the Bragg positions of a-MoSi2 (a) and b-MoSi2 (b).

Table 1Results of chemical analysis of a- and b-MoSi2 powders.

Sample Mo Si Na O Total

a-MoSi2ReagentA Co. Ltd. wt.% 61.5 (1) 37.9 (1) 0.07 (1) 0.78 (1) 100.3 (4)

2. Experimental procedure

Mo sheets (Nilaco, 99.95%, 100 mm in thickness), Si powder(Koujundo Kagaku, 99.999%,<75 mm) and Na (Nippon Souda, 99.95%) were used as starting materials. Mo sheets (0.70 g,5 � 5 � 0.1 mm3) and Si powder (0.42 g) were weighed in a molarratio of Mo:Si ¼ 1:2.05 in air. They were put into a sintered BNcrucible (416� 13mm3 in inner volume, Showa Denko, 99.5%) withNa metal in an Ar gas-filled glove box (MBraun, O2, H2O < 1 ppm).Themolar amount of Na (0.7 g) was about 4 times of that of Mo. Thecrucible was sealed in a stainless steel container (SUS316, 440 �40mm3) with a stainless steel cap. Details of this procedure and thearrangement of the crucible and sources can be found in Ref. [46].The heating temperatures for the preparation of b-MoSi2 and a-MoSi2 were 858 K and 1073 K, respectively. After heating for 12 h inan electric furnace, the samples were cooled therein by shutting offthe power to the furnace. The container was cut open and thesamplewas taken out of the container in air. Na and a trace of NaeSileft in the crucible were removed by reaction with 2-propanol andethanol. After several washings with distilled water, the samplewas dried at 353 K in air.

The powder samples for XRD were prepared by grinding the as-prepared coarse powder samples with a pestle and mortar. TheXRD patterns of the powders were measured using CuKa radiation(l ¼ 1.5418 Å) with a pyrolitic graphite monochromator, a scintil-lation counter, and a diffractometer (Rigaku, RINT2200). Thediffraction data for the Rietveld refinement were collected at roomtemperature from 15� to 140� in 2q with a sampling step of0.02e0.03� for 7e8 s. The RIETAN-FP program [47] was used forRietveld analysis of the powder XRD patterns. The contents of Mo,Si, and Na in the sample were analyzed using inductively coupledplasma optical emission spectroscopy (ICP-OES). The oxygencontent in the samples was measured by the He carrier fusion-infrared absorption method.

b- and a-MoSi2 powders were put into separateWC dies (10 mmin inner diameter), where the inner surface of which was coatedwith a graphite sheet, and sintered at 773e823 K for 10 min undera pressure of 600 MPa with pulse-current pressure sinteringequipment (SPS Syntex Inc., Model: SPS-1050). The temperaturewasmeasured by means of a thermoelectric couple, which was set closeto the sample through a small hole in the die. Rectangular samplesof 2 � 1.5 � 8 mm3 were cut from the sintered bodies. The Seebeckcoefficients and electrical resistivity of the samples were measuredin Ar atmosphere by a thermoelectric power (ΔE)etemperaturedifference (ΔT) method and by a direct current 4-probe method,respectively.

molar ratio 1 2.11 (1) 0.005 (1) 0.076 (1)B Co. Ltd. wt.% 62.7 (2) 35.8 (2) <0.03 1.02 (1) 99.5 (5)

molar ratio 1 1.95 (2) <0.002 0.098 (1)Present

workwt.% 62.1 (2) 37.1 (1) 0.05 (1) 0.61 (2) 99.9 (6)molar ratio 1 2.04 (2) 0.003 (1) 0.059 (3)

b-MoSi2Present

workwt.% 63.4 (2) 37.1 (1) <0.03 0.21 (1) 100.7 (5)molar ratio 1 2.00 (2) <0.002 0.020 (1)

3. Results and discussion

3.1. Synthesis and crystal structure

Fig. 1 shows the powder XRD patterns of the samples preparedat 1073 and 858 K with Na. The XRD reflections of the stabletetragonal phase a-MoSi2 were only observed in the pattern of thesample prepared at 1073 K. The XRD pattern of the sampleprepared at 858 K were observed to have boarder peaks than those

of a-MoSi2. All peaks were indexed on a hexagonal lattice of themetastable phase b-MoSi2.

Table 1 shows the results of chemical analysis for the powders ofa- and b-MoSi2 prepared in the present study and for commerciallyavailable a-MoSi2 powders (Company A: 99.9%, Company B: 99.5%).The chemical compositions of Mo and Si in the prepared MoSi2powders were generally consistent with the stoichiometry of 1:2.The oxygen contents in the prepared a- and b-MoSi2 powders werebelow 0.61 wt.%. The sodium in the b-MoSi2 powder was under thedetection limit of 0.03 wt.%. It is conceivable that some oxygen andsodium were deposited as oxides, hydroxides, and carbonates onthe surface and at the boundary of grains, and that others wereincluded in the grains.

Table 3Cell parameters and volumes of MoSi2 and atomic coordinates of Si.

a (Å) c (Å) V (Å3) z (a)/y (b) References

a-MoSi2exp. 3.20427 (2) 7.84652 (7) 80.5629 (10) 0.33515 (15) Present work

3.2112 (1) 7.8249 (3) 80.69 0.33543 (5) Christensen(1993) [31]

3.2064 (2) 7.8478 (8) 80.68 0.3353 (2) Harada et al.(1998) [32]

3.2056 (3) 7.8450 (4) 80.61 0.33528 (5) Tanaka et al.(2001) [33]

calc. 3.222 7.883 81.83 0.3355 Mattheiss(1992) [35]

3.219 7.880 81.65 0.3352 Pankhurst et al.(2004) [34]

b-MoSi2exp. 4.6016 (3) 6.5700 (3) 120.476 (10) 0.1658 (2) Present work

4.59 6.55 119.5 Wang et al.(2002) [36]

4.605 6.559 120.5 Nicolet & Lau(1983) [37]

4.609 6.558 120.6 d’Heurle et al.(1980) [30]

4.596 6.55 119.82 PDF#17-0917calc. 4.622 6.646 122.96 0.1642 Mattheiss

(1992) [35]4.615 6.618 122.07 0.1640 Pankhurst et al.

(2004) [34]

T. Yamada, H. Yamane / Intermetallics 19 (2011) 908e912910

Rietveld analysis of powder XRD was performed by applyinga preferred-orientation correction to the (110) plane of a-MoSi2 andto the (00.1) plane of b-MoSi2. After refinement of the latticeparameters, partial profile relaxation was applied to some reflec-tion peaks below 2q ¼ 65� in the XRD patterns. The calculated XRDpatterns by the Rietveld analysis and the differences between theobserved and calculated patterns are shown in Fig. 1. The results ofthe refined lattice parameters, the atomic coordinates, and reli-ability factors are given in Table 2. The final Rietveld refinementresulted in Rwp ¼ 8.56 and RF ¼ 0.37% for the a-MoSi2 powder. Theobtained lattice parameters and the atomic coordinates, z, of the Siatom of a-MoSi2 agreed with the previous reported values [31e33](Table 3).

The initial structural model used for the Rietveld refinement ofb-MoSi2 was the C40 structure (space group: P6222) with the Siatom coordinate (y ¼ 0.1640) being optimized by ab-initio calcu-lation [34]. The final Rietveld refinement resulted in Rwp ¼ 9.37 andRF¼ 0.63% for the b-MoSi2 powder. The refined lattice constants area ¼ 4.6016(3) and c ¼ 6.5700 (3) Å, which are close to the onesreported in the experimental studies by Wang et al., Nicolet & Lau,and d’Heurle et al. [30,36,37]. The refined y coordinate of the Siatom is 0.1658(2), slightly different from the ideal position in thehexagonal closed-packed lattice, y¼ 1/6, and the values (y¼ 0.1640and 0.1642) presented by the ab-initio calculations [34,35]. Themolar volume of b-MoSi2 calculated from the refined latticeconstants is 0.32% smaller than that of a-MoSi2. A similarpercentage of the lattice volume difference (0.34%) was shown inthe result of the calculation study by Pankhurst et al. [34].

Stacking faults on (0001) in the thin films of b-MoSi2 have beenobserved by transmission electron microscopy [36,48]. Thebroadened diffraction peaks of the b-MoSi2 powder could be causednot only by low crystal quality and lattice strain, but also by suchstacking faults in the structure.

3.2. Thermoelectric properties

The a- and b-MoSi2 powders were sintered at 823 K for 10 minwith an applied pressure of 600 MPa. The sintered bulk sampleprepared from the a-MoSi2 powder was a single phase of a-MoSi2,while the one from the b-MoSi2 powder was composed of b-MoSi2

Table 2Crystallographic data and structure refinement for a- and b-MoSi2.

a-MoSi2(prepared at 1073 K)

b-MoSi2(prepared at 858 K)

Formula MoSi2 MoSi2Crystal system tetragonal hexagonalSpace group I4/mmm (A 139) P6222 (A 180)Lattice parameters a (Å) 3.20420 (4) 4.6016 (3)

c (Å) 7.84870 (19) 6.5700 (3)V Å3 80.5818 (24) Å3 120.476 (10)

Z 2 3Density (calc.) (g/cm3) 6.270 6.290Reliability factors Rwp 0.1163 0.0823

Rp 0.0792 0.0584RI 0.0396 0.0107RF 0.0174 0.0058

Goodness-of-fit S 2.0012 1.4053

Atomic coordinate atom Mo Si Mo1 Si1

site 2a 4e 3c 6iocc. 1.0 1.0 1.0 1.0x 0 0 1/2 0.1658 (2)y 0 0 0 0.3316a

z 0 0.3352 (4) 0 0B/Å2 0.109 (12) 0.193 (13) 0.291 (11) 0.367 (15)

a (Si1, y) ¼ 2 � (Si1, x).

with a small amount of a-MoSi2. The sintered bulk sample of thesingle phase b-MoSi2 was prepared at 773 K. The relative densitiesof the bulk a- and b-MoSi2 samples were 67% and 65% of thetheoretical ones, respectively.

The temperature dependences of the electrical resistivities (r)and Seebeck coefficients (S) measured for the sintered samples areshown in Fig. 2. The r value of the a-MoSi2 sample was 0.74 mU cmat room temperature and increased with temperature in a rangefrom 300 to 725 K. Similar positive temperature dependences of rhave been reported for an a-MoSi2 single crystal and polycrystallinethin films of a-MoSi2 in previous reports [38,39]. However, thevalue of r at 300 K is higher than the reported values(10e150 mU cm) [38,39]. The higher r is probably due to the lowrelative density of the a-MoSi2 sintered body and its grainboundary resistivity. The r of the b-MoSi2 sample was 2.5 mU cm at300 K, which was 3 times larger than that of the a-MoSi2 sample. Alinear increase of r was observed in the measured temperaturerange.

Krontiras et al. prepared polycrystalline a- and b-MoSi2 thinfilms with a thickness of 80 nm [41]. They reported that the room-temperature resistivities of the a- and b-MoSi2 thin films were 63and 430 mU cm, respectively, and that the temperature depen-dences of rmeasured for the a- and b-MoSi2 thin films in the rangeof 4.2e300 K were positive and negative, respectively. On the otherhand, de Vries et al. reported [42] that the room-temperatureresisitivities of polycrystalline a- and b-MoSi2 films with a thick-ness of 200 nm were 60 and 400 mU cm, respectively, which weresimilar to those reported by Krontiras [41]. However, de Vries et al.observed the positive temperature dependences of resistivity forboth films at 4.2e320 K [42]. The sintered bulk samples of a- and b-MoSi2 prepared in the present study showed positive temperaturedependences of r as observed for the thicker films by de Vries et al.[42], although the r values of both sintered samples were largerthan those of the films [41,42].

The Seebeck coefficients (S) of the sintered a-MoSi2 samplewereþ0.8 toþ4.5 mV/K in the temperature range of 330e725 K. Thevalues were in a typical order of magnitude for metals. In contrast,the S value of the sintered b-MoSi2 sample was 60 mV/K at 330 K,

4.0

3.0

2.0

1.0

0

Elec

trica

lres

istiv

ity,

/ 10

-3cm

700600500400300Temperature, T / K

100

80

60

40

20

0

Seeb

eck

coef

ficie

nt, S

/V

K-1

700600500400300Temperature, T / K

b

a

Fig. 2. Electrical resistivities, r (a) and Seebeck coefficients, S (b) of the polycrystallinesintered samples of a-MoSi2 (squares) and b-MoSi2 (circles).

T. Yamada, H. Yamane / Intermetallics 19 (2011) 908e912 911

and reached 89 mV/K at 725 K. The maximum thermoelectric powerfactor calculated with the formula S2/r was 2.2 � 10�6 W cm�1 K�2

at 725 K.It has been reported that the r value of CrSi2 single crystals was

0.5e1 mU cm at room temperature and increased to a maximumvalue of 2e3 mU cm at 500e600 K, and then decreased withincreasing temperature [43e45]. The low temperature region witha positive temperature coefficient of r corresponds to the exhaus-tion region, and the high-temperature regionwith a negative one isthe intrinsic region. It has also been reported that S of CrSi2 singlecrystals showed a maximum value of þ125 to þ200 mV/K at550e700 K [43e45].

b-MoSi2 is considered to be a narrow-gap semiconductor witha band gap of 0.035 or 0.07 eV by theoretical calculations [34,35],in contrast to a-MoSi2 which is a semimetal [49]. While therelative density of the b-MoSi2 sample (65%) in the present studymay not be sufficient to discuss the behavior of the electricalresistivity, the positive values of S for the sintered b-MoSi2 sampleindicate that the holes are the majority carriers. This is consistentwith the positive Hall coefficients measured at 300 K in thestudies of b-MoSi2 thin films [41,42]. The positive temperaturecoefficient of r may correspond to the exhaustion temperatureregion as reported for CrSi2. However, it is not possible to measurethe r of the b-MoSi2 sample at higher temperatures to confirmwhether a negative temperature coefficient of r appears in theexhaustion region because of an irreversible transformation fromb- to a-MoSi2.

4. Summary

The present investigation can be summarized as follows:

1. b-MoSi2 powder was prepared by heating Mo sheets with Sipowder and Na at 858 K for 12 h. The sample has the stoi-chiometric composition with Mo:Si ¼ 1:2.00(2). The contentsof oxygen and sodium were 0.21(1) and less than 0.03 wt.%,respectively.

2. The lattice parameters of b-MoSi2 and the atomic coordinate ofSi atomwere experimentally determined to be a ¼ 4.6016(3) Åand c ¼ 6.5700(3) Å, and y ¼ 0.1658(2) with the hexagonalspace group P6222. The broad diffraction peaks of the b-MoSi2sample suggested the existence of lattice strain and/or stackingfaults in the crystallites of the sample.

3. The b-MoSi2 powder was sintered at 773 K and 600 MPa for10 min. The density of the b-MoSi2 sintered bulk sample was4.10 g/cm3 and 65% of the theoretical one. The resistivity of thesintered sample was 2.5 mU cm at 300 K, and a positivetemperature dependence of the resistivity was observed at300e725 K. The Seebeck coefficients were þ60 mV/K at 330 Kand þ89 mV/K at 725 K. The maximum thermoelectric powerfactor was 2.2 � 10�6 W cm�1 K�2 at 725 K.

Acknowledgments

This work was performed under the Inter-university Coopera-tive Research Program of the Institute for Materials Research,Tohoku University and was supported in part by a Grant-in-Aid forYoung Scientists (A) (20685015) from the Ministry of Education,Culture, Sports, Science and Technology, Japan. We would like tothank Mr. Akira Okubo, Dr. Tetsuya Ashino, and Mr. Fuyuki Saka-moto, who all belong to Tohoku University, for sintering of thesamples with the pulse-current pressure sintering equipment (Mr.Okubo) and for chemical quantitative analysis of elements in thesamples (Dr. Ashino and Mr. Sakamoto).

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