Thermoelectric power and phase transition of polycrystalline As2Te3 under pressure

Journal of Physics and Chemistry of Solids 66 (2005) 1744–1747 www.elsevier.com/locate/jpcs

Thermoelectric power and phase transition of polycrystalline As2Te3 under pressure T.J. Scheidemantela,b,*, J.F. Mengb, J.V. Baddingb b

a Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

Received 10 March 2005; revised 14 July 2005; accepted 18 July 2005

Abstract The pressure dependence of the thermoelectric power of monoclinic As2Te3 is measured up to 10 GPa using a Mao-Bell diamond anvil cell. The thermoelectric power never reaches an absolute value greater than the ambient pressure value of 242 mV/K. Evidence of a phase transition is present between 6 and 8 GPa where the thermoelectric power reaches an absolute value of 225 mV/K after passing through a minimum of Sz75 mV/K. X-ray diffraction experiments confirm that the resulting structure is b-As2Te3, which is isostructural with Bi2Te3 and Sb2Te3. q 2005 Elsevier Ltd. All rights reserved. PACS: 72.20.Pa; 61.10Nz; 61.50.Ks Keywords: Semiconductor; X-ray diffraction; Phase transition; Transport properties

1. Introduction The efficiency of a thermoelectric refrigerator or power generator depends on its geometry and on the figure of merit Z,

ZZ

S2 ; rk

(1)

of the materials from which the device is constructed. S is the Seebeck coefficient or thermoelectric power, r is the electrical resisitivity, and k is the thermal conductivity, which has two components, an electronic contribution and a lattice contribution. Z has units of inverse temperature. A dimensionless figure of merit (ZT), the product of Z and the temperature at which the device operates, is more often quoted. * Corresponding author. Address: Department of Physics, The Pennsylvania State University, 104 Davey Laboratory, PMB 150, University Park, PA 16802, USA. Tel.: C1 814 865 7260; fax: C1 814 865 3604. E-mail address: [email protected] (T.J. Scheidemantel).

0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.07.006

Heavily doped alloys of semiconductors can have high figures of merit and hence much effort is aimed at improving their transport properties to enhance Z. The best bulk, room temperature thermoelectric materials are alloys containing Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 [1,2]. Sb2Se3 is orthorhombic (Pnma), while the remaining three binary compounds have a rhombohedral structure with space group  Solid solutions containing these constituents also have R3m. this same rhombohedral structure over a broad range of concentrations. The thermoelectric properties of these materials have been investigated intensively. Other group V–VI compounds, such as arsenic telluride, have not, likely because they are not isostructural with Bi2Te3 and not as soluble in the above alloys. Several methods, such as the synthesis of new materials, the design of quantum structures, and combinatorial synthesis techniques, are all employed in the search for new or improved thermoelectric materials [3–8]. Pressure tuning offers an alternative means to search for improvements in transport properties. If an improved property is observed under pressure, it provides a target for synthesis at ambient pressure. Pressure can be changed more rapidly than a new material can be synthesized, allowing the phase space of materials interaction parameters to be rapidly explored [9].

T.J. Scheidemantel et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1744–1747

Fig. 1. The thermoelectric power of As2Te3 is shown up to 10 GPa. The behavior between 6 and 8 GPa is indicative of a structural transition.

Few investigations have been performed on crystalline As2Te3 as a thermoelectric material candidate [10–13]. Arsenic telluride has a monoclinic structure with space group C2/m (a-As2Te3) [14,15]. a-As2Te3 has a moderately high thermoelectric power, a low thermal conductivity for a crystalline material, but a poor electrical conductivity. These result in a figure of merit at room temperature that is much lower, ZTw0.16, than currently used thermoelectric alloys, ZTw1.0 [1,2,10,11]. A high pressure phase of arsenic telluride (b-As2Te3) is of the Bi2Te3-structure type [16,17]. This phase has also been observed in samples quenched from high-temperatures [18]. b-As2Te3 will likely have a large thermoelectric power because it has the same crystal structure as Bi2Te3 and a similar electronic structure [12]. Pressure tuned Sb1.5Bi0.5Te3 exhibited a substantial increase in its thermoelectric power and figure of merit at 2 GPa [19]. To attempt to reproduce this result, it is natural to consider alloying with arsenic, which is smaller than antimony, bismuth, or tellurium. Using conventional synthesis techniques, however, less than 1% of As2Te3 is soluble in Bi2Te3 [13]. Recent experiments though, show that As2Te3 may be more soluble in alloys of Bi2Te3 and Sb2Te3 than in Bi2Te3 alone [20]. High pressure synthesis techniques might also provide a path to alloys with higher concentrations of arsenic. Before investigating the thermoelectric properties of alloys of As2Te3 and Bi2Te3, it is important to understand the behavior of pure As2Te3 under pressure. Here, we report an investigation of the thermoelectric power of As2Te3 measured from 0.1 MPa to 10 GPa. The high pressure phase diagram of As2Te3 has only be studied up to 1.7 GPa [16,17]. A simple extrapolation of the data in Fig. 1 in Ref. [16] allows us to predict a structural phase transition near PZ7 GPa at 298 K.

2. Methods Thermoelectric power measurements as a function of pressure were made in a Mao-Bell diamond anvil cell (DAC) at ambient temperature. The details of the technique

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have been described previously [21]. The accuracy has been verified by measurements on several standard materials including bismuth and CePd3. Small (less than 1 cm) polycrystalline pieces of As2Te3 were obtained from Alfa Asar (99.999%). X-ray analysis confirmed a monoclinic structure with space group C2/m. Samples for the DAC were cut to approximately 500 mm! 50 mm!50 mm. This is smaller than usually needed for the 1 mm culet diamond anvils utilized in these experiments, but the samples were very soft, and cutting them smaller prevented contact with the gasket, due to deformation, while increasing the pressure. The pressure chamber was filled predominantly with monoclinic ZrO2 but a small amount of the CsI was placed above and beneath the sample. CsI becomes transparent near 1 GPa and remains transparent beyond the 10 GPa attained in these experiments allowing visualization of the sample. The zirconia is a good thermal insulator, protecting the sample from the diamonds, which have a very high thermal conductivity; however, it has been shown that even if the sample were in direct contact with the diamon anvil, the correct thermoelectric power is measured [22]. The monoclinic zirconia is also soft and flows quite well, especially above 2 GPa; nevertheless, the stress conditions are expected to have a significant uniaxial component [22]. Such a sample configuration and the resulting stress conditions were deliberately chosen to mimic those that gave large improvements in stress tuned Sb1.5Bi0.5Te3 [19]. The details of the stress conditions are expected to have a substantial effect on the thermoelectric power. Pressure was measured by means of ruby fluorescence [23]. Type K thermocouple junctions with 12.5 or 25 mm leads were embedded in the sample for the thermoelectric power measurements. Resistance measurements verified good electrical contact, implying good thermal contact. This is expected since, the thermocouples become deeply embedded in the sample due to the pressure from the diamonds. The temperature gradient was induced by means of an infrared (YLF) laser focused to a narrow line (using a cylindrical lens) to eliminate transverse thermal gradients. Large samples, 5 mm!1 mm!0.5 mm, were cut for measurements outside the DAC. These samples for measurements at ambient pressure were made by pressing two rectangular pieces of As2Te3 together with thermocouples between them. This ensured good thermal and electrical contact. These samples were heated axially on one end to further eliminate the effects of transverse thermal gradients. All voltages were measured simultaneously using two nanovolt meters connected to a personal computer for calculation of the thermoelectric power. The X-ray diffraction system adapted to the DAC was developed in our lab and the details of the apparatus are described elsewhere [24]. The diffraction patterns were collected using direct exposure film, which was developed using conventional techniques. The patterns were then scanned into a personal computer, fitted, and collapsed into

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T.J. Scheidemantel et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1744–1747

Table 1 Interplanar spacings at 4 GPa (a-As2Te3) and 8 GPa (b-As2Te3) b-As2Te3

a-As2Te3 h

K

l

dobs

dcalc

h

k

l

dobs

dcalc

0 1 5 6 0 3

0 1 1 0 2 1

3 2 0 1 0 4

3.29 3.00 2.33 2.27 2.01 1.98

3.31 3.07 2.30 2.30 2.01 1.96

1 0 1 1

0 1 0 1

4 5 10 0

3.06 2.87 2.23 1.97

3.07 2.93 2.22 1.95

a two-dimensional diffraction pattern from which interplanar lattice spacings and hence, lattice parameters could be calculated [25].

3. Results and discussion Ambient pressure and temperature X-ray diffraction analysis confirmed that the sample was polycrystalline monoclinic a-As2Te3. An ambient pressure thermoelectric power of SZK242 mV/K was measured for samples which were then cut for high pressure experiments. This agrees with the value reported for n-type samples [10]. Different samples sometimes gave different results indicating different carrier concentrations and even different majority carriers. The presence of p-type and n-type material within the same polycrystalline lump of As2Te3, and different values of the thermoelectric power among samples prepared by several different methods have previously been reported [10,11,13,26]. The most reproducible results came from samples cut from the same large ingot, which had the thermoelectric power at ambient pressure and temperature reported above. The pressure dependence of the thermoelectric power of several samples were measured. Here, we focus on the most reproducible trend. The absolute value of the thermoelectric power decreases rapidly from Sz250 mV/K at ambient pressure to Sz100 mV/K near 2 GPa (Fig. 1). It then decreases with a much lower slope to Sz75 mV/K near 6 GPa. This is followed by a very rapid increase to a thermoelectric power near the ambient pressure value at 8 GPa. The change in the thermoelectric power near 2 GPa is likely due to changes in the stress conditions due to yielding

of the monoclinic zirconium oxide and the resulting partial decrease in the uniaxial stress component. Previous X-ray diffraction studies of the Kondo insulator NdxCe3KxPt3Sb4 in our laboratory also showed behavior consistent with the zirconium oxide yielding at 2 GPa [22]. Such yielding is often observed in non-hydrostatic pressure media. Above 2 GPa, no such effects were noted. The structure near 6 GPa is more pronounced than at 2 GPa. Here, the magnitude of the thermoelectric power increases significantly over a range of 2 GPa reaching a maximum when the thermoelectric power is K225 mV/K at 8 GPa. As suggested earlier, this behavior could indicate a structural phase transition, which is expected near 7 GPa. The considerable increase in the magnitude of the thermoelectric power is further suggestive of a transition to the rhombohedral structure, which is expected to have a high thermoelectric power. To explore the possibility of a structural phase transition, X-ray diffraction analysis was conducted. Up to 4 GPa, the X-ray diffraction patterns showed no departure of the sample from the monoclinic phase, aAs2Te3. It is most probable then, that the changes in thermoelectric power near 2 GPa were caused by radically changing stress components due to the flowing pressure medium. The interplanar lattice spacings and calculated lattice parameters for the monoclinic phase at 4 GPa are shown in Tables 1 and 2, respectively. The volume corresponds to a unit cell that is 1.1% smaller than that found at ambient pressure [15]. Between 4 and 8 GPa, there were both phases, monoclinic and rhombohedral, present. This was not unexpected because of the non-hydrostatic conditions in the DAC. At 8 GPa, the sample was again single phase, with only b-As2Te3 present (Tables 1 and 2). The volume of the rhombohedral cell corresponds to a unit

Table 2 Unit cells at 4 GPa (a-As2Te3) and 8 GPa (b-As2Te3) Phase

Structure

˚) Lattice parameters (A

a-As2Te3

Monoclinic C2/m

b-As2Te3

Rhombohedral  R3m

aZ14.00G0.09 bZ4.02G0.06 cZ9.94G0.09 aZ10.02G0.04

˚ 3) Volume (A bZ87.0G0.28

558.8

aZ22.5G0.18

129.2

T.J. Scheidemantel et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1744–1747

cell that is 8% smaller than that found at atmospheric pressure [26]. Using our thermoelectric power data and the ambient pressure electrical conductivity (which agrees with the published measurement of n-type As2Te3 in Ref. [27]) scaled by two-probe relative resistance measurements (via the thermocouples), we were able to approximate the power factor as a function of pressure. Further, using the Wiedemann-Franz law (k/szconstant) to approximate the electronic contribution to the thermal conductivity, we were able to make a rough estimate of the figure of merit. Because no data is available for the lattice thermal conductivity of bAs2Te3, we assumed a pressure independent value of 4 W/ m K using a simple linear trend among the lattice thermal conductivities of Bi2Te3, Sb2Te3, and b-As2Te3. The accuracy of this estimate is limited by this assumption and possible error in the magnitude of the resistivity at high pressure. These approximations lead to a local maximum ZT at PZ8.3 GPa where ZTz0.12.

4. Conclusions We have measured the thermoelectric power of crystalline arsenic telluride up to 10 GPa using a Mao-Bell DAC. Approximating the thermal conductivity at 8.35 GPa, we estimated the high pressure dimensionless figure of merit to be 0.12, which is close to that of other group V–VI  semiconductors with the rhombohedral structure ðR3mÞ, in their pure undoped form. X-ray diffraction experiments confirmed that the sample did indeed undergo a structural phase transition. The resulting phase, b-As2Te3, is isostructural with Bi2Te3 and Sb2Te3.

Acknowledgements This work was funded in part by NSF Grant No. DMR02-05125.

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