Electron microscopy

Specimens were prepared by dispersing an ethanol suspension of finely powdered specimen on copper grids coated with holey carbon film which were mounted on a double tilt holder with maximum tilt angle of ±30°. Selected-area electron diffraction (SAED) and high-resolution

transmission electron microscopy (HRTEM) were carried out on a FEI Titan 80–300 equipped with a field emission gun operating at 300 kV. In addition, scanning transmission electron microscopy (STEM) using a high-angle annular dark field detector (HAADF) was performed to obtain Z contrast images. TEM images were recorded using a Gatan UltraScan 1000 (2k×2k) camera, for HAADF-STEM, a Fischione detector with an inner semi-angle of 32 mrad was used. For the simulation of high resolution images, the multi-slice method as implemented in the EMS program package was used.[60]

3.4.4.6 Thermoelectric properties

Both commercial and in-house-built facilities were used to determine the temperature dependence of the electrical and thermal conductivities as well as the Seebeck coefficient, which were investigated in the range from room temperature up to approx. 500 °C under vacuum. To avoid Peltier influences on the measurement, the electrical conductivity was measured by a four- point-probe setup using an AC method. The Seebeck coefficient was measured using a small temperature gradient across the sample while slowly changing the environment temperature in order to obtain Seebeck coefficients for each mean sample temperature. Type-R thermocouples attached directly to the sample’s surface were used for both temperature measurements and the Seebeck voltage was picked up via the Pt lines. The thermal conductivity was calculated from measurements of the thermal diffusivity by a laser-flash apparatus (Netzsch LFA 427), the heat capacity determined by differential scanning calorimetry (Netzsch DSC 404) and the samples’ density measured using a Mohr’s balance. Experimental errors are estimated at 2% for the electrical conductivity, 5% for the Seebeck coefficient and 7% for the thermal conductivity. The electronic contribution of the thermal conductivity was calculated according to the Wiedemann- Franz law for non-degenerate semiconductors.

3.4.5 References

[1] D. Bonnet, P. Meyers, J. Mater. Res. 1998, 13, 2740.

[2] M. A. Green, J. Mater. Sci. - Mater. Electron. 2007, 18, S15. [3] D. Lencer, M. Salinga, M. Wuttig, Adv. Mater. 2011, 23, 2030. [4] S. Raoux, W. Wojciech, D. Ielmini, Chem. Rev. 2009, 110, 240. [5] W. Bensch, M. Wuttig, Chem. Unserer Zeit 2010, 44, 92. [6] A. V. Shevelkov, Russ. Chem. Rev 2008, 77, 1.

[8] G. J. Snyder, E. S. Toberer, Nat. Mater. 2008, 7, 105.

[9] D. L. Medlin, G. J. Snyder, Curr. Opin. Colloid Interface Sci. 2009, 14, 226.

[10] M. S. Dresselhaus, G. Chen, M. Y. Tang, R. Yang, H. Lee, D. Wang, Z. Ren, J.-P. Fleurial, P. Gogna, Adv. Mater. 2007, 19, 1.

[11] M. G. Kanatzidis, Chem. Mater. 2010, 22, 648.

[12] Č. Drašar, A. Hovorková, P. Lošťák, H. Kong, C.-P. Li, C. Uher, J. Appl. Phys. 2008,

104, 023701.

[13] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Nature 2001, 413, 597. [14] J. R. Szczech, J. M. Higgins, S. Jin. J. Mater. Chem. 2011, 21, 4037.

[15] S. K. Bux, J.-P. Fleurial, R. B. Kaner, Chem. Commun. 2010, 46, 8311. [16] P. F. P. Poudeu, M. G. Kanatzidis, Chem. Commun. 2005, 21, 2672.

[17] K. Kifune, Y. Kubota, T. Matsunaga, N. Yamada, Acta Crystallogr. Sect. B 2005, 61, 492.

[18] C. S. Barrett, P. Cucka, K. Haefner, Acta Crystallogr. 1963, 16, 451-453. [19] T. L. Anderson, H. B. Krause, Acta Crystallogr. Sect. B 1974, 30, 1307-1310. [20] M. N. Schneider, O. Oeckler, Z. Anorg. Allg. Chem. 2008, 634, 2557-2561.

[21] L. E. Shelimova, O. G. Karpinsky, M. A. Kretova, V. I. Kosyakov, V. A. Shestakov, V. S. Zemskov, F. A. Kuznetsov, Inorg. Mater. 2000, 36, 768.

[22] O. G. Karpinsky, L. E. Shelimova, M. A. Kretova, J.-P. Fleurial, J. Alloys Compd. 1998,

268, 112.

[23] T. Matsunaga, R. Kojima, N. Yamada, K. Kifune, Y. Kubota, M. Takata, Appl. Phys. Lett. 2007, 90, 161919.

[24] M. N. Schneider, M. Seibald, O. Oeckler, Dalton Trans. 2009, 2004. [25] M. N. Schneider, O. Oeckler, Z. Anorg. Allg. Chem. 2010, 636, 137.

[26] O. Oeckler, M. N. Schneider, F. Fahrnbauer, G. Vaughan, Solid State Sci. 2011, 13, 1157. [27] M.N. Schneider, T. Rosenthal, C. Stiewe, O.Oeckler, Z. Kristallogr. 2010, 225, 463. [28] H. Lind, S. Lidin, Solid State Sci. 2003, 5, 47.

[29] M. N. Schneider, M. Seibald, P. Lagally, O. Oeckler, J. Appl. Crystallogr. 2010, 43, 1012.

[30] B. Legendre, C. Hancheng, S. Bordas and M. T. Clavaguera-Mora, Thermochim. Acta 1984, 78, 141.

[31] V. I. Kosyakov, V. A. Shestakov, L. E. Shelimova, F. A. Kuznetsov and V. S. Zemskov,

[32] S. Bordas, M. T. Clavaguera-Mora, B. Legendre and C. Hancheng, Thermochim. Acta 1986, 107, 239.

[33] A. Stegherr, Philips Res. Repts. Suppl. 1969, 6, 1. [34] R. Wulf, Acta Crystallogr. Sect. A 1990, 46, 681.

[35] B. L. Henke, E. M. Gullikson, J.C. Davis, Atomic Data and Nuclear Data Tables 1993,

54, 181.

[36] C. T. Chantler, K. Olsen, R. A. Dragoset, J. Chang, A. R. Kishore, S. A. Kotochigova, D. S. Zucker, X-Ray Form Factor, Attenuation and Scattering Tables, Version 2.1, National Institute of Standards and Technology, Gaithersburg MD, 2005.

[37] V. Petricek, M. Dusek, L. Palatinus, JANA2006 - The crystallographic computing system, Institute of Physics, Praha, Czech Republic, 2006.

[38] T. Chattopadhyay, J. X. Boucherle, H. G. von Schnering, J. Phys. C: Solid State Phys. 1987, 20, 1431.

[39] T. Matsunaga, N. Yamada, Y. Kubota, Acta Crystallogr. Sect. B 2004, 60, 685. [40] H. Böttner, G. Chen, R. Venkatasubramanian, MRS Bull. 2006, 31, 211.

[41] F. R. Harris, S. Standridge, C. Feik, D. C. Johnson, Angew. Chem. Int. Ed. 2003, 42, 5296.

[42] O. Oeckler, M. N. Schneider, T. Rosenthal, C. Stiewe, Z. Anorg. Allg. Chem. 2010, 636, 2095.

[43] J. Oestreich, Ph.D. Thesis, University of Konstanz, Germany, 2001.

[44] P. P. Konstantinov, L. E. Shelimova, E. S. Avilov, M. A. Kretova, V. S. Zemskov, Inorg. Mater. 2001, 37, 662.

[45] V. A. Kulbachinskiĭ, A. Y. Kaminskiĭ, P. M. Tarasov, P. Lostak, Phys. Solid State 2006,

48, 833.

[46] J. L. Cui, H. Fu, X. L. Liu, D. Y. Chen, W. Yang, Curr. Appl. Phys. 2009, 9, 1170.

[47] M. Kuwahara, O. Suzuku, N. Takethosi, T. Yagi, P. Fons, J. Tominaga, T. Baba, Jpn. J. Appl. Phys. 2007, 46, 6863.

[48] J. W. G. Bos, H. W. Zandbergen, M.-H. Lee, N. P. Ong, R. J. Cava, Phys. Rev. B 2007,

75, 195203.

[49] D. Pinisetty, R. V. Devireddy, Acta Mater. 2010, 58, 570.

[50] T. C. Chong, L. P. Shi, X. S. Miao, P. K. Tan, R. Zhao, Z. P. Cai, Jpn. J. Appl. Phys. 2000, 39, 737.

[51] T. C. Chong, L. P. Shi, R. Zhao, P. K. Tan, J. M. Li, H. K. Lee, X. S. Miao, A. Y. Du, C. H. Tung, Appl. Phys. Lett. 2006, 88, 122114.

[52] H. Yang, T. C. Chong, R. Zhao, H. K. Lee, J. Li, K. G. Lim, L. Shi, Appl. Phys. Lett. 2009, 94, 203110.

[53] M. Hase, Y. Miyamoto, J. Tominaga, Phys. Rev. B 2009, 79, 174112. [54] WINXPOW, Version 2.12, Stoe & Cie. GmbH, Darmstadt, 2005.

[55] A. Coelho, TOPAS -Academic, V. 4.1; Coelho Software, Brisbane, 2007. [56] SAINT, v6.01, Bruker AXS, Madison USA 1999.

[57] XPREP, Version 6.12, Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin,USA, 1996.

[58] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112. [59] SADABS, v2.03, Bruker AXS, Madison USA 1999. [60] P. Stadelmann, Ultramicroscopy 1987, 21, 131.

4 From phase-change materials to thermo-

electrics

4.1 Overview

The rational synthesis of new solid-state compounds with special properties is an idea that often resounds throughout the community of solid-state scientists. However, the discovery of novel materials remains challenging as the potential of theoretical approaches for the a priori

prediction of new compounds and their properties is still rather limited. The search for new materials with specific properties is often tedious and involves a lot of experimental trial and error. Nevertheless, various concepts often serve as guides during such a search. The synthesis of novel materials might be inspired by nature.[1-3] _{New preparative routes might be bio-inspired}

and for example copy biomineralization processes under laboratory conditions. They can also be geo-inspired by concepts and findings of modern mineralogy, e.g. the natural formation of textures on variable length scales, e.g. by precipitation, spinodal decomposition or phase transitions, may be mimicked to obtain nano- or microstructured materials. The formation of nano- and microstructures by partial decomposition or exsolution is also an intriguing approach to lower the thermal conductivity of thermoelectrics as demonstrated by examples such as AgPbmSbTe2+m (LAST-m), NaPbmSbTe2+m (SALT-m) or AgSbTe2 and Pb2Sb6Te11.[4-8] In

addtion to nature, technology may also provide inspiration concerning the search for new materials. For example, the properties of well-characterized and understood compounds that are applied for a specific purpose can be optimized for new applications. Such approaches include various doping experiments with additional elements, intercalation or change of the nano-/ microstructure.

In this context, compounds whose composition are comparable to those of phase-change materials, especially GeTe-rich phases (GeTe)n(Sb2Te3) (n = 12), have been shown to exhibit

interesting thermoelectric properties as discussed in Chapter 4.2. They are small-bandgap semiconductors that exhibit moderate thermal conductivities and therefore meet two basic requirements for thermoelectrics (cf. Chapter 1). For phase-change materials a moderate electrical conductivity allows for switching between amorphous and crystalline modifications by electrical heating and ensures that stored information can be read applying read-out currents. For thermoelectrics the electrical part of the thermal conductivity remains for low small-bandgap

semiconductors while simultaneously enough electrical conductivity to use the thermoelectric effect is present. Whereas in phase-change materials a small thermal conductivity enables the writting of sharp recording marks when the material is locally heated, in thermoelectrics the small thermal conductivity ensures that the temperature gradient necessary to use the thermoelectric effect is sustained during application. The highly disordered crystalline phases of phase-change materials exhibit simple average structures which are metastable with respect to the formation of long-periodic layered structures. Partial stabilization yields pronounced nano- or microstructures which further reduce the thermal conductivity (see also Chapter 5). Thin-film samples of metastable crystalline modifications are suitable to study their phase-change characteristics but less suitable for thermoelectric investigations or detailed real-structure analysis. Therefore, bulk material is required to confirm the hypothesis that the chemical systems used as phase-change materials are promising thermoelectrics, both concerning the structural characterization as well as for physical measurements. In this work, various synthetic approaches have been compared.

GeTe-rich compounds on the pseudobinary section GeTe - Sb2Te3 exhibit a stable rocksalt-type

structure at elevated temperatures,[9,10] which can be investigated in situ. Such high-temperature modifications of (GeTe)n(Sb2Te3) compounds exist for n ≥ 3, e. g. Ge3Sb2Te6 (n = 3).

21R-GeSb2Te4 (n = 1) and 9P-Ge2Sb2Te5 (n = 2) do not show phase transitions to a cubic high-

temperature phase before melting. The transition temperatures between cubic rocksalt-type and trigonal layered phases decrease with increasing n, as will be discussed in more detail in Chapter 5. Rietveld refinements on diffraction patterns of Ge3Sb2Te6 (n = 3, transition temperature 575

°C) and Ge12Sb2Te15 (n = 12, transition temperature 475 °C) collected at 600 °C confirm that

the high-temperature modifications exhibit rocksalt-type average structures. Single crystals were obtained by chemical transport reactions using iodine or SbI3 as transport agent or by crystal

formation during the annealing process in the stability range of the high-temperature modifications (cf. Chapter 4.2, Chapter 5.4 and Chapter 5.5). Above the transition temperatures to the cubic high-temperature phases, single crystals (n = 12 cf. Chapter 4.2 and n = 6, = 12, = 15 cf. Chapter 5.5) do not exhibit pronounced diffuse scattering, i. e. little short-range order: Te atoms occupy the anion positions whereas Ge atoms, Sb atoms as well as cation vacancies are randomly distributed on cation sites. Thus, the high-temperature phases are characterized by a large concentration of cation vacancies. Upon slow cooling, the stable trigonal layered structures are obtained. Formally, this involves the ordering of cation defects in 2D extended defect layers which are parallel. The stacking sequence of the Te layers changes and there are significant structural distortions around the resulting van der Waals gaps. These separate the rocksalt-type

slabs in the ordered structures described in Chapters 1 and 2, respectively. Rapid quenching partially prevents the formation of stable trigonal layered structures for compounds that exhibit cubic high-temperature modifications. Quenching melts (in air or liquid nitrogen) with n < 3, i. e. compounds with no cubic high-temperature phase, yields the corresponding trigonal layered structures (n = 1: 21R-type, n = 2: 9P-type) with a pronounced degree of stacking faults. As shown for GeBi2Te4 (n = 1, cf. Chapter 4.3), whose stable trigonal layered phase corresponds to

a 21R-type.[11] even melt-spinning experiments with very high cooling rates do not yield metastable samples with simple average structures. Although powder diffracton shows the normal 21R-type structure electron microscopy reveals that very small domains are present. Quenching of Ge3Sb2Te6 in air or liquid nitrogen yields a mixture of a phase with cubic metrics

and a highly disordered layered trigonal phase with broad reflections. The cubic phase vanishes at ~270 °C and long-range ordered 33R-Ge3Sb2Te6 forms at ~420 °C. Recent results [12] indicate

that melt-spinning impedes the formation of the layered phase and yields primarily the (pseudo-)cubic modification. In contrast, rapid quenching of the high-temperature modification of Ge12Sb2Te15 in air or water yields a homogenous highly disordered phase that is kinetically

inert up to 325 °C when a trigonal layered structure (with pronounced stacking disorder) is formed (also refer to Chapter 5.2). Whereas the quenched phases are pseudo-cubic according to powder diffraction, single-crystal diffraction as well as electron microscopy reveal nanoscale twin domains with trigonal structure that are associated with a cubic to rhombohedral phase transition. This multiple twinning is accompanied by short-range order of cation defects in finite intersecting layers that leads to characteristic diffuse scattering which will be discussed in Chapter 5.

Quenched samples of (GeTe)n(Sb2Te3) (n = 3, 4.5, 7, 12, 19, cf. Chapter 4.2 and Chapter 5.3)

exhibit rather high thermoelectric figures of merit ZT. For n = 12 and 19, a maximum value of 1.3 is reached at 450 °C. This is comparable to modern thermoelectrics.[13-17] This implies that metastable GeTe-rich Ge-Sb-Te materials may be promising for thermoelectric applications. However, at elevated temperatures the nanostructure of the materials is not long-time stable as diffusion processes are activated above ~ 300 °C in such materials (cf. Chapter 5). Below 300 °C, the thermoelectric figure of merit of compounds (GeTe)n(Sb2Te3) (n = 3, 4.5, 7, 12, 19) is

about one magnitude smaller than the maximum values at 450 °C. A detailed discussion of the corresponding structure-property relationships will be given in Chapter 5.

The cubic to rhombohedral phase transition is naturally accompanied by a volume and density change, respectively. Other phase transitions of, for instance, metastable cubic Ge2Sb2Te5

temperature) followed by the formation of a body-centred phase at  30 GPa. In contrast, stable 9P-Ge2Sb2Te5 remains crystalline upon compression and transforms to an orthorhombic

structure at 17 GPa before a body-centred phase is obtained at  33 GPa.[18-21] These findings imply that pressure is an interesting parameter for synthetic approaches especially when it is combined with high temperatures and quenching steps. This assumption is corroborated by high- pressure experiments on GeBi2Te4. According to Rietveld refinement, a sample of GeBi2Te4

obtained by quenching a stoichimetric melt under a constant pressure of 12 GPa (i. e. by switching of the furnace) exhibits a CuPt-type structure (cf. Chapter 1.2). Electron microscopy reveals a pronounced nanostructure which explains why the "average structure" can only be described with very prolate atomic displacement ellipsoids. At ambient pressure, the sample transforms into 21R-GeBi2Te4 above 200 °C. This indicates that a highly disordered high-

pressure high-temperature phase is partially retained. Variation of the thermal treatment applied under constant pressure of 12 GPa allows one to alter the domain sizes and orientations as well as the defect layer arrangements. Although the small volume of high-pressure samples impedes the complete thermoelectric characterization of the thermoelectric properties, the dependency of their electrical conductivity on the micro-/nanostructure is very pronounced. The characteristics of the electrical conductivity changes from metallic to semiconducting behavior with decreasing domain size and a more random orientation distribution of domains. Scattering of the electrons at the domain or grain boundaries becomes predominant and the electrical conductivity decreases as the mean free path of electrons decreases. These results show that metastable germanium bismuth tellurides and germanium antimony tellurides are intriguing precursors for thermoelectrics with extended nanostructures, and also exhibit interesting properties themselves without further processing.

References for chapter 4.1

[1] E. Dujardin, S. Mann, Adv. Mater. 2002, 14, 775.

[2] L. A. Estroff, A. D. Hamilton, Chem. Mater. 2001, 13, 3227. [3] W. Depmeier, Cryst. Res. Technol. 2009, 44, 1122.

[4] A. C. Bruce, J. K. Matthew, L. H. Joel, H. Mi-Kyung, C. Duck-Young, G. K. Mercouri,

Adv. Funct. Mater. 2009, 19, 1254.

[5] C. J. Vineis, A. Shakouri, A. Majumdar, M. G. Kanatzidis, Adv. Mater.2010, 22, 3970. [6] P. F. P. Poudeu, J. D'Angelo, A. D. Downey, J. L. Short, T. P. Hogan, M. G. Kanatzidis,

[7] T. Ikeda, V. A. Ravi, L. A. Collins, S. M. Haile, G. J. Snyder, J. Electron. Mater. 2007,

36, 716.

[8] S. N. Zhang, T.J. Zhu, S. H. Yang, C. Yu, X. B.Zhao, J. Alloys Compds.2010, 499, 215. [9] T. Matsunaga, N. Yamada, Y. Kubota, Acta Crystallogr. Sect. B 2004, 60, 685.

[10] T. Matsunaga, H. Morita, R. Kojima, N. Yamada, K. Kifune, Y. Kubota, Y. Tabata, J.-J. Kim, M. Kobata, E. Ikenaga, K. Kobayashi, J. Appl. Phys. 2008, 103, 093511.

[11] L. E. Shelimova, O. G. Karpinskii, P. P. Konstantinov, E. S. Avilov, M. A. Kretova, V. S. Zemskov, Inorg. Mater. 2004, 40, 451.

[12] T. Schröder, publication in preparation.

[13] A. V. Shevelkov, Russ. Chem. Rev. 2008, 77, 1.

[14] K. Nielsch, J. Bachmann, J. Kimling, H. Böttner, Adv. Energy Mater.2011, 1, 713. [15] M. G. Kanatzidis, Chem. Mater. 2010, 22, 648.

[16] G. J. Snyder, E. S. Toberer, Nat. Mater. 2008, 7, 105.

[17] J. R. Sootsman, D. Y. Chung, M. G. Kanatzidis, Angew. Chem. Int. Ed. 2009, 48, 8616. [18] A. V. Kolobov, J. Haines, A. Pradel, M. Ribes, P. Fons, J. Tominaga, Y. Katayama, T.

Hammouda, T. Uruga, Phys. Rev. Lett. 2006, 97, 035701.

[19] Y. Q. Cheng, M. Xu, H. W. Sheng, Y. Meng, X. D. Han, E. Ma, Appl. Phys. Lett. 2009,

95, 131904.

[20] M. Krbal, A. V. Kolobov, J. Haines, P. Fons, C. Levelut, R. Le Parc, M. Hanfland, J. Tominaga, A. Pradel, M. Ribes, Phys. Rev. Lett. 2009, 103, 115502.

[21] M. Krbal, A. V. Kolobov, P. Fons, J. Haines, A. Pradel, M. Ribes, A. A. Piarristeguy, C. Levelut, R. Le Parc, V. Agafonov, M. Hanfland, J. Tominaga, Phys. Rev. B 2011, 83, 024105.