Tuning the Vacancy Concentration in Lithium Germanium Antimony

Article Cite This: Chem. Mater. 2018, 30, 7970−7978

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Tuning the Vacancy Concentration in Lithium Germanium Antimony TelluridesInfluence on Phase Transitions, Lithium Mobility, and Thermoelectric Properties Stefan Schwarzmüller,‡ Matthias Jakob,‡ Markus Nentwig,‡ Thorsten Schröder,† Alexander Kuhn,§ Andre Düvel,§ Paul Heitjans,§ and Oliver Oeckler*,‡

Chem. Mater. 2018.30:7970-7978. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/13/18. For personal use only.

‡

Institute for Mineralogy, Crystallography and Materials Science; Faculty of Chemistry and Mineralogy, Leipzig University, Scharnhorststraße 20, 04275 Leipzig, Germany † Department of Chemistry, Ludwig Maximilian University Munich, Butenandtstraße 5-13 (D), 81377 Munich, Germany § Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstraße 3-3a, 30167 Hanover, Germany S Supporting Information *

ABSTRACT: In the solid solution series Li 2−x Ge 3+1/2x Sb 2 Te 7 and Li2−xGe11+1/2xSb2Te15 (0 ≤ x ≤ 2), the heterovalent substitution gradually changes the vacancy concentration on the cation position from 0% (for x = 0) to 14.3% and 6.67%, respectively. Fewer vacancies extend the stability range of the rocksalt-type high-temperature phase to lower temperatures, which is favorable for thermoelectric applications. Further differences in thermoelectric properties correlate with the Li/Ge ratio. The phononic part of thermal conductivity decreases with increasing Li content and all Li-containing compounds exhibit enhanced thermoelectric figures of merit zT compared to their Li-free parent phases with a maximum zT value of 1.9 for LiGe3.5Sb2Te7 at 450 °C. 7Li solid state NMR reveals high Li mobility at elevated temperatures. Thus, lithium germanium antimony tellurides can be considered as new member of phononliquid electron-crystal (PLEC) thermoelectric materials with superior thermoelectric properties.

1. INTRODUCTION Presently, a large variety of highly efficient thermoelectric materials like PbTe, Bi2Te3, half-Heusler phases, clathrates, Zintl phases, metal oxides, and so forth are available for power generation applications.1−5 To the same extent, germanium antimony tellurides (GST materials) (GeTe)nSb2Te3 have emerged as a promising class of thermoelectric materials.6−9 Their properties can be tuned by a broad variety of homovalent substitutions, e.g., Sn2+, Cd2+, or Mn2+ for Ge2+,10−12 Se2− for Te2−,13 or In3+ for Sb3+.14 These materials are characterized by high concentrations of cation vacancies, which are completely disordered in NaCl-type high-temperature (HT) phases.15−18 At lower temperatures, vacancies tend to order and form layers. These correspond to van der Waals gaps once the ordering is complete and the anion layer stacking changes from “cubic” ABC to “hexagonal” ABA around the gaps. This leads to a homologous series of layered compounds for integer values of n.19,20 Quenching the HT phases, however, yields nanodomain structures due to incomplete vacancy ordering as a consequence of long diffusion pathways and strain induced by the rearrangement of the anion arrangement in domain structures.21 These domain structures exhibit promising thermoelectric properties, which is also true for the highly disordered high-temperature phases, but not for the layered phases with van der Waals gaps.6 Thus, the latter should be avoided. Furthermore, GST materials are used as phase-change materials (PCMs) due to their fast crystallization © 2018 American Chemical Society

kinetics and the combination of high electrical and low thermal conductivity.22,23 The latter properties are also essential for thermoelectric materials, in addition to high Seebeck coefficients. Both in PCMs and in thermoelectric materials, vacancy ordering plays a key role.6,13,18 The cubic HT phases are unstable at lower temperatures due to the high vacancy concentrations present, which result in incomplete coordination polyhedra for most atoms. The vacancy concentration can be reduced by heterovalent cation substitution. Replacing, e.g., Ge2+ with twice the amount of Li+ reduces the number of vacancies. Compounds without structural vacancies are the limit of such solid solution series and can then be described as (GeTe)y(LiSbTe2)2.24 These phases do not exhibit the phase transitions typical for pristine GST materials and represent solid solutions where y need not be integer and can adopt all values ≥0. For small values of y, these solid solutions are cubic (average NaCl-type structure or closely related variants with a small part of the Ge atoms in tetrahedral sites) over the whole temperature range from room temperature (RT) to ca. 500 °C. For y > ca. 6, they exhibit the phase transition of GeTe, with a NaCl-type HT phase and GeTe-type low-temperature (LT) phase. Similar to the layered phases of GST materials, this phase is rhombohedral; however, Received: August 25, 2018 Revised: October 20, 2018 Published: October 23, 2018 7970

DOI: 10.1021/acs.chemmater.8b03609 Chem. Mater. 2018, 30, 7970−7978

Article

Chemistry of Materials

Figure 1. Experimental (black dots) and calculated (light gray) powder diffraction patterns and difference plots (dark gray, bottom histogram) for Li2Ge3Sb2Te7 (top left), Li2Ge11Sb2Te15 (top right), LiGe3.5Sb2Te7 (bottom left), and LiGe11.5Sb2Te15 (bottom right) with peak positions as black vertical lines; for details of the Rietveld refinements see Tables 1 and 2 compounds NaPbmSbTem+2,29 brittle, bright shiny metallic samples were obtained. They are slightly sensitive to moisture and become covered with a black film when kept at air for several days. Thus, they were stored under argon atmosphere. 2.2. Chemical Analysis. The chemical compositions were confirmed by ICP-OES (Varian Vista RL) and SEM-EDX (scanning electron microscope LEO 1530 Gemini, Zeiss, Germany with energydispersive X-ray detector, Oxford Instruments, U.K.). Detailed results are given in Tables S1−S4 in the Supporting Information, SI. 2.3. Diffraction Methods. For RT powder X-ray diffraction (PXRD), a Huber G670 diffractometer (Guinier geometry with imaging-plate detector and integrated read-out system) with Cu-Kα1 radiation (Ge(111) monochromator, λ = 1.54051 Å) was used. Powder samples were fixed between two Mylar foils with vacuum grease and prepared on flat sample holders in a glovebox. Subsequent Rietveld analysis applying a fundamental parameter approach was performed with the program TOPAS.30 Preferred orientation was considered by spherical harmonics of the fourth order. For hightemperature powder diffraction (HT-PXRD), a similar Huber G670 diffractometer with Mo-Kα1 radiation (λ = 0.70930 Å) and a ceramic heating fork was used. Samples for HT-PXRD were sealed in silica glass capillaries under dry argon atmosphere and rotated during measurement. 2.4. Thermoelectric Characterization. Electrical conductivity σ and Seebeck coefficient S were measured simultaneously under static He atmosphere with a Linseis LSR-3 instrument. NiCr/Ni and Ni contacts were used in a bipolar setup (i.e., continuous reversal of the polarity of the thermocouples). Thermal diffusivity was measured under static He atmosphere with a Linseis LFA1000 laser flash setup equipped with an InSb detector. Heat loss and finite pulse corrections were performed applying Dusza’s model.31 At each temperature, the values from 5 measured points were averaged. Thermal conductivity κ was then calculated by multiplying the values with the Dulong-Petit heat capacity and the X-ray density obtained from PXRD lattice parameters as M · No · V−1 · NA−1, with M = molar mass, No = number of formula units per unit cell, V = unit cell volume, NA = Avogadro constant. The latter is similar to measured densities (see Table S5 in the SI). For GST materials, also lithium-containing ones, the DulongPetit approximation yielded values close to measured ones.6,24 The phononic part of thermal conductivity κph was calculated with a

it does not contain van der Waals gaps, but is simply a distorted variant of the NaCl type (the term “rhombohedral phase” should therefore be avoided to characterize certain modifications). The phononic part of the thermal conductivity of these Li-containing compounds is in the same range as that of GST materials with vacancies. This led to the assumption that Li atoms can be viewed as “pseudo-vacancies” that efficiently scatter phonons.24 Yet, the low atomic mass of Li and the relatively low bonding energy of its monovalent ion in tellurides may offer the additional possibility of atom mobility. This would render Li-substituted GST a new member of the intriguing class of PLEC (phonon−liquid electron−crystal) thermoelectric materials with very low thermal conductivity (κ) at high temperature.25−27 This concept can be seen as an extension of the PGEC (phonon−glass electron−crystal) concept, which focuses on disorder and “rattling” atoms.28 Lithium mobility should be more pronounced in compounds with a certain amount of vacancies. Thus, it seems promising to investigate compounds that lie between GST materials (GeTe)nSb2Te3 and vacancy-free solid solutions (GeTe)y(LiSbTe2)2. According to the simple principle “Ge2+ + vacancy = 2 Li+ ”, the vacancy concentration can easily be tuned.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Due to the exothermic reaction of Li and Te and the low molar mass of Li, LiGe was used as precursor. It was synthesized from the elements (Li, 99.9% from Alfa Aesar and Ge, 99.999% from Smart Elements or Haines & Maassen) in boron nitride or niobium crucibles sealed in silica ampules. The desired compounds (e.g., 2 g) were synthesized by heating stoichiometric mixtures of LiGe and the pure elements (Ge, see above; Sb, 99.999% from VEB Spurenelemente Freiberg; Te, 99.999% from Alfa Aesar) to 950 °C for 90 min in graphitized silica glass ampules under Ar atmosphere. The ampules were quenched at air, then annealed for 1 day at 550 °C and subsequently slowly cooled in the furnace (i.e., the furnace was switched off). Similar to polycrystalline ingots of the related 7971


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Table 1. Structure Parameters of Li2Ge3Sb2Te7, LiGe3.5Sb2Te7, Li2Ge11Sb2Te15, and LiGe11.5Sb2Te15 Measured at RT: Wyckoff Positions, Atomic Coordinates, Occupancy Factors, and Isotropic Displacement Parameters xyz

structure type

compound

atom

Wyckoff position

NaCl

Li2Ge3Sb2Te7 LiGe3.5Sb2Te7 Li2Ge3Sb2Te7 LiGe3.5Sb2Te7 Li2Ge11Sb2Te15 LiGe11.5Sb2Te15 Li2Ge11Sb2Te15 LiGe11.5Sb2Te15

Li/Ge/Sb

4a

000

Te

4b

1/2 1/2 1/2

Li/Ge/Sb

3a

000

Te

3a

0 0 0.5229(6) 0 0 0.5201(8)

GeTe

occupancies

Beq (in Å2)

0.29/0.43/0.29 0.15/0.54/0.31 1

2.32(9) 2.81(13) 0.28(5) 0.77(8) 1.27(13) 1.58(14) 0.88(7) 0.41(7)

0.13/0.73/0.13 0.07/0.79/0.14 1

Table 2. Details of the Rietveld Refinements for Li2Ge3Sb2Te7, LiGe3.5Sb2Te7, Li2Ge11Sb2Te15, and LiGe11.5Sb2Te15 at RT compound crystal system/space group formula units per unit cell reflections parameters/thereof background formula mass (in g mol−1) F(000) cell parameters (in Å) cell volume (in Å3) X-ray density (in gcm−3) wavelength (in Å) 2θ range (in °) Rp/Rwp RBragg goodness of fit

Li2Ge3Sb2Te7

Li1Ge3.5Sb2Te7

cubic/Fm3̅m (no. 225) 4/7 15 21/12 20/12 195.50 199.70 81.14 83.00 a = 6.0429(2) a = 6.0185(4) 220.67(2) 218.01(5) 5.88 6.08 1.540596 (Cu−Kα1) 20 ≤ 2θ ≤ 100 0.0135/0.0210 0.0200/0.0363 0.0496 0.0119 0.76 1.35

Lorenz number L as a function of S according to L = (1.5 + exp(−|S|/ 116 μVK−1) · 10 8 WΩK−2.32 Considering all measurement uncertainties (≤10% for S and σ, ∼5% for κ), the given zT values may exhibit an absolute uncertainty of up to 20%. 2.5. Solid State NMR. Static 7Li NMR spectra were recorded with a Bruker MSL 400 spectrometer connected to an Oxford cryomagnet with a nominal field of 9.4 T, corresponding to a 7Li Larmor frequency of 155.4 MHz. The measurements were performed with a commercial high-temperature NMR probe (Bruker) with the powdered samples being sealed in glass ampules. The measuring temperature was varied in the range from −25 °C to 350 °C. Heating and cooling was performed with a gas flow of adjusted temperature circulating around the ampule in the NMR coil. For all experiments, the saturation recovery sequence was used, followed by an adjustable waiting time and a single π/2 pulse (2 μs). For each spectrum, 16 scans were accumulated. The waiting time was set to 6 times the spin−lattice relaxation time T1 to obtain fully relaxed spectra. 7Li magic angle spinning (MAS) NMR spectra were measured with a Bruker AVANCE III spectrometer connected to a cryomagnet with a nominal field of 14.1 T using a commercial Bruker MAS probe with a spinning frequency of 25 kHz. The spectra were referenced against an aqueous solution of LiCl.

Li2Ge1̂ 1Sb2Te15

Li1Ge11.5Sb2Te15

trigonal/R3m (no. 160) 3/15 31 25/12 198.02 82.67 a = 4.2061(2), c = 10.5990(4) 162.39(2) 6.07

24/12 199.98 83.53 a = 4.2149(2), c = 10.5057(6) 161.64(2) 6.16

0.0122/0.0194 0.0514 0.68

0.0241/0.0421 0.0100 1.63

All powder patterns could be explained well by the structure models (Figure 1). Further refinement details and the structure parameters obtained are listed in Tables 1 and 2, respectively. Due to a high concentration of cation vacancies of 1/7 = 14.3% in hypothetical NaCl-type Ge4Sb2Te7, this compound rather adopts a 39R layered structure (space group R3̅m, a = 4.207(1), c = 72.79(1) Å) with pronounced vacancy ordering.33,34 In lithium-containing compounds derived from Ge4Sb2Te7, the vacancy concentration is lower, which leads to more favorable coordination polyhedra in a NaCl-type structure, which accordingly is stabilized.24 Yet the diffraction pattern of LiGe3.5Sb2Te7 shows reflection broadening and small shoulders at the reflections which may be due to a partial phase transition. In contrast to Ge4Sb2Te7, hypothetical NaCltype Ge12Sb2Te15 has a vacancy concentration of only 1/15 = 6.67% and thus forms no extended vacancy layers. However, due to the high GeTe content, it adopts the GeTe-type structure with randomly distributed vacancies, which is maintained for the lithium-containing derivatives. For the NaCl-type structure, lengths and strengths are equal for all bonds. The lattice parameter increases isotropically upon lithium incorporation in line with cubic symmetry. In contrast, the lattice parameters of the GeTe type increase anisotropically. The structure predominantly expands along [001] when more lithium is present and the number of vacancies is thus reduced. In Li2−xGe11+x/2Sb2Te15 (x = 0, 1, 2) compounds, the lattice distortion represented by the c/a ratio, which equals 2.449 in the undistorted cubic NaCl-type, increases from 2.468 for x = 2 (i.e., Ge12Sb2Te15)14 via 2.492 for x = 1 to 2.5199 for x = 0. 3.2. High-Temperature Powder Diffraction. The interplay of vacancies and GeTe content strongly influences the

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. According to previous results from combined X-ray and neutron scattering data for Li-containing GST compounds (GeTe)y(LiSbTe2)2 without vacancies,24 both NaCl-type and GeTe-type phases exhibit random Ge/ Li/Sb disorder on the cation site. This was also assumed in the Rietveld refinements for compounds with additional vacancies. Individual displacement parameters were refined for each Wyckoff sites; thus, all cations were constrained to have the same displacement. Site occupancies were fixed according to the sum formulas corresponding to the nominal compositions. 7972


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Figure 2. Temperature-dependent PXRD (heating and subsequent cooling from bottom to top) for Li2Ge3Sb2Te7, Li1Ge3.5Sb2Te7, Li2Ge11Sb2Te15, and LiGe11.5Sb2Te15; phase transition temperatures are marked with horizontal lines and given in Table 3 (reflections from the furnace are marked with an asterisk; the powder diffraction pattern of Li2Ge11Sb2Te15 is the same as shown in ref 24).

Table 3. Phase Transition Temperatures from a Layered Low Temperature Phase to a NaCl-Type HT Phase (during Heating) compounds derived from Ge4Sb2Te7

transition temperature

compounds derived from Ge12Sb2Te15

transition temperature

Ge4Sb2Te710 LiGe3.5Sb2Te7 Li2Ge3Sb2Te7

490 °C distortions between 200 and 400 °C small distortion at 380 °C

Ge12Sb2Te156 LiGe11.5Sb2Te15 Li2Ge11Sb2Te15

475−500 °C 230 °C 210 °C

GeTe-rich compounds show a different behavior. Both vacancy-free Li2Ge11Sb2Te15 and vacancy-containing LiGe11.5Sb2Te15 show a phase transition at 210 and 230 °C, respectively. This transition is similar to those of Ge12Sb2Te15 and pure GeTe; it takes place between a GeTe-type RT phase and a NaCl-type HT phase and typically does not involve a significant extent of vacancy ordering. The coordination spheres change from a 3 + 3 fold coordination in a severely distorted octahedron of the GeTe type to regular octahedral coordination in the NaCl type. In general, the NaCl-type HT phases with high vacancy concentrations exhibit more unfavorable incomplete coordination polyhedra. Hence, the phase transition toward the cubic HT phase takes place at lower temperatures for compounds with fewer vacancies (Table 3). In addition, the formation of a NaCl-type phase becomes more favorable due to the higher ionicity of the Li-containing materials. 3.3. Thermoelectric Characterization. The thermoelectric properties of pristine GST materials in comparison with Li-containing ones, both with and without remaining vacancies (Figure 3) show a good cycling stability over 3 heating cycles up to 450 °C (Figures S4−S8 in the SI). Small hystereses are attributed to different temperatures (under-

thermal behavior of the compounds discussed in this contribution as shown by temperature-dependent PXRD (Figure 2). Vacancy-free, GeTe-poor Li2Ge3Sb2Te7 adopts a NaCl-type structure over the whole temperature range from RT to 600 °C. It is a thermodynamically stable compound up to temperatures where decomposition by Te evaporation or melting takes place. However, the thermoelectric properties (section 3.3) indicate a slightly hysteretic phase transition. Rietveld refinements assuming a rhombohedral unit cell indicate slight changes of the c/a ratio at the corresponding temperatures (cf. Figure S2 in the SI). In earlier studies, neutron powder diffraction suggested that a small amount of Ge occupies tetrahedral voids at RT.24 Thus, the fraction of cations in tetrahedral voids might slightly change with temperature. Vacancy-containing GeTe-poor LiGe3.5Sb2Te7 exhibits a similar thermal behavior. It also seems to adopt a NaCl-type structure over the whole temperature range. However, evaluating the c/a ratio of a hypothetical rhombohedral unit cell indicates a somewhat more significant deviations from the ideal c/a ratio between 200 and 400 °C (cf. Figure S3 in the SI). Thus, the additional vacancies in LiGe3.5Sb2Te7 compared to Li2Ge3Sb2Te7 induce a more pronounced distortion of the cubic unit cell. 7973


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Figure 3. Thermoelectric properties of Li-containing variants derived from Ge4Sb2Te7 (left) and Ge12Sb2Te15 (right); for Ge12Sb2Te15, the values from ref 35 were used; the curves were averaged from reproducible heating and cooling cycles, discarding the first cycle.

cooling) for phase transitions or element ordering during heating and cooling, respectively. PXRD patterns of the samples before and after thermoelectric measurements are not significantly different (Figure S1 in the SI). For the discussion here, the values of the transport properties were averaged from reproducible heating and cooling cycles, discarding the first cycle, since this can be significantly different due to relaxation of strain and coarsening effects.6,21,35 From the measured transport properties σ, S, and κ, derived quantities such as the phononic part of thermal conductivity κph, quality factor B (eq 1) and figure of merit zT = S2σT/κ were calculated. The zT value for an optimal carrier concentration (zTopt) was estimated applying an effective mass model (details of the calculations are described in the SI and can also be found in the references):36−41 2 i k y σE B = jjjj B zzzz 0 T k e { κph

This is due to more favorable intrinsic properties of Ge12Sb2Te15 (B = 0.62 compared to B = 0.29 for Ge4Sb2Te7, cf. Table 4 and Figures S9−S14 in the SI) and an already optimal electron chemical potential in Ge12Sb2Te15 (zTopt = zTmeasured). Table 4. Calculated Thermoelectric Properties of LiContaining Variants Derived from Ge4Sb2Te7 and Ge12Sb2Te15 at 450 °C Ge4Sb2Te7 LiGe3.5Sb2Te7 Li2Ge3Sb2Te7 Ge12Sb2Te15 LiGe11.5Sb2Te15 Li2Ge11Sb2Te15

(1)

S (η) (kB / e)2 Bln(1 + e η)

+ L (η)

B

zTopt

zTmeasured

0.80 0.54 0.51 0.98 0.90 0.65

0.29 1.01 0.68 0.62 0.66 0.92

0.80 1.91 1.48 1.39 1.46 1.80

0.52 1.90 1.48 1.39 1.45 1.45

3.3.1. Li2−xGe11+x/2Sb2Te15. With increasing lithium content (i.e., decreasing x) in Li2−xGe11+x/2Sb2Te15 (x = 0, 1, 2), κph decreases and B increases. For LiGe11.5Sb2Te15, the optimal zT value of 1.46 is reached. For Li2Ge12Sb2Te15, the electron chemical potential η is shifted away from an optimum and thus zTopt is not realized, but could probably be obtained via further doping. The vacancy concentration cannot be directly correlated with thermoelectric properties since the change of the Li/Ge ratio probably constitutes the dominant effect on the band structure, and the carrier concentration. However, the decreased phase transition temperature for LiGe11.5Sb2Te15 compared to Ge12Sb2Te15 leads to a shift of local maxima to lower temperatures in the measurements of σ and S as a function of temperature (Figure 3). 3.3.2. Li2−xGe3+x/2Sb2Te7. Both lithium-containing derivatives (x = 0, 1) exhibit decreased σ and κ and enhanced S

2

zT (η) =

κph

(2)

This approach allows one to separate the intrinsic material properties that are independent of doping, i.e., the intrinsic electrical conductivity σE0 and κphcombined in the material quality factor B (eq 1)from those depending on optimal doping (i.e., on the electrons’ chemical potential η, (eq 2)). The thermoelectric properties of pristine GST materials (GeTe)nSb2Te3 strongly depend on their GeTe content n. Whereas a high GeTe content (e.g., Ge12Sb2Te15) is favorable for high zT values at elevated temperatures, low GeTe contents (e.g., Ge4Sb2Te7) usually exhibit poor thermoelectric properties, which can be improved via Mn or Sn substitution.10,12 7974


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Chemistry of Materials compared to the parent compound Ge4Sb2Te7 (x = 2). This is in line with the reported values σ = 0.04 S cm−1 and S = 465 μV K−1 for LiSbTe2 at RT.42 With increasing lithium content (i.e., decreasing x) in Li2−xGe3+x/2Sb2Te7 (x = 0, 1, 2), κph decreases in the temperature range from 250 °C to 450 °C. Both lithium-containing compounds reach the maximal possible zTopt value and are thus optimally doped. However, B is higher for LiGe3.5Sb2Te7 at 450 °C (possibly due to a higher intrinsic σE0). Thus, a zT value of 1.9 is obtained. Furthermore, the phase transitions involve changes of slopes for Li2Ge3Sb2Te7 (best visible for S) and LiGe3.5Sb2Te7 (mostly visible for zT). In both series of compounds, κph decreases with increasing lithium content. This can be due to alloy scattering or enhanced scattering at possible mobile lithium atoms in case of matching hopping time scales,43 which would render Li containing GST a new phonon-liquid electron-crystal (PLEC) material.25−27 For thermoelectric applications, not only a high peak zT value (zTmax) is desired, but also good thermoelectric performance over a wide temperature range. This becomes evident from eq 3 for the overall thermoelectric efficiency of a thermoelectric module ηTE:44,45 ηTE =

1 + zTavg − 1 TH − TC · T TH 1 + zTavg + TC

H

(3)

with TH and TC being the hot and cold side temperature and zTavg the average zT value in the applied temperature gradient. Figure 4 shows the average zT values in an intermediate

Figure 5. Temperature dependent 7Li NMR spectra (νL = 155.53 MHz) measured under static conditions at an external magnetic flux density of 9.4 T.

Figure 4. Average zT values in the temperature range from 75 °C to 450 °C.

Figure 6. 7Li NMR line widths vs temperature: motional narrowing.

temperature range between 75 and 450 °C. LiGe3.5Sb2Te7 and Li2Ge3Sb2Te7 not only exhibit high zTmax values but also zTavg values above unity, making them interesting candidates for application. 3.4. Solid-State NMR. In order to investigate possible Li mobility for LiGe3.5Sb2Te7 and Li2Ge3Sb2Te7, 7Li solid state NMR was performed. Figure 5 shows the central transitions of the static 7Li NMR spectra, each one recorded at a different temperature. The NMR lines of both samples exhibit motional narrowing. At low temperatures, the central transition is broadened due to static homonuclear dipolar interaction between the 7Li spins and heteronuclear dipolar interaction between the 7Li and the 123Te/125Te nuclei. At higher temperatures, as the Li spins become mobile on the time scale of their interaction (jump rate > rigid lattice line width), then the interaction is averaged and thus the line narrows. Figure 6 shows the change of the full width at half-maximum ( f whm) of the NMR lines with temperature. For Li2Ge3Sb2Te7, the compound without cation vacancies,

motional narrowing sets in at 75 °C while this is the case already at 25 °C for LiGe3.5Sb2Te7, i.e., the compound with vacancies in the Li substructure. At the respective onset temperature, according to the condition above, the average Li jump rate exceeds 1000 s−1. This value would correspond to a translational Li diffusion coeffient of the order of 10−17 m2s−1 and a Li ion conductivity of the order of 10−7 Sm−1, estimated using the Einstein−Smoluchowski and the Nernst−Einstein relation, respectively.46 The Li ions are more mobile in the vacancy-containing compound, which goes along with the expectations. For both compounds, motional narrowing occurs in a relatively wide temperature range, especially for LiGe3.5Sb2Te7. Even at 350 °C, the narrowing is not complete. This is often observed in disordered materials where the Li spins reside in different environments with different energy barriers for the jump process.47−49 Then, the Li atoms residing on individual sites become mobile at different temperatures. 7975



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Although both compounds are crystalline, the cation substructure is indeed disordered.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03609. Elemental analyses (ICP-OES and SEM-EDX), resulting weights and densities for samples used for thermoelectric measurements, PXRD before and after thermoelectric measurements, temperature dependent evolution of lattice parameters for Li2 Ge 3 Sb 2 Te 7 and LiGe3.5Sb2Te7, three measurement cycles of thermoelectric properties for all compositions, information concerning the modeling of thermoelectric data, and calculated zT(η) vs electron chemical potential η at 450 °C for all compositions investigated (PDF)

■

Figure 7. 7Li MAS NMR spectra recorded at B0 = 14.1 T and a spinning frequency of 25 kHz and νL = 233.30 MHz.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paul Heitjans: 0000-0003-1563-9176 Oliver Oeckler: 0000-0003-0149-7066

In the 7Li MAS NMR spectra (νrot = 25 kHz) of both compounds recorded at 14.1 T (Figure 7) the lines are relatively broad ( fwhm = 2.63 ppm for LiGe3.5Sb2Te7 and 2.13 ppm for Li2Ge3Sb2Te7) although the rotation rate is much faster than the dipolar interaction rate. Therefore, in this case, the fwhm reflects the ensemble of 7Li spins residing in different environments due to disorder which isas expectedslightly more pronounced in the case of LiGe3.5Sb2Te7.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Dr. Holger Kohlmann (Leipzig University for allowing the use of his glovebox and Jaroslava Obel (LMU Munich) for ICP-OES analyses. A PhD scholarship for S.S. from the Studienstiftung des deutschen Volkes is gratefully acknowledged.

4. CONCLUSIONS

■ ■

According to the simple principle “Ge2+ + vacancy = 2 Li+”, the vacancy concentration in lithium germanium antimony tellurides can be adjusted. Together with the GeTe content, the vacancy concentration acts as guiding principle to predetermine the structure that is formed at low temperature. All phases discussed in this contribution have a NaCl-type HT phase in common, whose existence range can be shifted toward lower temperatures by fewer vacancies or more lithium, respectively. However, as much as it is desirable for semiconducting materials in general to tune the vacancy concentration as single variable, it always correlates with further parameters, like composition, temperature, or synthesis conditions. Thus, the thermoelectric properties can be better understood with respect to the Li/Ge ratio than just based on the vacancy concentration. While the change in electron chemical potential might be favorable in the case of Ge4Sb2Te7 → Li2Ge3Sb2Te7 or disadvantageous in the case of Ge12Sb2Te15 → Li2Ge11Sb2Te15, the phononic part of thermal conductivity (κph) of all compounds’ cubic HT-phases decreases with increasing lithium content. This might be related to the mass difference between lithium and germanium on the one hand, but, in addition, also on lithium mobility, which could be detected by 7Li solid state NMR. Yet, the ionic conductivity is much smaller than the electronic conductivity, which is favorable for thermoelectric materials: there is almost no bipolar transport and no pronounced tendency toward solidstate electrolysis

DEDICATION This work is dedicated to Professor Bernt Krebs on the occasion of his 80th birthday. REFERENCES

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Tuning the Vacancy Concentration in Lithium Germanium Antimony

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