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Thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7: A new silicide Zintl phase with the Zr2Fe12P7 structure type Kurt Silsby, Fan Sui, Xiaowei Ma, Susan M. Kauzlarich, and Susan E Latturner Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02784 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015
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Thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7: A new silicide Zintl phase with the Zr2Fe12P7 structure type Kurt Silsbya, Fan Suib, Xiaowei Maa, Susan M. Kauzlarichb, Susan E. Latturner*a a
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32308
b
Department of Chemistry, University of California, Davis, CA 95616
Abstract: Ba1.9Ca2.4Mg9.7Si7 was grown as large crystals from the reaction of silicon with barium and calcium in Mg/Al flux. This compound is a charge-balanced Zintl phase with the Zr2Fe12P7 structure type in hexagonal space group P-6 (a = 11.196(2) Å, c = 4.4595(9) Å); barium occupies the Zr sites, the lighter alkaline earths (Mg, Ca) mix on the Fe sites, and silicon occupies the phosphorus sites. An isostructural germanide (Ba1.2Sr0.9Mg11.9Ge7, a = 11.064(1) Å, c = 4.3709(6) Å)) was similarly synthesized from reactions of germanium with barium and strontium in Mg/Al flux. Density of states calculations indicate these phases are semimetals, in agreement with their charge-balanced nature. Measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient were carried out from 300 – 1000 K to investigate the thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7. This compound is an n-type semimetal with low thermal conductivity (2 W/m·K), moderate Seebeck coefficient (-200 µV/K at 900 K) and a thermoelectric figure of merit ZT of 0.35 at 900 K.
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Introduction Materials that demonstrate the thermoelectric effect exhibit an electric potential when a temperature gradient is applied across them, or conversely produce a temperature gradient when an electric potential is applied. Thermoelectric materials with high efficiency at elevated temperatures are of great interest as power sources (for instance, harnessing waste heat from the engine to power automobile electronics or using the heat released by the decay of a radioactive source module to power deep space probes such as New Horizons).1-3 The thermoelectric behavior of a material depends on its electrical resistivity ρ, thermal conductivity κ, and Seebeck coefficient S. The overall thermoelectric figure of merit ZT is temperature dependent, and is optimized for compounds with high Seebeck coefficients, low electrical resistivity, and low thermal conductivity (ZT = [S2/ρ·κ]T). Optimization is difficult given that the three parameters ρ, κ, and S are interrelated and all dependent on number of carriers; highest ZT values are found for small band gap semiconductors or semimetals with complex structures containing heavy atoms.1,2 Workhorse thermoelectric materials include doped Bi2Te3 phases for low temperature applications such as Peltier coolers, and SiGe alloys or PbTe solid solutions for high temperature uses such as power generation from waste heat.4 In addition to their electronic properties, another factor that must be taken into account for potential widespread use of thermoelectric materials is the relative abundance and toxicity of their constituent elements. 5 Unfortunately, many of the benchmark compounds contain tellurium, bismuth (both low-abundant), or lead (toxic). Zintl phases based on tetrelides such as silicon, germanium, or tin are showing promise as potential alternatives to tellurides. Zintl phases are salt-like compounds formed when highly electropositive metals (alkali, alkaline earth, Eu, Yb) react with main group metalloids. The metalloids accept electrons and may also form M-M bonds to fill their valence shells; the resulting anions are charge-balanced by the surrounding metal cations.6 These compounds are therefore typically small band gap semiconductors. In recent years, such materials have been investigated as potential thermoelectric materials (Ba8-ySryAl14Si32, Ba8Ga16Ge30 and SrMgSi) and as magnetoresistive phases (EuMgSn).7-11 Magnesium silicide (Mg2Si) has been of particular interest as a possible high temperature thermoelectric material. While its thermoelectric figure of merit ZT is only about 0.15 at 700 K, partial substitution of tin onto the silicon site modifies the electronic 2 ACS Paragon Plus Environment
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properties and lowers thermal conductivity sufficiently to result in an impressive ZT of 1.2 at 700 K for Mg2Si0.4Sn0.6.12-14 In our exploration of the use of magnesium-based fluxes as growth media for complex intermetallics, we have synthesized a new silicide Zintl phase, Ba1.9Ca2.4Mg9.7Si7, and an isostructural germanide, Ba1.2Sr0.9Mg11.9Ge7. While magnesium has a relatively high melting point, considerable vapor pressure, and is corrosive toward common crucible materials, these factors can be mitigated by using a mixed Mg/Al flux in sealed steel ampoules. A 50:50 Mg/Al mixture melts at 450 °C.15 This mixture is an excellent solvent for most main group elements and all alkaline earth and rare earth metals. We have grown a wide variety of complex aluminide intermetallics in this flux mixture, including R3Fe(Al/Mg)4Tt2 and R5Mg5Fe4Al12Si6 (R = rare earth metal).16,17 The aluminum in the Mg/Al flux is much less reactive if heavy alkaline earth metals (A = Ca, Sr, Ba) are used as reactants instead of rare earth elements. (Mg/Al)/A/Tt reactions yield Zintl phases such as CaMgSi18 and the title compound. Ba1.9Ca2.4Mg9.7Si7 and Ba1.2Sr0.9Mg11.9Ge7 form in the Zr2Fe12P7 structure type; the Zr and Fe sites are occupied by alkaline earth cations, and the phosphide sites by tetrelide anions, leading to an overall charge-balanced stoichiometry. Mixing of alkaline earth cations is observed on several sites. The complex structure and site mixing leads to low thermal conductivity; measurements of the resistivity, thermal conductivity, and Seebeck coefficient of Ba1.9Ca2.4Mg9.7Si7 indicate this compound may have potential as a thermoelectric material. Since Zr2Fe12P7 is a member of a family of pnictides that form a homologous series of complex hexagonal structures,19,20 the formation of silicides with this structure type indicates that additional A/Mg/Tt Zintl phases with increasing structural complexity may be isolable.
Experimental Procedure Synthesis. Reactants were used as received: Mg and Al metal slugs (99.95 % and 99.99 % respectively) from Alfa Aesar; Ba rod (99+ %) and Ca shot (99.5%) from Alfa Aesar; Si (99+ %) and Ge (99.999 %) powders and Sr (99 %) chunks from Strem Chemicals. The elements were initially weighed out in Mg/Al/Tt/Ba/A (Tt = Si or Ge, A = Sr or Ca) ratios of 15:15:4:1:1 mmol 3 ACS Paragon Plus Environment
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and loaded into stainless steel crucibles in an argon-filled glove box. The crucibles were then sealed shut by arc welding under argon, then enclosed in evacuated (30 mTorr) fused silica tubes. The samples were then heated from room temperature to 950 °C in 10 hours, held at 950 °C for 10 hours, cooled to 750 °C in 80 hours, and held at 750 °C for 24 hours. The reaction ampoules were then removed from the furnace, flipped, and centrifuged to separate the remaining Mg/Al flux from the crystals formed in the reaction. Other reaction ratios were used to determine if a better yield could be achieved. The yield of Ba1.9Ca2.4Mg9.7Si7 was optimized with a Mg/Al/Si/Ba/Ca reaction ratio of 15:15:4:1:2 mmol; the germanide Ba1.2Sr0.9Mg11.9Ge7 was optimized at a Mg/Al/Ge/Ba/Sr reaction ratio of 15:15:4:1:1 mmol. Elemental Analysis. SEM-EDS analysis was performed using a JEOL 5900 scanning electron microscope equipped with PGT Prism energy dispersion spectroscopy software. A 30 kV acceleration voltage was used. Selected crystals were arranged on double-sided carbon tape adhered to an aluminum sample puck. Each crystal was cleaved to expose inner portions to avoid erroneous readings due to traces of residual flux observable on the surface. Several spots on each crystal were analyzed for 60 s at each location. Analysis of Ba1.9Ca2.4Mg9.7Si7 samples indicated an average ratio of 13% Ba, 11% Ca, 39% Mg, 34% Si, 3% Al; Ba1.2Sr0.9Mg11.9Ge7 samples averaged 5% Ba, 3% Sr, 46% Mg, 39% Ge, 5% Al. No incorporation of metals from the stainless steel crucible was observed. Small amounts of aluminum (3-5%) are consistently seen due to the aluminum sample holder and traces of Mg/Al flux inclusions in the samples. Crystallographic studies After elemental analysis, small pieces were cleaved from larger crystals and mounted on glass fibers using epoxy. Single-crystal X-ray diffraction data were collected at 293 K in a stream of nitrogen using a Bruker APEX 2 CCD diffractometer with a Mo Kα radiation source. Absorption corrections were applied to the datasets using the SADABS program.21 Refinement of the structures was performed using the SHELXTL package.22 The structures were solved in hexagonal space group P-6; collection and refinement parameters for both phases are listed in Table 1. Assignment of atomic positions was straightforward for the heavy alkaline earth sites (Ba,Sr). For the refinement of the Ba1.9Ca2.4Mg9.7Si7 structure, the positions of magnesium and silicon were determined by comparison with the previously reported analog compound 4 ACS Paragon Plus Environment
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(Sr2.2Mg11.8Ge7)23 and by consideration of cation and anion packing and bond lengths. Calcium incorporation was confirmed by SEM-EDS, and this element can possibly substitute on either Mg sites or heavy AE sites. To explore this, the occupancy of each site was investigated; the Mg4 site was found to be well over 100% occupied, and the Ba1 and Ba2 sites were slightly less than fully occupied (97% occupancy). These three sites were therefore refined as mixed occupied with calcium, with resulting proportions listed in Table 2. Powder X-ray diffraction data were collected on powder samples which were ground from bulk samples of flux-grown products using a PANalytical X’Pert PRO with a Cu Kα radiation source. The resulting pattern was compared to that calculated based on the single crystal structure. Further details can be found in the CIF files in Supporting Information, along with information on the germanide analog Ba1.2Sr0.9Mg11.9Ge7.
Table 1. Crystallographic data collection parameters for the Zr2Fe12P7-type phases. Ba1.9Ca2.4Mg9.7Si7
Ba1.2Sr0.9Mg11.9Ge7
Formula weight
789.58
1040.96
Crystal System
Hexagonal
Hexagonal
P-6
P-6
a (Å)
11.196(2)
11.064(1)
c (Å)
4.4595(9)
4.3709(6)
1
1
484.1(1)
463.3(1)
2.709
3.731
-14 ≤ h ≤ 14, -14 ≤ k ≤ 14,
-14 ≤ h ≤ 14, -14 ≤ k ≤ 13,
-5 ≤ l ≤ 5
-5 ≤ l ≤ 5
5248
5300
848 / 48
834 / 48
5.206
16.586
0.0134 / 00356
0.0100 / 0.0237
Space group
Z Volume (Å3) 3
Density, calc (g/cm ) Index ranges
Reflections collected Unique data/parameters µ (mm-1) R1/wR2
a -
-3
Residual peak/hole (e A ) a
0.88 / -1.27 2
0.45 / -0.51
2 2
2 2
R1 = Σ||Fo|-|Fc||/Σ|Fo|; wR2 = [Σ[w(Fo - Fc ) ]/Σ[w(Fo ) ]]
1/2
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Table 2. Atomic coordinates and isotropic thermal parameters for Ba1.9Ca2.4Mg9.7Si7. Atom
Wyckoff Site
x
y
z
Occ
Ueqa
Ba(1)/Ca(1)
1c
1/3
2/3
0
0.960(4) / 0.040(4)
0.0106(1)
Ba(2)/Ca(2)
1f
2/3
1/3
½
0.965(3) / 0.035(3)
0.0107(1)
Si(1)
3j
0.4159(1)
0.3067(1)
0
1
0.0084(2)
Si(2)
3k
0.1186(1)
0.4067(1)
½
1
0.0089(2)
Si(3)
1a
0
0
0
1
0.0086(3)
Mg(1)
3j
0.4261(1)
0.0564(1)
0
1
0.0112(2)
Mg(2)
3j
0.1591(1)
0.2812(1)
0
1
0.0099(2)
Mg(3)
3k
0.3924(1)
0.4387(1)
½
1
0.0108(2)
Ca(4)/Mg(4)
3k
0.21754(8)
0.10468(8)
½
0.765(9) / 0.235(9)
0.0111(2)
a
Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
Electronic structure calculations Density of states (DOS) calculations were carried out using the TB-LMTO-ASA technique with the Stuttgart TB-LMTO 4.7 software package.24,25 The structural model used for Ba1.9Ca2.4Mg9.7Si7 is based on the unit cell dimensions and atomic coordinates derived from single crystal diffraction data. However, full occupancy of the 1c and 1f sites by barium was assumed, and the Ca(4)/Mg(4) site was modeled as fully occupied by calcium. The stoichiometry of the resulting ordered model is Ba2Ca3Mg9Si7. The germanide Ba1.2Sr0.83Mg11.97Ge7 contains Ba/Sr mixing on the barium sites, so calculations on the two ordered models Ba2Mg12Ge7 and Sr2Mg12Ge7 were carried out for comparison. Empty spheres were added by the program where appropriate to fill the unit cell volume. A 12×12×12 κ-point mesh was used and integrated using the tetrahedron method. The basis sets consisted of 6s/5d/4f for Ba, 5s/5p/4d for Sr, 4s/4p/3d for Ca, 4s/4p for Ge, 3s/3p for Mg, and 3s/3p for Si. Thermal studies Thermal stability analysis was carried out on Ba1.9Ca2.4Mg9.7Si7 to determine appropriate temperatures for further processing. Data were collected on a TA Instruments Q600 system, which allows for simultaneous analysis of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Crystals of Ba1.9Ca2.4Mg9.7Si7 were ground into powder which was pressed into the bottom of an alumina sample cup in the instrument; an empty 6 ACS Paragon Plus Environment
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alumina sample cup was used as a DSC reference. A flow of argon gas at 100 mL/min was used to prevent oxidation of the sample during heating. The sample and reference were heated in two cycles; the first cycle heated at 10°C/min to 800 °C and then cooled at 5°C/min to 50°C, and the second cycle heated at 10°C/min to 1000 °C and cooled at 5°C/min to 50°C. Transport measurements Low temperature measurements of electrical resistivity were carried out on a single crystal of Ba1.9Ca2.4Mg9.7Si7 using a Quantum Design PPMS system from 2 K to 300 K. Platinum wires were affixed to the crystal using silver paste. Identical measurements were carried out on a bar cut from a spark plasma sintered pellet (see below) of the same material for comparison. Five-probe Hall measurements were performed on the this bar-shaped sample using the PPMS at 273 K, scanning the applied magnetic field from 70000 Oe to -70000 Oe. The Hall coefficient was determined from the average value of the measured Hall coefficients at these two opposite magnetic field directions for accuracy. Thermoelectric measurements Samples of Ba1.9Ca2.4Mg9.7Si7 for thermoelectric measurements were prepared by ball milling 1.13 grams of crystals into fine powder and then pressing 12.7 mm diameter pellets with a high-density graphite die (POCO) under vacuum ( 1, a value of around 0.4 for an undoped compound indicates that Ba1.9Ca2.4Mg9.7Si7 shows promise as a potential high temperature thermoelectric material. Conclusions Tetrel elements react with alkaline earth metals in Mg/Al flux to form a variety of complex A/Mg/Tt Zintl phases which grow as large crystals. Many of the resulting products are complex semimetals or semiconductors and may be of interest as thermoelectric materials, particularly since they do not contain elements that are scarce or toxic. The title compound Ba1.9Ca2.4Mg9.7Si7 has low thermal conductivity and a high Seebeck coefficient, and a complex crystal structure which is amenable to extensive substitution and doping on several sites. Further isovalent substitution may result in even lower thermal conductivity; it is able to incorporate all of the heavy alkaline earth elements, and Eu2+ substitution is also feasible. The isolation of silicide and germanide variants indicates that formation of solid solutions such as A2+xMg12-xSi7yGey,
as well as incorporation of small amounts of tin on the tetrelide sites, might also be
possible. Ba1.9Ca2.4Mg9.7Si7 is at or slightly above the optimal resistivity range for thermoelectric materials; the majority of high ZT materials have resistivities in the 1 – 10 µΩ·m range.5 Carrier concentration (and thus the electrical resistivity and Seebeck effect) could be controlled by careful aliovalent substitution (such as P for Si or Li for Mg). It is possible that some aliovalent substitution is already present in these flux-grown samples from incorporation of traces of aluminum from the flux; reactions without Al are being explored. Further complexity in the A/Mg/Tt system may be achievable, given the observed compositional flexibility of both the A2Mg12Tt7 and A5+xMg18-xTt13 phases.23, 26-28 Tetrelide Zintl phase analogs of other members of the Rn(n-1)T(n+1)(n+2)Pn(n+1)+1 series should also be semimetals with good thermoelectric properties. Isolation of hypothetical products such as A6Mg20Tt13 20 ACS Paragon Plus Environment
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(Zr6Ni20P13-type) or A12Mg30Tt21 (La12Rh30P21-type) will require careful control of reactant ratios and activities. This is difficult to achieve in metal flux chemistry, so different synthetic methods (such as spark plasma sintering of stoichiometric mixtures under pressure, to minimize volatility of alkaline earth metals; or use of metal silicide reactants for arc-melting reactions) may be more fruitful.
Acknowledgements This research was supported by funding from the National Science Foundation (Division of Materials Research) through grant numbers DMR-11-06150 and DMR-14-10214, and by the FSU Department of Chemistry and Biochemistry (S.E.L); and by funding from DMR-14-05973 (S.M.K.). This research made use of the scanning electron microscope facilities of the FSU Physics Department.
Supporting Information Differential scanning calorimetry data for Ba1.9Ca2.4Mg9.7Si7; band structure calculated for Ba2Ca3Mg9Si7; Hall coefficient measurements for Ba1.9Ca2.4Mg9.7Si7 at 273 K; temperature dependence of ZT calculated using resistivity data from the 6.5 kN SPS pellet; crystallographic data for Ba1.9Ca2.4Mg9.7Si7 and Ba1.2Sr0.9Mg11.9Ge7 as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.
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