Influence of Thermally Activated Solid-State Crystal ... - ACS Publications

Article pubs.acs.org/cm

Influence of Thermally Activated Solid-State Crystal-to-Crystal Structural Transformation on the Thermoelectric Properties of the Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) System Gnu Nam,† Woongjin Choi,† Hongil Jo,‡ Kang Min Ok,‡ Kyunghan Ahn,*,§ and Tae-Soo You*,† †

Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea § School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The solid-solution Zintl compounds with the mixed cations of Ca2+and Yb2+ in the Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba5Al2Bi6-type and Ca5Ga2Sb6-type phases have been characterized for seven compounds with 2.5 ≤ x ≤ 5.0 and three compounds with 1.0 ≤ x ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al2Sb8] double chains and isolated space-filling Ca2+/Yb2+ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca1.5Yb3.5Al2Sb6 from the Ba5Al2Bi6-type to the Ca5Ga2Sb6-type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al2Sb8] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba5Al2Bi6-type and Ca5Ga2Sb6-type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca5Ga2Sb6-type phase should mainly be attributed to an extended bond distance of the bridging Sb−Sb in the [Al2Sb8] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca1.5Yb3.5Al2Sb6.

■

INTRODUCTION Thermoelectric (TE) materials and the devices based on these materials have been considered as one of the smart approaches to reduce global energy consumption by recovering the wasted heat from various heat sources and converting it into electricity.1−4 The performance of TE materials is represented by the TE figure of merit ZT, expressed by σS2T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity.5 High σ and S as well as low κ are necessary to maximize ZT, but this is quite elusive to achieve because of the interdependence of these three parameters σ, S, and κ. Thus, the decoupling of these parameters is critical for improving ZT, and there are two © 2017 American Chemical Society

main research directions to maximize this value for TE materials: (1) substantially low lattice thermal conductivities κlatt can be successfully achieved by nanostructuring,6−9 hierarchical architecturing,10−12 and searching for TE materials having an intrinsically low κlatt,13−15 and (2) the power factor (PF = σS2) can be significantly enhanced by electronic band structure engineering, including resonance level effects and band convergence.16−18 Among various candidate materials for TE applications, the Zintl phase is a relatively novel material Received: December 14, 2016 Revised: January 10, 2017 Published: January 10, 2017 1384

DOI: 10.1021/acs.chemmater.6b05281 Chem. Mater. 2017, 29, 1384−1395

Article

Chemistry of Materials Table 1. SXRD Data and Structure Refinement Results for the Ca5−xYbxAl2Sb6 (1.40(2) ≤ x ≤ 3.45(1)) System empirical formula structure type crystal system space group unit cell dimensions (Å)

volume (Å3) dcalcd (g/cm3) data/restraints/parameters R indicesa (I > 2σ(I)) R indicesa (all data) goodness of fit on F2 largest diff. peak/hole (e/Å3)

Ca3.60(2)Yb1.40Al2Sb6 Ca5Ga2Sb6 orthorhombic Pbam (No. 55) a = 11.962(6) b = 13.929(8) c = 4.427(2) 737.50(70) 5.273 922/0/44 R1 = 0.0321 wR2 = 0.0752 R1 = 0.0454 wR2 = 0.0824 1.134 1.672/−1.568

Ca2.78(2)Yb2.22Al2Sb6 Ca5Ga2Sb6 orthorhombic Pbam (No. 55) a = 12.032(1) b = 14.023(1) c = 4.453(1) 751.37(3) 5.655 972/0/45 R1 = 0.0214 wR2 = 0.0536 R1 = 0.0225 wR2 = 0.0541 1.218 1.467/−1.110

Ca1.58(2)Yb3.42Al2Sb6 Ba5Al2Bi6 orthorhombic Pbam (No. 55) a = 7.325(1) b = 22.904(2) c = 4.409(1) 739.82(11) 6.459 1040/0/44 R1 = 0.0300 wR2 = 0.0624 R1 = 0.0327 wR2 = 0.0643 1.212 2.813/−2.643

Ca1.55(1)Yb3.45Al 2Sb6 Ca5Ga2Sb6 orthorhombic Pbam (No. 55) a = 12.001(1) b = 13.993(1) c = 4.446(1) 746.51(3) 6.419 1274/0/45 R1 = 0.0217 wR2 = 0.0394 R1 = 0.0284 wR2 = 0.0408 1.127 1.298/−1.553

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo2 − Fc2)]/∑[w(Fo2)2]}1/2, where w = 1/[σ2Fo2 + (A − P)2 + (B − P)], in which P = (F02 + 2Fc2)/3 and A and B are weight coefficients. a

changes in TE performance of the title Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) system, and the theoretical approaches based on the tight-binding linear muffin-tin orbital (TB-LMTO) calculations. Density of states (DOS) and crystal orbital Hamilton population (COHP) analyses successfully prove the correlation between two title structure types and the atomic orbital states differentiating the electrical transport properties of these phases. The site preference between Ca2+ and Yb2+ over three available cationic sites is also elucidated using spatial as well as electronic factors.

that can intrinsically fulfill the above-mentioned requirements via its complex crystal structure and semiconducting behavior, both of which eventually result in enhanced ZT.2,19−22 Recently, the A5M2Pn6 series has been extensively investigated as Zintl-phase TE materials,23−31 and two major structure types have been identified: the Yb5M2Sb6 series (M = Al, Ga, In; Ba5Al2Bi6-type structure)23−25 having metallic behavior and the A5M2Sb6 series (A = Ca, Sr, Eu; M = Al, Ga, In; Ca5Ga2Sb6-type structure)26,27,29−31 showing semiconducting behavior. In particular, p-type doping for A or M in the A5M2Sb6 system (A = Ca, Eu; M = Al, Ga, In; Ca5Ga2Sb6type)32−37 has successfully enhanced the maximum ZT value, and the substitution of Sr for Yb in the Yb5Al2Sb6 (Ba5Al2Bi6type) system24 has also been conducted. However, despite the possible ZT improvement via the Ca2+ and Yb2+ cation mixing, which had already been proven to increase phonon scattering by the mass contrast of cations in the CaxYb1−xZn2Sb2 system38 without sacrificing too much in σ and S, only a few attempts at cation-mixing reactions have been reported for the A5M2Sb6 system.24 Our investigation starts from this basic idea of introducing the Ca2+ and Yb2+ mixed cations to produce the solid-solution Ca5−xYbxAl2Sb6 system with a reduced κlatt and to study the correlation between the Ca/Yb ratio and the resultant crystal structure. Therefore, a total of 10 particular compositions with different Ca/Yb ratios were successfully synthesized by hightemperature reactions and crystallized in either one of two very similar but slightly different structure types, the Ba5Al2Bi6 type39 or the Ca5Ga2Sb6 type,39 despite the very similar sizes (r(Ca2+) = 1.00 Å vs r(Yb2+) = 1.02 Å)40 and divalent states of the two cations. Intriguingly, for the first time we discovered the solid-state crystal-to-crystal structural transformation of the Yb-rich compounds from their original Ba5Al2Bi6-type phase to the Ca5Ga2Sb6-type phase during the annealing process at the elevated temperature. In addition, along with this structural transformation, the metallic conductivity of the Ba5Al2Bi6-type phase is switched to the semiconducting behavior of the Ca5Ga2Sb6-type phase, which eventually results in the improved ZT value. In this article, we discuss the experimental investigations, including the synthesis, crystal-to-crystal structural transformation under various annealing conditions, and resultant

■

EXPERIMENTAL SECTION

Synthesis. All of the sample preparation processes were conducted inside an Ar-filled glovebox with O2 and H2O contents below 0.1 ppm or under vacuum. The reactant elements used were purchased from Alfa Aesar, and the list is as follows: Yb (ingot, 99.9%), Ca (shot, 99.5%), Al (piece, 99.9%), and Sb (shot, 99.9%). The slightly tanned surfaces of Yb and Ca were cleaned by scraping with a scalpel or a metal brush in a glovebox before use in reactions. Ca2.78(2)Yb2.22Al2Sb6 was originally obtained from the conventional high-temperature reaction. The reactant mixture with a Ca:Yb:Al:Sb ratio of 3:2:2:6 was loaded and sealed in a Nb ampule (length = 4 cm, diameter = 1 cm) under a partial Ar atmosphere, and then the ampule was sealed again inside an evacuated fused-silica jacket acting as a secondary container to prevent the Nb ampule from oxidation. The reactant mixture was heated to 1323 K at a rate of 5 K/h using a muffle furnace, kept there for 24 h, and then cooled to room temperature at a rate of 10 K/h. The reaction produced irregular-shaped small single crystals with metallic luster. Once the target compound Ca2.78(2)Yb2.22Al2Sb6 was successfully characterized, we tried to synthesize several more compounds in the Ca5−xYbxAl2Sb6 solid-solution series with various Ca/Yb ratios (x = 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.3, 4.5, 5.0) by arc melting. Unlike the previously reported Yb5Al2Sb6 system,23 a series of targeted compounds were successfully synthesized without using any additional Ge or Si, which were reported as catalysts to form the targeted compounds. Each reaction produced a single-phase product of either the Ba5Al2Bi6-type or Ca5Ga2Sb6-type phase, depending on the Ca/Yb ratio. Powder X-ray diffraction (PXRD) patterns of 10 assynthesized title compounds are provided in Figure S1. All of the products were air- and moisture-stable for at least up to 3 weeks. In order to grow the larger single crystals for single-crystal X-ray diffraction (SXRD) measurements, the Yb-rich Ca1.5Yb3.5Al2Sb6 (nominal composition) was annealed at 1023 K for 2 weeks, and we discovered the solid-state structural transformation from its original Ba5Al2Bi6-type phase to the Ca5Ga2Sb6-type phase. Five other Yb-rich 1385


Article

Chemistry of Materials

Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters (Ueqa) from the SXRD Refinements for the Ca5−xYbxAl2Sb6 (1.40(2) ≤ x ≤ 3.45(1)) System

a

atom

Wyckoff site

M1b M2b M3b Al Sb1 Sb2 Sb3

4g 4g 2a 4h 4h 4h 4g


4g 4g 2a 4h 4h 4h 4g


4g 4g 2a 4h 4h 4h 4g


4g 4g 2a 4h 4h 4h 4g

occupation (Ca2+/Yb2+)

x

Ca3.60(2)Yb1.40Al2Sb6 (Ca5Ga2Sb6 Type) 0.71(1)/0.29 0.0921(1) 0.69(1)/0.31 0.3261(1) 0.81(1)/0.19 0 1 0.3308(3) 1 0.0240(1) 1 0.1565(1) 1 0.3390(1) Ca2.78(2)Yb2.22Al2Sb6 (Ca5Ga2Sb6 Type) 0.53(1)/0.47 0.0916(1) 0.51(1)/0.49 0.3266(1) 0.71(1)/0.29 0 1 0.3306(1) 1 0.0239(1) 1 0.1564(1) 1 0.3387(1) Ca1.58(2)Yb3.42Al2Sb6 (Ba5Al2Bi6 Type) 0.66(1)/0.34 0.2662(1) 0.69(1)/0.31 0.4479(1) 0.73(1)/0.27 0 1 0.3141(5) 1 0.0133(1) 1 0.1992(1) 1 0.0230(1) Ca1.55(1)Yb3.45Al2Sb6 (Ca5Ga2Sb6 Type) 0.28(1)/0.72 0.0911(1) 0.26(1)/0.74 0.3273(1) 0.46(1)/0.54 0 1 0.3305(2) 1 0.0244(1) 1 0.1561(1) 1 0.3380(1)

y

z

Ueqa (Å2)

0.2477(1) 0.0134(1) 0 0.2118(3) 0.4001(1) 0.0943(1) 0.3211(1)

0 0 0 1 /2 1 /2 1 /2 0

0.0145(4) 0.0155(4) 0.0166(7) 0.0159(7) 0.0147(2) 0.0140(2) 0.0140(2)

0.2478(1) 0.0141(1) 0 0.2123(2) 0.4001(1) 0.0944(1) 0.3212(1)

0 0 0 1 /2 1 /2 1 /2 0

0.0095(2) 0.0118(2) 0.0098(3) 0.0106(4) 0.0094(1) 0.0086(1) 0.0081(1)

0.2523(1) 0.0888(1) 0 0.3782(2) 0.3122(1) 0.4944(1) 0.1358(1)

0 0 0 1 /2 1 /2 1 /2 0

0.0062(2) 0.0070(2) 0.0060(3) 0.0071(7) 0.0053(2) 0.0097(2) 0.0054(2)

0.2477(1) 0.0146(1) 0 0.2127(1) 0.3998(1) 0.0942(1) 0.3220(1)

0 0 0 1 /2 1 /2 1 /2 0

0.0063(1) 0.0076(1) 0.0060(2) 0.0071(4) 0.0064(1) 0.0061(1) 0.0059(1)

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. bM = Ca2+/Yb2+ mixed site.

Table 3. Selected Bond Distances for the Ca5−xYbxAl2Sb6 (1.40(2) ≤ x ≤ 3.45(1)) System bond distance (Å) structure type

atomic pair

Ca3.60(2)Yb1.40Al2Sb6

Ca2.78(2)Yb2.22Al2Sb6

Ca1.58(2)Yb3.42Al2Sb6

Ca1.55(1)Yb3.45Al2Sb6

Ca5Ga2Sb6 and Ba5Al2Bi6

Al−Sb1 Al−Sb2 Al−Sb3 (×2) Sb1−Sb1 Sb2−Sb2 M1−Sb2 (×2) M1−Sb1 (×2) M2−Sb1 (×2) M2−Sb3 M3−Sb2 (×4) M3−Sb3 (×2) M1−Sb1 (×2) M1−Sb1 (×2) M2−Sb2 (×2) M2−Sb3 M3−Sb2 (×4) M3−Sb3 (×2)

2.789(4) 2.651(4) 2.688(3) 2.837(2) − 3.171(2) 3.175(2) 3.456(2) 3.329(2) 3.182(1) 3.150(2) − − − − − −

2.811(2) 2.670(2) 2.702(1) 2.859(1) − 3.193(1) 3.191(1) 3.470(1) 3.358(1) 3.202(1) 3.170(1) − − − − − −

2.673(4) 2.792(4) 2.703(2) − 2.930(1) − − − − − − 3.212(1) 3.190(1) 3.448(1) 3.293(1) 3.120(1) 3.114(1)

2.809(2) 2.671(2) 2.699(1) 2.866(1) − 3.188(1) 3.179(1) 3.460(1) 3.347(1) 3.191(1) 3.160(1) − − − − − −

Ca5Ga2Sb6 Ba5Al2Bi6 Ca5Ga2Sb6

Ba5Al2Bi6

also annealed at 1023 °C for 2 weeks, but only Ca2Yb3Al2Sb6 and CaYb4Al2Sb6 (nominal compositions) underwent the solid-state

compounds having x = 3.0, 4.0, 4.3, 4.5, and 5.0, all of which originally adopted the Ba5Al2Bi6-type phase via the arc-melting synthesis, were 1386


Article


Figure 1. Combined ball-and-stick and polyhedral representations of the crystal structures of the (a) Ba5Al2Bi6-type and (b) Ca5Ga2Sb6-type phases viewed along the c-axis direction. The identical imaginary unit cells are highlighted by rhombus boxes in both phases. The tetrahedral [AlSb4] moieties and the bridging Sb−Sb bonds are illustrated as blue polyhedra and orange dumbbells, respectively. Color codes for atoms are as follows: M, gray; Al, blue; Sb, orange. structural transformation to the Ca5Ga2Sb6-type phase. To find the proper range of annealing temperature and duration to activate this type of structural transformation, three as-synthesized samples of Ca1.5Yb3.5Al2Sb6 were annealed at 673 and 873 K for 2 weeks and at 1023 K for 1 month. The maximum annealing temperature was set at 1023 K since the product began to decompose beyond 1073 K according to thermogravimetric analysis (TGA) (Figure S2). X-ray Diffraction. A total of 10 title compounds in the Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) solid-solution system were characterized by PXRD, and four of those were also characterized by SXRD. PXRD patterns were obtained at room temperature using a Bruker D8 diffractometer equipped with an area detector and monochromatic Cu Kα1 radiation (λ = 1.54059 Å). The collection step size was set at 0.05° in the range of 15° ≤ 2θ ≤ 85° with a total exposure time of 1 h. Initially, the phase purities of the title compounds were checked by comparing the collected powder patterns with the simulated patterns of either one of the two structure types, and then all of the peaks in each powder pattern were indexed using the program Rietica41 to verify the lattice parameters of each unit cell. SXRD data were collected using a Bruker SMART APEX2 CCD-based diffractometer equipped with Mo Kα1 radiation (λ = 0.71073 Å). First, several silvery, lustrous, irregular-shaped single crystals were selected from each batch of crushed products, and their crystal qualities were briefly checked by a rapid scan. After this, the best crystals were chosen for full data collection using Bruker’s APEX2 program.42 Data reduction, integration, and unit cell parameter refinements were conducted using the SAINT program,43 and SADABS was used to perform semiempirical absorption corrections based on equivalents.44 The entire sets of reflections of four selected compounds were wellmatched with the orthorhombic crystal system, and the space group Pbam was eventually chosen for four crystal structures adopting either the Ba5Al2Bi6-type or Ca5Ga2Sb6-type phase. Detailed crystal structures were solved by direct methods and refined to convergence by full matrix least-squares methods on F2. The refined parameters include the scale factor, the atomic positions with anisotropic displacement parameters, extinction coefficients, and occupancy factors of the Ca/ Yb mixed site. During the last stage of a refinement cycle, atomic positions were standardized using STRUCTURE TIDY.45 Important crystallographic data, atomic positions with atomic displacement parameters (ADPs), and selected interatomic distances are shown in Tables 1−3. Further details about each crystal structure can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (49) 7247−808−666; email: crysdata@fiz-karlsruhe.de) under depository numbers CSD432095 for Ca3.60(2)Yb1.40Al2Sb6, CSD-432096 for Ca2.78(2)Yb2.22Al2Sb6,

CSD-432097 for Ca 1.58(2) Yb 3.42 Al 2 Sb 6 , and CSD-432098 for Ca1.55(1)Yb3.45Al2Sb6. Electronic Structure Calculations. To understand the overall electronic structure, including the chemical bonding in each structure type, and the correlation between the phase transition and the atomic orbital contributions, a series of tight-binding linear muffin-tin orbital (TB-LMTO) calculations with the atomic sphere approximation (ASA) were carried out using the Stuttgart TB-LMTO47 program.46−50 For a practical reason, the idealized chemical composition “CaYb4Al2Sb6” was exploited for two hypothetical structural models adopting either the Ba5Al2Bi6-type (model 1) or the Ca5Ga2Sb6-type (model 2) structure, and the total electronic energy of each model was calculated. The lattice parameters for the two models were extracted from the SXRD results for Ca 1 . 5 8 ( 2 ) Yb 3 . 4 2 A l 2 Sb 6 and Ca1.55(1)Yb3.45Al2Sb6 for models 1 and 2, respectively. In particular, the lattice parameters of model 2 were slightly adjusted to make the total unit cell volume the same as that of model 1 (see Table S1). To apply the idealized composition “CaYb4Al2Sb6” for the two models, we assigned a Ca atom only at the M3 site (Wyckoff 2a site), where the largest Ca occupancy was observed among three mixed-cationic sites according to the SXRD results. The local density approximation was used for exchange and correlation, and all relativistic effects were taken into account using a scalar relativistic approximation except spin−orbit coupling. In the ASA method, space is filled with overlapping Wigner− Seitz (WS) atomic spheres.46−50 The symmetry of the potential is considered spherical inside each WS sphere, and a combined correction is used to take into account the overlapping part. The WS radius for each atomic sphere was calculated by requiring that the overlapping potential be the best possible approximation to the full potential and was determined by an automatic procedure.51 This overlap should not be too large because the error in kinetic energy introduced by the combined correction is proportional to the fourth power of the relative sphere overlap. The WS radii used for both structure types are as follows: Ca = 2.14 Å, Yb = 2.14 Å, Al = 1.55 Å, and Sb = 1.72 Å. The basis sets included 4s, 4p, and 3d orbitals for Ca; 6s, 6p, and 5d orbitals for Yb; 3s, 3p, and 3d orbitals for Al; and 5s, 5p, 5d, and 4f orbitals for Sb. The Yb 6p, Ca 4p, Al 3d, and Sb 5d and 4f orbitals were treated by the Löwdin downfolding technique.52 The kspace integrations were conducted by the tetrahedron method,53 and the self-consistent charge density was obtained using 343 irreducible k points in the Brillouin zone for both structure types. Thermogravimetric Analysis. The thermal stability of the Ca1.5Yb3.5Al2Sb6 was investigated by TGA using a TA Instruments SDT2960 thermal analyzer. The sample (about 20 mg) was enclosed in an alumina crucible, heated under a continuous nitrogen flow from 1387


Article


Figure 2. Local coordination environments of the [Al2Sb8] units with eight surrounding M sites in (a) Ca1.58(2)Yb3.42Al2Sb6 (Ba5Al2Bi6 type) and (b) Ca1.55(1)Yb3.45Al2Sb6 (Ca5Ga2Sb6 type). Atomic labels and important bond distances are also provided.

Figure 3. Schematic illustration of the two different packing patterns of cationic and anionic polyhedra observed in the (a) Ba5Al2Bi6-type and (b) Ca5Ga2Sb6-type phases. Color codes for polyhedra are as follows: M1 site, yellow; M2 site, red; M3 site, green; Al site, blue. The Sb−Sb bonds bridging the [Al2Sb8] units are highlighted as thick orange bars.

■

room temperature to 1273 K at a rate of 10 K/min, and then cooled to room temperature naturally. Electrical Transport Properties. Three samples of Ca1.5Yb3.5Al2Sb6 (nominal composition), which adopted either the Ba5Al2Bi6-type or Ca5Ga2Sb6-type phase as a result of the different heat post-treatments, were cut and polished into a rectangular shape (3 mm × 3 mm × 9 mm) for the electrical conductivity measurements. The longer direction coincides with the direction in which the electrical conductivity was measured. The electrical conductivity σ and the Seebeck coefficient S were measured simultaneously under a helium atmosphere from room temperature to 700 K using a ULVAC-RIKO ZEM-3 instrument system. Thermal Conductivities. Thermal diffusivity (D) was directly measured using three disk-shaped samples of Ca1.5Yb3.5Al2Sb6 under an inert atmosphere from room temperature to 700 K by a flash diffusivity method using a Netzsch LFA 457 MicroFlash instrument. In the flash diffusivity method, the front face of a disk-shaped sample is irradiated by a short laser burst, and the resultant temperature increase on the rear face is recorded and analyzed by an IR detector. The thermal conductivity (κ) was calculated from the equation κ = DCpρ, where ρ and Cp are the density and heat capacity of the sample, respectively. In this work, the Dulong−Petit value (3R/atom, where R is the gas constant) was used for Cp. The total thermal conductivity κtot was assumed to be the sum of the lattice (κlatt) and electronic (κelec) thermal conductivities. κelec was expressed using the Wiedemann− Franz law (κelec = LσT), where L is the temperature-dependent Lorenz number. An L value was estimated using the single parabolic band (SPB) model54 from the temperature-dependent Seebeck coefficient, which was based on the assumption of acoustic phonon scattering. Finally, κlatt was calculated from the relationship κlatt = κtot − κelec.

RESULTS AND DISCUSSION Crystal Structures. The Zintl phase Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) solid-solution system with Ca2+/Yb2+ mixed cations was synthesized by arc melting and crystallized in two slightly different structure types according to both PXRD and SXRD analyses. Among the title solid-solution compounds, three compounds with x = 1.0, 1.5, and 2.0 (nominal compositions) crystallized in the Ca5Ga2Sb6-type structure39 whereas seven compounds having x = 2.5, 3.0, 3.5, 4.0, 4.3, 4.5, and 5.0 (nominal compositions) adopted the Ba5Al2Bi6-type structure,39 as shown in Figure 1. Both structure types belong to the orthorhombic space group Pbam (Pearson code oP26, Z = 2) and include seven crystallographically independent atomic sites in each unit cell: three Ca2+/Yb2+ mixed sites, one Al site, and three Sb sites (see Table 2). PXRD patterns of the 10 title compounds are displayed in Figure S1. The overall crystal structures of the two distinctive phases are closely related to each other with respect to structural building blocks, such as (1) the [Al2Sb8] units formed by the dimerized tetrahedral [AlSb4] moieties and (2) the isolated Ca2+/Yb2+ mixed cations filling the space between those [Al2Sb8] units. In particular, the [Al2Sb8] units in both structure types are further connected to each other along the c-axis direction by sharing four corner Sb atoms, and the connection results in the infinite one-dimensional (1D) double chains of 1∞[Al2(Sb4Sb4/2)].24,26 There also exist several structural differences between the two phases, including (1) the local geometries of the [Al2Sb8] units 1388


Article


Figure 4. Three different cationic sites observed in Ca1.55(1)Yb3.45Al2Sb6 (Ca5Ga2Sb6 type). Selected interatomic distances are also provided. Color codes for atoms are as follows: M, gray; Sb, orange.

Figure 5. Two collected PXRD patterns of Ca1.5Yb3.5Al2Sb6 adopting (a) the Ba5Al2Bi6-type structure before the phase transition and (b) the Ca5Ga2Sb6-type structure after the phase transition are compared with their simulated PXRD patterns.

The different geometrical arrangements of the 1D anionic double chains and the Ca2+/Yb2+ mixed cations in the two title phases can be represented by the schematic packing patterns of three cationic polyhedra as illustrated in Figure 3. It is remarkable that the slightly different Sb−Sb bond distances and the distinctive geometrical arrangements of the structural units in the Ba 5 Al 2 Bi 6 -type and Ca 5 Ga 2 Sb 6 -type phases are responsible for the different TE properties, including the metallic or semiconducting conduction, as will be shown below in Thermoelectric Properties. Further details about the correlation between this structural transformation and the corresponding changes in electrical transport properties

and (2) the geometrical arrangements between the 1D 1 ∞[Al2(Sb4Sb4/2)] double chains and the Ca/Yb mixed cations. For instance, the Sb−Sb bond distance bridging the dimerized [AlSb4] moieties is shortened from 2.930(1) to 2.866(1) Å in Ca1.58(2)Yb3.42Al2Sb6 (Ba5Al2Bi6 type) and Ca1.55(1)Yb3.45Al2Sb6 (Ca5Ga2Sb6 type), respectively (see Figure 2). In addition, to compensate for this particular bond-shortening effect without sacrificing too much of the local coordination geometry, the Sb−M and Sb−Al bond distances become slightly elongated from 3.120 (1) and 2.792(4) Å, respectively, to 3.179(2) and 2.809 (2) Å, respectively, in each compound. 1389


Article


for 2 weeks, we tried to find out the proper range of annealing temperature and duration to activate this type of structural transformation. Therefore, we attempted to vary the annealing conditions by using five as-synthesized Ca1.5Yb3.5Al2Sb6 samples at annealing temperatures ranging from 673 to 1023 K with durations ranging from 1 week to 1 month. As shown in Figure S3, the five PXRD patterns indicate that all of the samples successfully underwent the structural transformation, and therefore, even temperatures as low as 673 K and durations as short as 1 week of annealing are enough to activate this type of structural transformation. The correlation between the annealing duration and the TE properties will be further discussed in Thermoelectric Properties. Lastly, we also examined the minimum Ca requirement for the structural transformation. Thus, four Yb-rich compounds with x = 4.0, 4.3, 4.5, and 5.0 were prepared and annealed at 1023 K for 2 weeks. According to the PXRD patterns (Figure S4), Ca0.7Yb4.3Al2Sb6, Ca0.5Yb4.5Al2Sb6, and Yb5Al2Sb6 maintained their original Ba5Al2Bi6-type phase even after annealing, whereas CaYb4Al2Sb6 turned into the Ca5Ga2Sb6-type structure. Detailed structural information, including the phase boundary and phase transition temperature between the Ca5Ga2Sb6type and Ba5Al2Bi6-type phases, should be determined to optimize the TE performance of the solid-solution Ca 5−xYb xAl 2Sb 6 system as well as to construct phase equilibrium between two phases. Therefore, on the basis of the above-mentioned series of experimental investigations, a tentative crystallographic phase diagram of the Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) system can be generated, as illustrated in Figure 6. The semiconducting Ca5Ga2Sb6-type structure can be

observed in several Yb-rich compounds will be discussed on the basis of a comprehensive theoretical investigation in Electronic Structures and Chemical Bonding. Interestingly, a particular site preference between Ca2+ and Yb2+ was observed during the structure refinement of the three Ca5Ga2Sb6-type compounds, where Ca2+ preferred the M3 site and Yb2+ preferred the M2 site, as displayed in Table 2. One simple way of understanding this type of site preference55,56 is to consider the size factor criterion, which is based on the size match between a central cation and the surrounding local coordination environment.57,58 As illustrated in Figure 4, the Ca5Ga2Sb6-type phase contains three different types of cationic sites. Both the M1 and M3 sites are surrounded by six Sb atoms forming two slightly distorted octahedra, whereas the M2 site is coordinated by seven Sb atoms building a two-edge-capped square pyramid. Therefore, on the basis of the size factor criterion, the relatively larger seven-coordinate M2 site should be preferred by the relatively larger Yb2+ cation rather than Ca2+ (r(Ca2+) = 1.00 Å vs r(Yb2+) = 1.02 Å).40 Moreover, between two six-coordinated octahedral sites, the M3 site is slightly more symmetric than the M1 site in terms of the M−Sb distances (see Figure 4) and the Sb−Sb−Sb−M torsion angles (0.00° vs 0.75°). Therefore, one can assess that the more symmetric M3 site should be more suitable for the relatively smaller Ca2+ cation in the Ca5Ga2Sb6-type phase. Solid-State Structural Transformation. As briefly mentioned in the previous section, for the first time we have discovered the solid-state structural transformation of the Ybrich compound Ca1.5Yb3.5Al2Sb6 (nominal composition) from its original Ba5Al2Bi6-type phase to the Ca5Ga2Sb6-type phase via annealing for 2 weeks at 1023 K. Two PXRD patterns collected before and after the phase transition are displayed in Figure 5, and the SXRD data sets for Ca1.58(2)Yb3.42Al2Sb6 (Ba5Al2Bi6 type) and Ca1.55(1)Yb3.45Al2Sb6 (Ca5Ga2Sb6 type) in Table 1 confirm the two distinct crystal structures. Interestingly, this type of structural transformation is observed only for some Yb-rich Ba5Al2Bi6-type compounds, and it is irreversible. In addition, no Ca-rich Ca5Ga2Sb6-type compound underwent the structural transformation to the Ba5Al2Bi6-type structure. This type of irreversible structural transformation can be rationalized by the relatively more symmetric Ca5Ga2Sb6-type crystal structure. In Figure 1, the identical imaginary unit cell (a red-shaded rhombus box) can be selected for each structure type. However, the arrangement of these unit cells is relatively simpler and more symmetric in the Ca5Ga2Sb6-type phase than in the Ba5Al2Bi6-type phase. For instance, in the Ca5Ga2Sb6type phase, these unit cells can be located above and below each other, forming a kind of 1D “strip” along the b-axis direction. In addition, two such neighboring 1D strips are displaced up or down by one-fourth of the length of a unit cell along the b-axis direction. In the Ba5Al2Bi6-type phase, the identical unit cells also form a similar type of 1D strip along the a-axis direction. However, the 1D strips located above and below the central strip not only are displaced left or right but also face in the opposite direction from the central one, as if a glide plane exists between two neighboring 1D strips. Therefore, we speculate that the postannealing process provides enough thermal activation energy to transform the kinetically more stable Ba5Al2Bi6-type phase with a relatively lower local symmetry into the thermodynamically more stable Ca5Ga2Sb6type phase with a higher local symmetry. Since we serendipitously discovered the initial solid-state structural transformation from the annealing process at 1023 K

Figure 6. Phase diagram of the solid-solution Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) system. A total of 10 arc-melted compounds before annealing and a total of 11 compounds after annealing are displayed according to their structure types. The dashed lines indicate the tentative phase boundaries between the Ca5Ga2Sb6-type and Ba5Al2Bi6-type phases.

expanded from ca. x = 2 up to x = 4 for Ca5−xYbxAl2Sb6 via heat post-treatment (T ≥ 673 K), whereas non-heat-treated samples showed a metallic Ba5Al2Bi6-type structure at x ≥ 2.5. However, the phase transition boundaries drawn using dashed lines should be considered only a preliminary result because their location may be changed as more data become available. Electronic Structures and Chemical Bonding. To understand the correlations between the structural transformation and the total electronic energies of the two structure 1390


Article


Figure 7. DOS, PDOS, and COHP curves of CaYb4Al2Sb6 adopting the (a) Ba5Al2Bi6-type or (b) Ca5Ga2Sb6-type phase: total DOS (bold black outline), Yb PDOS (gray region), Ca PDOS (purple region), Sb PDOS (orange region), and Al PDOS (green region). The PDOS of the p orbitals of (a) Sb2 or (b) Sb1 and (a) Ca3 or (b) Yb1 are also displayed for each phase. The two COHP curves in each panel represent interatomic interactions of the bridging Sb−Sb and the (a) Sb2−Ca3 or (b) Sb1−Yb1 in each model. Each inset shows a smaller energy range near EF from −2 to 1 eV. EF (dashed vertical line) is the energy reference (0 eV).

Second, we performed density of states (DOS) calculations using the two above-mentioned structure models, and the total DOS (TDOS) and partial DOS (PDOS) curves were thoroughly analyzed to understand the individual atomic orbital distributions over the entire energy range. As shown in Figure 7, the overall shapes of the two TDOS curves are quite similar. In particular, the valence band region can roughly be divided into two sections for both structure types: (1) the lower section between ca. −11 and −9 eV includes strong contributions from the s orbitals of Al and Sb and (2) the higher section between ca. −7 and 0 eV contains major contributions from the p orbitals of Al and Sb with some addition of Yb. On the other hand, the two TDOS curves also nicely represent the different electrical conductivities of the two phases: the TDOS of the Ba5Al2Bi6-type phase (Figure 7a) indicates metallic conduction with a noticeable DOS value at the Fermi level (EF), whereas that of the Ca5Ga2Sb6-type phase (Figure 7b) entails semiconducting behavior with a small band gap at EF. Since the structural transformation from the Ba5Al2Bi6-type structure to the Ca5Ga2Sb6-type structure provoked switching of the electrical conduction behavior, we tried to further understand the correlation between the band-gap size and the structural transformation. According to the PDOS curves, the largest contribution to the valence band just below EF mostly comes from the bonding states of the bridging Sb atoms in the [Al2Sb8] units, with some additions of cations in both structure types. However, the antibonding states of the bridging Sb

types as well as between the overall electronic structure and the individual atomic orbital distribution, a series of theoretical calculations using the TB-LMTO-ASA method46−50 were performed sequentially. First, we tried to rationalize the experimentally observed structural transformation of Ca 1.5 Yb 3.5 Al 2 Sb 6 from the Ba5Al2Bi6-type structure to the Ca5Ga2Sb6-type structure by comparing the total electronic energies of the two structure types and eventually to verify which structure type would be the more energetically favorable for the given composition. Therefore, two hypothetical structural models were designed, one adopting the Ba5Al2Bi6-type structure and the other the Ca5Ga2Sb6-type structure, and for practical reasons an idealized composition of “CaYb4Al2Sb6” was applied for both models. Except for some intrinsic structural differences that were already described in the previous section, the rest of calculation conditions involving WS radii of atoms and total numbers of irreducible k points were kept identical for the two models. Then a series of calculations was performed, and the total electronic energy of the Ca5Ga2Sb6-type model was lower than that of the Ba5Al2Bi6-type model by 0.66 eV/formula unit. Thus, the resultant energy difference implies that the Ca5Ga2Sb6-type phase is the more energetically favorable structure type for the Yb-rich CaYb4Al2Sb6 compound, which is in good agreement with the observed structural transformation toward the Ca5Ga2Sb6-type structure. 1391


Article


Figure 8. Temperature-dependent (a) electrical conductivity σ and (b) Seebeck coefficient S of Ca1.5Yb3.5Al2Sb6 with three different annealing conditions measured over the temperature range of 300−700 K. The σ values of Ca5Al2Sb6 and Yb5Al2Sb6/0.5Ge from the literature are also displayed in (a).

composition (Yb-rich vs Ca-rich). In addition, this result also proves that the longer heat treatment further decreases the electrical conductivity, probably due to a significant reduction in the number of defect states, if any, between the valence band and the conduction band. In Figure 8b, the temperature dependence of the Seebeck coefficients S is nicely displayed. All three samples show positive Seebeck coefficients within the measured temperature range, indicating that holes are the major charge carriers (ptype conduction). In particular, the Seebeck coefficient of compound 1 is ca. 20 μV/K at 300 K and increases almost linearly with increasing temperature up to ca. 50 μV/K at 700 K. The absolute values of the Seebeck coefficients are relatively small, which is another indication of metallic conduction of the sample. A similar variation of S persists for compound 2 having the 2 weeks of annealing, probably as a result of the presence of defect states as mentioned just above. However, compound 3 having the 1 month of annealing shows Seebeck coefficients ranging between ca. 120 and 180 μV/K, which are significantly improved compared with those of the other two compounds. Since compounds 2 and 3 both underwent the complete structural transformation after annealing at 1023 K according to the PXRD and SXRD analyses, we can rule out the possibility of an intermediate state having a mixture of the two title phases. Therefore, besides the possibility of the presence of defect states, it can be presumed that the longer annealing duration can help electrons be transferred from the electropositive atoms to the electronegative atoms in Ca 1.5Yb3.5Al2Sb6 more completely. The corresponding power factors (PFs) of the three compounds are plotted as functions of temperature in Figure S5a. The room temperature PFs are 0.11, 0.002, and 0.03 μW cm−1 K−2 for compounds 1, 2, and 3, respectively. However, these values increase with increasing temperature and eventually reach the maximum values of 0.62, 0.36, and 0.77 μW cm−1 K−2, respectively, at 700 K. The relatively small PFs of the three compounds can be attributed to relatively quite low S values for compounds 1 and 2 and the relatively low σ values for compound 3. It is highly expected that especially compound 3 could show a significantly enhanced PF when the proper doping is applied to enhance the σ value.

atoms, which are located at the bottom of the conduction band in both structure types, significantly cross over EF in the case of the Ba5Al2Bi6-type phase, resulting in a disappearance of the band gap (Figure 7a). On the other hand, those in the case of the Ca5Ga2Sb6-type phase stay within the conduction band above EF. These different behaviors of the antibonding states in the two different structure types can be rationalized by the bridging Sb−Sb bond distance. The relatively shorter Sb−Sb bond distance in the Ca5Ga2Sb6-type phase should provide a stronger bonding interaction than that in the Ba5Al2Bi6-type phase, which eventually generates the relatively larger separation between the valence band and the conduction band. Therefore, this phenomenon can explain the semiconducting behavior of the Ca5Ga2Sb6-type phase with a small band gap. Thermoelectric Properties. To understand the influence of the solid-state structural transformation and the annealing duration on the TE properties of the two title phases, various physical property measurements were conducted for three selected Yb-rich Ca1.5Yb3.5Al2Sb6 samples: compound 1, the Ba5Al2Bi6-type phase with no annealing; compound 2, the Ca5Ga2Sb6-type phase after 2 weeks of annealing at 1023 K; and compound 3, the Ca5Ga2Sb6-type phase after 1 month of annealing at 1023 K. The electrical conductivities σ of compounds 1, 2, and 3 are ca. 300, 75, and 2 S/cm, respectively, at room temperature, as displayed in Figure 8a. In particular, σ of compound 1 (Ba5Al2Bi6 type) decreases with increasing temperature, indicating the typical metallic conduction behavior. However, compounds 2 and 3 (Ca5Ga2Sb6 type) obtained after the annealing process both show typical semiconducting behaviors, where σ increases as the temperature increases as a result of thermal excitation of carriers. These characteristics are in good agreement with two previous reports: metallic conduction of Yb5Al2Sb6 adopting the Ba5Al2Bi6-type phase24 and semiconducting behavior of Ca5Al2Sb6 adopting the Ca5Ga2Sb6type phase.26 For comparison, we have added temperaturedependent σ data of Yb5Al2Sb6/0.5Ge24 and Ca5Al2Sb626 in Figure 8a. Therefore, such an interesting physical behavior reveals that the electrical transport properties of three title compounds are solely influenced by the structure type (Ca5Ga2Sb6-type vs Ba5Al2Bi6-type) rather than the chemical 1392


Article


Figure 9. Temperature-dependent (a) thermal conductivity κtot and (b) figure of merit ZT of Ca1.5Yb3.5Al2Sb6 with three different annealing conditions measured over the temperature range of 300−700 K.

The room-temperature total thermal conductivities κtot of compounds 1, 2, and 3 are ca. 4.0, 1.4, and 1.1 W/mK, respectively, as shown in Figure 9a. Since the electronic thermal conductivity κelec can be estimated from the Wiedemann−Franz law (κelec = LσT), the lattice thermal conductivity κlatt can be calculated using the simple equation κlatt = κtot − κelec. Here the temperature-dependent Lorenz factors L were evaluated from the temperature-dependent Seebeck coefficients using the single parabolic band (SPB) model.54 The estimated temperature-dependent L values are shown in Figure S6. The temperature-dependent L value of compound 3 ranges from 1.83 × 10−8 to 1.66 × 10 −8 V 2/K2 , indicative of a nondegenerate value, whereas compounds 1 and 2 exhibit degenerate L values ranging between 2.44 × 10−8 and 2.17 × 10−8 V2/K2. The calculated room-temperature κlatt values of the title compounds are ca. 3.8, 1.3, and 1.1 W/mK, respectively (see Figure S5b), indicating that the majority of the heat is transported by lattice vibrations in the form of phonons. For comparison, we have included the temperature-dependent κlatt data for Yb5Al2Sb6/0.5Ge,24 Yb5Al2Sb6,59 and Ca5Al2Sb626 in Figure S5b as well. It is noticeable that there are certain variations of room-temperature κlatt values among the Ba5Al2Bi6-type compounds, including compound 1 (ca. 3.8 W/mK), Yb5Al2Sb6/0.5Ge24 (ca. 3.1 W/mK), and Yb5Al2Sb659 (ca. 2.4 W/mK), as plotted in Figure S5b. Such differences should be attributed to the various preparation methods and the slightly different chemical compositions among the three compounds: arc-melted Ca1.5Yb3.5Al2Sb6 in our work versus arcmelted Yb5Al2Sb6/0.5Ge24 versus ball-milled and spark-plasmasintered (BM-SPS) Yb5Al2Sb6.59 In particular, the lowest κlatt value, for BM-SPS Yb5Al2Sb6,59 could originate from the intensified grain boundary scattering, which is more effective for phonon scattering. It is quite remarkable that a simple annealing process can make a material with the same stoichiometry far more thermally insulating as a result of the structural transformation. Lastly, the TE figure of merit ZT as a function of temperature is plotted in Figure 9b. Overall, the ZT values of all three compounds increase with increasing temperature and show their maximum values at 700 K. In particular, it is observed that the 1 month annealing process for compound 3 boosts ZT up to 6 times higher than that of compound 1 with no annealing process, although the absolute value is still relatively low. Thus, compound 3 with optimal doping could show a significantly enhanced ZT value due to the

combination of relatively high PF and low κlatt values. Experimental attempts to synthesize the optimally doped compounds are in progress and will be the topic of our next article.

■

CONCLUSION A total of 10 Zintl phases belonging to the Ca5−xYbxAl2Sb6 (1.0 ≤ x ≤ 5.0) solid-solution system have been synthesized by arc melting. Ca-rich compounds crystallized in the Ca5Ga2Sb6-type structure, whereas Yb-rich compounds adopted the Ba5Al2Bi6type structure. The structural differences, including those in the bond distance of the bridging Sb−Sb bonds in the [Al2Sb8] units and the geometrical arrangements of several structural building moieties, are responsible for the different types of conduction (metallic or semiconducting) of the two structure types. The thermally activated solid-state crystal-to-crystal structural transformation of the Yb-rich Ca1.5Yb3.5Al2Sb6 from the Ba5Al2Bi6-type phase to the Ca5Ga2Sb6-type phase was observed for the first time in this work, and it was regarded that the postsynthetic annealing process provided enough activation energy to transform the kinetically stable Ba5Al2Bi6-type phase into the thermodynamically stable Ca5Ga2Sb6-type phase. Total electronic energy comparison between two structural models adopting either the Ba5Al2Bi6-type or Ca5Ga2Sb6-type structure proves that the Ca5Ga2Sb6-type model is more energetically favorable than the Ba5Al2Bi6-type model for the Yb-rich CaYb4Al2Sb6 composition. The relatively shorter bridging Sb−Sb bond distance in the Ca5Ga2Sb6-type phase provokes a larger separation between the valence band and the conduction band and eventually results in opening of the band gap, unlike the Ba5Al2Bi6-type phase. Therefore, we can conclude that the electrical transport properties of the title compounds can directly be assessed by the structure type, which is determined by the synthesis route, rather than by the chemical composition. In most systems, this kind of direct assessment is impossible because of the competing effects of the composition and the structure type. To further improve the overall TE performance of the title system, band structure engineering60 using p-type doping for particular elements influencing the orbital states near EF has been under investigation and will be a topic of our next article in the near future. In addition, phase separation of the metallic Ba5Al2Bi6-type phase and the semiconducting Ca5Ga2Sb6-type phase and the introduction of particular impurity phases into 1393


Article


thermoelectrics with all-scale hierarchical architectures. Nature 2012, 489, 414−418. (11) Luo, Y.; Yang, J.; Liu, M.; Xiao, Y.; Fu, L.; Li, W.; Zhang, D.; Zhang, M.; Cheng, Y. Multiple heteroatom induced carrier engineering and hierarchical nanostructures for high thermoelectric performance of polycrystalline In4Se2.5. J. Mater. Chem. A 2015, 3, 1251−1257. (12) Zhao, L.-D.; Hao, S.; Lo, S.-H.; Wu, C.-I.; Zhou, X.; Lee, Y.; Li, H.; Biswas, K.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures. J. Am. Chem. Soc. 2013, 135, 7364−7370. (13) Brown, S. R.; Kauzlarich, S. M.; Gascoin, F.; Snyder, G. J. Yb14MnSb11. New high efficiency thermoelectric material for power generation. Chem. Mater. 2006, 18, 1873−1877. (14) Wang, S.; Tan, X.; Tan, G.; She, X.; Liu, W.; Li, H.; Liu, H.; Tang, X. The realization of a high thermoelectric figure of merit in Gesubstituted β-Zn4Sb3 through band structure modification. J. Mater. Chem. 2012, 22, 13977−13985. (15) Bouyrie, Y.; Candolfi, C.; Dauscher, A.; Malaman, B.; Lenoir, B. Exsolution Process as a Route toward Extremely Low Thermal Conductivity in Cu12Sb4‑xTexS13 Tetrahedrites. Chem. Mater. 2015, 27, 8354−8361. (16) Ahn, K.; Han, M.-K.; He, J.; Androulakis, J.; Ballikaya, S.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. Exploring resonance levels and nanostructuring in the PbTe-CdTe system and enhancement of the thermoelectric figure of merit. J. Am. Chem. Soc. 2010, 132, 5227− 5235. (17) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of electronic bands for high performance bulk thermoelectrics. Nature 2011, 473, 66−69. (18) Heremans, J. P.; Wiendlocha, B.; Chamoire, A. M. Resonant levels in bulk thermoelectric semiconductors. Energy Environ. Sci. 2012, 5, 5510−5530. (19) Chemistry, Structure, and Bonding of Zintl Phases and Ions: Selected Topics and Recent Advances; Kauzlarich, S. M., Ed.; VCH: New York, 1996. (20) Toberer, E. S.; Cox, C. A.; Brown, S. R.; Ikeda, T.; May, A. F.; Kauzlarich, S. M.; Snyder, G. J. Traversing the Metal-Insulator Transition in a Zintl Phase: Rational Enhancement of Thermoelectric Efficiency in Yb14Mn1−xAlxSb11. Adv. Funct. Mater. 2008, 18, 2795− 2800. (21) Toberer, E. S.; May, A. F.; Snyder, G. J. Zintl Chemistry for Designing High Efficiency thermoelectric Materials. Chem. Mater. 2010, 22, 624−634. (22) Nam, G.; Jang, E.; Jo, H.; Han, M.-K.; Kim, S.-J.; Ok, K. M.; You, T.-S. Cationic Site-Preference in the Yb14‑xCaxAlSb11 (4.81 ≤ x ≤ 10.57) Series: Theoretical and Experimental Studies. Materials 2016, 9, 553−565. (23) Kim, S.-J.; Ireland, J. R.; Kannewurf, C. R.; Kanatzidis, M. G. Yb5In2Sb6: A New Rare Earth Zintl Phase with a Narrow Band Gap. J. Solid State Chem. 2000, 155, 55−61. (24) Todorov, I.; Chung, D. Y.; Ye, L.; Freeman, A. J.; Kanatzidis, M. G. Synthesis, Structure and Charge Transport Properties of Yb5Al2Sb6: A Zintl Phase with Incomplete Electron Transfer. Inorg. Chem. 2009, 48, 4768−4776. (25) Subbarao, U.; Sarkar, S.; Gudelli, V. K.; Kanchana, V.; Vaitheeswaran, G.; Peter, S. C. Yb5Ga2Sb6: A Mixed Valent and Narrow-Band Gap Material in the RE5M2X6 Family. Inorg. Chem. 2013, 52 (23), 13631−13638. (26) Toberer, E. S.; Zevalkink, A.; Crisosto, N.; Snyder, G. J. The Zintl Compound Ca5Al2Sb6 for Low-Cost Thermoelectric Power Generation. Adv. Funct. Mater. 2010, 20, 4375−4380. (27) Yan, Y. L.; Wang, Y. X. Crystal structure, electronic structure, and thermoelectric properties of Ca5Al2Sb6. J. Mater. Chem. 2011, 21, 12497−12502. (28) Luo, D. B.; Wang, Y. X.; Yan, Y. L.; Yang, G.; Yang, J. M. The High Termopower of the Zintl Compound Sr5Sn2As6 over a Wide Temperature Range: First-Principles Calculations. J. Mater. Chem. A 2014, 2, 15159−15167.

the title system in order to maximize ZT have also been under investigation.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05281. PXRD patterns of the 10 title compounds, TGA diagram of Ca1.5Yb3.5Al2Sb6, structural information on the two hypothetical models, PXRD patterns of the five samples annealed at different temperatures, PXRD patterns of the four samples having various Ca contents, and temperature-dependent PF and κlatt values (PDF) Crystallographic data for Ca3.60(2)Yb1.40Al2Sb6 (CIF) Crystallographic data for Ca2.78(2)Yb2.22Al2Sb6 (CIF) Crystallographic data for Ca1.58(2)Yb3.42Al2Sb6 (CIF) Crystallographic data for Ca1.55(1)Yb3.45Al2Sb6 (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82 (43) 261-2282. Fax: +82 (43) 267-2289. *E-mail: [email protected]. Phone: +82 (2) 880-1530. ORCID

Kang Min Ok: 0000-0002-7195-9089 Tae-Soo You: 0000-0001-9710-2166 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the NRF funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A1A1A05027845).

■

REFERENCES

(1) DiSalvo, F. J. Thermoelectric cooling and power generation. Science 1999, 285, 703−706. (2) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105−114. (3) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of electronic bands for high performance bulk thermoelectrics. Nature 2011, 473, 66−69. (4) Zeier, W. G.; Zevalkink, A.; Gibbs, Z. M.; Hautier, G.; Kanatzidis, M. G.; Snyder, G. J. Thinking like a chemist: Intuition in thermoelectric materials. Angew. Chem., Int. Ed. 2016, 55, 6826−6841. (5) Slack, G. A. In CRC Handbook of Thermoelectrics; Rowe, D. M., Ed.; CRC Press: Boca Raton, FL, 1995; pp 19−25. (6) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Enhanced thermoelectric performance of rough silicon nanowires. Nature 2008, 451, 163−167. (7) He, J.; Kanatzidis, M. G.; Dravid, V. P. High performance bulk thermoeelctrics via a panoscopic approach. Mater. Today 2013, 16, 166−176. (8) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634−638. (9) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features. Adv. Mater. 2010, 22, 3970−3980. (10) Biswas, K.; He, J.; Blum, I. D.; Wu, C.-I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-performance bulk 1394


Article


(52) Dronskowski, R.; Blöchl, P. E. Crystal orbital Hamilton populations (COHP): Energy-resolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 1993, 97, 8617−8624. (53) Blöchl, P. E.; Jepsen, O.; Andersen, O. K. Improved tetrahedron method for Brillouin-zone integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16223−16233. (54) May, A.; Toberer, E. S.; Saramat, A.; Snyder, G. J. Characterization and analysis of thermoelectric transport in n-type Ba8Ga16‑xGe30+x. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 125205. (55) Jeon, B.-Y.; Jeon, J.; Lee, J.; Kim, J.; You, T.-S. Experimental and theoretical investigations for site-preference and anisotropic size change of RE11Ge4In6‑xMx (RE = La, Ce; M = Li, Ge; x = 1, 1.96). J. Alloys Compd. 2015, 620, 269−276. (56) Jeon, B.-Y.; Nam, G.; Lee, D. W.; Ok, K. M.; You, T.-S. Ce11Ge3.73(2)In6.27: Solid-state synthesis, crystal structure and sitepreference. J. Solid State Chem. 2016, 236, 195−202. (57) Misra, S.; Miller, G. J. Gd5−xYxTt4 (Tt = Si or Ge): Effect of Metal Substitution on Structure, Bonding, and Magnetism. J. Am. Chem. Soc. 2008, 130, 13900−13911. (58) Nam, G.; Jeon, J.; Kim, Y.; Kang, S. K.; Ahn, K.; You, T.-S. Combined effect of chemical pressure and valence electron concentration through the electron-deficient Li substitution on the RE4LiGe4 (RE = La, Ce, Pr, and Sm) system. J. Solid State Chem. 2013, 205, 10−20. (59) Aydemir, U.; Zevalkink, A.; Ormeci, A.; Wang, H.; Ohno, S.; Bux, S.; Snyder, G. J. Thermoelectric properties of the Zintl phases Yb5M2Sb6 (M = Al, Ga, In). Dalton Trans. 2015, 44, 6767−6774. (60) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125−6135.

(29) Park, S.-M.; Choi, E. S.; Kang, W.; Kim, S.-J. Eu5In2Sb6, Eu5In2‑xZnxSb6: Rare earth Zintl Phases with Narrow Band Gaps. J. Mater. Chem. 2002, 12, 1839−1843. (30) Zevalkink, A.; Pomrehn, G. S.; Johnson, S.; Swallow, J.; Gibbs, Z. M.; Snyder, G. J. Influence of the Triel Elements (M = Al, Ga, In) on the Transport Properties of Ca5M2Sb6 Zintl Compounds. Chem. Mater. 2012, 24, 2091−2098. (31) Zevalkink, A.; Takagiwa, Y.; Kitahara, K.; Kimura, K.; Snyder, G. J. Thermoelectric properties and electronic structure of the Zintl phase Sr5Al2Sb6. Dalton Trans. 2014, 43, 4720−4725. (32) Zevalkink, A.; Toberer, E. S.; Bleith, T.; Flage-Larsen, E.; Snyder, G. J. Improved carrier concentration control in Zn-doped Ca5Al2Sb6. J. Appl. Phys. 2011, 110, 013721. (33) Johnson, S. I.; Zevalkink, A.; Snyder, G. J. Improved thermoelectric properties in Zn-doped Ca5Ga2Sb6. J. Mater. Chem. A 2013, 1, 4244−4249. (34) Zevalkink, A.; Swallow, J.; Snyder, G. J. Thermoelectric Properties of Mn-Doped Ca5Al2Sb6. J. Electron. Mater. 2012, 41, 813−818. (35) Chanakian, S.; Zevalkink, A.; Aydemir, U.; Gibbs, Z. M.; Pomrehn, G.; Fleurial, J.-P.; Bux, S.; Snyder, G. J. Enhanced Thermoelectric Properties of Sr5In2Sb6 via Zn-doping. J. Mater. Chem. A 2015, 3, 10289−10295. (36) Chanakian, S.; Aydemir, U.; Zevalkink, A.; Gibbs, Z. M.; Fleurial, J.-P.; Bux, S.; Snyder, G. J. High temperature thermoelectric properties of Zn-doped Eu5In2Sb6. J. Mater. Chem. C 2015, 3, 10518− 10524. (37) Zevalkink, A.; Chanakian, S.; Aydemir, U.; Ormeci, A.; Pomrehn, G.; Bux, S.; Fleurial, J.-P.; Snyder, G. J. Thermoelectric properties and electronic structure of the Zintl phase Sr5In2Sb6 and the Ca5‑xSrxIn2Sb6 solid solution. J. Phys.: Condens. Matter 2015, 27 (1), 015801. (38) Gascoin, F.; Ottensmann, S.; Stark, D.; Haile, S. M.; Snyder, G. J. Zintl phases as thermoelectric materials: Tuned transport properties of the compounds CaxYb1‑xZn2Sb2. Adv. Funct. Mater. 2005, 15, 1860− 1864. (39) Codier, G.; Schäfer, H.; Stelter, M. Perantimonidogallates and -indates: calcium perantimonidogallate (Ca5Ga2Sb6), calcium perantimonidoindate (Ca5In2Sb6), and strontium perantimonidoindate (Sr5In2Sb6). Z. Naturforsch. B 1985, 40, 5−8. (40) Shannon, R. D.; Prewitt, C. T. Acta. Effective ionic radii in oxides and fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925−946. (41) Hunter, B. A.; Howard, C. J. Rietica; Australian Nuclear Science and Technology Organization: Menai, Australia, 2000. (42) APEX2; Bruker AXS: Madison, WI, 2007. (43) SAINT; Bruker AXS: Madison, WI, 2002. (44) Sheldrick, G. M. SADABS; University of Göttingen: Göttingen, Germany, 2003. (45) Gelato, L. M.; Parthé, E. J. Appl. Crystallogr. 1987, 20, 139−143. (46) Andersen, O. K. Linear methods in band theory. Phys. Rev. B 1975, 12, 3060−3083. (47) Andersen, O. K.; Jepsen, O. Explicit, first-principles tightbinding theory. Phys. Rev. Lett. 1984, 53, 2571−1574. (48) Lambrecht, W. R. L.; Andersen, O. K. Linear methods in band theory. Minimal basis sets in the linear muffin-tin orbital method: Application to the diamond-structure crystals carbon, silicon, and germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34, 2439−2449. (49) Jepsen, O.; Burkhardt, A.; Andersen, O. K. The TB-LMTO-ASA Program, version 4.7; Max-Planck-Institut fur Festkorperforschung: Stuttgart, Germany, 1999. (50) Andersen, O. K.; Jepsen, O.; Glötzel, D. Canonical Description of the Band Structures of Metals. In Highlights of Condensed Matter Theory; Bassani, F., Fumi, F., Tosi, M., Eds.; Elsevier: Amsterdam, 1985. (51) Jepsen, O.; Andersen, O. K. Calculated electronic structure of the sandwich d1 metals LaI2 and CeI2: Application of new LMTO techniques. Z. Phys. B: Condens. Matter 1995, 97, 35−47. 1395


Influence of Thermally Activated Solid-State Crystal ... - ACS Publications

Recommend Documents