Eutectoid Flux Growth and Physical Properties of Single Crystal

Nov 14, 2014 - Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75080, United States. ‡ Department of Physics and Astronomy...
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Eutectoid Flux Growth and Physical Properties of Single Crystal Ln117Ni54−ySn112−z (Ln = Gd−Dy) Luis E. Reyes,† Roy N. McDougald, Jr.,† Gregory T. McCandless,† Mojammel Khan,‡ David P. Young,‡ and Julia Y. Chan*,†,‡ †

Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75080, United States Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, United States

‡

S Supporting Information *

ABSTRACT: Ln117Ni53−ySn112−z (Ln = Gd−Dy) have been grown via the eutectoid flux growth method, characterized using single crystal Xray diffraction, and determined to have a face centered cubic unit cell with lattice parameters of a = 30.070(4), 29.862(5), and 29.823(4) Å for the Gd, Dy, and Tb analogues. The compounds contain over 1100 atoms per unit cell with a complex bonding network and multiple magnetic sublattices. In addition, disorder is prevalent throughout the structure. These physical characteristics are ideal when searching for ultralow thermal conductivity materials. Magnetic susceptibility and electrical properties are presented, and all analogues exhibit positive Curie−Weiss constants, suggesting ferromagnetic interactions in each compound, in addition to a spin-glass component to the magnetic behavior. There are several structural factors that promote low κL values in a material. Materials with unit cells that contain a large number of atoms and a complex bonding network and are composed of high atomic mass atoms typically possess low values of κL.1−3 The large number of atoms (N) in the unit cell lowers the fraction of vibrational modes that efficiently carry heat to 1/N.1 The complex bonding networks and high atomic mass atoms aid in the reduction of κL by scattering phonons and reducing atomic vibrational modes.1−3 The introduction of disorder is frequently employed as an avenue for the reduction of κL. Disorder, such as partial occupancy, vacancies, substitution and doping, and rattling atoms, has been shown to reduce κL.2,7 Control over the degree or type of disorder present in a crystal system is required to investigate the lower limits of κL, but a correlation to the structure and growth conditions is often lacking.8 The matter is complicated further by the ever increasing unit cell complexity of novel materials designed for thermoelectrics. The Dy117Co57Sn112-type structure has a large face-centered cubic unit cell with lattice parameter a = 30.159(3) Å and contains 1140 atoms with relatively high atomic mass.4 Ln117Co53−ySn112−z (Ln = La−Lu, except Pm, Eu, and Yb) 4,9−16 have been reported to crystallize in the Dy117Co57Sn112 structure-type. Extremely low κL of 2.8 mW/ (cm·K) at 300 K was reported on single crystalline Gd117Co56Sn112.4 Such a low value of κL is due to its large

1. INTRODUCTION A key parameter affecting the performance of thermoelectric materials is the lattice thermal conductivity. The dimensionless figure of merit (ZT) of a thermoelectric is defined as ZT = S2T/ (ρκT), where S is the Seebeck coefficient (μV/K), ρ resistivity (mΩ·cm), and κT is total thermal conductivity (mW/(cm·K)).1 Maximizing ZT requires a large Seebeck coefficient, low ρ, and low κT. The problem that arises when attempting to maximize ZT is that the constants are interrelated.2,3 For example, as the Seebeck coefficient increases so does the electrical resistivity of the material, negatively impacting ZT. To reduce κT, the use of amorphous materials or substitutional disorder can be implemented, but both methods generally lead to increases in electrical resistivity.2,4 Although the reduction of κT has resulted in several high ZT thermoelectrics, materials with intrinsically low κT provide another avenue to accessing high ZT values, thereby circumventing the experimental difficulties that accompany introducing disorder into a crystal system. The κT of a material consists of contributions from the lattice (κL) and electronic (κe) components. While κe is proportional to 1/ρ in good thermoelectric materials, such as small band gap semiconductors, κL is usually much larger than κe and dominates κT, making low κL materials highly desirable.1 Yb14MnSb113 and SnSe5 are exemplary materials that possess relatively high values of ZT, as well as intrinsically low κL conductivity. Yb14MnSb11 was determined to have a maximum ZT value of ∼0.79 with κL of ∼4.8 mW/(cm·K) at 1223 K.6 Anisotropic measurements of SnSe revealed a ZT value of 2.6(3) with κL of 2.3 mW/(cm·K) at 973 K, when measured along the b-axis of the orthorhombic unit cell. © XXXX American Chemical Society

Received: September 10, 2014 Revised: November 11, 2014

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dx.doi.org/10.1021/cg501364h | Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Crystallographic Data for Ln117Ni53−ySn112−z (Ln = Gd−Dy) formula cryst syst space group a (Å) V (Å−3) Z cryst dimensions (mm3) temp (K) density (g cm−3) θ range (deg) μ (mm−1) no. of collected reflns no. of unique reflns Rint Δρmax (e Å−3) Δρmin (e Å−3) GOF R1(F) for Fo2 > 2σ(Fo2)a Rw(Fo2)b a

Crystal Data Gd117Ni52.9(3)Sn107.4(1) Tb117Ni51.1(2)Sn103.6(1) cubic cubic Fm3̅m Fm3̅m 30.070(4) 29.862(5) 27191(7) 26629(8) 4 4 0.10 × 0.10 × 0.05 0.20 × 0.20 × 0.08 298(2) 298(2) 8.366 8.453 2.71−30.49 3.05−30.50 41.263 43.597 Data Collection and Refinement 585608 79579 2095 2056 0.0411 0.0567 6.316 6.197 −6.339 −8.199 2.534 3.492 0.0378 0.0353 0.1312 0.1270

Dy117Ni52.6(2)Sn112 cubic Fm3̅m 29.823(4) 26526(6) 4 0.16 × 0.08 × 0.08 298(2) 8.863 2.73−30.47 46.409 58162 2048 0.0591 6.710 −9.099 2.641 0.0334 0.1135

R1(F) = ∑||Fo| − |Fc||/∑|Fo|. bRw(Fo2) = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2.

2. EXPERIMENTAL SECTION

unit cell, its crystal structure (that consists in part of a complex Co/Sn bonding network), and the high atomic mass of its atoms. 1−3 Physical properties have been reported for Ln117Co57−ySn112−z (Ln = Ce, Pr, Sm, and Gd−Dy),4,9−13 and with many unique magnetic sites, exotic magnetism such as cluster spin glass and Kondo behavior have been reported for Pr117Co54.5Sn115.2 and Ce2CoSn2, respectively (Ce2CoSn2 is expected to adopt a variant of the Dy117Co57Sn112 -type structure).10,12 For the compounds that follow Curie−Weiss behavior, Ln117Co57Sn112 (Ln = Pr and Gd−Dy) reported positive θw values ranging from 16 to 40 K that do not follow de Gennes scaling or scale with respect to ionic radii.4,11,13 Growth and characterization of new analogues may aid in forming a complete picture of the complex magnetism in this family of compounds. Single crystal experiments conducted on Ln117Co57−ySn112−z (Ln = Sm, Gd−Dy)4,9,13 have determined that each analogue varies slightly in its disorder. Disorder, such as partial occupancies and large atomic displacement parameters, has been reported for these isostructural analogues. The type of disorder that affects thermal conductivity greatest can be discerned by coordinating structural analysis to the subsequent variations in the thermal conductivity. It is our goal to determine the role of disorder and its effects on thermal conductivity, and the Dy117Co57Sn112-type structure is ideal for discerning the effects of disorder in highly complex crystalline materials. In this work, we report the crystal growth, structure, and physical properties of single crystal Ln117Ni53−ySn112−z (Ln = Gd−Dy). To the best of our knowledge, these are the first nickel containing compounds to be synthesized adopting the Dy117Co57Sn112-type structure. Structural disorder will be described and compared in detail for each of the synthesized analogues. Additionally, magnetic susceptibility, magnetization, and electrical properties will be presented.

2.1. Synthesis. High purity lanthanide (Ln) ingots, nickel powder, and tin shot (all >99.9 wt % purity, metal basis) were used as received. Single crystals of Ln117Ni53−ySn112−z (Ln = Gd−Dy) were grown using the eutectoid flux growth technique. This technique provides an alternative to a low-melting metal flux by targeting the eutectic point of the Ni−Sn binary phase diagram. In particular, initial molar ratios target the relatively low temperature (∼500−700 °C) melting eutectic point formed between lanthanides (Gd, Dy, or Tb) and nickel at a molar ratio of ∼32 atom % nickel.17 Molar ratios of 13:5:8 (Gd:Ni:Sn), 13:5:8 (Tb:Ni:Sn), or 13:5:13 (Dy:Ni:Sn) were added to an alumina crucible, loaded into a silica tube, and capped by an inverted alumina crucible. Subsequently, the silica tube was evacuated, backfilled to ∼0.2 atm with Ar, and sealed. The ampule was placed in a high temperature muffle furnace and heated at a rate of 100 °C/h to 1260 °C and allowed to dwell for 48 h. After dwelling, the furnace was slowly cooled to 1200 °C at a rate of 1 °C/h, followed by a second slow cooling to 1050 °C at a rate of 4 °C/ h and subsequent quenching to room temperature producing an ingot at the bottom of the crucible. Ln117Ni53−ySn112−z (Ln = Gd−Dy) crystals were etched from the flux matrix by reacting with a mixture of H2O/EtOH in a 1:9 volume ratio. The flux matrix is highly reactive in water; therefore ethanol is mixed with the water to reduce reactivity and reduce the stress on the crystals. The flux matrix is composed of a high concentration of rare earth, which is known to react in water and form rare earth hydroxides and H2 gas. Powder X-ray diffraction (XRD) of the resulting decomposed matrix shows the material to be amorphous. The Ln117Ni53−ySn112−z (Ln = Gd−Dy) crystals were dark gray or black in color with smooth, lustrous, undulating surfaces, as well as some surfaces with shearing patterns. The targeted phases were obtained for all compounds, with larger single crystals found in the Dy analogue, and crystal size decreases as Dy > Gd > Tb. The majority of the crystals had a volume of ∼1 mm3, which is large enough for magnetic and transport measurements but too small for accurate measurements of the thermal conductivity with our current equipment. 2.2. Elemental Analysis. Energy-dispersive X-ray spectroscopy (EDS) using a LEO 1530 VP scanning electron microscope (SEM) with an accelerating voltage of 19 keV was performed on single crystals of Ln117Ni53−ySn112−z (Ln = Gd−Dy). Data were collected on a minimum of eight different locations on a single crystal. The average atomic percent was used to determine the composition of B

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Table 2. Atomic Coordinates and Atomic Displacement Parameters for Gd117Ni52.9(3)Sn107.4(1) atom

site

x

y

z

Ueq (Å2)

occ.

Gd1 Gd2 Gd3 Gd4 Gd5 Gd5′ Gd6 Gd6′ Gd7 Gd8 Ni1 Ni2 Ni3 Ni4 Ni5 Ni6 Ni7 Sn1 Sn1′ Sn2 Sn3 Sn4 Sn5 Sn6 Sn7 Sn8 Sn9 Ni9′

96k 96k 96k 96j 48i 48i 24e 24e 8c 4a 96k 96k 32f 32f 4b 32f 24e 96k 96k 96k 48i 48h 48g 32f 24e 24e 32f 48i

0.067289(16) 0.179979(15) 0.200197(16) 0.25327(2) 0.12370(5) 0.14660(9) 0.33881(9) 0.29997(18) 1/4 0 0.16776(5) 0.07980(10) 0.39148(12) 0.30610(10) 1/2 0.0564(3) 0.3952(5) 0.07307(3) 0.06761(5) 0.10711(2) 0.21110(4) 0.14552(3) 1/4 0.14609(3) 0.10711(13) 0.21129(9) 0.44719(8) 1/2

0.067289(16) 0.179979(15) 0.200197(16) 0.10445(2) 0.12370(5) 0.14660(9) 0 0 1/4 0 0.16776(5) 0.07980(10) 0.39148(12) 0.30610(10) 1/2 0.0564(3) 0 0.07307(3) 0.06761(5) 0.10711(2) 0.21110(4) 0.14552(3) 1/4 0.14609(3) 0 0 0.44719(8) 0.4269(4)

0.15464(2) 0.40435(2) 0.06822(2) 0 1/2 1/2 0 0 1/4 0 0.23157(6) 0.01467(14) 0.39148(12) 0.30610(10) 1/2 0.0564(3) 0 0.32395(5) 0.35129(7) 0.24198(4) 1/2 0 0.14324(5) 0.14609(3) 0 0 0.44719(8) 0.4269(4)

0.00908(18) 0.00827(18) 0.00795(18) 0.00746(18) 0.0244(5) 0.0116(11) 0.0194(7) 0.0199(14) 0.0116(5) 0.0311(10) 0.0147(4) 0.0200(9) 0.0217(16) 0.0033(13) 0.010* 0.009(4) 0.013(5) 0.0035(2) 0.0035(2) 0.0144(2) 0.0153(3) 0.0072(3) 0.0149(3) 0.0102(3) 0.0157(10) 0.0223(5) 0.0354(10) 0.037(4)

1 1 1 1 0.734(4) 0.266(5) 0.690(5) 0.310(5) 1 1 1 0.50 0.701(13) 0.538(11) 0.78(2) 0.231(13) 0.224(14) 0.588(2) 0.412(2) 1 1 1 1 1 0.566(8) 1 0.749(8) 0.251(8)

*

Atomic displacement parameters fixed for final refinement. 2.5. Modeling Structural Disorder. The structure has several atomic sites with disorder of either partial occupancy or site displacement. Partially occupied atomic sites are identified by unusually large atomic displacement parameters (ADPs) in comparison to previously published Dy117Co57Sn112-type structures. Partial occupancy values were determined by allowing occupancies to refine freely. Split atomic sites are two atomic sites whose occupancies are correlated, where the sum of their occupancies is equal to one. Split atomic sites are a result of either dynamic (continuous) disorder, in which there is actually movement of an atom, or static (discrete) disorder, where an atom is distributed differently among unit cells. A split atomic site, denoted with ′, is usually in proximity (less than ∼2 Å) to the initial (higher occupancy) atomic site. Because of their proximity to one another, if both sites were occupied at once, the interatomic distances would be unrealistically short. As a result, both sites are not occupied at the same time, which requires the sum of both occupancies to be no greater than one. Indications of split atomic sites can be unusual atomic displacement parameters that are large or prolated. Additionally, the existence of residual electron density in proximity (less than ∼1 Å) to an atomic site suggests the need for split atomic sites and is seen frequently when refining this series of compounds. To account for a split site, the residual electron density is added to the model as a new atomic site. The site occupancy factors of the new atomic site (′), as well as the original atomic site, are allowed to refine independently. If the sum of their resulting occupancies is approximately one, the sites are most likely split, and their occupancies are constrained to unity. For every split site reported, the site occupancy factors were refined freely prior to constraining each pair to unity. Due to the proximity of the original atomic site to the new split atomic site, the ADPs of both are initially locked to be equal to one another and later unlocked after the occupancy refinement is stable. In some cases, the ADPs are required to remain locked.

Gd117(7)Ni43(7)Sn91(7), Tb117(7)Ni38(6)Sn110(7), and Dy117(8)Ni38(9)Sn113(7) when normalized to the lanthanide stoichiometry attained from single crystal diffraction. Error bars were determined using the standard deviation of the EDS data. 2.3. Physical Properties. Physical properties were measured on single-crystal fragments. All magnetic measurements of Gd117Ni52.9(3)Sn107.4(1), Tb117Ni51.1(2)Sn103.6(1), and Dy117Ni52.6(2)Sn112 were conducted using a Quantum Design magnetic property measurement system (MPMS). The temperature-dependent susceptibility data were measured under zero-field-cooled (ZFC) and fieldcooled (FC) conditions between 3 and 300 K with an applied field of 0.1 T. Field-dependent magnetization data were measured at 3 K with applied fields up to 7 T for each compound. The electrical resistance was measured on a single crystal of each analogue using the standard four-probe method in a Quantum Design physical properties measurement (PPMS) system from 3 to 290 K. Gold wires (1 mil diameter) were attached to the sample with a conductive epoxy (Epotek H20E). 2.4. Single Crystal X-ray Diffraction. Single crystal X-ray diffraction was conducted on crystal fragments with dimensions of approximately 0.10 × 0.10 × 0.05 mm3 for the Gd, 0.20 × 0.20 × 0.08 mm3 for the Tb, and 0.16 × 0.08 × 0.08 mm3 for the Dy analogues. The crystals were attached to a glass fiber using epoxy and mounted onto a goniometer of a Bruker Kappa D8 Quest CMOS diffractometer with Mo Kα radiation (λ = 0.71073 Å) and a IμS microfocus X-ray source. Single crystal data were collected at 298 K for all samples. SADABS was used to apply a multiscan absorption correction to highly redundant data. Crystallographic parameters are provided in Table 1. Direct methods were used for the initial structure model. SHELXL9718 was used to refine the model of each compound, and data were corrected with extinction coefficients and refined with anisotropic displacement parameters. The obtained structural model was compared with the crystallographic data of Gd117Co56Sn112.4 C

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Figure 1. (a) The continuous Ni/Sn bonding framework within the within Gd117Ni52.9(3)Sn107.4(1) crystal structure. The repeating cage-centered unit is highlighted in dark gray for clarity. (b) The Sn3- (orange), Sn4- (blue), Sn6- (green), and Sn8-centered (yellow) polyhedral networks. To model Gd117Ni52.9(3)Sn107.4(1), the site occupancies of Sn9 (32f(x,x,x)x = 0.44719(8)) were shared with Ni9′ (48i(1/2,x,x)x = 0.4269(4)) and constrained to 1.0. The Ni9′ site was not found in other analogues. The occupation of both Sn9 and Ni9′ sites results in an unrealistic distance of 1.808(8) Å, which is shorter than the sum of their metallic radii, thus confirming that only one of the two sites can be occupied at once. Initially, Ni9′ was modeled as Sn, but the atomic site was unstable; Ni was then used and found to improve the model. The Ln5 (48i(x,x,1/2)x ≈ 0.124) and Ln6 (24e(x,0,0)x ≈ 0.339) atomic sites are disordered in the Gd and Tb analogues (where Ln = lanthanide). Ln5 and Ln6 were refined as split positions Ln5/Ln5′ and Ln6/Ln6′, because of their one-directional atomic displacement parameters and the presence of residual electron density in their proximity ( 0. However, each sample passes through a shallow minimum in the resistance below 50 K, which is common in rare earth intermetallics where the low temperature transport is dominated by Kondo scattering. The resistance of the Tb and Dy samples continues to increase with decreasing temperature, while the Gd sample displays a sharp drop in resistance below ∼11 K, indicating the onset of magnetic ordering. The decrease in the resistance is consistent with a decrease in the spin-disorder scattering.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.Y.C. acknowledges Office of Basic Energy Sciences, US Department of Energy, Grant Nos. DE-FG02-08ER46528 and NSF-DMR1358975. D.P.Y. acknowledges funding from NSFDMR1306392.

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4. CONCLUSION We have discovered that flux growth synthesis with excess main group metals is a productive method to grow single crystals of novel intermetallics.21 In this paper, we report a route to grow single crystals of the rare earth-rich stannides using a Ln−Ni eutectoid to stabilize the Ln117Ni54−ySn112−z (Ln = Gd−Dy) phase. These materials are of particular interest because of their complex unit cells, which may lead to very low values of lattice thermal conductivity, and thus the growth of high quality single crystals is necessary to determine the material’s intrinsic properties. To the best of our knowledge, these are the first nickel-containing compounds to be synthesized in the Dy117Co57Sn112-type structure. Structural disorder surrounding the 4b(1/2,1/2,1/2) position is found to vary within the synthesized series and is linked to the presence of an additional atomic position Ni9′ (48i(1/2,x,x)x = 0.4269(4)) not seen in previous analogues. The compounds are determined to possess positive θw values, which suggest FM fluctuations, as well as divergence in the ZFC and FC magnetic susceptibility. This I

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