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Metal flux growth of complex alkaline earth/rare earth metal silicides with a homologous series of metal phosphide structure types Guillermo Vasquez, and Susan E. Latturner Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02916 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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Chemistry of Materials

Metal flux growth of complex alkaline earth/rare earth metal silicides with a homologous series of metal phosphide structure types

Guillermo Vasquez and Susan E. Latturner* Department of Chemistry and Biochemistry Florida State University Tallahassee, FL 32306 [email protected]

Abstract Three new metal silicides were grown as competing products from reactions of silicon with barium and ytterbium in Mg/Al flux; the dominant product is determined by heating profile as well as by reactant ratio. The compounds Ba2Yb0.88Mg11.12Si7, Ba5Yb2.26Mg16.73Si12, and Ba20Yb4.7Mg61.3Si43 all exhibit hexagonal crystal structures that are analogous to ternary metal phosphides (Zr2Ni12P7, Ho5Ni19P12, and Ho20Ni66P43, respectively). The structures feature building blocks comprised of silicon anions surrounded by nine metal cations; these tricapped trigonal prismatic Si@(Mg/Yb/Ba)9 polyhedra share faces to form the overall structures. Magnetic susceptibility measurements show Pauli paramagnetic behavior for all three compounds, indicating the ytterbium is divalent. The silicides have a 2:1 ratio of metal cations to silicide anions and are therefore charge-balanced. This should result in semiconducting or semimetallic behavior; the latter is supported by electronic structure calculations. The formation of these compounds indicates that a large family of complex silicides is accessible from reactions of silicon and divalent metals (Ca, Sr, Ba, Eu, Yb) in magnesium-based fluxes.

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Introduction Semiconducting and semimetallic metal silicides are of great interest for a variety of potential applications. They are integral to microelectronics, with metal silicide films and interfaces acting as ohmic contacts, gate electrodes, and diffusion barriers.1 Rare earth silicides such as EuSi2 are being investigated for magnetoresistance and spintronics applications.2 Solar cell technology also relies heavily on silicon and silicides, with narrow band gap compounds such as FeSi2 explored for use as photovoltaic materials.3 Magnesium silicide and substituted variants are gaining great interest as alternatives to the currently used thermoelectric compounds Bi2Te3 and PbTe; the silicides contain non-toxic, earth abundant elements, and Mg2Si0.4Sn0.6 has a ZT of 1.2 at 700 K.4 Metal fluxes have proven to be a useful medium for the synthesis of silicide phases. Flux reactions involve the use of a low melting metal as a solvent; refractory elements such as silicon dissolve in the molten metal and react at temperatures well below their melting point, allowing for the formation of complex intermetallics and the growth of large crystals.5 Metal silicide compounds grown from flux reactions include Ba8Al14Si31 (from Al flux), SmNiSi3 (from Ga flux), YbCrxSi2-x (from In flux), and TiSi2, Mo5Si3, WSi2 from Cu flux.6-9 Mixed metal flux systems are of current interest as reaction media for synthesizing new silicide phases. In addition to forming lower-melting eutectics, mixed fluxes add an additional methodology for controlling the reaction chemistry. In certain cases, both components of the flux are incorporated into the products, as was observed with the formation of Gd5Mg5Fe4Al12Si6 and Yb3-δFeAl4-xMgxSi2 from reactions in Mg/Al flux mixtures.10, 11 However, in other cases only one of the constituent elements is incorporated into the products, as is seen in the formation of CaMgSi and Ba1.9Ca2.4Mg9.7Si7 from the same flux.12,13 In this work, reactions of silicon with barium and ytterbium in Mg/Al flux have produced three new silicide phases, Ba2Yb0.88Mg11.12Si7, Ba5Yb2.26Mg16.73Si12, and Ba20Yb4.7Mg61.3Si43. All three products have hexagonal crystal structures that are analogous to ternary metal phosphides (Zr2Ni12P7, Ho5Ni19Si12, and Ho20Ni66P43, respectively).14 Barium occupies the rare earth sites and magnesium or Mg/Yb mixtures occupy the transition metal sites; the phosphide sites are occupied by silicide anions. The common structural building blocks and similar formation energetics of the three products make it difficult to favor one phase over another 2 ACS Paragon Plus Environment

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during the synthesis process. Magnetic susceptibility data and electronic structure calculations indicate these compounds are charge-balanced semi-metals, in accordance with their overall 2:1 ratio of divalent metal cations to silicide anions. The complex structures, site mixing and incorporation of heavy elements indicates that these materials have the potential for low thermal conductivity and may be of interest as thermoelectric compounds, particularly Ba20Yb4.7Mg61.3Si43. The family of hexagonal ternary metal phosphides is extensive; the fact that three new charge-balanced silicides with structures analogous to metal phosphides can be isolated using flux chemistry indicates that many more silicide members of this family will be discovered using this synthetic method. Experimental Section Synthesis The elemental reactants were used as received: magnesium and aluminum slugs (99.95% and 99.99%, Alfa Aesar); ytterbium chunks (99.9%, Alfa Aesar); barium rod (99+%, Alfa Aesar); and silicon powder (99+ %, Strem). The elements were initially weighed out in Mg/Al/Si/Ba/Yb ratios of 15:15:3:2.5:0.5 mmol and loaded into stainless steel crucibles under argon. The crucibles were sealed shut by arc-welding under argon and were then placed in silica tubes and flame-sealed under vacuum (30 mTorr). The ampules were heated from room temperature to 950°C in 10 h, held at 950°C for 10 h, and cooled to 750°C in 80 h. The reaction ampules were then removed from the furnace, inverted, and centrifuged for 1 min to decant the remaining Mg/Al melt. The steel crucibles were cut open and the crystals adhered to the walls of the crucible were scraped out. Other reaction ratios and heating profiles were used to determine if a better yield could be achieved. The yield of Ba20Yb4.7Mg61.3Si43 was optimized at a Mg/Al/Si/Ba/Yb ratio of 21:9:3:1.5:0.5 heated to 950°C in 10 h, held at 950°C for 10 h, and cooled to 750°C in 40 hrs. The yield of the structurally related competing phase Ba5Yb2.29Mg16.71Si12 increased with a slower cooling rate (see discussion). Ba2Yb0.88Mg11.12Si7 was synthesized with a Mg/Al/Si/Ba/Yb ratio of 15:15:3:3:0.5 heated to 950°C in 10 h, held at 950°C for 10 h, and cooled to 750°C in 80 hrs. Variation of the concentration of Ba was explored; counterintuitively, Ba2Yb0.88Mg11.12Si7 predominated in Ba rich reaction (3 mmols). As the Ba concentration decreased a mixture of all three phases would form with Ba5Yb2.29Mg16.71Si12 predominating. At the lowest concentration of Ba (0.55mmols) the 3 ACS Paragon Plus Environment


BaMgAlSi2 byproduct (BaAl4 type) would predominate. When varying the concentration of Yb a mixture of Al3Yb2 and BaMgAlSi2 would predominate at higher concentration of Yb (4-2 mmols). At the lowest concentration of Yb (0.5 mmol) Ba5Yb2.29Mg16.71Si12 and BaMgAlSi2 formed as the major products. Elemental Analysis Products were analyzed by SEM-EDS using a FEI Nova 400 NanoSEM with energy dispersive spectroscopy (EDS) capabilities. Samples of product crystals were mounted onto aluminum pucks using double-sided carbon tape and analyzed using a 30 kV acceleration voltage. The surface of the pucks were completely covered with carbon tape to eliminate Al artifacts produced by the puck. Analysis of the surface of some of the crystals showed residual flux droplets in some areas, so measurements were taken where no flux was present. Analysis of Ba20Yb4.7Mg61.3Si43 crystals indicated an average ratio of 18(2)% Ba, 3(1)% Yb, 45(1)% Mg, 34(2)% Si; Ba5Yb2.29Mg16.71Si12 samples averaged 17(3)% Ba, 8(3) Yb, 41(3) % Mg, 35(2) % Si; and Ba2Yb0.85Mg11.15Si7 samples averaged 14(3)% Ba, 6(2) Yb, 43(4) % Mg, 37(2) % Si. No incorporation of elements from the steel crucibles was observed. Diffraction studies Samples of Ba20Yb4.7Mg61.3Si43, Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7 were examined under a microscope to select crystals for diffraction studies. Suitable pieces were cut from larger crystals and were mounted in Dual-Thickness MicroLoopsTM (MiTeGen) using Parabar oil. Single-crystal X-ray diffraction data was collected at 293K using a Bruker APEX 2 CCD diffractometer with a Mo Kα radiation source. Absorption corrections were applied to the data sets using the SADABS program. Refinements of structures were performed using the SHELXTL package.15,16 The Ba20Yb4.7Mg61.3Si43 structure was solved in the hexagonal P63/m (No. 176) space group. The Ba5Yb2.29Mg16.71Si12 structure was initially refined in the P-6 (No.174) space group but use of the ADDsym program in the PLATON software suite indicated the presence of additional symmetry elements and converted the structure to space group P-62m (No.187).17 The structure of Ba2Yb0.85Mg11.15Si7 was solved in the P-6 space group. Further crystallographic information for these compounds are shown in Table 1. Powder X-ray diffraction data were collected on powder samples obtained from ground bulk samples of flux-


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grown products using a PANalytical X-Pert PRO with a Cu Kα radiation source. The resulting patterns were compared to those calculated based on the single crystal structures.

Table 1. Crystallographic data collection parameters for the three Ba/Yb/Mg/Si phases. Chemical formula

Ba2Yb0.88Mg11.12Si7

Ba5Yb2.26Mg16.73Si12

Ba20Yb4.70Mg61.30Si43

Mr (g/mol)

893.91

1826.26

6258.16

Crystal system, space group

Hexagonal, P-6

Hexagonal, P-62m

Hexagonal, P63/m

Temperature (K)

273

273

273

a, c (Å)

11.0830(6), 4.4150(3)

14.8223(8), 4.5174(3)

27.928(4), 4.4808(6)

V (Å3)

469.65(6)

859.51(11)

3026.6(9)

1

1

1

Dx (g/cm )

3.161

3.529

3.433

Radiation type

Mo Kα1 (λ = 0.71073)

Mo Kα1 (λ = 0.71073)

Mo Kα1 (λ = 0.71073)

µ (mm-1)

9.30

12.52

10.74

Rint

0.022

0.023

0.076

2θ values (°)

2θmax = 57°, 2θmin = 2.1°

2θmax = 57°, 2θmin = 3.2°

2θmax = 50.2°, 2θmin = 3.4°

No. of reflections

853

838

2055

No. of parameters

46

46

141

R1, wR2

0.0107, 0.0245

0.0105, 0.0257

0.0331, 0.0777

∆〉max, ∆〉min (e Å-3)

0.47, -0.52

0.58, -0.55

3.98, -1.35

Z 3

a

2

2 2

2 2 1/2

R1=Σ(|Fo|- |Fc|) / Σ |Fo|; wR2=[ Σ [w(Fo - Fc ) ]/ Σ (w|Fo| ) ] .

Atomic positions, refined occupancies, and bond lengths for all three structures are given in Supporting Information (Table S1 and S2). Assignments of atomic positions for barium were straightforward in each case. The positions of magnesium and silicon atoms were determined by comparison with the previously reported parent structure types (Ho20Ni66P43, Ho5Ni19P12, and Zr2Fe12P7) and by consideration of cation and anion packing and bond lengths. All three structures exhibit Yb/Mg mixing on specific positions. For the Ba5Yb2.29Mg16.71Si12 structure, the Yb5 position (3g Wyckoff site) refined as 80% occupied, but assigning the site as barium or magnesium resulted in an occupancy greater than 100%. Taking into consideration the bond lengths of the site and the occupancy, the position was therefore assigned as mixed occupied with Mg, with the refinement indicating a 76.4%/23.6% Yb/Mg mixing. Likewise, in the



Ba2Yb0.85Mg11.15Si7 structure the Yb4 position (3j Wyckoff site) showed partial occupancy; the position was therefore assigned as mixed occupied with Mg, with the refinement indicating a 29.5%/70.5% Yb/Mg mixing. The Ba20Yb4.7Mg61.3Si43 structure proved far more complex, with a larger unit cell, higher number of unique crystallographic sites, and mixed occupancies and split sites. One of the eight unique silicon sites, Si8 on a 2a Wyckoff site, was too close to its symmetry equivalents and refined as close to 50% occupied. Mixing on the Si site is unlikely to occur since all other possible candidates are cationic and the half occupancy is similar to that of the Ni8 site in the Ho20Ni66P43 structure.14 The occupancy was therefore fixed at 50%. The nearby surrounding atoms are consequently also disordered. Yb11 and Yb12 (both on 6h Wyckoff sites) represent a 50/50 split site; occupancy of these positions depends on whether the Si8 closest to them is occupied (in which case Yb11 is not occupied and Yb12 is) or not (in which case Yb11 is occupied and Yb12 is not; see discussion). Constraining the site-splitting as 50/50 and subsequent refinement indicated electron density less than is expected for Yb. Based on bond lengths to silicon (similar for Mg and Yb, significantly smaller than Ba), both split sites were refined as mixed occupied by Yb and Mg. Yb11 refined as 19% Yb / 31% Mg, and Yb12 as 4% Yb / 46% Mg. An additional Yb/Mg mixed site was found elsewhere in the structure, refining at 55.2% Yb and 44.8 % Mg. Electronic Structure Calculations Density of states calculations were carried out using the Stuttgart TB-LMTO-ASA software package, based on atomic positions and space groups determined from single crystal Xray diffraction experiments.18,19 Calculations were not possible for the Ba20Yb4.7Mg61.3Si43 structure. Modelling the disorder (split sites and mixed occupancies) in this compound requires the use of a supercell with a doubled c-axis, resulting in a structure with too many atoms for the software to handle. The Ba5Yb2.29Mg16.71Si12 structure also contains disorder, but only in the form of Yb/Mg mixed occupancy on one site (76.4%/23.6% Yb/Mg). This site was modelled as completely filled by Yb, resulting in a stoichiometry of Ba5Yb3Mg16Si12 for the model compound. The Ba2Yb0.85Mg11.15Si7 structure also has one Yb/Mg mixed occupancy site (29.5%/70.5% Yb/Mg). This site was modelled as completely filled by Mg, resulting in a stoichiometry of Ba2Mg12Si7 for the model compound used in the calculations. Empty spheres 6 ACS Paragon Plus Environment

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were added by the program where appropriate to fill the unit cell volume. An 18 x 18 x 6 k-point mesh was used for both hexagonal structures; these were integrated using the tetrahedron method. The basis set for Ba5Yb3Mg16Si12 consisted of Ba 6s/6p/5d/4f, Yb 6s/6p/5d/4f, Mg 3s/3p/3d and Si 3s/3p/3d orbitals. The Ba 6p, Yb 6p, Mg 3d and Si 3d orbitals were downfolded. The basis set for Ba2Mg12Si7 consisted of Ba 6s/6p/5d/4f, Mg 3s/3p/3d, Si 3s/3p/3d. The Ba 6p, Mg 3d and Si 3d orbitals were downfolded. Magnetic Properties Magnetic susceptibility measurements were carried out using a Quantum Design SQUID magnetic property measurement system. Small single crystals of Ba20Yb4.7Mg61.3Si43 were ground into powder which was weighed and then placed into a gelatin capsule. This capsule was placed into the SQUID sample holder. Temperature dependence data were collected from 1.8 – 300 K at an applied field of 500 G. A sample of Ba2Yb0.85Mg11.15Si7 was prepared similarly and measured under identical conditions. For Ba5Yb2.29Mg16.71Si12, a large single crystal was weighed and then sandwiched between kapton tape; this was then placed in the SQUID sample holder. Data were collected from 1.8 K – 300 K at an applied field of 1000 G.

Results and Discussion Synthesis Reactions of tetrels (Tt = Si, Ge, Sn, Pb) with heavy alkaline earth metals (Ca, Sr, Ba) and divalent rare earth metals (Eu or Yb) in Mg/Al flux produce crystals of complex Zintl phases which may prove to be promising thermoelectric materials. Observed products of our ongoing survey of (Mg/Al)/A/Tt reactions (A = Ca, Sr, Ba, Eu, Yb) include CaMgSi, Ba2Ca2Mg10Si7, BaSrMg12Ge7, A8-xMg15+xSi13 (A = Sr, Ba, or Eu), and EuMgTt (Tt = Sn, Pb).12,13,20,21 For the most part aluminum acts as an inert solvent component in these flux systems, facilitating the reaction by lowering the melting point and viscosity of magnesium but usually not incorporating itself into the product compounds. This behavior is in contrast to what occurs when the divalent alkaline earth or rare earth metal reactants are substituted with trivalent rare earths (R = La – Nd, Sm, Gd – Tm, Y). Reactions of trivalent rare earths with silicon in Mg/Al flux lead to



compounds that incorporate both flux elements and iron from the crucible, forming pentenary intermetallics such as Gd5Mg5Fe4Al12Si6 and Yb3-δFeAl4-xMgxSi2.10,11 (a)

(b)

Figure 1. a) Microscope image of crystals of Ba5Yb2.29Mg16.71Si12 grown from Mg/Al flux, on 1mm grid paper. b) SEM image of Ba20Yb4.7Mg61.3Si43 grown from Mg/Al flux.

Ba20Yb4.7Mg61.3Si43 was initially observed as a low yield byproduct of reactions of barium, ytterbium and silicon in an Al/Mg flux. The major products were BaMgAlSi2 (with the BaAl4 structure type) and Ba5Yb2.29Mg16.71Si12. Ba20Yb4.7Mg61.3Si43 grows from Mg/Al flux as thin silver metallic needles averaging 0.1 mm in diameter and up to 1 mm and Ba5Yb2.29Mg16.71Si12 forms as large silver hexagonal rods up to 0.6 mm in diameter and 3 mm long, as shown in Figure 1; however, this latter phase has also been seen to grow as needle crystals. Ba2Yb0.85Mg11.15Si7 is generally seen a minor product and has been seen to grow as either a silver rod or needle crystal. All three silicide compounds are very air stable and show no sign of oxidation even after several weeks of exposure to moist air. Different reaction stoichiometries were explored to optimize the yield of the Ba20Yb4.7Mg61.3Si43 phase, which is of particular interest due to its highly complex structure. Ultimately a 21:9:3:2.5:0.5 Mg/Al/Si/Ba/Yb stoichiometry that is more magnesium-rich was chosen to avoid the BaMgAlSi2 8 ACS Paragon Plus Environment

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byproduct. The heating profile was also changed. A shorter cooling period from 950 to 725C° in 10 hours favored the growth of Ba20Yb4.7Mg61.3Si43 (as seem in PXRD data in Figure 2) in up to 30% yield based on silicon; however, the Ba5Yb2.29Mg16.71Si12 phase was still prevalent in some reactions indicating that the two competing phases are energetically similar and form under very similar conditions. A longer cooling period of 80 hours promoted more formation of the Ba5Yb2.29Mg16.71Si12 phase.

Theoretical Intensity (au)

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10

Experimental

20

30

40

50

60

70

2θ Theta Figure 2. Powder X-ray diffraction data for solid products of reactions of Ba, Yb, and Si in Mg/Al flux (Mg/Al/Si/Ba/Yb in 21:9:3:2.5:0.5 mmol ratio). Calculated pattern derived from the Ba20Yb4.7Mg61.3Si43 crystal structure is in blue while the data for powdered product are in black.

The co-existence of these three Ba/Yb/Mg/Si phases in the Mg/Al flux is not surprising, considering their similar building blocks (see below) and stoichiometries; these three competing products likely have very similar thermodynamic stabilities, leading to difficulty in favoring the formation of one over the other. The analogous phosphides (Ho2Ni12P7, Ho5Ni19P12 and Ho20Ni66P43) were also reported to occur as mixed phases albeit from high temperature reactions of stoichiometric ratios of reactants.22-24 The use of metal flux synthesis allows for manipulation of reactant concentrations, cooling rates, and quenching temperatures, potentially enabling the isolation of one product over another. Several families of compounds formed in metal flux reactions exhibit similar competition between products. For instance, La6(Mn/Ni/Al)13Sn and La11(Mn/Ni/Al)13Sn4-d both crystallize from reactions of Mn, Al, and Sn in La/Ni flux and possess identical transition metal layers and La/Sn slabs of different sizes. La11(Mn/Ni/Al)13Sn4d

only exists between 600 – 700 °C; quenching the reaction at higher temperatures yields only

La6(Mn/Ni/Al)13Sn.25 9 ACS Paragon Plus Environment


Attempts were made to synthesize the Ba20Yb4.7Mg61.3Si43 compound from a stoichiometric combination of the elements (without extra flux) using a more traditional solid state heating profile. The reaction mixture 30.65 mmol Mg, 21.5 mmol Si, 10 mmol Ba and 2.35 mmol Yb was sealed in a steel crucible under argon and then placed inside a quartz sleeve. The ampule was heated to 950°C in 10 h, held at 950°C for 10 h, and cooled to 25°C in 190 h. The product was a silver metallic chunk; when ground, it formed a silver powder. PXRD data indicated the formation of Si, Mg2Si and YbMg2; the title phase was not found (see Figure S1 in supporting information). Crystal Structures Ba20Yb4.7Mg61.3Si43 and the byproducts Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7 form under similar reaction conditions and all are structural analogs of ternary metal phosphides. A large number of RxTyPz compounds (R = rare earth/early transition metal, T = late transition metal) have been reported that feature a 2:1 metal to phosphorus ratio, with phosphorus sites surrounded by 9 metal sites in tricapped trigonal prismatic coordination. These P@(R,T)9 units pack together in a variety of different patterns, many of which form homologous series such as Rn(n-1)T(n+1)(n+2)Pn(n+1)+1.26,27 As a result, all of these phosphides have hexagonal symmetry with similar c-axis lengths. The flux-grown Ba/Yb/Mg/Si compounds are analogs of the Zr2Fe12P7, Ho20Ni66P43 and Ho5Ni19P12 structure types; all three are comprised of face-sharing Si@(Ba,Yb,Mg)9 tricapped trigonal prisms (resulting in very similar c-axis lengths in the 4.4 – 4.5Å range, see Table 1). Ba20Yb4.7Mg61.3Si43 has the Ho20Ni66P43 structure type in hexagonal space group P63/m; this is shown in Figure 3. Barium cations occupy the holmium sites and magnesium and ytterbium cations mix on the nickel sites. The silicon anions are positioned in the phosphorus sites. There are eight crystallographically unique Si anion sites; each of these is surrounded by 9 alkaline earth and rare earth ions (Si@(Ba, Yb, Mg)9). The Ba-Si bond lengths (3.438(2) 3.554(2)Å) in these tricapped trigonal prisms are within range of previously reported Ba-Si bond lengths (3.3729-3.7653Å) in BaSi2.28 The Mg-Si bond lengths (2.658(4) - 2.919(4)Å) are shorter, and similar to those in Mg2Si (2.7657Å) and Ba1.9Ca2.4Mg9.7Si7 (2.734(1) – 2.903(1)Å).29,13


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(a)

(b)

Figure 3. a) Ba20Yb4.7Mg61.3Si43 structure viewed along the c-axis. Yb/Mg mixed sites are in red, Mg sites are in yellow, Ba sites are in black and Si sites are shown as tricapped trigonal prisms in various shades of blue and purple. b) Tricapped trigonal prismatic coordination of four out of the eight different silicon sites, viewed perpendicular to the long trigonal prisms axis.

All the Si sites are fully occupied except for Si8 (located on a chain of 2a Wyckoff sites along the c-axis), shown in Figure 4. This site cannot be fully occupied as this would result in unrealistically short bond distances to its symmetry equivalents (2.242Å) and to a nearby cation site (2.017Å). Allowing the Si8 occupancy to freely refine results in a value close to 50%. Similar disorder is seen in the Ho20Ni66Si43 parent structure, which features partial occupancy by phosphorus on this 2a site.24 Constraining this site as half-occupied makes physical sense with regard to the Si-Si bonding as well as the observed partial occupancy of surrounding sites. The resulting disorder of the 2a site perturbs the surrounding cations which are partially occupied split sites with mixed Mg/Yb occupancy. The bond distance between the Si disordered site and the closest of the split Mg/Yb sites in the same ab plane (2.017(1)Å) indicates that if the half occupied silicon is present, then this Mg/Yb site is not. The overall occupancy of the split sites were therefore constrained to each be 0.50, which allowed for refinement of their Mg/Yb mixing. The closest site (2.017(1)Å from the silicon site) refined as 38.5% Yb and 61.5% Mg; the one slightly further away (2.557(1) Å from silicon) refined as 7.1% Yb and 92.9% Mg. This latter distance is slightly shorter than reported Mg-Si bond lengths but still physically reasonable. The 11 ACS Paragon Plus Environment


unreasonable distance of 2.017Å is not actually observed, since the silicon site at that distance will not be occupied. Instead, the next Si8 site (further down the c-axis) will be present, at a distance of 3.015Å (see Figure 4). This is very close to the range of Yb-Si bonds (2.8962.9898Å) reported for YbSi2.30 The resulting stoichiometry of Ba20Yb4.7Mg61.3Si43 is comprised of 15.5 mol% Ba, 3.6%Yb, 47.5% Mg, and 33.3% Si, in reasonable agreement with the composition indicated by elemental analysis measurements. Given the environment of Si sites, they can be viewed as monoatomic anions with a -4 charge; this is balanced by the Ba, Mg, and presumably divalent Yb cations resulting in a charge balanced stoichiometry of (Ba2+/Yb2+/Mg2+)86(Si4-)43. This compound is therefore a Zintl phase and is expected to be a small band-gap semiconductor. (a)

(b)

Figure 4. Coordination of the disordered silicon site in Ba20Yb4.7Mg61.3Si43. Si sites are in blue, Yb/Mg sites are in dark and light red. (a) Positions of all disordered sites. Green arrow indicates the position of the split sites diagonal to the central Si atom. Blue arrow indicates the position of the split sites planar to the central Si atom. (b) Actual coordination when partial occupancies are taken into account. Each of these two models is 50% occupied.

The other two hexagonal Ba/Yb/Mg/Si phases found in this work--Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7--are shown in Figure 5. They are isostructural with ternary rare earth transition metal phosphides Ho5Ni19P12 and Zr2Fe12P7, respectively. The silicon anions occupy the phosphorus sites of the parent structures, barium cations occupy the rare earth or early 12 ACS Paragon Plus Environment

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transition metal sites, and magnesium and ytterbium cations are positioned on the late transition metal sites. Both of these structures are significantly simpler than that of Ba20Yb4.7Mg61.3Si43. They each have only 3 crystallographically unique silicon sites, coordinated by 9 alkaline earth and rare earth ions, forming tricapped trigonal prisms (Si@(Ba, Yb, Mg)9) that share faces. Both compounds have only one disordered site, occupied by a mixture of ytterbium and magnesium. For Ba5Yb2.29Mg16.71Si12 (space group P-62m) this is a 3g Wyckoff site, which refines as 76.4% Yb / 23.6% Mg. The distance between this mixed site and neighboring silicon sites (3.006(2) 3.202(1)Å) is similar to Yb-Si distances reported for YbSi2 (2.896-2.9898Å), but are too short to accommodate barium and longer than Mg-Si bonds reported for Mg2Si (2.7657Å).29,30 For Ba2Yb0.85Mg11.15Si7 (space group P-6) the mixed site is a 3j site which refines as 29.5% Yb / 70.5% Mg. The distance from this site to neighboring silicon sites (2.920(2) - 3.0119(4)Å) is again within the range expected for Yb-Si bonds and slightly long for Mg-Si bonds. With overall ratios of (Ba2+/Yb2+/Mg2+)24Si12 and (Ba2+/Yb2+/Mg2+)14Si7, both compounds have a 2:1 ratio of divalent cations to Si4- anions, and are therefore charge-balanced Zintl phases and expected to be small bandgap semiconductors or semimetals. (a)

(b)

Figure 5. (a) Ba5Yb2.29Mg16.71Si12 structure viewed down the c-axis, and coordination of the three silicon sites shown as blue tricapped trigonal prism polyhedra. The Yb/Mg mixed sites are in red, Mg sites are in yellow, Ba sites are in black and Si sites are in blue. (b) Ba2Yb0.85Mg11.15Si7 structure and silicon coordination polyhedra.



Magnetic susceptibility All three Ba/Yb/Mg/Si compounds exhibit roughly temperature independent susceptibility, indicating they are Pauli paramagnetic (see Figure 6). This confirms that the ytterbium ions in these materials are divalent. Normalizing all their formulas to a 2:1 metal to silicon ratio (“M2Si”), the magnitude of the Pauli paramagnetism ranges from 10-4 to 10-6 emu/mol. This is somewhat low compared to other Pauli paramagnetic metal silicides such as Yb(Zn,Si)2 and YbZn2Si2 which have moments in the 10-3 emu/mol range.31 However, the Pauli paramagnetism of metals and metalloids is directly dependent on the number of carriers at the Fermi level, which is expected to be low for these semi-metallic compounds (vide infra). Ba20Yb4.7Mg61.3Si43 has the highest susceptibility, indicating that the phase likely has more electronic states at EF and will exhibit more metallic characteristics than the other two phases. 1.00E-04

χ (emu/mol M2Si)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8.00E-05 6.00E-05 Ba5Yb3Mg16Si12 Ba5Yb2.3Mg16.7Si12 4.00E-05

Ba20Yb4.7Mg61.3Si443 Ba20Yb4.7Mg16.3Si43 Ba2Yb0.9Mg11.1Si7 Ba2Yb0.87Mg11.15Si7

2.00E-05

0.00E+00 0

100

200

300

T (Kelvin) Figure 6. Temperature dependence of the magnetic susceptibility for Ba20Yb4.7Mg61.3Si43, Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7 normalized to susceptibility per M2Si unit (M = Ba/Yb/Mg).

Electronic Structure Density of states calculations were carried out on ordered model compounds Ba5Yb3Mg16Si12 and Ba2Mg12Si7; the data are shown in Figure 7. Both models exhibit similar pseudo-gaps at the Fermi level; however, there exists a small number of states at Ef. These compounds are therefore expected to be poor metals. The partial DOS diagrams indicate that the silicon orbitals make the predominant contribution to the valence bands below and at the Fermi level, which is expected for “anionic” silicide species. Of the alkaline earth elements, the states derived from barium are found above the Fermi level, whereas magnesium states contribute to 14 ACS Paragon Plus Environment

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both the valence and conduction bands. The rare earth element Yb has 4f states localized in a narrow band just below Ef. While DOS calculations were not possible for the highly disordered Ba20Yb4.7Mg61.3Si43, this charge-balanced compound can be expected to have a similar pseudogap with a small number of states at the Fermi level. a)

b)

c)

d)

Figure 7. Density of states data for model compounds of the phases Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7. The Fermi level is set at 0 eV. a) DOS of Ba2Mg12Si7. b) Partial density of states contributions for different elements in Ba2Mg12Si7. c) DOS of Ba5Yb3Mg19Si12. d) Partial density of states contributions for different elements in Ba5Yb3Mg19Si12.

It is notable that Ba2Yb0.85Mg11.15Si7 is isostructural (and isoelectronic) with Ba1.9Ca2.4Mg9.7Si7 which was also grown from Mg/Al flux. The density of states data reported for that compound, modelled as Ba2Ca3Mg9Si7, featured a similar pseudogap at EF. 15 ACS Paragon Plus Environment


Ba1.9Ca2.4Mg9.7Si7 was found to exhibit semimetallic behavior, with a room temperature resistivity of 8 µΩ·m and a carrier concentration of 5.8 × 1019 cm-3, similar to values reported for Mg2Si0.4Sn0.6.13,4,32 It is likely that the electronic properties of the Ba/Yb/Mg/Si compounds reported here will be comparable. The incorporation of an additional heavy element (ytterbium) should increase the mass fluctuation scattering of phonons, so the thermal conductivity of the title phases will likely be even lower than the 2 W/mK reported for Ba1.9Ca2.4Mg9.7Si7.13 The large unit cell and disorder in Ba20Yb4.7Mg61.3Si43 will further lower the thermal conductivity; this phase is therefore of greatest interest as a potential thermoelectric material. For measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, additional work on optimizing the synthesis to allow each phase to be isolated in pure form is required. Residual traces of the flux must also be eliminated; preliminary explorations of heating at 500°C with a thermal gradient under high vacuum (10-5 Torr) indicates this is effective in removing excess magnesium.33

Conclusions A large amount of research is currently being carried out on Mg2Si-based thermoelectric compounds, most of which are doped solid solutions of cubic Mg2Si and Mg2Sn with the relatively simple anti-fluorite structure type. Substitution of the silicon with other tetrelides via high temperature synthesis routes such as spark plasma sintering maintains this simple cubic structure.32 Likewise, high temperature synthesis of ternaries derived from substituting magnesium for other divalent elements typically yield compounds with simple structures, exemplified by EuMgSi with the TiNiSi structure type.34 It is intriguing that our exploration of the Mg2Si/Ba2Si/Yb2Si phase space using Mg/Al flux chemistry yields compounds that are not merely (Mg/Ba/Yb)2Si solid solutions with small unit cells, but are instead a family of semimetals with a series of complex hexagonal structures. This opens up a new arena of metal silicide compounds. It is likely that Mg/Al flux reactions using other tetrelides (Tt = Ge, Sn) and other divalent metals (A = Ca, Sr, Eu) will also produce analogs of other members of the extensive RxTyPz metal phosphide structural family with semimetallic behavior, as we initially postulated in our report on Ba1.9Ca2.4Mg9.7Si7 (Zr2Ni12P7 structure type).13 In the Ba/Yb/Mg/Si system, Ba20Yb4.7Mg61.3Si43 is of particular interest, given the inherent disorder in the structure 16 ACS Paragon Plus Environment

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and large number of unique crystallographic sites that are likely amenable to substitution and doping. The complexity of the structure is matched by the complexity of the synthesis; further work is needed to determine the optimal heating profile that will eliminate the formation of the energetically similar Ba5Yb2.29Mg16.71Si12 and Ba2Yb0.85Mg11.15Si7 byproducts. In-situ calorimetry or diffraction studies on these flux reaction systems may shed light on the specific conditions required for crystallization for each phase.

Acknowledgements This research was supported by the Division of Materials Research of the National Science Foundation (grant number DMR-14-10214). This work utilized the scanning electron microscope facilities of the Biological Sciences Imaging Resource (BSIR) in the FSU Biology Department; we thank Dr. Eric Lochner for his assistance with this equipment.

Supporting Information Tables of atomic positions and bond lengths for all three Ba/Yb/Mg/Si compounds; powder X-ray diffraction data for the product of attempted stoichiometric synthesis. Further crystallographic information is available as deposited CIF files.

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