Competing Phases, Complex Structure, and Complementary

Jul 25, 2013 - ABSTRACT: Four new intermetallic phases R3-δFeAl4-xMgxSi2 (R = Yb, Dy) and. R3-δFeAl4-xMgxGe2 (R = Er, Y) were synthesized in Mg/Al ...
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Competing Phases, Complex Structure, and Complementary Diffraction Studies of R3‑δFeAl4‑xMgxTt2 Intermetallics (R = Y, Dy, Er, Yb; Tt = Si or Ge; x < 0.5) Xiaowei Ma,† Jeffrey B. Whalen,†,‡ Huibo Cao,§ and Susan E. Latturner*,† †

Department of Chemistry and Biochemistry and ‡National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306, United States § Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Four new intermetallic phases R3‑δFeAl4‑xMgxSi2 (R = Yb, Dy) and R3‑δFeAl4‑xMgxGe2 (R = Er, Y) were synthesized in Mg/Al (1:1 mol ratio) molten flux. These phases have a new structure type in tetragonal space group P4/mbm (a = 13.3479(9) Å, c = 4.0996(3) Å, Z = 4, and R1 = 0.0176 for Yb2.77FeAl3.72Mg0.28Si2). The structure features iron in trigonal prismatic coordination by aluminum atoms. The prisms share trigonal faces to form chains running along the c-axis, similar to the chains seen in several related structures, including that of the previously reported competing phases R5Mg5Fe4Al12Si6 (R = Gd, Dy, and Y). Occupancies of Mg, Al, and Si sites in Yb2.77FeAl3.72Mg0.28Si2 were determined by single crystal X-ray and neutron diffraction, bond length analysis, and comparison to atom positions and bond lengths in the isostructural germanides. Electronic structure calculations indicate these phases are polar intermetallics with pseudogaps near the Fermi level. The magnetic properties of these phases are determined by the rare earth ions. Y3‑δFeAl4‑xMgxGe2 is Pauli paramagnetic; the Yb3+ cations in Yb2.77FeAl3.72Mg0.28Si2 exhibit Curie−Weiss behavior with no ordering in the temperature range observed. Er3‑δFeAl4‑xMgxGe2 and Dy3‑δFeAl4‑xMgxSi2 order antiferromagnetically at TN = 2.8 and 4.0 K, respectively; the former undergoes a spin reorientation at ∼4400 G according to the ac field dependence of magnetization. KEYWORDS: metal flux, intermetallics, antiferromagnetism, neutron diffraction, silicide

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Flux reactivity is of particular interest if mixed metal fluxes are used; we have recently explored syntheses in mixtures such as RE/Ni, RE/Co (RE = La, Ce, Nd, etc), Mg/Al, and Ca/Li. Some metal combinations form low-melting eutectics at specific elemental ratios. For instance, a 24/76 mol ratio of Ce and Co melts at 424 °C; a 12/88 ratio of Al and Si melts at 577 °C; and a 81/19 ratio of Au and Si, while too expensive to be used as a flux, exhibits a low eutectic melting point of 363 °C.17 Eutectic formation allows for lowered reaction temperatures, which favor the formation of kinetically stabilized phases. However, flux mixtures introduce the additional complication of one or both flux metals potentially being incorporated into the final products. The Mg−Al phase diagram reveals a broad low temperature range (∼470 °C) with 40−60% Mg content.17 The reaction of Ca and Si in a 1:1 mol ratio Mg/Al flux yields CaMgSi; in this case, Mg is a reactive flux component and Al is inert.18 On the other hand, we recently reported the growth of R5Mg5Fe4Al12Si6 (R = Y, Gd, Dy) in the same flux; both

INTRODUCTION

Exploratory synthesis in molten metal solvents has resulted in the discovery of a large number of new intermetallic silicides and germanides, a class of materials of interest for their wide ranging applications.1−4 Low melting metals most commonly used as reaction media include Al, Ga, In, Sn, and Pb.5 Aluminum flux has been proven to be a particularly productive growth medium; complex silicide phases synthesized in molten aluminum include Gd1.33Pt3Al7Si,6 R(AuAl2)nAl2(AuxSi1‑x)2 (R = La−Gd, Yb),7 Ba8Al14Si31,8 R8Ru12Al49Si9(AlxSi12‑x) (R = Pr, Sm; x ≈ 4),9 and RFe4Al9Si6 (R = Tb, Er).10 The crystals can be isolated readily by soaking the product in a strong basic solution to etch away the flux after the reaction. Heavier group III elements gallium and indium are also good flux synthesis media. Gallium often incorporates into products to yield gallides such as RE4FeGa12‑xGex (RE = Y, Ce, Sm, Gd, or Tb), RE3Ga9Ge (RE = Y, Ce, or Gd), RE3Ni3Ga8Ge3 (RE = Sm, Gd), and Yb3Ga4Ge6.11−14 In comparison to the usually reactive fluxes Al and Ga, molten indium is often an inert solvent; for instance, germanides β-RENiGe2 (RE = Dy, Er, Yb or Lu) and α-CaGe2 can be isolated from excess indium flux.15,16 © 2013 American Chemical Society

Received: June 18, 2013 Revised: July 10, 2013 Published: July 25, 2013 3363

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Table 1. Crystallographic Data and Collection Parameters for Yb2.77FeAl3.72Mg0.28Si2, Dy3‑δFeAl4‑xMgxSi2, Y3‑δFeAl4‑xMgxGe2, Er3‑δFeAl4‑xMgxGe2, and Yb5Fe4Al17Si6 Yb2.77FeAl3.72Mg0.28Si2, XRD data

Yb2.77FeAl3.72Mg0.28Si2, neutron data

cryst syst space group cell edges (Å)

tetragonal tetragonal P4/mbm P4/mbm a = 13.3479(9),b c = 4.0996(3)b

V, Å3 Z ρ (g/cm3) 2θ (max) radiation T (K) reflns unique reflns data/params μ (mm−1) R(int) R1/wR2a (I > 2(I)) R1/wR2 (all data) largest diff peak and hole (e Å−3)

730.41(9) 4 6.35 56.55° Mo Kα 290 7778 533 533/39 37.78 0.0384 0.0172/0.0385 0.0182/0.0388 1.36/−0.90

a

730.41(9) 4 6.35 80° 1.5424 Å 290 404 138

0.0313 0.0475 0.08160

Dy3‑δFeAl4‑xMgxSi2 tetragonal P4/mbm a = 13.527(4), c = 4.142(1) 758.0(4) 4 6.19 56.03° Mo Kα 290 8116 550 550/40 31.77 0.0366 0.0131/0.0220 0.0155/0.223 0.98/−0.72

Y3‑δFeAl4‑xMgxGe2 tetragonal P4/mbm a = 13.680(1), c = 4.1507(3) 776.8(1) 4 4.91 56.40° Mo Kα 290 8382 566 566/38 31.94 0.0572 0.0187/0.0393 0.0231/0.0405 0.61/−0.72

Er3‑δFeAl4‑xMgxGe2 tetragonal P4/mbm a = 13.562(4), c = 4.120(1) 757.8(4) 4 7.09 56.52° Mo Kα 290 8148 557 557/38 42.88 0.0346 0.0152/0.0292 0.0156/0.0293 1.22/−0.99

Yb5Fe4Al17Si6 tetragonal P4/mmm a = 11.433(8), c = 4.040(3) 528.2(7) 1 5.40 56.52° Mo Kα 290 5908 424 424/35 25.60 0.0385 0.0170/0.0378 0.0190/0.0383 0.83/−0.90

R1 = (|Fo| − |Fc|)/|Fo|; wR2 = [[w(Fo2 − Fc2)2]/(w|Fo|2)2]1/2. bUnit cell parameters for Yb2.77FeAl3.72Mg0.28Si2 determined by X-ray diffraction data. loaded into stainless steel crucibles in an Ar-filled glovebox. The steel crucibles were welded shut under argon and then sealed into fused silica tubes under vacuum (30 mTorr). All reaction ampules were placed in a muffle furnace and heated from room temperature to 950 °C in 10 h, held at 950 °C for 5 h, cooled to 750 °C in 80 h, and then held at 750 °C. While at this temperature, the reaction ampules were quickly removed from the furnace, flipped, and centrifuged to decant excess Mg/Al molten flux off the product crystals which adhere to the crucible walls. Reactions with varying reactant ratios were attempted to optimize yield and crystal size. After the optimal ratios were determined, reactions were carried out using the same preparation method in niobium crucibles, to avoid possible contamination from the elements found in steel crucibles. Two additional phases were prepared in this work, using aluminum flux. Yb5Fe4Al17Si6 was produced from reactions carried out to explore the effect of eliminating Mg from the flux. Crystals of Yb5Fe4Al17Si6 were obtained from reacting Al/Si/Fe/Yb (30/2/1/1 mmol) in steel crucibles with the same heating and centrifuge procedure described for the title phases. The known ternary phase YbAl2Si2 was then synthesized to be used as a standard to determine accurate Al/Si ratios in SEM-EDS measurements (vide infra).21 To grow crystals of this phase, Al, Si, and Yb were loaded into an alumina crucible with a mmol ratio of 20/2/1. The alumina crucible was then sealed in a fused silica tube, and the reaction was carried out using a different heating profile: the ampule was heated to 950 °C in 10 h, held at 950 °C for 5 h, and then cooled to room temperature in 80 h. Once the reaction was finished, the alumina crucible was soaked overnight in a 5 M NaOH solution to remove the flux from the product YbAl2Si2 crystals. Elemental Analysis. SEM-EDS analysis was performed using a JEOL 5900 scanning electron microscope (30 kV acceleration voltage) equipped with PGT Prism energy dispersion spectroscopy software. Selected crystals were arranged on double-sided carbon tape adhered to an aluminum sample puck. Each crystal was cleaved to expose inner portions to acquire more accurate elemental analysis of the bulk sample and avoid erroneous readings due to residual flux coating on the surface. Several spots on each crystal were analyzed for 60 s at each location. Magnesium analysis was hindered by the very small amount of this element present in the R3‑δFeAl4‑xMgxTt2 phases (x < 0.5) and some overlap of the Mg Kα peak with that of Al; a distinct small Mg Kα peak was observed for the Y3‑δFeAl4‑xMgxGe2 analogue, but not for the others. The flux-grown YbAl2Si2 crystals were used as an external reference to enable more accuracy in analysis of Si and Al contents of

magnesium and aluminum were incorporated into the product.19 The present work demonstrates the usefulness of complementary diffraction techniques in the analysis of the complex structure of new quinary phases grown in Mg/Al flux. Both flux elements are incorporated when reacted with iron, late rare earths, and either silicon or germanium. The product silicides (R3‑δFeAl4‑xMgxSi2 with R = Yb or Dy) and germanides (R3‑δFeAl4‑xMgxGe2 with R = Er or Y) crystallize in a new tetragonal structure type. Accurate determination of the Mg, Al, and Si site occupancies required a combination of single crystal neutron and X-ray diffraction studies, which also enabled the observation of Mg/Al mixed occupancy on one site. Observing and understanding the mixing behavior of these elements is of great interest for optimizing the properties of lightweight alloys and adventitious precipitates that form therein, such as the “π phase” Al9FeMg3Si5, a compound with a structure which has proven difficult to characterize.20 Electronic structure calculations on model compounds Y 3 FeAl 3.5 Mg 0.5 Ge 2 and Y3FeAl4Ge2 indicate that the change in valence electron count resulting from Mg incorporation may induce vacancies on one or both rare earth sites. The title phases form in the midst of a rich phase space, with the large excess of flux allowing the syntheses to forego formation of many known binary and ternary phases. Indeed, the major competing phase is another quinary phase featuring similar iron-centered building blocks, the previously reported R5Mg5Fe4Al12Si6 (R = Y, Gd, Dy).19 The formation of R3‑δFeAl4‑xMgxSi2 instead of R5Mg5Fe4Al12Si6 can be promoted by adjusting the R:Tt ratio in the synthesis; the title phases also incorporate a much smaller amount of Mg into their structure.

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EXPERIMENTAL METHODS

Synthesis. Reactants were used as received: Mg and Al metal slugs (99.95%), Fe (99+%) and Ge (99.999%) powders from Alfa Aesar; Si (99+%), Yb, Er, and Y (99.9%) powders from Strem Chemicals; Yb slugs and Dy powder (99.9%) from Metall. The elements were initially weighed out in Mg/Al/Tt/Fe/R ratios of 15/15/2/1/1 mmol and 3364

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Table 2. Atom Positions and Isotropic Thermal Parameters for Yb2.77FeAl3.72Mg0.28Si2 Yb1 Yb2 Fe1 Si1 Al1/Mg1 Al2 Al3 Al4

Wyckoff site

x

y

z

occa

Uisoa

8j 4h 4h 8i 2a 8i 4g 2c

0.17837(2) 0.66751(2) 0.12263(7) 0.0971(1) 0 0.3493(1) 0.2038(1) 0

0.05979(2) 0.16751(2) 0.62263(7) 0.1872(1) 0 0.0064(1) 0.7038(1) 0.5

0.5 0.5 0.5 0 0 0 0 0.5

0.92(2) 0.93(6) 1 1 0.44(9)/0.56(9) 1 1 1

0.0167(18) 0.020(3) 0.022(3) 0.024(3) 0.024(3) 0.024(3) 0.024(3) 0.024(3)

a

The occupancy and isotropic thermal parameters were determined from the refinement of single crystal neutron diffraction data. Atom positions were determined from the refinement of single crystal X-ray diffraction data. the silicide phases. For instance, the EDS data for YbAl2Si2 exhibit an Al/Si atomic ratio of 44.2%/26.0% versus 42.9%/13.1% for Yb 3‑δ FeAl 4‑x Mg x Si 2 . This indicates that the Al:Si ratio in Yb3‑δFeAl4‑xMgxSi2 should be 2:1 considering the similar backscatter coefficients of aluminum and silicon in EDS measurement.22 X-ray Diffraction. For all phases studied in this work, powder Xray diffraction data were collected on a PANalytical X’Pert PRO with a Cu Kα radiation source, and single crystal diffraction data were collected at room temperature on a Bruker APEX2 single crystal diffractometer with a Mo Kα radiation source. Selected crystal samples were broken into suitable size, and small spheroid fragments were mounted on glass fibers for diffraction. Data were processed using the SAINT and SADABS programs.23 Space group assignment was accomplished by XPREP, and refinement of the structure was performed using SHELXTL. 24 The structures of the four R3‑δFeAl4‑xMgxTt2 title phases were solved in tetragonal space group P4/mbm; Yb5Fe4Al17Si6 was solved in tetragonal space group P4/ mmm. Crystallographic data and collection parameters for all five phases are shown in Table 1; Tables 2 and 3 show atom positions and

occupied but the 8j site consistently dropped to 97−99%. Although the variation from 100% was small and occupancy values can be affected by other factors such as absorption effects, allowing the occupancy to refine consistently yielded more appropriate thermal parameters for the 8j rare earth site. Neutron Diffraction. To gain more insight into the occupancies of the Mg, Al, and Si sites, a large single crystal (1 × 1 × 4 mm3) of Yb3‑δFeAl4‑xMgxSi2 was studied by neutron diffraction at the HB-3A four-circle single crystal diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. The data were collected at 300 K with neutron wavelength 1.5424 Å from a bent perfect Si-220 monochromator.25 The structure refinement was based on ∼400 reflections and completed using the program FULLPROF.26 To limit the number of refinable parameters, the atomic positions were fixed to those obtained from the single crystal X-ray diffraction study of this phase, and only the thermal displacement and occupancy parameters were refined. Thermal parameters of light elements (Al, Si, and Mg) were constrained to be equivalent. The neutron diffraction data collection conditions and refined structural parameters are shown in Tables 1 and 2. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra for Yb2.77FeAl3.72Mg0.28Si2 were obtained on a Physical Electronics PHI 5100 series XPS with a nonmonochromated dual anode (Mg and Al) source equipped with a single channel hemispherical energy analyzer. The Al Kα X-ray source (15 kV and 30 mA) was used. Single crystals were placed on carbon tape adhered to a XPS stage puck. To avoid the effect of impurities on the surface, the XPS spectra were collected after sputtering with Ar ions (5 kV) for 30 min. Electronic Structure Calculations. Density of states (DOS) and crystal orbital Hamilton population (COHP) calculations were carried out with the tight binding-linear muffin tin orbitals-atomic sphere approximation (TB-LMTO-ASA) program package.27 To avoid complications from partially filled shells and site mixing, model compounds were used. Yb3‑δFeAl4‑xMgxSi2 was modeled as Y3FeAl4Si2 (atomic positions determined by single crystal X-ray diffraction of Yb3‑δFeAl4‑xMgxSi2 were used, with x = 0 and Yb3+ replaced with Y3+ at 100% occupancy). Er3‑δFeAl4‑xMgxGe2 was modeled either as Y3FeAl4Ge2 (atomic positions for Er3‑δFeAl4‑xMgxGe2 used, with Er3+ replaced with Y3+, and x = 0) or Y3FeAl3.5Mg0.5Ge2 (with x = 0.5, corresponding to 100% Mg occupancy on the 2a site). No empty Wigner−Seitz spheres were needed to fill the empty space in the structure. The following radii of atomic spheres were used: r(Y) = 3.54/3.70 Å, r(Fe) = 2.55 Å, r(Mg) = 3.48 Å, r(Al) = 2.55/2.87/3.11 Å, r(Si) = 2.82 Å, r(Ge) = 2.92 Å. The basis set contains Y(5s, 4p), Fe(4s, 3d, 4p), Mg(3s, 3p), Al(3s, 3p), Si(3s, 3p), and Ge(4s, 4p), with Y(4d, 5p), Mg(3d), Al(3d), Si(3d), and Ge(4d) being downfolded. The calculation was made for 195 κ points in the irreducible Brillouin zone. Integration over the Brillouin zone was performed by the tetrahedron method.28 Magnetic Properties. Magnetic measurements were carried out on a Quantum Design SQUID Magnetic Property Measurement System. Large single crystals were selected and held between two strips of kapton tape, oriented with c-axis parallel to the applied field.

Table 3. Atom Positions and Isotropic Thermal Parameters for Yb5Fe4Al17Si6 Yb1 Yb2 Fe1 Al1 Al2 Al3 Al4 Si1 Si2

Wyckoff site

x

y

z

Ueqa

4n 1a 4l 4j 1d 8q 4k 4m 2f

0.26972(3) 0 0.2904(1) 0.2027(1) 1/2 0.1169(1) 0.3321(1) 0.1755(2) 0

1 /2 0 0 0.2027(1) 1 /2 0.3701(1) 0.3321(1) 0 1 /2

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

0.0087(1) 0.0123(1) 0.0081(2) 0.0121(4) 0.013(1) 0.0093(3) 0.0041(4) 0.0114(4) 0.0132(6)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. isotropic thermal parameters for Yb 2.77 FeAl 3.72 Mg 0.28 Si 2 and Yb5Fe4Al17Si6, respectively. Further data for the other three R3‑δFeAl4‑xMgxTt2 analogues can be found in Supporting Information (Tables S1−S4 and CIF data). During the refinement of the silicide structures, assignments of rare earth and iron sites were straightforward; all lighter element sites were initially assigned as aluminum. Allowing the occupancies of these sites to vary was not informative (because of very similar X-ray scattering factors, the sites appeared fully occupied whether assigned as Mg, Al, or Si). Assignments were modified on the basis of bond length considerations and elemental analysis. In the final refinement cycles, occupancies of all sites were allowed to vary. All appeared fully occupied (100 ± 1%), with the exception of one or both of the rare earth sites in the R3‑δFeAl4‑xMgxTt2 compounds. The occupancies of both the 8j and 4h sites in Yb3‑δFeAl4‑xMgxSi2 were significantly lower than 100% (89% and 97%, respectively); for the other analogues, the 4h sites were fully 3365

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Magnetic susceptibility temperature dependence data were collected between 1.8 and 300 K at 100 G. Field-dependent magnetization data were collected at 1.8 K in fields up to 7 T. Field dependence studies of ac magnetization for Er3‑δFeAl4‑xMgxGe2 and Dy3‑δFeAl4‑xMgxSi2 were performed with 1 Hz frequency and 3 × 10−4 amplitude of the ac field under a dc bias field up to 3 T.

block: a chain of face-sharing FeAl6 trigonal prisms, highlighted in Figure 2. Reactions with dysprosium involve a competition

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RESULTS AND DISCUSSION Synthesis. R3‑δFeAl4‑xMgxTt2 compounds were grown from reactions of iron with either silicon or germanium and R = Y, Dy, Er, or Yb in Mg/Al flux in stainless steel crucibles. The phases form as air-stable silver rectangular crystals up to 5 mm in length; Figure 1 shows the SEM image of a

Figure 1. SEM image of a representative Yb2.77FeAl3.72Mg0.28Si2 crystal.

Yb2.77FeAl3.72Mg0.28Si2 crystal. Visual inspection indicates that very little flux residue is left on the crystal surface after centrifugation. The yield of this phase is optimized with a reactant ratio of Mg/Al/Si/Fe/Yb = 15/12/3/1/2 (yield ∼40%). Varying the ratio lowers the yield and leads to formation of competing byproducts such as YbAl2, Fe5Si3, and Mg2Si. Attempts to carry out the reaction in Nb crucibles produced a Yb/Fe/Al/Si quaternary phase instead (P3̅1c, a = 8.50(1) Å, c = 18.41(2) Å), the detailed structure of which is not yet solved. Its SEM image (Figure S1, Supporting Information) indicates that the crystals form in a twisted rod shape and are likely twinned. Failure to reproduce the synthesis of Yb3‑δFeAl4‑xMgxSi2 in Nb crucibles may indicate that incorporation of trace impurities from the steel is needed to form this compound (although the Dy analogue can be grown in Nb crucibles). Nevertheless, single phase products were obtained in steel crucibles, as indicated by the powder X-ray diffraction data in Figure S2 (Supporting Information). No incorporation of impurities from the steel was observed in the EDS analysis, although due to the low sensitivity of this technique, trace contamination cannot be ruled out. Attempts to synthesize R3‑δFeAl4‑xMgxSi2 analogues with other rare earth metals confirm that the structure is only stable for smaller rare earth ions. Reactions with early rare earths (R = La−Sm) produce RFe2Al8‑xMgx (x ≤ 1), quaternary variants of the RFe2Al8 structure;29 studies of their properties are in progress. Syntheses with R = Gd and Y produce the previously reported R5Mg5Fe4Al12Si6 compounds.19 It is notable that RFe 2 Al 8‑x Mg x , R 5 Mg 5 Fe 4 Al 12 Si 6 , and the title phases R3‑δFeAl4‑xMgxSi2 all contain an identical structural building

Figure 2. Structures of (a) the title phases R3‑δFeAl4‑xMgxSi2, (b) the R5Fe4Al17‑xMgxSi6 compounds Yb5Fe4Al17Si6 and Dy5Mg5Fe4Al12Si6, and (c) RFe2Al8‑xMgx phases (R = La−Nd), all viewed down the c-axis. Capped trigonal prismatic coordinations of iron atoms are shown as red polyhedra. Rare earth, iron, aluminum, magnesium, and silicon atoms are purple, red, cyan, green, and blue spheres, respectively.

between two possible products: Dy3‑δFeAl4‑xMgxSi2 and Dy5Mg5Fe4Al12Si6. Having a Si:Dy ratio above 1 makes dysprosium the limiting reactant and favors Dy5Mg5Fe4Al12Si6; if this ratio is 1 or lower, Dy3FeAl4‑xMgxSi2 is produced. This presents an example of control over the synthesis of compositionally and structurally related phases by careful adjusting of the reactant ratio. The Dy3‑δFeAl4‑xMgxSi2 analogue can be grown in steel or niobium crucibles, with the yield optimized at a reactant ratio of Mg/Al/Si/Fe/Dy = 15/15/2/ 1/2 (∼40% yield). Germanium analogues of the R3‑δFeAl4‑xMgxSi2 phases were sought to aid in structural analysis; Al and Si have very similar X-ray scattering factors, but Al and Ge can easily be distinguished in a structural refinement. Attempts to synthesize Yb3‑δFeAl4‑xMgxGe2 from the reaction of Yb/Fe/Ge in Mg/Al flux yielded the known ternary phase YbMgGe instead.30 The 3366

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analogous Dy/Fe/Ge reaction in Mg/Al flux produces DyFe4Al8‑xMgx, a substituted variant of DyFe4Al8.31 On the other hand, substitution of Yb by Y or Er (reactions of Y/Fe/ Ge or Er/Fe/Ge in Mg/Al flux) generated Y3‑δFeAl3.5Mg0.5Ge2 and Er3‑δFeAl4‑xMgxGe2; the optimal reactant ratio for both is Mg/Al/Ge/Fe/R = 15/15/1/1/2 (producing ∼70% and 45% yields, respectively). The powder patterns of the products indicated the presence of trace amounts of ErFe4Al8, YFe6Ge6, and YAlGe3 byproducts. These rare earths do not form the silicide R3‑δFeAl4‑xMgxSi2; reactions of Er/Fe/Si in Mg/Al flux lead to Er2Fe3Al9‑xSix, a quaternary analogue of Nd2Co3Al9.32 The extent of magnesium incorporation in the title phases was difficult to determine. EDS analysis typically indicated little to no Mg in the samples, and the slight overlap between the Al Kα and Mg Kα peaks (as well as the possibility of residual flux on the samples) puts the accuracy of the analysis into question. Neutron diffraction data does support the presence of Mg on one site in the Yb analogue (see structure description). However, the question remains whether magnesium is actually needed for the synthesis of this phase. To test this, a reaction of silicon, iron, and ytterbium in aluminum flux was prepared. Aluminum has a melting point of 660 °C, and crystalline products were isolated from the flux by centrifugation at 750 °C though the yield was quite poor. Single crystal diffraction analysis reveals that the reaction leads to a different phase, Yb5Fe4Al17Si6, isostructural to R5Mg5Fe4Al12Si6 (R = Dy, Gd, Y). Hence, the presence of magnesium is necessary to promote the formation of the R3‑δFeAl4‑xMgxSi2 phases; it is incorporated in small amounts (x = 0.5 or less) on a particular site to stabilize the structure. Attempts to synthesize the title phases from stoichiometric mixtures of the elements led to binary and ternary aluminide phases instead, likely due to loss of the volatile magnesium during the reaction. Structure. The R3‑δFeAl4‑xMgxTt2 title phases crystallize with a new structure type in tetragonal space group P4/mbm, shown in Figure 2a. The unit cell sizes scale as expected with the sizes of the R3+ cations and the incorporation of silicon versus germanium; Yb2.77FeAl3.72Mg0.28Si2 has the smallest unit cell and Y3‑δFeAl4‑xMgxGe2 the largest (see Table 1). Yb5Fe4Al17Si6 has the same structure as R5Mg5Fe4Al12Si6 (R = Gd, Dy, Y), which forms in space group P4/mmm and is shown in Figure 2b. 19 Magnesium is not incorporated in Yb5Fe4Al17Si6; all the Mg sites in the structure (4j and 1d Wyckoff sites) are instead occupied by Al atoms. A common building block in the R3‑δFeAl4‑xMgxTt2, R5Mg5Fe4Al12Si6, and RFe2Al8‑xMgx structures is the chain of iron-centered aluminum trigonal prisms (shown as red polyhedra in Figures 2 and 3), linked by sharing trigonal faces along the c-axis. Accordingly, the c-axis unit cell parameter for all three structure types is around 4 Å, defined by the length of the FeAl6 prism. The trigonal prisms are also connected to form dimers or chains within the ab-plane by monocapping Al or Si atoms. In Yb5Fe4Al17Si6 this linkage forms dimers along the a- or b-axes; in R3‑δFeAl4‑xMgxTt2, the dimers are at an angle to these axes. While the iron atoms in Yb5Fe4Al17Si6 are coordinated by a mixture of Al and Si atoms, the iron site in the R3‑δFeAl4‑xMgxTt2 structure is surrounded only by Al (8i, 4g, and 2c Wyckoff sites, see Figure 3a). In Yb2.77FeAl3.72Mg0.28Si2, the bond lengths between Fe and surrounding sites range from 2.31 to 2.60 Å (see Table 4); this falls in the Fe−Al bond range 2.3−2.8 Å observed in other intermetallic phases such as EuFe2Al8 and Al2FeSi.33,34 The iron−iron distance along the caxis is the length of the c-axis parameter (about 4 Å in both

Figure 3. Coordination environments of atoms in the R3‑δFeAl4‑xMgxTt2 structure. (a) Monocapped trigonal prismatic coordination of iron sites (red) which share trigonal faces to form chains. (b) Coordination of the 2a Wyckoff site, occupied by Mg/Al mixture. (c) Coordination of tetrel atoms in the 8i Wyckoff site. (d) Coordination of the rare earth ion in the 4h Wyckoff site. (e) Coordination of the rare earth ion in the 8j Wyckoff site. (f) Positioning of rare earth ions in an ab-plane (viewed down c-axis of unit cell); dashed lines indicate distances of less than 3.75 Å (Al and Fe atoms also in this plane removed for clarity).

structures and all analogs), and longer than that across the bridge of the dimers. The determination of Al and Si siting in R3‑δFeAl4‑xMgxSi2 (R = Yb or Dy) was aided by the synthesis of the germanide analogues R3‑δFeAl4‑xMgxGe2 (R = Er or Y). In the structural refinements of the germanides, Ge was clearly located on the 8i site, with the lighter Al atoms on 2a, 8i, 4g, and 2c sites; all atomic sites display occupancies close to 1. The tetrel atom is bonded to three nearby Al (or Al/Mg) sites in trigonal planar coordination; it is also surrounded by a trigonal prism of rare earth atoms along the same trigonal axis. The bonds to the 8i site in Yb2.77FeAl3.72Mg0.28Si2 are shorter than would be expected for an Al atom (for instance, the bonds to neighboring Yb ions range from 2.87 to 2.97 Å, compared to the Yb−Al bonds in the structure which are all longer than 3.1 Å; see Table 4), supporting the assignment of silicon to this site. Silicon (or germanium) is also the most electronegative element in the compound and will be stabilized by the surrounding trigonal prism of rare earth cations. 3367

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siting and mixed occupancies in Ba 8 Al 1 4 Si 3 1 and Pr8Ru12Al49Si9(AlxSi12‑x).8,9 The R3‑δFeAl4‑xMgxSi2 structure has 5 light-element sites and 3 possible elements filling them. Refinement of the neutron diffraction data indicates that the 2a site has a neutron scattering factor in between that of Al and Mg. The site is not likely to contain silicon (due to bond length considerations and comparison to the germanide analogues), and while it could be refined as a partially occupied Mg site, the X-ray data refinement does not support this. Therefore, it was refined as a mixed site, containing a mixture of 56% Mg and 44% Al (see Table 3); this would lead to a stoichiometry of Yb3‑δFeAl3.72Mg0.28Si2. The possibility of mixtures on the other light element sites in the structure was also investigated. Of particular interest were the 8i silicon site (which was confirmed to be 100% silicon) and the 2c aluminum site. The latter position is the bridging site across the iron centered dimers, centering the Fe−Al−Fe linkage. The observed Fe−Al bond length of 2.315(1) Å is on the short end of the expected Fe−Al bond lengths in intermetallics; this could indicate that this 2c position is actually filled by smaller silicon atoms (as it is in the corresponding dimers in the R5Mg5Fe4Al12Si6 structure). However, assigning this site as silicon resulted in poorer refinement parameters and 50% larger thermal displacements, indicating aluminum is more likely to occupy this site. The neutron diffraction data also confirm partial occupancy of the rare earth sites in Yb3‑δFeAl3.72Mg0.28Si2. Rare earth atoms occupy two sites in the structure (8j and 4h Wyckoff sites). Figure 3d shows the coordination of the 4h rare earth site, which is coordinated by 4 Al, 2 Mg/Al, 4 Si, and 1 Fe atom. Each 4h site in R3‑δFeAl4‑xMgxTt2 also has 4 neighboring R3+ ions in the ab-plane at distances of 3.6429(5) Å for the Yb analogue, with 2 more at a longer distance of 4.0996(3) Å along the c-axis (not shown). The 8j sites are coordinated by 7 Al, 4 Si, and 3 Fe atoms, with 4 neighboring R3+ ions at distances of 3.6429(5) and 3.5510(5) Å, as seen in Figure 3e. The short R3+−R3+ distances in the ab-plane, and the fact that the positions form triangular motifs (shown in Figure 3f), may have implications for their magnetic behavior (vide infra). Refinement of the neutron diffraction data indicates that the 8j and 4h sites are 92% and 93% occupied, respectively, leading to an overall stoichiometry of Yb2.77FeAl3.72Mg0.28Si2. Since slight partial occupancy (97−99%) of the 8j site was consistently observed in refinements of the X-ray data for all the analogues, a general formula of R3‑δFeAl4‑xMgxTt2 for these phases is indicated, with δ < 0.3. Electronic Structure Calculations. To investigate the electronic effects of Mg substitution on the 2a site, total and partial density-of-states (DOS) data were calculated for model compounds Y3FeAl4Si2, Y3FeAl4Ge2 (both with 100% Al on the 2a site), and Y3FeAl3.5Mg0.5Ge2 (with 100% Mg occupancy on the 2a site). To avoid complications from partially occupied fshells, Yb and Er ions are substituted with Y, reasonable considering their similar ionic radii and same oxidation number of +3. The DOS diagrams are shown in Figure 4. All three model compounds exhibit similar DOS characteristics over the whole energy range. The rare earth cations have the main contribution to the states above the EF, while orbitals from Fe, Al, and Si (or Ge) are dominant below the EF, leaving a pseudogap at or near the EF. This is expected for stable polar intermetallic structures. Y3FeAl4Si2 and Y3FeAl4Ge2 both have a valence electron count (VEC, per formula unit) of 37 e−; replacing the aluminum on the 2a site with magnesium results in a VEC of 36.5 e− for Y3FeMg0.5Al3.5Ge2, which shifts the

Table 4. Bond Lengths in Yb2.77FeAl3.72Mg0.28Si2 and Yb5Fe4Al17Si6 Yb2.77FeAl3.72Mg0.28Si2 bond Yb(1)−Fe(1) Yb(1)−Si(1) × 4 Yb(1)−Mg(1) × 2 Yb(1)−Al(2) × 2 Yb(1)−Al(3) × 4 Yb(1)−Yb(1) Yb(2)−Si(1) × 4 Yb(2)−Al(2) × 4 Yb(2)−Al(3) × 2 Yb(2)−Al(4) × 2 Yb(2)−Yb(1) Fe(1)−Al(2) × 4 Fe(1)−Al(3) × 2 Fe(1)−Al(4) × 2 Mg(1)−Si(1) × 4 Al(2)−Si(1) Al(2)−Al(2) × 2 Al(2)−Al(3) Al(2)−Al(4) × 8 Al(3)−Si(1) × 2

bond distance, Å 2.7856(7) 2.932(1)/ 2.876(1) 3.2414(3) 3.149(1) 3.220(4) 3.5510(5) 2.974(1) 3.104(1) 3.179(2) 3.1620(5) 3.6429(5) 2.599(1) 2.559(1) 2.315(1) 2.815(1) 2.565(2) 2.965(4)/ 2.726(4) 2.729(3) 2.874(1) 2.668(2)

Yb5Fe4Al17Si6 bond

bond distance, Å

Yb(1)−Al(1) Yb(1)−Al(2) × 2

3.485(3) 3.319(1)

Yb(1)−Al(3) × 4 Yb(1)−Al(4) × 4 Yb(1)−Si(2) Yb(2)−Al(1) × 4 Yb(2)−Si(1) × 8 Yb(2)−Fe(1) Fe(1)−Al(1) × 2 Fe(1)−Al(3) × 4 Fe(1)−Si(1) × 2 Fe(1)−Si(2) Al(1)−Al(3) × 4 Al(1)−Al(4) × 2 Al(2)−Al(4) × 4 Al(3)−Al(3) × 2 Al(3)−Al(4)

3.056(2) 2.876(1) 3.084(2) 3.277(3) 2.848(2) 3.320(2) 2.525(2) 2.588(1) 2.410(2) 2.396(2) 2.951(2) 2.910(2) 2.714(2) 2.971(3)/ 2.674(3) 2.499(2)

Al(3)−Si(1) Al(3)−Si(2) × 2 Si(1)−Si(1) × 2

2.595(3) 2.842(2) 2.838(4)

The assignments described above lead to a stoichiometry of R3‑δFeAl4Tt2 and leave open the question of the amount and location of magnesium substitution in the structure. Due to similarities in size and electronegativity, Mg will be likely to substitute onto aluminum sites in the structure, rather than on tetrel or rare earth positions. The 2a site at the center and corners of the unit cell is the most likely position for Mg incorporation. This site is surrounded by a cube of eight rare earth atoms and also coordinated to four tetrel atoms. The coordination environment is similar to that of the 1d site in the R5Mg5Fe4Al12Si6 structure, which was also assigned as a Mg site.19 It features longer bond lengths to neighboring atoms than seen for the other aluminum sites, and is therefore suited to incorporate the larger magnesium atoms. Refining this position as occupied by Mg instead of Al does not affect the overall R-factor since these elements have very similar X-ray scattering factors. However, for all four R3‑δFeAl4‑xMgxTt2 analogues, the thermal parameters for this position are higher than expected if assigned as Al, but drop to values more in line with the other light elements in the structure when refined as a Mg site. If this site has 100% Mg occupancy, the resulting stoichiometry is R3‑δFeAl3.5Mg0.5Tt2 and the phase is better denoted by its empirical formula R6−2δFe2Al7MgTt4 (Z = 2). However, due to the likelihood of mixed Mg/Al occupancy (see below), it is instead written as R3‑δFeAl4‑xMgxTt2 (Z = 4) to maintain resemblance to the simpler R3‑δFeAl4Tt2 formula. Neutron diffraction studies were carried out on a large crystal of Yb3‑δFeAl4‑xMgxSi2 to confirm atom siting and occupancies and determine the extent of substitution. While Mg, Al, and Si have nearly identical X-ray scattering coefficients, their neutron scattering coefficients are different enough to enable distinction of these elements (coherent neutron scattering length is smallest for Al at 3.449 fm, then Si at 4.149 fm, and largest for Mg at 5.375 fm).35 This feature was used to analyze Al and Si 3368

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Figure 4. Total and partial density of states data for Y3FeAl4Si2 (model compound for R3‑δFeAl4‑xMgxSi2 with no Mg in 2a site), Y3FeAl4Ge2 (model for R3‑δFeAl4‑xMgxGe2 with no Mg), and Y3FeAl3.5Mg0.5Ge2 (model for R3‑δFeAl4‑xMgxGe2 with 100% Mg on 2a site).

Figure 5. Partial density of states data for Y, Fe, Mg, Al, and Ge in Y3FeAl3.5Mg0.5Ge2. Note the different DOS scales for various elements.

Fermi level of this phase out of the pseudogap. The resulting DOS values at E F for Y 3 FeAl 4 Si 2 , Y 3 FeAl 4 Ge 2 , and Y3FeAl3.5Mg0.5Ge2 are 5.2, 6.2, and 15.8 states/eV cell, respectively. This indicates that incorporation of Mg has a slight electronic destabilizing effect. This may induce the vacancies that are consistently observed on the adjacent 8j rare earth sites (see Figure 3c); these vacancies and associated local distortions may act to stabilize the structure. Similar behavior has been observed in intermetallics such as Nb1‑δB2, LaZn1‑δAs2, and La21‑δMn8Te7C12.36−38 The partial DOS data for each element in Y3FeMg0.5Al3.5Ge2 are compared in Figure 5. The region just below the Fermi level is dominated by bands derived from iron 3d orbitals, with smaller contributions from aluminum 3p and yttrium 4d states. The chains of iron-centered trigonal prisms of aluminum may therefore make the dominant contribution to the conductivity of this phase, which is likely to be highly anisotropic. Transport measurements along each axis are needed to confirm this, but are hindered by the rod-shaped crystals. The fact that the Fe dorbitals are below Ef and essentially filled is in agreement with the diamagnetic behavior of iron in these compounds. Some hybridization is observed between states derived from Mg porbitals and states from Y d-orbitals, further supporting a link between the presence of Mg and vacancies on the adjacent RE sites. To further investigate the bonding within the iron/aluminum chains, orbital interactions between the iron and each of the three surrounding aluminum sites were analyzed by calculation of crystal orbital Hamilton populations (COHP) for model compounds Y 3 FeAl 4 Si 2 and Y 3 FeAl 4 Ge 2 (based on Yb3‑δFeAl4‑xMgxSi2 and Er3‑δFeAl4‑xMgxGe2 respectively, with x = 0). The COHP data for the silicide and germanide are very similar, as shown in Figure 6. As discussed previously, the iron atom is coordinated by a trigonal prism of aluminum (8i and 4g Al sites); these prisms are linked in the ab-plane by a bridging Al atom (2c site), which forms an Fe−Al−Fe chain with unusually short bond lengths in the range 2.313(7)−2.332(2)

Figure 6. Calculated COHP data for the three Fe−Al bonds in Yb3FeAl4‑xMgxSi2 and Er3FeAl4‑xMgxGe2 phases.

Å. Of the three unique Fe−Al bonds, those between the iron and the aluminum in the 4g sites appear to be most optimized, changing from bonding to antibonding at the Fermi level. The Fe−Al bonds involving the 2c and 8i aluminum sites are predominantly nonbonding in the vicinity of EF, with the bond to the 2c aluminum atoms exhibiting slight antibonding character just below EF. Despite the short bond length, this Fe−Al bond is not as stable as the other two. It is derived predominantly from interactions between Fe 3d and Al 3p 3369

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orbitals, indicated by the narrow range of bonding states from −1 to −1.5 eV (corresponding to the Fe d-orbital states). The interactions with the other two Al sites involve the Fe 3p orbitals, and are bonding over a broader and lower energy range. This indicates that the iron−aluminum interactions running in the c-axis direction (along the chains of face-sharing prisms) are stronger than the linkage between the prisms in the ab-plane. Magnetic Properties. Figure 3f highlights the coplanar positioning of the two rare earth sites in the R3‑δFeAl4‑xMgxTt2 structure; the rare earth ions form a tiling pattern composed of squares, triangles, and distorted octagons. The presence of triangles of ions (featuring short R3+−R3+ distances) is of particular interest, since positioning of paramagnetic ions in triangular patterns leads to competing magnetic coupling forces and is a key feature in many geometrically frustrated magnetic systems such as pyrochlores.39 The temperature-dependent magnetic susceptibility and inverse magnetic susceptibility data for Yb 2 .7 7 FeAl 3 .7 2 Mg 0. 28 Si 2 , Er 3 ‑δ FeAl 4 ‑x Mg x Ge 2 , and Dy3‑δFeAl4‑xMgxSi2 are presented in Figure 7. Yb2.77FeAl3.72Mg0.28Si2 exhibits paramagnetic behavior over the entire measured temperature range; no magnetic ordering was observed, and no splitting between field-cooled and zero-field cooled data. The inverse susceptibility versus temperature follows the Curie−Weiss law well; the fit yields a magnetic moment of 4.64(2) μB per ytterbium ion and a Weiss constant θ = −4.8(1) K. The observed ytterbium moment is in good agreement with the theoretical value of 4.5 μB for Yb3+ ions; a +3 oxidation state is also supported by XPS data (see Supporting Information Figure S3). This indicates that the iron electrons are delocalized and do not contribute to the magnetic moment of these phases. This is further evidenced by susceptibility data for Y3‑δFeAl4‑xMgxGe2, which exhibits temperature independent Pauli paramagnetism as expected for an intermetallic phase with no magnetic ions (see Supporting Information Figure S4). Figure 7b,c shows the magnetic susceptibility data for Er3‑δFeAl4‑xMgxGe2 and Dy3‑δFeAl4‑xMgxSi2. Both phases exhibit Curie−Weiss behavior at high temperature and antiferromagnetic ordering at low temperature, with Neel temperatures (TN) of 2.8 and 3.8 K, respectively. The very low ordering temperatures of Er and Dy phases (and the lack of ordering in the Yb phase) are somewhat surprising given the short distances between the rare earth ions. Fitting the inverse susceptibilities (above 150 K) of Er and Dy phases to the Curie−Weiss law yields effective magnetic moments of 9.57(3) μB per Er3+ ion (and θ = 17.3(9) K), and 10.63(1) μB per Dy3+ ion (and θ = −15.0(7) K). The magnetic moments are in agreement with the theoretical values for Er3+ ions (9.6 μB) and Dy3+ ions (10.6 μB). The Weiss constants are significantly higher than the observed ordering temperatures for these phases. This, and the triangular arrangements of the rare earth ions, indicates the possibility of competing magnetic interactions or geometric frustration (as does the fact that Er3‑δFeAl4‑xMgxGe2 orders antiferromagnetically but has a positive Weiss constant, which usually indicates ferromagnetic coupling forces). However, the ratio of θ to TN is not as high as would be expected for a spin glass (θ/TN is usually 10 or higher for spin glasses;39 observed ratios are 6.2 and 3.9 for the Er and Dy phases studied here). Also, magnetically frustrated systems often exhibit differences in their field-cooled versus zero fieldcooled susceptibilities; little to no FC/ZFC splitting is seen in the data for the R3‑δFeAl4‑xMgxTt2 phases.

Figure 7. Temperature dependence of magnetic susceptibilities (filled symbols) and inverse magnetic susceptibilities (unfilled symbols) for (a) Yb 2.77 FeAl 3.72 Mg 0.28 Si 2 , (b) Er 3‑δ FeAl 4‑x Mg x Ge 2 , and (c) Dy3‑δFeAl4‑xMgxSi2 at 100 G. Insets show low temperature magnetic susceptibility behavior.

Field dependence of magnetization data collected at 1.8 K for the Yb, Er, and Dy phases is shown in Figure 8. At low applied fields, Yb2.77FeAl3.72Mg0.28Si2 exhibits the linear field depend-

Figure 8. Field dependence of magnetization for Yb2.77FeAl3.72Mg0.28Si2, Dy3‑δFeAl4‑xMgxSi2, and Er3‑δFeAl4‑xMgxGe2 at 1.8 K. 3370

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ence typical of a paramagnet, although fields above 1.5 T appear sufficient to induce some spin alignment and saturation. The antiferromagnetically ordered Dy3‑δFeAl4‑xMgxSi2 exhibits similar behavior, saturating above 2 T. However, the magnetization at this field is well below that expected for Dy3+ moments, indicating a metamagnetic transition might occur at higher fields than are available in this study. Er3‑δFeAl4‑xMgxGe2 exhibits more complex magnetization data. A metamagnetic transition is observed for the Er analogue at ∼4400 G. The antiferromagnetically ordered spins undergo an evident reorientation, possibly to a canted ferromagnetic state, which slowly saturates with increasing field. The spin reorientation for Er3‑δFeAl4‑xMgxGe2 is further confirmed by a sharp peak at ∼4400 G in the field-dependent ac magnetization curve at 1.8 K, while the data for Dy3‑δFeAl4‑xMgxSi2 do not show evidence of any distinct transition up to 3 T (see Figure 9). The

Ce−Sm), likely because those syntheses had a higher concentration of silicon in the flux.10,40

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, XPS data, X-ray powder diffraction patterns, and crystallographic data for R3‑δFeAl4‑xMgxSi2 phases in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 850-644-4074. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by funding from the National Science Foundation (Division of Materials Research) through Grant DMR-11-06150. This work made use of the XPS and SEM Facilities of the FSU Physics Department. The research at ORNL’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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REFERENCES

(1) Miglio, L.; Heurle, F. Silicides: Fundamentals and Applications; World Scientific Publishing Co. Pte. Ltd: Erice, Italy, 1999. (2) Allen, L. H.; Besser, P.; Burnette, J. E.; Cheng, S. L. Silicide Technology for Integrated Circuits; The Institution of Electrical Engineers: London, U.K., 2004. (3) Tang, J.; Wang, C. Y.; Hung, M. H.; Jiang, X.; Chang, L. T.; He, L.; Liu, P. H.; Yang, H. J.; Tuan, H. Y.; Chen, L. J.; Wang, K. L. ACS Nano 2012, 6, 5710−5717. (4) Zaimaa, S.; Nakatsukaa, O.; Kondoa, H.; Sakashitaa, M.; Sakaib, A.; Ogawa, M. ECS Trans. 2007, 6, 197−205. (5) Kanatzidis, M. G.; Pöttgen, R.; Jeitschko, W. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (6) Latturner, S. E.; Kanatzidis, M. G. Inorg. Chem. 2002, 41, 5479− 5486. (7) Latturner, S. E.; Kanatzidis, M. G. Inorg. Chem. 2008, 47, 2089− 2097. (8) Condron, C. L.; Martin, J.; Nolas, G. S.; Piccoli, P. M.; Schultz, A. J.; Kauzlarich, S.M.. Inorg. Chem. 2006, 45, 9381−9386. (9) Sieve, B.; Chen, X. Z.; Henning, R.; Brazis, P.; Kannewurf, C. R.; Cowen, J. A.; Schultz, A. J.; Kanatzidis, M. G. J. Am. Chem. Soc. 2001, 123, 7040−7047. (10) Sieve, B.; Gray, D. L.; Henning, R.; Bakas, T.; Schultz, A. J.; Kanatzidis, M. G. Chem. Mater. 2008, 20, 6107−6115. (11) Zhuravleva, M. A.; Wang, X.; Schultz, A. J.; Bakas, T.; Kanatzidis, M. G. Inorg. Chem. 2002, 41, 6056−6061. (12) Zhuravleva, M. A.; Kanatzidis, M. G. J. Solid State Chem. 2003, 173, 280−292. (13) Zhuravleva, M. A.; Pcionek, R. J.; Wang, X.; Schultz, A. J.; Kanatzidis, M. G. Inorg. Chem. 2003, 42, 6412−6424. (14) Zhuravleva, M. A.; Salvador, J.; Bilc, D.; Mahanti, S. D.; Ireland, J.; Kannewurf, C. R.; Kanatzidis, M. G. Chem.Eur. J. 2004, 10, 3197−3208. (15) Salvador, J. R.; Gour, J. R.; Bilc, D.; Mahanti, S. D.; Kanatzidis, M. G. Inorg. Chem. 2004, 43, 1403−1410. (16) Tobash, P. H.; Bobev, S. J. Solid State Chem. 2007, 180, 1575− 1581. (17) Massalski, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprzak, L. Binary Alloy Phase Diagrams, 2nd ed.; ASM International: Materials Park, OH, 1990; Vol. 1−3.

Figure 9. Alternating current magnetization of Er3‑δFeAl4‑xMgxGe2 and Dy3‑δFeAl4‑xMgxSi2 in the applied dc magnetic field at 1.8 K.

triangular positioning and short distances between the magnetic ions in the R3‑δFeAl4‑xMgxTt2 phases may produce several nearly degenerate magnetic ground states, but their energies are different enough so that one state is favored over another at sufficiently low temperature or high field.

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CONCLUSION Reactions in metal flux mixtures promote formation of complex multinary phases instead of potential binary and ternary intermetallic products. This has been demonstrated by the formation of four new quinary phases Yb2.77FeAl3.72Mg0.28Si2, Dy3‑δFeAl4‑xMgxSi2, Er3‑δFeAl4‑xMgxGe2, and Y3‑δFeAl4‑xMgxGe2 from reactions of Si or Ge with Fe and late rare earth elements (Yb, Er, Dy, or Y) in mixed Mg/Al flux. Single crystal neutron diffraction studies were vital in determining the siting of the Mg, Al, and Si atoms in the structure, even enabling the determination of the Mg/Al ratio on a mixed site. While the magnesium content of these phases is low (x < 0.5), it is needed to stabilize the structure; in the absence of Mg, other compounds such as Yb 5 Fe 4 Al 17 Si 6 will form. The R3‑δFeAl4‑xMgxTt2 structure is highlighted by a chain of facesharing iron-centered trigonal prisms of aluminum, a feature also seen in a flux-grown phases R5Mg5Fe4Al12Si6 and RFe2Al8‑xMgx, which form when different rare earths are used in the synthesis. The size of the rare earth ion determines how these chains of FeAl6 prisms pack together to form the overall structure. The stability of this structural building block was confirmed by DOS and COHP calculations. This FeAl6 trigonal prism does not appear in other multinary phases grown from Al flux such as RFe4Al9Si6 (R = Tb, Er) and R4Fe2+xAl7‑xSi8 (R = 3371

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dx.doi.org/10.1021/cm401976s | Chem. Mater. 2013, 25, 3363−3372