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Structural Characterization of Intermetallic Compounds by 27Al Solid State NMR Spectroscopy Christopher Benndorf,†,‡,§ Hellmut Eckert,*,‡,∥ and Oliver Janka*,†,⊥ †

Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 28/30, 48149 Münster, Germany ‡ Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 28/30, 48149 Münster, Germany § Institut für Mineralogie, Kristallographie und Materialwissenschaften, Universität Leipzig, Scharnhorststraße 20, 04275 Leipzig, Germany ∥ Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP 13566-590, Brazil ⊥ Institut für Chemie, Carl von Ossietzky Universität Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26129 Oldenburg, Germany CONSPECTUS: Intermetallic compounds are of broad interest for solid state chemists, condensed matter physicists, and material scientists due to their intriguing crystal chemistry, their physical properties, and their potential applications, ranging from lab curiosities to everyday objects. To characterize and understand the properties of new compounds and novel materials, the availability of structural information, particularly single-crystal X-ray diffraction data, is a mandatory prerequisite. Especially when it comes to the formation of compounds with deficient or mixed site occupancies, superstructures, or representatives crystallizing in other, thus far unknown structure types, a complementary method for structural analysis is of great value. Solid state nuclear magnetic resonance spectroscopy has been a valuable tool in many areas of chemistry, being an element-selective, site-specific, and inherently quantitative tool for detailed structural characterization. Magic-angle spinning conditions eliminate or reduce the effect of anisotropic interactions in the solid state, producing high-resolution spectra. Until recently, 27Al NMR studies of intermetallic aluminum compounds have been relatively sparse and mostly limited to binary systems. In this Account, we will summarize the current state of the art of high-resolution 27Al NMR in intermetallic compounds focusing on recent research efforts in our laboratories and the interpretation of NMR parameters in terms of the structural details of the compounds investigated. Besides theoretical aspects of 27Al NMR spectroscopy, short paragraphs on experimental details and the crystal chemistry of the discussed compounds are given. In the main part of this Account, we focus on three key aspects: (i) crystal structure validation, (ii) structural disorder and mixed site occupancies, and (iii) the electronic structure, all of which can be investigated by spectroscopic means. For the first part, we have chosen the ternary equiatomic compounds CaAuAl (TiNiSi type), BaAuAl (LaIrSi type), and Ba3Pt4Al4 (own type). Structural disorder and mixed site occupancies have been probed in the ScTAl series (T = Cr, Ru, Ag, Re) crystallizing in the TiNiSi, HfRhSn, and MgZn2-type structures. Also Na2Au3Al and the Heusler compounds, Sc(T0.5T′0.5)2Al (T = T′ = Ni, Pd, Pt, Cu, Ag, Au), have been used for structure validation purposes, based on the number and signal area ratios of the resonances observed and on the comparison between experimental and theoretically calculated nuclear electric quadrupolar interaction parameters. Electronic structure information available from 27Al magnetic shielding will be discussed based on experimental data obtained for the RET5Al2 series (RE = Y, Lu; T = Pd, Pt), the extended RE10TAl3 series (RE = Y, Lu; T = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt), and the ordered Heusler compounds ScT2Al (T = Ni, Pd, Pt, Cu, Ag, Au).

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INTRODUCTION

compounds can be used as outstanding magnetocaloric materials,4 while Nb3Sn is used for the fabrication of commercial superconducting devices.5 Ni3Al superalloy,6,7 γTiAl, and α2-Ti3Al find use in aviation, aeronautics, space craft, or the automotive sector.8 Single crystals required for some

Intermetallic compounds are of broad interest for solid state chemists, condensed matter physicists, and material scientists due to their intriguing crystal chemistry and physical properties. Nd2Fe14B,1 SmCo5,2 and Sm2Co173 are examples for new generation hard magnetic materials and among the strongest permanent magnets available at the moment. Al−Ni−Co alloys are used for fridge magnets, for example. Gd5Si2Ge2 and related © 2017 American Chemical Society

Received: March 29, 2017 Published: June 7, 2017 1459

DOI: 10.1021/acs.accounts.7b00153 Acc. Chem. Res. 2017, 50, 1459−1467

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Accounts of Chemical Research

perturbation theory (Figure 1). To first order, the resonance frequency of central |1/2⟩ ↔ |−1/2⟩ transition remains

applications can be grown from metal fluxes.9 Schmitt and coworkers recently reported on the targeted growth of rare-earth intermetallics with synergistic magnetic and electrical properties.10 To characterize and understand the properties of intermetallic compounds, the availability of structural information, particularly single-crystal X-ray diffraction data, is a mandatory prerequisite. Especially when it comes to the formation of compounds with deficient or mixed site occupancies, superstructures, or representatives crystallizing in new structure types, a complementary method for structural analysis is of great value. NMR spectroscopy is an element-selective, sitespecific, and inherently quantitative tool for detailed structural characterization. 11 With a 100% natural abundance, a moderately high gyromagnetic ratio of γ = 6.976 × 107 rad· T−1·s−1, and a medium-sized nuclear electric quadrupole moment, eQ = 0.15 × 10−28 m2, the 27Al nucleus (I = 5/2) is an excellent candidate for NMR spectroscopic experiments.12,13 Indeed, 27Al NMR has found widespread use for the structural characterization of aluminum-based melts and glasses,14 polyoxoanions,15 zeolites,15−18 catalysts,17 and metal−organic frameworks.15 Overall, very few review articles focusing on NMR studies of intermetallic compounds have appeared, and these few tend to be limited to studies of individual nuclei (69,71Ga,19 45Sc,20 25Mg21) in specific groups of intermetallic compounds. Until a few years ago, 27Al NMR studies of intermetallic aluminum compounds had been relatively sparse21−32 and mostly limited to binary systems. This situation has changed in the wake of recent invigorated research efforts on the crystal chemistry of intermetallic aluminum compounds.

Figure 1. Effect of the nuclear electric quadrupolar interaction upon the 27Al (I = 5/2) Zeeman energy levels. First- and second-order energy level shifts are schematically represented for one particular crystalline orientation.

unaffected, whereas those of the other four Zeeman transitions become orientation dependent. In polycrystalline materials, these satellite transitions produce broad powder patterns. Quadrupole couplings with energies exceeding 1/20 of the Zeeman energy need to be analyzed using second-order perturbation theory. In this regime the central |1/2⟩ ↔ |−1/2⟩ transition is anisotropically broadened as well. In addition, magnetic shielding anisotropies and dipolar interactions with the magnetic moments of proximal nuclei have an influence upon the spectra. These latter contributions can be eliminated by applying the technique of magic angle sample spinning (MAS). Under this condition, the broad satellite transitions give rise to a spinning sideband manifold, whose envelope approximates the line shape under static conditions. In the presence of second-order perturbations, the broadening of the central transition is not completely removed by MAS, resulting in characteristic line shapes as well. The quadrupolar interaction parameters CQ and ηQ can be extracted from such MAS NMR spectra with the help of line shape simulations (Figure 2). For CQ values up to ∼2−3 MHz, they are most reliably measured from the intensity profiles of the satellite spinning sideband patterns. For larger CQ values, this becomes difficult because the intensity distributions get distorted owing to off-resonance effects and probe bandwidth limitations. In this regime (CQ > 3 MHz), the quadrupole coupling parameters can be obtained by simulating the central transition line shapes, which are influenced by second-order perturbation effects. The experimental results can be compared with quantum-chemically calculated values based on crystallographic input, using standard DFT software such as VASP, GAUSSIAN, or WIEN2k. As discussed below, this comparison is an important method of validation for crystal structures proposed by single crystal or powder X-ray diffraction methods. Another important observable available from 27Al MAS NMR spectra is the resonance frequency of the signal center of the central |1/2⟩ ↔ |−1/2⟩ transition, δ(ν0). It is comprised of an isotropic magnetic shielding contribution, δms, and a negative contribution from second order quadrupolar effects.33 The latter contribution is dependent on the relative strength of the quadrupolar and Zeeman interactions. For spin-5/2 nuclei such as 27Al, δ(ν0i) measured in ppm at the resonance frequency ν0i is given by

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FUNDAMENTAL ASPECTS AND EXPERIMENTAL METHODOLOGY OF 27AL MAS NMR SPECTROSCOPY At the magnetic flux density B0, the 27Al nuclei possess six Zeeman states with energies Em = −mγ ℏB0

(1)

where m = 5/2, 3/2, 1/2, −1/2, −3/2, or −5/2 are the orientational quantum numbers. The five allowed transitions (Δm = ±1) occur at the frequency ω = γ(B0 + Bint )

(2)

where Bint comprises different additional local fields sensed by the nuclei owing to the effect of various anisotropic internal interactions of the 27Al nuclei with their local magnetic and electronic environments. In particular, the Zeeman transition frequencies are affected by the interaction of the 27Al nuclear electric quadrupole moment eQ with the electric field gradient (EFG) created by the local charge and electronic environment of the nuclei. The anisotropic character of the EFG is described by a traceless second rank tensor, whose principal axis direction is defined by its maximal component, eq. Experimental spectra are analyzed in terms of the nuclear electric quadrupolar coupling constant, CQ = e2qQ, characterizing the magnitude of the quadrupole interaction, and the asymmetry parameter, ηQ (0 ≤ ηQ ≤ 1) describing the deviation of the EFG from cylindrical (axial) symmetry. These parameters give important information on the local coordination symmetry and the electronic environment of the nuclei. The effect of the quadrupolar interaction on the Zeeman levels is described by standard 1460


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calculations of absolute magnetic shielding values in intermetallic compounds are still very much a matter of active investigation. As discussed further below, useful insights can be obtained, however, for series of isotypic compounds by comparing experimental values of δms with s-electron densities at the Fermi edge obtained from band structure calculations.32

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EXPERIMENTAL ASPECTS The intermetallic aluminum compounds discussed in this Account were synthesized from the pure elements either by arcmelting34 under an atmosphere of purified argon or by reaction in refractory metal containers heated by resistive or induction furnaces.35 Homogenization of the specimens can be achieved by annealing the material within sealed evacuated silica ampules in tube furnaces for several days. Annealing of the as-cast buttons is also possible in induction furnaces.36 Detailed experimental procedures can be found in the original literature. Standard characterization of the samples was performed by powder and single crystal X-ray diffraction and metallography in combination with EDX. For recording high-fidelity solid state 27Al NMR spectra, magnetic flux densities within the range 4.7 to 14.1 T, and spinning rates of 15 kHz are usually sufficient for intermetallic compounds; however, improved resolution is often obtained using higher rotation speeds. Samples are typically diluted with nonmetallic powders (boron nitride, silica, NaCl, etc.) in a 1:1 mass ratio, to avoid Joule heating and probe detuning effects. As the 27Al nutation frequencies may be ill-defined as a result of incomplete excitation of the satellite transitions, the use of short pulses corresponding to small flip angles (≤20°) is advisable. The wide excitation windows afforded by short pulse lengths are also important for recording satellite transition spinning sideband manifolds, which often extend beyond a spectral window of 1 MHz. To avoid distortions in these profiles, the magic angle must be set very accurately, and spinning speed variations must be kept to a minimum (within ±1 Hz). A 1 M aqueous solution of Al(NO3)3 serves as the reference standard for all the spectra reported here.

Figure 2. Effect of the electric field gradient asymmetry parameter ηQ on the MAS NMR line shape of half-integer quadrupolar nuclei with second-order quadrupolar perturbations.

δ(ν0i) = δms − (SOQE)2

6000 (ν0i)2

(3)

where the quantity SOQE (second order quadrupolar effect) is ⎛ η 2⎞ given by CQ ⎜1 + Q3 ⎟ . Thus, both parameters δms and ⎝ ⎠ SOQE can be determined by measuring δ(ν0) at two different frequencies (magnetic field strengths). The magnetic shielding contribution results from multiple magnetic screening mechanisms arising from the electronic environment: δms = δdia + δorb + δCurie + δ K

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CRYSTAL CHEMISTRY AND STRUCTURAL DATA In most of the presented examples, the aluminum atoms contribute to a polyanionic framework, which is formed with the respective late transition metal. The alkali-metal, alkalineearth-metal, or rare-earth-metal cations are found in cavities of the polyanions. Depending on the transition metal content, the Al atoms form covalent heteroatomic bonds (e.g., Ba3Pt4Al4,37 ScPtAl and ScAuAl,38 or BaAuAl39) or are isolated in cases of transition metal or rare-earth rich phases (RET5Al2,40 RE10TAl3,41 or Na2Au3Al42). The local symmetries surrounding the Al atoms in the respective crystal structures have been depicted along with the NMR data in the following paragraphs. This structural information is available without a detailed discussion. For the TiNiSi,43−46 MgZn2,47−49 and LaIrSi50,51 type compounds as well as the Heusler phases,52−54 several papers have been published regarding the crystal chemistry, physical properties, and structural relations. The crystal chemistry of the other presented examples (Ba3Pt4Al4,37 RET5Al2,40 RE10TAl3,41 ScAuAl,38 and Na2Au3Al42) has been discussed in great detail in the original publications. Furthermore, we would like to point out some new textbooks on intermetallics.55,56

(4)

The term δdia is a diamagnetic effect caused by electron circulation within closed shells. The term δorb arises from the orbital angular momentum of excited electronic states admixed into the electronic ground state. The term δCurie describes interactions arising from unpaired electrons with Curie-type paramagnetism, whereas the Knight shif t, δK, is a deshielding effect caused by the Fermi contact interaction of the nuclei with spin polarized electron density near the Fermi edge. While the Curie-type contribution can be identified by its inverse temperature dependence, the remaining three contributions cannot be separated by experimental means. In nonmetallic compounds, the sum δdia + δorb is known as chemical shif t. For metals, the Knight shift contribution tends to dominate, even though theoretical studies have suggested that this is not always the case. At the present time, exact theoretical 1461


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Figure 3. Experimental 27Al MAS NMR spectra of CaAuAl (left) and BaAuAl (right) recorded at a spinning frequency of νrot = 28 kHz and an external magnetic flux density of B0 = 9.4 T. Signals caused by impurities are marked with asterisks. The Al coordination environments are depicted. Adapted with permission from ref 39. Copyright 2017 Wiley VCH.

Figure 4. (left) Experimental 27Al MAS NMR spectrum of Ba3Pt4Al4 (top) and simulated spectra (middle) based on theoretical EFG calculations. The bottom part shows the simulated satellite transition MAS sideband intensity profiles for Al1 (CQ = 2.55 MHz, ηQ = 0.81, orange) and Al2 (CQ = 3.10 MHz, ηQ = 0.34, green). The coordination environments of the Al1 and Al2 atoms are depicted. (right) Enlargement of the spinning-sideband pattern. Adapted with permission from ref 37. Copyright 2017 American Chemical Society.

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CRYSTAL STRUCTURE VALIDATION The most basic contribution of solid state NMR to the chemistry of intermetallic compounds is crystal structure validation, by confirming the number of crystallographic sites and their relative multiplicities and by comparing experimental nuclear electric quadrupolar coupling parameters with corresponding values calculated quantum chemically from single crystal data input. Figure 3 shows an illustrative example for the compounds CaAuAl (TiNiSi type) and BaAuAl (LaIrSi type).39 Although both compounds are equiatomic, the large difference in 27Al quadrupolar coupling is striking and in excellent agreement with quantum chemical predictions. In the case of CaAuAl, the interaction is so weak that the quadrupolar coupling parameters are best extracted from the satellite spinning sideband intensity profiles. In contrast, for BaAuAl, the central transition shows strong second-order anisotropic broadening indicating a large electric field gradient at the site of the aluminum atoms. The ηQ value of zero indicates an axial field gradient, as expected from the point symmetry (.3.) of the Wyckoff site occupied by aluminum. The compound Ba3Pt4Al4 crystallizes in an orthorhombic structure featuring two crystallographically distinct Al local environments.37 While both of them can be described as approximately tetrahedral, they feature different degrees of distortion, corresponding to Strobel distortion57 parameters of 0.40 for Al1 and 2.32 for Al2. Experimentally, we observe two equally intense signals at 1031 and 762 ppm. From the spinning sideband profile analysis, it is evident that the 1031 ppm site is characterized by the larger quadrupolar coupling constants and

thus must be assigned to the more distorted coordination sphere of the Al2 species. This assignment is consistent with theoretical electric field gradient calculations using the VASP software. Figure 4 shows good agreement between the experimental sideband intensity profiles and those simulated based on these theoretical calculations. In particular, the results confirm that the EFGs of Al1 and Al2 are characterized by substantially different asymmetry parameters. Crystal structure validation of this kind has also been done for numerous other crystalline aluminum-based intermetallic compounds. Table 1 illustrates excellent agreement between experimental and theoretical values of CQ and ηQ. Site differentiation and characterization based on 27Al quadrupolar coupling parameters has also been used in earlier work.21,23,24,26,29,31,58 Table 1. Experimental and Calculated Quadrupolar Coupling Constants (in MHz) and Asymmetry Parameters in Intermetallic Aluminum Compounds compound 38

ScPtAl CaAuAl39 BaAuAl39 ScAuAl38 Na2Au3Al42 Ba3Pt4Al437

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structure type

CQ,exp/CQ,theor

ηQ,exp/ηQ,theor

TiNiSi TiNiSi LaIrSi HfRhSn Mo3Al2C own type

6.28/6.23 3.20/3.30 13.2/12.4 4.42/4.41 8.04/7.02 >2.0/2.55 >3.0/3.10

0.63/0.70 0.98/0.97 0.00/0.00 0.70/0.70 0.00/0.00 0.80/0.81 0.30/0.34

Al1 Al2


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Figure 5. 27Al MAS NMR spectra of ScCrAl, ScRuAl, ScAgAl, and ScReAl recorded at two different magnetic field strengths. The two coordination environments are depicted. Adapted with permission from ref 38. Copyright 2016 De Gruyter.

Figure 6. 27Al solid state MAS NMR spectra and simulations (green, ordered Na2Au3Al; orange, nonordered Na2Au4−xAlx (x < 1)) of the phases Na2Au4−xAlx (x = 1, 0.75, and 0.5) recorded at a spinning frequency of 28 kHz and external magnetic fields of B0 = 4.7 T (left) and 7.05 T (right). The Al coordination environment is depicted. Adapted with permission from ref 42. Copyright 2017 American Chemical Society.

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simulations. Rather, average SOQE and δms values can be determined via eq 3 by measuring the center of gravity of the signals at two different magnetic field strengths (Figure 5). Furthermore, such field-dependent studies also contain information on the dominant line broadening mechanism. A distribution of isotropic magnetic shielding values results in MAS NMR spectra whose widths (in Hz) increase with increasing magnetic field strengths, whereas a distribution of second-order quadrupolar broadening effects will cause the line widths (in Hz) to decrease with increasing field strength. In the latter case, one normally observes rather asymmetric peak shapes, with a pronounced “tailing” toward lower frequencies. Based on such measurements, the spectra of the compounds ScCrAl, ScAgAl, and ScReAl were shown to be dominated by a distribution of δms, whereas in the case of the Ru compound second-order quadrupolar broadening effects prevail.

STRUCTURAL DISORDER AND MIXED SITE OCCUPANCIES

Order−disorder phenomena have been investigated for compounds of the ScTAl series (T being a transition metal).38 In ScPtAl (TiNiSi type) and ScAuAl (HfRhSn type), the Al atoms are exclusively positioned on the 4c/6g Wyckoff sites, giving rise to well-defined 27Al NMR spectra. In contrast, the compounds ScCrAl, ScRuAl, ScAgAl, and ScReAl crystallize in the disordered MgZn2 type structure, in which the Al and transition metal atoms are extensively mixed on their respective 2a and 6h sites and thus can be found in multiple local environments. This situation results in strongly broadened line shapes, arising from a distribution of both magnetic shielding and nuclear electric quadrupole coupling effects. In such cases, the spectra cannot be analyzed with line shape 1463


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Table 2. Values of δms (in ppm relative to 1 M aqueous solution of Al3+) in Intermetallic Aluminum Containing Compounds compound

structure type

LaAl232 YAl232 LuAl232 Cu1‑xAl226 TiAl222 AuAl242 ZrNiAl30 BaAuAl39 ScAuAl38 ScRuAl38 ScAgAl38 ScReAl38 CaAuAl39 ScPtAl38 Co4Al1331 Fe4Al1331 Ru4Al1331 TiAl3

MgCu2

HfGa2 CaF2 ZrNiAl LaIrSi HfRhSn MgZn2

TiNiSi Co4Al13 Fe4Al13 TiAl3 (DO22)

VAl3 NbAl3 TaAl3 ScAl3 ZrAl3 HfAl3 YPd5Al240 LuPd5Al240 YPt5Al240 LuPt5Al240 TiAl22 AlB228 Ba3Pt4Al437 LaCu3Al227

Cu3Au ZrAl3 (DO23)

anti-ZrNi2Al5

CuAu AlB2 Ba3Pt4Al4 PrNi2Al3

δms

compound

structure type

δms

600 364 386 1500 330/610 571 393 511 405 425 496 409 782 409 609 −175 278 410/27023 335/25322 −170/−10023 −97/−13022 350/26023 580/54023 756 455/384/68722 147/146/22324 145/232/30424 623 634 277 273 −170 880 762/1031 650

ScNi2Al59 ScPd2Al59 ScCu2Al59 ScAg2Al59 ScAu2Al59 Sc(Ni0.5Pd0.5)2Al59 Sc(Ni0.5Cu0.5)2Al59 Sc(Ni0.5Ag0.5)2Al59 Sc(Ni0.5Au0.5)2Al59 Sc(Pd0.5Cu0.5)2Al59 Sc(Pd0.5Ag0.5)2Al59 Sc(Pd0.5Au0.5)2Al59 Sc(Ag0.5Au0.5)2Al59 Y10FeAl341 Y10CoAl341 Y10NiAl341 Y10RuAl341 Y10RhAl341 Y10PdAl341 Y10OsAl341 Y10IrAl341 Y10PtAl341 Lu10FeAl341 Lu10CoAl341 Lu10NiAl341 Lu10RuAl341 Lu10RhAl341 Lu10PdAl341 Lu10OsAl341 Lu10IrAl341 Lu10PtAl341 Ti3Al22 Mg17Al1221 Na2Au3Al42

Heusler-phase

851 826 426 445 649 850 620 647 615 556 636 735 440 192 329 310 391 320 205 353 280 215 422 483 506 511 460 353 476 439 433 200 1300 902

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Similar field-dependent work has been carried out on the solid solutions Na2Au4−xAlx (Figure 6). These results suggest substantially smaller CQ values than observed in the ordered Na2Au3Al phase.42 Again, the dominant line broadening mechanism is a distribution of magnetic shielding effects. It is further evident that the isotropic magnetic shifts increase with increasing Au/Al ratios. Analogous disordering effects are noted for the mixed Heusler phases of composition Sc(T0.5T′0.5)2Al.59 Their 27Al NMR signals are generally found at resonance frequencies intermediate of those of their respective ScT2Al and ScT′2Al endmembers. The mixing of the two different transition metal atoms on the 8c site results in statistically distributed Al coordination spheres, producing broadened line shapes dominated by δms distributions. With one exception, the peak centers are close to those calculated from the average resonance shifts of the endmembers. Line-broadening effects in the mixed Sc(T0.5T′0.5)2Al phases are found to be particularly severe when the differences in the δms values of the endmembers are large. All of these results indicate that the δms values in these intermetallic compounds are strongly influenced by local contributions generated by the short-range order atomic environments.

anti-Co2Al5

Mg3Cd Mg17Al12 Mo3Al2C

ELECTRONIC STRUCTURE

Electronic structure information is predominantly available from the magnetic shielding effect, δms, given by eq 4. In metallic Al, the experimental value δms = 1640 ppm60 is close to the calculated value of δK considering only the direct and the core polarization contributions (1510 + 114 = 1624 ppm).61 The excellent agreement suggests that the sum δorb+ δdia of metallic Al and of the hydrated Al3+ ion reference are very close to each other. Unfortunately, this conclusion cannot be generalized to intermetallic Al compounds, where δms values typically range from 200 to 800 ppm (Table 2). As these values are significantly smaller than in elemental Al and a few other intermetallics, they indicate significantly weaker Fermi contact interactions. At the same time, the contribution of δorb may be significantly enhanced due to strong covalency effects. With the theoretical treatment of such systems still under active development, useful insights have been sought by comparing δms values within series of closely related isotypic compounds. For such series, it may be a valid assumption that there are no large variations in δorb so that variations of δms may be attributable to the effect of the elemental constituents upon the s-electron densities of states at the Fermi level. The latter may be influenced by the valence electron concentration contributed by the metal or the different abilities of the atoms involved to 1464


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Accounts of Chemical Research attract electron density toward their cores. To describe the latter effect, the Pauling electronegativity concept62 has proven useful in some cases. An example is the isostructural series RET5Al2 (RE = Y, Lu, T = Pd, Pt; Table 2), which crystallizes in the inversely occupied ZrNi2Al5 structure type. The Al atoms are on the Wyckoff site 4e (point symmetry 4mm) and are cuboctahedrally surrounded by 8 T and 4 Al atoms. As illustrated by Table 2, the Pd compounds (EN = 2.20) resonate at significantly higher frequencies than the Pt compounds (EN = 2.28). Possibly this effect arises from the stronger electronwithdrawing character of the more electronegative Pt atoms, reducing the s-electron density of states at the Fermi level sensed by the 27Al nuclei. In contrast, the RE atoms, which are not part of the first coordination sphere of the aluminum atoms, have essentially no effect on the 27Al magnetic shielding. An inspection of the magnetic shielding trends in the Heusler phases reveals, however, that this electronegativity model is too simple. While δms decreases, as expected, in the series ScPd2Al → Sc(Pd0.5Au0.5)2Al → ScAu2Al, the trends involving the silvercontaining compounds are found opposite to those expected from the electronegativity of Ag (EN = 1.96). More promising results were obtained in a recent theoretical study using the gauge-including projector augmented wave (GIPAW) method.63 These calculations, which yield the orbital and spin contributions from the same converged ground-state spinpolarized electronic structure in the magnetic field64,65 constitute the first theoretical study of 27Al magnetic shieldings in intermetallic compounds. As illustrated in Figure 7, the

Figure 8. Experimental 27Al δms values in Y10TAl3 (orange circles) and Lu10TAl3 (green circles). Reproduced from ref 41. Coyright 2017 The Royal Society of Chemistry.

compound resonates about 150 ppm downfield of the Y compound, and identical trends are seen regarding the effect of the transition metal. The resonance shifts of the compounds with the iron and cobalt group elements Co, Ru, Rh, Os, and Ir, excluding iron, exhibit a decrease from the involved 3d to the 5d members. (In the case of the iron containing samples the experimental shift may be affected by elemental iron at the grain boundaries). In contrast, in the triad of the nickel group compounds in both cases the Pd containing samples are characterized by the lowest resonance shifts. Considering the triads with transition metals related to the 4d and 5d periods, respectively, a monotonic decrease of δms is observable with increasing valence electron concentration (e.g., the series Ru → Rh → Pd and Os → Ir → Pt). A similar trend was noted previously for the experimental 29Si magnetic shielding effects within a series of isotypic ScTSi silicides.67 Within the 3d series, this trend is also followed for Y10CoAl3 and Y10NiAl3, but not for Lu10CoAl3 and Lu10NiAl3. Again, GIPAW calculations may prove useful for separating the respective δ orb and δK contributions in these compounds, leading to a more detailed fundamental understanding of these trends.

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AUTHOR INFORMATION

Corresponding Authors

*Hellmut Eckert. E-Mail: [email protected]. *Oliver Janka. E-Mail: [email protected].

Figure 7. Correlations between theoretical and experimental chemical shifts in the Heusler-phases ScT2Al. The red straight line through the data represents a hypothetical perfect agreement, while the dashed and solid black lines were derived from linear fits considering only the best values computed by different methods. Reproduced with permission from ref 66. Copyright 2017 American Chemical Society.

ORCID

Hellmut Eckert: 0000-0002-6536-0117 Oliver Janka: 0000-0002-9480-3888 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript and declare no competing financial interest.

calculated δms values are found in excellent agreement with the experimental data.59 Variations in both δorb and δK are responsible for the differences in δms in this series of compounds. Interesting trends are also observed within the isotypic RE10TAl3 (RE = Y, Lu; T = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt) series. The Al atoms occupy a site with point symmetry mm2, with 10 rare-earth and 2 Al atoms in the first coordination sphere. As the Al nuclei experience sizable quadrupolar interactions, eq 3 has to be used to extract δms from fielddependent measurements. Figure 8 shows the effect of the RE and T elements upon δms. For a given T element, the Lu 66

Notes

The authors declare no competing financial interest. Biographies Christopher Benndorf received the doctoral degree in chemistry from the Westfälische Wilhelms-Universität (WWU) Münster, Germany, in 2017. His thesis combined aspects of synthetical solid state chemistry, crystallography, and solid state NMR spectroscopy of intermetallic compounds in the groups of Prof. Rainer Pöttgen and Prof. Hellmut Eckert. In January 2017, he subsequently started a habilitation at the 1465


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(13) Stone, N. J. Table of nuclear electric quadrupole moments. At. Data Nucl. Data Tables 2016, 111−112, 1−28. (14) Putnis, A. Quantification of disorder in silicate melts, glasses and crystals using NMR spectroscopy. Phys. Chem. Miner. 1996, 23, 247− 247. (15) Haouas, M.; Taulelle, F.; Martineau, C. Recent advances in application of 27Al NMR spectroscopy to materials science. Prog. Nucl. Magn. Reson. Spectrosc. 2016, 94−95, 11−36. (16) Bell, A. T. NMR applied to zeolite synthesis. Colloids Surf., A 1999, 158A, 221−234. (17) Engelhardt, G. Characterization of Zeolite Catalysts and Related Materials by Multinuclear Solid-State NMR Spectroscopy. In Surface Science: Principles and Current Applications; MacDonald, R. J., Taglauer, E. C., Wandelt, K. R., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 1996; pp 321−330. (18) Blumenfeld, A. L.; Fripiat, J. J. Characterization of Brønsted and Lewis acidity in zeolites by solid-state NMR and the recent progress in the REDOR technique. Magn. Reson. Chem. 1999, 37, S118−S125. (19) Haarmann, F.; Koch, K.; Jeglič, P.; Pecher, O.; Rosner, H.; Grin, Y. NMR Spectroscopy of Intermetallic Compounds: An Experimental and Theoretical Approach to Local Atomic Arrangements in Binary Gallides. Chem. - Eur. J. 2011, 17, 7560−7568. (20) Eckert, H.; Pöttgen, R. 45Sc Solid State NMR Spectroscopy − A Complementary Tool to X-ray Crystallography for Structure Determination of Intermetallic Compounds. Z. Anorg. Allg. Chem. 2010, 636, 2232−2243. (21) Bastow, T. J.; Smith, M. E. Observation of precipitation in MgAl alloys by 25Mg and 27Al NMR. J. Phys.: Condens. Matter 1995, 7, 4929. (22) Bastow, T. J.; Forwood, C. T.; Gibson, M. A.; Smith, M. E. Local site symmetry and electronic structure of trialuminide and related intermetallic alloys probed by solid-state NMR. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 2988−2997. (23) Lue, C.-S.; Chepin, S.; Chepin, J.; Ross, J. H. NMR study of trialuminide intermetallics. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 7010−7014. (24) Lue, C. S.; Xie, B. X.; Horng, S. N.; Su, J. H.; Lin, J. Y. Stability and local electronic properties of Al3M (M = Zr, Hf): An NMR study. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 195104. (25) Ratai, E.; Bruins, P.; Hernandez, C. J.; Kauzlarich, S. M.; Augustine, M. P. Magnetic Resonance Study of a Series of Phosphorus-Containing Zintl Compounds: Ca14AlP11, Ca14MnP11, and Eu14MnP11. Chem. Mater. 2002, 14, 2467−2475. (26) Haarmann, F.; Armbrüster, M.; Grin, Y. Local Ordering in the Intermetallic Compound Cu1−xAl2 Studied by NMR Spectroscopy. Chem. Mater. 2007, 19, 1147−1153. (27) Bandyopadhyay, B.; Majumder, M.; Ghoshray, A.; Ghoshray, K. 27 Al and 63Cu NMR studies on intermetallic Kondo compound CeCu3Al2. Phys. B 2010, 405, 4691−4695. (28) Pavarini, E.; Baek, S. H.; Suh, B. J.; Borsa, F.; Bud’ko, S. L.; Canfield, P. C. NMR relaxation rates and Knight shifts in MgB2 and AlB2: theory versus experiments. Supercond. Sci. Technol. 2003, 16, 147. (29) Celotto, S.; Bastow, T. J. Study of precipitation in aged binary Mg−Al and ternary Mg−Al−Zn alloys using 27Al NMR spectroscopy. Acta Mater. 2001, 49, 41−51. (30) Nowak, B.; Hayashi, S. 27Al NMR Study in ZrNiAl. Solid State Nucl. Magn. Reson. 2000, 18, 59−69. (31) Jeglič, P.; Vrtnik, S.; Bobnar, M.; Klanjšek, M.; Bauer, B.; Gille, P.; Grin, Y.; Haarmann, F.; Dolinšek, J. M-Al-M groups trapped in cages of Al13M4 (M = Co,Fe,Ni,Ru) complex intermetallic phases as seen via NMR. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 104201. (32) Yuan, C. C.; Yang, Y. F.; Xi, X. K.; Cui, J.; Xiang, J. F. Solid state 27 Al NMR investigation of Knight shift tensors of LnAl2 (Ln = Y, La and Lu) and correlation with DFT calculations. Solid State Nucl. Magn. Reson. 2012, 41, 28−31. (33) Samoson, A. Satellite transition high-resolution NMR of quadrupolar nuclei in powders. Chem. Phys. Lett. 1985, 119, 29−32.

Institute for Mineralogy, Crystallography and Materials Science of the Universität Leipzig, Germany. Hellmut Eckert obtained his doctoral degree in chemistry at the Westfälische Wilhelms-Universität Münster, Germany, in 1982. Following postdoctoral positions at Rutgers University and Caltech, he became a Professor of Chemistry at the University of California, Santa Barbara (1987−1995) before being appointed to a C-4 Professorship at the Institute of Physical Chemistry of the WWU Münster. Since 2011, he is also Professor Titular at the Sao Carlos Physics Institute of the University of Sao Paulo, Brazil. He received the Haber prize of the Deutsche Bunsengesellschaft in 1989 and the George Morey Award of the American Ceramic Society in 2016. His research focuses on the development and application of modern solid state NMR techniques for the structural analysis of disordered materials. Oliver Janka received his doctoral degree in chemistry from the University of Stuttgart, Germany, in 2010 working on rare-earth containing fluorides in the group of Prof. Thomas Schleid. Afterwards he spent two years as a postdoctoral fellow at the University of California, Davis, in the group of Prof. Susan Kauzlarich where he started crystallographic work on intermetallics. Currently he is a junior group leader at the WWU Münster, Germany, and temporary lecturer for inorganic chemistry at the Carl von Ossietzky Universität in Oldenburg, Germany. His research focuses on the structural chemistry and the physical properties of intermetallic aluminum compounds with complementary NMR spectroscopic studies to support the crystallography of these materials.

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ACKNOWLEDGMENTS H.E. acknowledges the Deutsche Forschungsgemeinschaft (DFG) for financial support under the Grant Ec168/14-1.

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REFERENCES

(1) Croat, J. J.; Herbst, J. F.; Lee, R. W.; Pinkerton, F. E. High-energy product Nd-Fe-B permanent magnets. Appl. Phys. Lett. 1984, 44, 148− 149. (2) Strnat, K.; Hoffer, G.; Olson, J.; Ostertag, W.; Becker, J. J. A Family of New Cobalt-Base Permanent Magnet Materials. J. Appl. Phys. 1967, 38, 1001−1002. (3) Ray, A.; Strnat, K. Easy directions of magnetization in ternary R2(Co,Fe)17 phases. IEEE Trans. Magn. 1972, 8, 516−518. (4) Gschneidner, K. A.; Pecharsky, V. K. Magnetocaloric Materials. Annu. Rev. Mater. Sci. 2000, 30, 387−429. (5) Matthias, B. T.; Geballe, T. H.; Geller, S.; Corenzwit, E. Superconductivity of Nb3Sn. Phys. Rev. 1954, 95, 1435−1435. (6) Davis, J. R. ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys; ASM International: Materials Park, OH, 2000. (7) Sikka, V. K. Intermetallic-Based High-Temperature Materials at Corrosion, NACE International., San Antonio, TX, 1999. (8) Peters, M.; Leyens, C. Titan und Titanlegierungen; Wiley-VCH: Weinheim, 2002. (9) Kanatzidis, M. G.; Pöttgen, R.; Jeitschko, W. The Metal Flux: A Preparative Tool for the Exploration of Intermetallic Compounds. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (10) Schmitt, D. C.; Drake, B. L.; McCandless, G. T.; Chan, J. Y. Targeted Crystal Growth of Rare Earth Intermetallics with Synergistic Magnetic and Electrical Properties: Structural Complexity to Simplicity. Acc. Chem. Res. 2015, 48, 612−618. (11) Duer, M. J. Introduction to Solid-State NMR Spectroscopy; WileyBlackwell: Oxford, 2004. (12) Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Granger, P.; Hoffman, R. E.; Zilm, K. W. Further conventions for NMR shielding and chemical shifts. Pure Appl. Chem. 2008, 80, 59−84. 1466


Article


(53) Heusler, F. Heusler Phasen. Verh. Dt. Phys. Ges. 1903, 5, 219. (54) Webster, P. J. Heusler alloys. Contemp. Phys. 1969, 10, 559−577. (55) Pöttgen, R.; Johrendt, D. Intermetallics - Synthesis, Structure, Function; De Gruyter: Berlin, Boston, 2014. (56) Dshemuchadse, J.; Steurer, W. Intermetallics; Union of Crystallography; Oxford University Press: Oxford, 2016. (57) Strobel, S.; Choudhury, A.; Dorhout, P. K.; Lipp, C.; Schleid, T. Rare-Earth Metal(III) Oxide Selenides M4O4Se[Se2] (M = La, Ce, Pr, Nd, Sm) with Discrete Diselenide Units: Crystal Structures, Magnetic Frustration, and Other Properties. Inorg. Chem. 2008, 47, 4936−4944. (58) Pecher, O.; Mausolf, B.; Peters, V.; Lamberts, K.; Korthaus, A.; Haarmann, F. Unravelling Local Atomic Order of the Anionic Sublattice in M(Al1−xGax)4 with M = Sr and Ba by Using NMR Spectroscopy and Quantum Mechanical Modelling. Chem. - Eur. J. 2016, 22, 17833−17842. (59) Benndorf, C.; Niehaus, O.; Eckert, H.; Janka, O. 27Al and 45Sc NMR Spectroscopy on ScT2Al and Sc(T0.5T′0.5)2Al (T = T′ = Ni, Pd, Pt, Cu, Ag, Au) Heusler Phases and Superconductivity in Sc(Pd0.5Au0.5)2Al. Z. Anorg. Allg. Chem. 2015, 641, 168−175. (60) Andrew, E. R.; Hinshaw, W. S.; Tiffen, R. S. More precise determination of the Knight shift of aluminium. Phys. Lett. A 1973, 46, 57−58. (61) Shyu, W.-M.; Das, T. P.; Gaspari, G. D. Direct and CorePolarization Contributions to the Knight Shift in Metallic Aluminum. Phys. Rev. 1966, 152, 270−278. (62) Jensen, W. B. Electronegativity from Avogadro to Pauling: Part 1: Origins of the Electronegativity Concept. J. Chem. Educ. 1996, 73, 11. (63) Pickard, C. J.; Mauri, F. All-electron magnetic response with pseudopotentials: NMR chemical shifts. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 245101. (64) d’Avezac, M.; Marzari, N.; Mauri, F. Spin and orbial magnetic response in metals: Susceptibility and NMR shifts. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 165122. (65) Laskowski, R.; Blaha, P. NMR Shielding in Metals Using the Augmented Plane Wave Method. J. Phys. Chem. C 2015, 119, 19390− 19396. (66) Ferreira, A. R.; Reuter, K.; Scheurer, C. Toward Routine GaugeIncluding Projector Augmented-Wave Calculations for Metallic Systems: The Case of ScT2Al (T = Ni, Pd, Pt, Cu, Ag, Au) Heusler Phases. J. Phys. Chem. C 2016, 120, 25530−25540. (67) Harmening, T.; Eckert, H.; Fehse, C. M.; Sebastian, C. P.; Pöttgen, R. 45Sc Solid State NMR studies of the silicides ScTSi (T=Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt). J. Solid State Chem. 2011, 184, 3303−3309.

(34) Pöttgen, R.; Gulden, T.; Simon, A. Miniaturisierte Lichtbogenapperatur für den Laborbedarf. GIT Labor-Fachzeitschrift 1999, 43, 133−136. (35) Kußmann, D.; Hoffmann, R.-D.; Pöttgen, R. Syntheses and Crystal Structures of CaCuGe, CaAuIn, and CaAuSn − Three Different Superstructures of the KHg2 Type. Z. Anorg. Allg. Chem. 1998, 624, 1727−1735. (36) Pöttgen, R.; Lang, A.; Hoffmann, R.-D.; Künnen, B.; Kotzyba, G.; Müllmann, R.; Mosel, B. D.; Rosenhahn, C. The stannides YbPtSn and Yb2Pt3Sn5. Z. Kristallogr. - Cryst. Mater. 1999, 214, 143−150. (37) Stegemann, F.; Benndorf, C.; Bartsch, T.; Touzani, R. S.; Bartsch, M.; Zacharias, H.; Fokwa, B. P. T.; Eckert, H.; Janka, O. Ba3Pt4Al4 - Structure, Properties, and Theoretical and NMR Spectroscopic Investigations of a Complex Platinide Featuring Heterocubane [Pt4Al4] Units. Inorg. Chem. 2015, 54, 10785−10793. (38) Radzieowski, M.; Benndorf, C.; Haverkamp, S.; Eckert, H.; Janka, O. On new ternary equiatomic scandium transition metal aluminum compounds ScTAl with T = Cr, Ru, Ag, Re, Pt, and Au. Z. Naturforsch., B: J. Chem. Sci. 2016, 71b, 553−566. (39) Benndorf, C.; Stegemann, F.; Seidel, S.; Schubert, L.; Bartsch, M.; Zacharias, H.; Mausolf, B.; Haarmann, F.; Eckert, H.; Pöttgen, R.; Janka, O. Equiatomic AEAuX (AE=Ca-Ba, X=Al-In) Intermetallics: A Systematic Study of their Electronic Structure and Spectroscopic Properties. Chem. - Eur. J. 2017, 23, 4187−4196. (40) Benndorf, C.; Stegemann, F.; Eckert, H.; Janka, O. New transition metal-rich rare-earth palladium/platinum aluminides with RET5Al2 composition: structure, magnetism and 27Al NMR spectroscopy. Z. Naturforsch. 2015, 70b, 101−110. (41) Benndorf, C.; Eckert, H.; Janka, O. Ternary rare-earth aluminium intermetallics RE10TAl3 (RE = Y, Ho, Tm, Lu; T = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) with an ordered anti-Co2Al5 structure. Dalton Trans. 2017, 46, 1083−1092. (42) Stegemann, F.; Benndorf, C.; Zhang, Y.; Bartsch, M.; Zacharias, H.; Fokwa, B. P. T.; Eckert, H.; Janka, O. Network Formation by Condensed Tetrahedral [Au3Al] Units in Na2Au3Al: Crystal and Electronic Structure, Spectroscopic Investigations, and Physical Properties of an Ordered Ternary Auride. Inorg. Chem. 2017, 56, 1919−1931. (43) Janka, O.; Niehaus, O.; Pöttgen, R.; Chevalier, B. Cerium intermetallics with TiNiSi-type structure. Z. Naturforsch., B: J. Chem. Sci. 2016, 71b, 737−764. (44) Landrum, G. A.; Hoffmann, R.; Evers, J.; Boysen, H. The TiNiSi Family of Compounds: Structure and Bonding. Inorg. Chem. 1998, 37, 5754−5763. (45) Nuspl, G.; Polborn, K.; Evers, J.; Landrum, G. A.; Hoffmann, R. The Four-Connected Net in the CeCu2 Structure and Its Ternary Derivatives. Its Electronic and Structural Properties. Inorg. Chem. 1996, 35, 6922−6932. (46) Pöttgen, R. Coloring, Distortions, and Puckering in Selected Intermetallic Structures from the Perspective of Group-Subgroup Relations. Z. Anorg. Allg. Chem. 2014, 640, 869−891. (47) Ohba, T.; Kitano, Y.; Komura, Y. The charge-density study of the Laves phases, MgZn2 and MgCu2. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, 40, 1−5. (48) Stein, F.; Palm, M.; Sauthoff, G. Structure and stability of Laves phases part II − structure type variations in binary and ternary systems. Intermetallics 2005, 13, 1056−1074. (49) Stein, F.; Palm, M.; Sauthoff, G. Structure and stability of Laves phases. Part I. Critical assessment of factors controlling Laves phase stability. Intermetallics 2004, 12, 713−720. (50) Matar, S. F.; Pöttgen, R.; Nakhl, M. Electronic structure and chemical bonding in LaIrSi-type intermetallics. Z. Naturforsch., B: J. Chem. Sci. 2017, 72b, 207−213. (51) Klepp, K.; Parthe, E. LaIrSi with an Ordered SrSi2 Derivative Structure. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, B38, 1541−1544. (52) Graf, T.; Felser, C.; Parkin, S. S. P. Simple rules for the understanding of Heusler compounds. Prog. Solid State Chem. 2011, 39, 1−50. 1467


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