The new equiatomic scandium transition metal aluminides ScTAl for T = Cr, Ru, Ag, Re, Pt, and Au were obtained by arc-melting of the elements followed by subsequent annealing for crystal growth. The samples were studied by powder and single crystal X-ray diffraction. The structures of three compounds were refined from single crystal X-ray diffractometer data: ScCrAl, MgZn2 type, P63/mmc, a = 525.77(3), c = 858.68(5) pm, R1 = 0.0188, wR2 = 0.0485, 204 F2 values, 13 variables, ScPtAl, TiNiSi type, Pnma, a = 642.83(4), b = 428.96(2), c = 754.54(5) pm, R1 = 0.0326, wR2 = 0.0458, 448 F2 values, 20 variables and ScAuAl, HfRhSn type, P6̅2c, a = 722.88(4), c = 724.15(4) pm, R1 = 0.0316, wR2 = 0.0653, 512 F2 values, 18 variables. Phase pure samples of all compounds were furthermore investigated by magnetic susceptibility measurements, and Pauli-paramagnetism but no superconductivity was observed down to 2.1 K for all of them. The local structural features and disordering phenomena have been characterized by 27Al and 45Sc magic angle spinning (MAS) and static NMR spectroscopic investigations.
Equiatomic ternary compounds RETAl with a rare-earth element, a transition metal and aluminum are known for their large structural diversity. Several different structure types are formed, with both ordered and disordered T and Al atom arrangements. For a large number of transition metals (T = Fe , Rh [2–6], Ir , Ni [8, 9], Pd [10, 11], Pt [10, 12], Cu [8, 13], Ag , Au , Zn ) the whole series of equiatomic RETAl compounds are known from RE = La–Nd, Sm, Gd–Lu, including Y. For other transition metals, e.g. T = Ru, only a single compound has been reported so far (CeRuAl [15, 16]). No equiatomic rare-earth compounds of the early transition metals have been prepared yet. Several review articles discussing the crystal chemistry as well as the solid-state physics aspects of these compounds in great detail have been published recently [17–19]. With scandium instead of a rare-earth element, so far equiatomic compounds with T = Fe , Co , Ni , Cu , Pd , and Ir  have been reported, all of them crystallizing in the MgZn2 type structure. The solid solutions Sc(Ni1–xAlx)2 have been investigated with respect to their hydrogen storage capabilities . Figure 1 shows a graphical overview of the ternary equiatomic Sc-T-Al system. We have synthesized and structurally characterized six new ScTAl compounds with T = Cr, Ru, Ag, Re, Pt, and Au and measured their magnetic susceptibilities. While for T = Cr, Ru, Ag and Re the hexagonal MgZn2 type structure has been found, ScPtAl crystallizes, like the rare-earth representatives, in the orthorhombic TiNiSi type structure. ScAuAl crystallizes in the hexagonal HfRhSn type structure, a superstructure of ZrNiAl, showing aurophilic interactions in the trigonal prismatic Au@Sc6 units. To characterize the local structural environments of the scandium and aluminum sites in these different structure types we have conducted detailed field dependent 27Al and 45Sc magic angle spinning (MAS) and static NMR measurements.
The ScTAl compounds were synthesized from the elements, using scandium ingots (Merck, 99.999%), chromium pieces (Alfa Aesar, 99.995%), rhenium (Starck, 99.9%) and ruthenium powder (Degussa, 99.9%), silver (Agosi AG, 99.5%) and gold drops (Agosi AG, 99.99%), platinum sheets (Agosi AG, 99.9%), and aluminum turnings (Koch Chemicals, 99.99%). Pieces of scandium were first arc-melted under an argon pressure of 800 mbar in a water cooled copper hearth . Before use, the rhenium powder was stirred for 30 min in boiling concentrated aqueous NaOH to remove oxidic impurities and washed three times with demineralized water and acetone. The transition metal powder was pressed into pellets with a diameter of 6 mm. All starting materials were weighed in the ideal 1:1:1 ratio (Sc:T:Al) and arc-melted under an argon pressure of 800 mbar. The obtained button was remelted and turned over several times to increase the homogeneity. Fragments of the crushed buttons were sealed in quartz tubes under vacuum and annealed at 800°C for 7 d or alternatively heated for 2 h in a high frequency furnace (Hüttinger Elektronik, Freiburg, Germany, Typ TIG 2.5/300) . The samples show metallic luster and are stable in air over months.
2.2 X-ray diffraction
The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800 scanner) with CuKα1 radiation using α-quartz (a = 491.30, c = 540.46 pm, Riedel-de-Häen) as an internal standard. Correct indexing of the diffraction lines was ensured through intensity calculations. The lattice parameters were obtained through least-squares fits  with standard deviations smaller than ±0.1 pm for all axes.
Irregularly shaped crystals of ScCrAl, ScPtAl and ScAuAl were obtained by mechanical fragmentation of the annealed arc-melted buttons. These fragments were glued to thin quartz fibers using beeswax. The crystal quality was tested by Laue photographs on a Buerger camera (white molybdenum radiation, imaging plate technique, Fujifilm BAS-1800). Intensity data sets of suitable crystals were collected at room temperature by use of a Stoe IPDS II diffractometer (graphite-monochromatized MoKα radiation; oscillation mode). Numerical absorption corrections were applied to the data sets. All relevant crystallographic data and details of the data collections and evaluations are listed in Table 1.
|Molar mass g mol−1||122.4||267.0||268.9|
|Calculated density Dx, g cm−3||3.96||8.52||8.17|
|Volume V, nm3||0.206||0.208||0.328|
|Crystal size, μm3||30 × 50 × 110||10 × 15 × 15||10 × 15 × 20|
|Transmission ratio, min/max||0.485/0.799||0.244/0.701||0.257/0.536|
|±hmax; ±kmax; ±lmax||8; 8; 13||9; 6; 11||11; 11; 11|
|θ range, deg||4.48–34.81||4.16–33.31||3.25–34.73|
|Absorption coefficient μ, mm−1||8.2||70.3||70.1|
|Reflections, unique||6484, 204||5635, 448||11197, 512|
|Reflections with |Fo| > 3 σ(Fo)||177||400||402|
|R1/R1 with |Fo| > 3 σ(Fo)||0.024/0.019||0.036/0.033||0.045/0.032|
|wR2/Goodness of Fit (GooF)||0.049/2.94||0.046/3.37||0.065/1.93|
|Extinction coefficient g||20(3)||62(4)||115(1)|
|Residual electron density|
|Δρ (max/min), e × 106 pm−3||2.27/–2.33||3.53/–2.54||4.31/–3.15|
Further details of the crystal structure investigations can be obtained from the Fachinformationszentrum (FIZ) Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: firstname.lastname@example.org), on quoting the deposition numbers given above.
2.3 Structure refinements
Careful analyses of the three data sets revealed space group P63/mmc for ScCrAl, Pnma for ScPtAl and P6̅2c for ScAuAl. Isotypism with MgZn2 in the case of ScCrAl, with TiNiSi for ScPtAl and with HfRhSn for ScAuAl was already evident from the Guinier powder patterns. The starting values for the atomic parameters were obtained using Superflip . The three structures were refined on F2 with anisotropic displacement parameters for all the atoms using the Jana2006  routine. Refinement of the occupancy parameters revealed the expected Cr/Al mixing for ScCrAl, but no mixing was observed in the cases of the other crystals. The mixed occupancy was refined as a least-squares variable in the final cycles, leading to the composition ScCr1.06(1)Al0.94(1). For simplicity reasons the ideal formula ScCrAl will be used in the text. All other sites were fully occupied within three standard deviations. The final difference Fourier synthesis revealed no significant residual peaks. The final positional parameters and interatomic distances are listed in Tables 2–5.
aMixed occupation refined to 49(1)% Cr, 51(1)% Al; bmixed occupation refined to 42(1)% Cr, 58(1)% Al. The anisotropic displacement factor tensor of the atoms is defined by: –2π2[(ha*)2U11+···+2hka*b*U12]. U12 = U23 = 0 for ScCr1.06(1)Al0.94(1) and ScPtAl.
|Sc–||d (pm)||(Cr/Al1)–||d (pm)||(Al/Cr)2–||d (pm)|
|Sc–||d (pm)||Pt–||d (pm)||Al–||d (pm)|
|Sc–||d (pm)||Au1–||d (pm)||Au2–||d (pm)||Al–||d (pm)|
2.4 EDX data
Semiquantitative energy dispersive X-ray (EDX) analyses on all bulk samples were carried out on a Leica 420i scanning electron microscope. The polycrystalline annealed pieces were embedded in a methylmethacrylate matrix and polished with diamond and SiO2 emulsions of different particle sizes. The experimentally observed compositions were close to the weighed ones and phase pure samples (as detectable within instrumental limitations) were observed after annealing. No impurity elements heavier than sodium (detection limit of the instrument) were observed.
2.5 Magnetic properties
Polycrystalline pieces of the annealed ingots were packed in kapton foil and attached to the sample holder rod of a vibrating sample magnetometer unit (VSM) for measuring the magnetization M(T,H) in a Quantum Design Physical Property Measurement System (PPMS). The samples were investigated in the temperature range of 2.1–300 K and with magnetic flux densities up to 10 kOe (1 kOe = 7.96 × 104 A m−1).
2.6 Solid state NMR spectroscopy
27Al and 45Sc solid state NMR spectra were recorded on Bruker DSX 500 (500 and 200 MHz), Bruker DSX 400 (400 MHz) and Bruker AVANCE III (300 MHz) spectrometers using static and magic-angle spinning (MAS) conditions. The samples were ground to fine powders under dry cyclohexane and mixed with an appropriate amount of boron nitride, to reduce the density and the electrical conductivity of the samples. The diluted samples were loaded into cylindrical ZrO2 rotors with diameters of 2.5 mm and 4 mm, respectively, and spun at the magic angle with frequencies of 12 kHz and 28 kHz, respectively, to record MAS NMR spectra. The conducted MAS experiments were conventional rotor-synchronized π/2-τ-π-τ spin-echo experiments with typical π/2 pulse lengths between 1.83–0.34 μs (27Al) and 1.44–0.22 μs (45Sc) and relaxation delays of 0.2 s at flux densities of 11.7, 9.4, 7.05 and 4.7 T. Static spectra were recorded at 7.05 T using the wideband uniform-rate smooth truncation QCPMG (WURST-QCPMG) sequence  with the WURST-80 pulse shape and 8-step phase cycling version.
Resonance shifts were referenced to 1 molar Al(NO3)3 and 1 molar ScCl3 solutions in H2O. The NMR spectra were recorded using the Bruker Topspin software , and the analysis was performed with the help of the Dmfit software . All NMR parameters extracted from the experimental spectra are listed in Table 6.
|Sample||δiso (ppm)||FWHM (kHz)||G/L||CQ,calcd./CQ,exp. (MHz)||ηQ,calcd./ηQ,exp.||p1 (μs)||νrot (kHz)||B0 (T)|
Theoretical electric field gradient calculations were conducted for ScPtAl and ScAuAl using the Wien2k code, a full-potential all electron method based on the LAPW + LO method . SCF calculations were done with Rmt parameters of 2.39–2.47 atomic units for Sc, 2.07–2.17 atomic units for Al and 2.50 atomic units for Pt and Au. Separation energies between the core and valence states were set to –6 Ry. A plane wave cutoff parameter value Rmtmin × Kmax of 7.00 was used in both cases. For describing the first Brillouin zone, 3 k-points were used initially, and this was successively increased up to 800 (ScPtAl) and 1500 (ScAuAl) k-points. 5692 (ScPtAl) and 5807 (ScAuAl) plane waves were used to describe the electronic state of the crystal structures. All calculated quadrupolar coupling constants CQ and asymmetry parameters ηQ are included in Table 6.
3 Results and discussion
3.1 Crystal structure
The synthesized ScTAl compounds were found to crystallize in different structure types, which will be discussed in this paragraph. ScPtAl crystallizes, like the REPtAl series [10, 12], in the orthorhombic TiNiSi type structure with space group Pnma and lattice parameters of a = 642.83(4), b = 428.96(2), and c = 754.54(5) pm. All atoms are located on the 4c site (.m.). The Pt position is surrounded by four Sc and two Al atoms in the shape of a tricapped trigonal prism, with one Sc and two Al atoms capping the rectangular faces (Fig. 2, top). The prisms are condensed via the triangular top and bottom faces to form strands running along the crystallographic b axis. These strands are connected over the Sc2 edges forming layers within the ab plane. The capping atoms are finally utilized to connect neighboring strands to create a three-dimensional network (Fig. 3). Due to this connection the prisms are shifted in height by y/2. The scandium atoms are located in cavities within the polyanionic [PtAl]δ– framework, which consists of corrugated hexagons (Fig. 2, bottom). As expected for a platinum compound, the atoms in the heteroatomic rhombs highlighted in cyan (Fig. 2, bottom) are arranged to achieve a maximum distance between the Pt atoms, being consistent with the observations of Nuspl et al. . This tilt can been attributed to the higher electronegativity of the Pt atoms, which move away from each other . The details about the crystal structure refinement are listed in Tables 1–5. For more detailed information about the crystal chemistry of TiNiSi type compounds we refer to the literature [33, 35].
ScAuAl is found to crystallize in the hexagonal HfRhSn type structure (P6̅2c, a = 722.88(4), and c = 724.15(4) pm) which has been reported to be a super structure of ZrNiAl (P6̅2m). Zumdick et al. discovered a doubling of the c axis in HfRhSn  and ZrRhSn  with the help of single crystal investigations. Amongst the aluminum compounds, so far only ZrPtAl  and HfPtAl  have been reported to crystallize in the HfRhSn type structure. This structure type had been assigned first based on powder diffraction measurements and later confirmed by single crystal diffraction. In Fig. 4 the Guinier powder data along with the calculated theoretical diffraction patterns of ScAuAl both in the HfRhSn and the ZrNiAl type are depicted. The small distortions in the structure result in changes of the intensities in the diffraction patterns. Data from single crystal investigations is given in Tables 1–5. Similar to the ZrNiAl type structure, one crystallographic Sc, one Al and two Au sites are found in the crystal structure of ScAuAl. However, one transition metal atom is shifted off the mirror plane compared to ZrNiAl, which results in a symmetry reduction from space group P6̅2m to P6̅2c. In ScAuAl the Au1 atoms are surrounded in an undistorted tricapped trigonal prismatic arrangement by six Al atoms with three Sc atoms over the rectangular faces (Fig. 5, top). The Au2 atoms are also surrounded by nine atoms but in the shape of distorted tricapped trigonal prisms (Fig. 5, bottom). Six Sc atoms form the prism with three Al atoms as capping atoms. Six of these Au2 prisms are condensed over the Sc edges to form large rings, being centered by the Au1 prisms. The shortest distances are found between Au and Al and range from 262 to 271 pm, consistent with the idea of a three-dimensional infinite [AuAl]δ– polyanion. These distances are in good agreement with Au–Al distances in REAuAl (RE = Y, La–Nd, Sm–Lu; TiNiSi type structure)  or REAu3Al7 (RE = Y, Ce–Nd, Sm, Gd–Lu; ScRh3Si7 type structure)  compounds. The Sc atoms are located in the hexagonal voids of the polyanionic framework and are slightly displaced, as seen in Fig. 6 (top). Due to this displacement the trigonal prism surrounding the Au2 atoms has two different triangular faces and also two different Sc–Au2 distances of 283 and 288 pm. The distortion is shown in Fig. 6 (bottom). The Au2 atoms exhibit a shorter (334 pm) and a longer distance (391 pm) to each other with the contracted triangular face in between the longer distance. The displacement of the Au2 atoms from the mirror plane of the undistorted structure was determined to be 14 pm. The driving force of this distortion is the formation of the shorter Au–Au distances due to aurophilic interactions between the gold atoms.
The four compounds ScTAl with T = Cr, Ru, Ag, and Re have been found to crystallize in the hexagonal MgZn2 type structure (P63/mmc). The lattice parameters are given in Table 7. MgZn2 contains one crystallographic Mg site (4f, 3m.), here occupied by Sc, and two Zn sites (2a, 3̅m. and 6h, mm2) with a ratio of 1:3. Therefore the T and Al atoms are mixed on their crystallographic sites thus achieving an equiatomic composition. The (T/Al)1 atoms form a hexagonal layered arrangement (highlighted in red), with one fourth of the atoms removed; the resulting triangles are condensed over vertices, surrounding the hexagonal voids (Fig. 7, top). This pattern is also known as Kagomé lattice. Those layers are located on heights of z = 1/4 and z = 3/4 and stacked in an A,B,A… sequence, forming face-sharing bi-tetrahedral units with the (T/Al)2 atoms located on z = 0, z = 1/2 and z = 1. The Sc atoms form a lonsdaleite like arrangement which interpenetrates the [TAl]δ– polyanionic framework (Fig. 7, bottom, the two differently membered rings of this framework are highlighted in blue).
aLattice parameters from single crystal diffraction.
3.2 Magnetic properties
All prepared samples have been investigated with respect to their physical properties. No superconductivity was found down to 2.1 K. As anticipated no magnetic ordering was observed for the six compounds. They show Pauli-paramagnetic behavior with small susceptibilities of 5.0(1) × 10−4 emu mol−1 (ScCrAl), 1.7(1) × 10−4 emu mol−1 (ScRuAl), 1.4(1) × 10−4 emu mol−1 (ScReAl), 1.4(1) × 10−4 emu mol−1 (ScAgAl), 5.6(1) × 10−5 emu mol−1 (ScPtAl) and 6.1(1) × 10−6 emu mol−1 (ScAuAl) at 300 K, consistent with no localized d electrons (no magnetic moment) at the transition metals. In Fig. 8 the susceptibilities measured between 3 and 300 K at 10 kOe are summarized.
3.3 27Al and 45Sc NMR spectroscopy
The ScTAl compounds have been investigated by 27Al and 45Sc solid state NMR spectroscopy. Merely the spectra of the ordered compounds ScPtAl and ScAuAl exhibit only one distinct signal for each nucleus, as expected from the proposed structures with only one Sc and Al site, whereas the non-ordered phases ScCrAl, ScRuAl, ScAgAl and ScReAl reveal strongly broadened resonances. Due to the interaction of the conduction electrons with the local magnetic moments of the 27Al and 45Sc nuclei a strong resonance shift, known as Knight shift, is observed in all spectra. In Fig. 9, both, the 27Al and 45Sc MAS spectra for ScPtAl and ScAuAl recorded at a magnetic flux density of 11.7 T are shown.
In addition to the dominant central +1/2 ↔ –1/2 Zeeman transitions, the spectra show wide spinning sideband manifolds, which originate from the outer ±1/2 ↔ ±3/2 and ±3/2 ↔ ±5/2 Zeeman transitions in the case of 27Al (I = 5/2), and from the outer ±1/2 ↔ ±3/2, ±3/2 ↔ ±5/2, and ±5/2 ↔ ±7/2 Zeeman transitions in the case of 45Sc (I = 7/2). All of these satellite transitions are anisotropically broadened by first-order nuclear electric quadrupolar perturbations. Since the coordination environments of the Al and Sc atoms are non-spherical, the quadrupole moments of the 27Al and 45Sc nuclei interact with the local electrical field gradients (EFG), which results in the specific line shape of the NMR signals and the resulting spinning sideband pattern. Due to limitations in probe excitation bandwidths, the intensity profiles of the spinning sideband manifolds are distorted, making a determination of the quadrupolar interaction parameters from these profiles unreliable. Rather, these parameters were obtained from the lineshape analysis of the central resonances studied at different magnetic field strengths. Figure 10 shows the central transitions of the MAS spectra recorded at 7.05 T with spinning frequencies of 28.0 kHz and the simulated spectra based on theoretical calculations. The platinum compound shows well resolved 27Al and 45Sc signals with typical line shapes caused by second-order quadrupolar perturbations. The nuclear electric quadrupole coupling constant CQ and the electric field gradient asymmetry parameters ηQ obtained via lineshape simulations agree very well with the theoretically calculated parameters. In contrast, the spectrum of ScAuAl shows very strong broadening effects and no typical line shape owing to second-order quadrupolar perturbations. Figure 11 shows additional static 27Al and 45Sc WURST-QCPMG spectra of this compound. The observed spikelet envelope is well-reproduced by a simulation based on the theoretically calculated quadrupole interaction parameters, even though a large linebroadening parameter must be used. Possibly, the broadening effects may be attributed to intrinsic disordering in the ScAuAl crystal structure, although no such effects could be noticed in the X-ray diffraction experiments.
Figures 12 and 13 show the 27Al and 45Sc MAS-NMR signals of ScCrAl, ScRuAl, ScAgAl and ScReAl. For these compounds, strong line broadening effects are noted, which can be explained by the mixed occupancies of Al and Cr, Ru, Ag or Re, respectively, on the two different crystallographic sites in the disordered, ternary MgZn2– type crystal structure. The effect upon the NMR signals is also caused by the mixing of the two different elements on the 2a and 6h sites. The dominant origin of the broadening effects in these spectra was investigated by examining the magnetic field dependence. If the broadening effect is dominated by a distribution of Knight shifts, the line width (in Hz) should increase linearly with increasing magnetic field strength, whereas in the case of dominant second-order quadrupolar perturbations the line width (in Hz) is predicted to decrease with increasing field strength.
Table 6 indicates that in the 27Al and 45Sc MAS-NMR spectra of ScAgAl and in the 45Sc MAS-NMR spectra of ScRuAl and ScReAl the Knight shift distribution is the dominant effect. However, for the 27Al resonances of the latter two compounds, and for the 27Al and 45Sc resonances of ScCrAl, the Knight shift distribution effect and the second-order quadrupolar broadening effects are of comparable magnitude (even though the Knight shift distribution effect tends to dominate in most cases). The spectra of the chromium compound show some other anomalies: the line broadening effects are much more pronounced and both resonances occur at significantly lower frequencies than for the other materials. Such low-frequency shifts have been previously observed for other chromium intermetallic compounds, and their specific origins are the subject of on-going investigations . Based on the magnetic data presented in the present study, we can, however, exclude contributions due to Curie-type paramagnetism.
It thus appears that (with the exception of the anomalous behavior in the Cr compound) the influence of the transition metal atoms on the 27Al Knight shifts is rather small compared to the situation of the 45Sc Knight shifts, indicating that the transition metal atom exercises very little influence on the unpaired electron spin densities of the aluminum atoms even for compounds that have different crystal structures. In contrast, these structural differences are much better resolved by the nuclear electric quadrupolar coupling parameters, which show excellent agreement with the theoretically predicted values.
Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.
We thank Dipl.-Ing. U. Ch. Rodewald for collecting the single crystal intensity data and Dr. Martin Peterlechner for the opportunity to use the PPMS.
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