On new ternary equiatomic scandium transition metal aluminum compounds ScTAl with T = Cr, Ru, Ag, Re, Pt, and Au

2.1 Synthesis

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 [24]. 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) [25]. 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 CuK_α1 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 [26] 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 MoK_α 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.

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 [27]. The three structures were refined on F² with anisotropic displacement parameters for all the atoms using the Jana2006 [28] 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 ScCr_1.06(1)Al_0.94(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.

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 [29] 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 [30], and the analysis was performed with the help of the Dmfit software [31]. All NMR parameters extracted from the experimental spectra are listed in Table 6.

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 [32]. SCF calculations were done with R_mt 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 R_mt^min × K_max 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 C_Q and asymmetry parameters η_Q are included in Table 6.

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. [33]. This tilt can been attributed to the higher electronegativity of the Pt atoms, which move away from each other [34]. 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].

Fig. 2:

Coordination environments surrounding the Pt (top) and Sc (bottom) atoms in ScPtAl. The heteroatomic rhombs are highlighted in cyan.

Fig. 3:

View onto the extended unit cell of TiNiSi type ScPtAl along the crystallographic b axis. The trigonal prismatic Pt coordinations are shifted in height by ^y/₂ and highlighted by different line thicknesses.

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 [36] and ZrRhSn [37] with the help of single crystal investigations. Amongst the aluminum compounds, so far only ZrPtAl [38] and HfPtAl [38] 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) [14] or REAu₃Al₇ (RE = Y, Ce–Nd, Sm, Gd–Lu; ScRh₃Si₇ type structure) [39] 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.

Fig. 4:

Experimental powder X-ray diffraction pattern in comparison with the calculated intensities for ScAuAl both in the ZrNiAl type structure and the HfRhSn type superstructure. The differences in the intensities between the two structures are clearly visible.

Fig. 5:

Coordination environments surrounding the Au1 (top) and Au2 (bottom) atoms in ScAuAl.

Fig. 6:

Coordination environment surrounding the Sc cations in the crystal structure of ScAuAl (HfRhSn type, top). Projection of the extended unit cell along the c axis (middle). The two different trigonal prismatic environments surrounding the Au atoms are emphasized. Fragments of the substructure of ScAuAl formed by the trigonal prisms of Sc and Au atoms (bottom). Relevant interatomic Au–Au distances are given in pm. Sc atoms are depicted in blue, Al atoms in open and Au in black circles, respectively.

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).

Fig. 7:

Crystal structure of MgZn₂ type ScTAl (T = Cr, Ru, Ag, and Re). The Kagomé lattices (top) and the lonsdaleite arrangement of the Sc atoms (bottom) are highlighted. Sc atoms are shown in blue, the mixed occupied T/Al in divided black/white circles. The occupation shown was obtained from single crystal data for ScCr_1.06(1)Al_0.94(1).

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.

Fig. 8:

Magnetic susceptibilities measured in zero-field-cooled mode at 10 kOe for ScCrAl (black, right scale), ScRuAl (green), ScReAl (red), ScAgAl (blue), ScPtAl (orange), and ScAuAl (purple). All samples show Pauli-paramagnetic behavior.

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.

Fig. 9:

²⁷Al and ⁴⁵Sc MAS NMR spectra of ScPtAl (top) and ScAuAl (bottom) recorded at B₀ = 11.7 T with a spinning frequency of 28.0 kHz. The simulations (red curves) are based on the calculated quadrupolar interaction parameters.

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 C_Q and the electric field gradient asymmetry parameters η_Q 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.

Fig. 10:

Central transitions of the ²⁷Al and ⁴⁵Sc MAS NMR spectra of ScPtAl (top) and ScAuAl (bottom) recorded at B₀ = 7.05 T with a spinning frequency of 28.0 kHz and simulated spectra (red curves) based on the theoretical quadrupolar interaction parameters.

Fig. 11:

Static ²⁷Al and ⁴⁵Sc WURST QCPMG spectra of ScAuAl recorded at B₀ = 7.05 T and simulations (red curves) of the central transitions based on the theoretical calculated quadrupolar interaction parameters. The bottom spectra show an enlarged view of the central transitions.

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.

Fig. 12:

²⁷Al and ⁴⁵Sc MAS NMR spectra and simulations (red lines) of ScTAl (T = Cr, Ru, Ag, and Re) recorded at B₀ = 4.7 T and a spinning frequency of 12 kHz. Signals caused by impurities are marked with asterisks.

Fig. 13:

²⁷Al and ⁴⁵Sc MAS NMR spectra and simulations (red lines) of ScTAl (T = Cr, Ru, Ag, and Re) recorded at B₀ = 9.4 T and a spinning frequency of 12 kHz. Signals caused by impurities are marked with asterisks.

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 [40]. 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.