Formation of the Sub-oxide Sc4Au2O1-x and the Drastically Negative 27Al NMR Shift in Sc2Al

During attempts to synthesize Sc₄AuAl in the cubic Gd₄RhIn-type structure, the solid solution Sc₂Au_0.5Al_0.5 in the PbCl₂-type structure formed instead. Subsequently, the solid solution Sc₂Au_1-xAl_x was investigated with respect to its existence range along with the structure types formed for different compositions with x = 0, 0.25, 0.5, 0.75, and 1. According to X-ray powder diffraction studies, Sc₂Al and nominal Sc₂Au_0.25Al_0.75 crystallized in the hexagonal Ni₂In-type structure (P6₃/mmc), while Sc₂Au_0.5Al_0.5, Sc₂Au_0.75Al_0.25, and Sc₂Au were found to crystallize in the orthorhombic PbCl₂-type structure (Pnma). The crystal structures of Sc₂Au and Sc₂Au_0.59(1)Al_0.41(1) were refined from single-crystal data (Sc₂Au: a = 648.0(1), b = 467.2(1), c = 835.2(2) pm, wR2 = 0.0382, 535 F² values, 25 variables; Sc₂Au_0.59(1)Al_0.41(1): a = 632.48(5), b = 472.16(3), c = 848.67(6) pm, wR2 = 0.0484, 540 F² values, 21 variables). Contamination with air during the synthesis of Sc₂Au led to the discovery of a compound adopting the cubic W₄Co₂C-type structure (stuffed cubic Ti₂Ni type). Using Sc₂O₃ as a defined oxygen source led to samples with high amounts of Sc₄Au₂O_1-x. All intermetallic compounds exhibited Pauli paramagnetic behavior in the investigated temperature range of 2.1 to 300 K, and no superconductivity was observed at low temperatures and low fields. Sc₂Au and Sc₂Al were investigated by ²⁷Al and ⁴⁵Sc solid-state NMR investigations. For Sc₂Al, one signal was found in the ²⁷Al NMR spectra in line with the crystal structure; however, an extremely negative resonance shift of δ = -673 ppm was observed. In both compounds, two Sc resonances were observed, in line with the proposed crystal structure. Finally, it was observed that the stability of Sc₂Au in air is limited. This was investigated via thermal analysis and (temperature-dependent) powder X-ray diffraction. DFT calculations helped in assessing charge analysis, electronic properties, and chemical bonding.

Raman and NMR spectroscopic and theoretical investigations of the cubic laves-phases REAl2 (RE = Sc, Y, La, Yb, Lu)

The cubic Laves-phase aluminides REAl₂ with RE = Sc, Y, La, Yb and Lu were prepared from the elements by arc-melting or using refractory metal ampoules and induction heating. They all crystallize in the cubic crystal system with space group Fd3̄m and adopt the MgCu₂ type structure. The title compounds were characterized by powder X-ray diffraction and spectroscopically investigated using Raman and ²⁷Al and in the case of ScAl₂ by ⁴⁵Sc solid-state MAS NMR. In both, the Raman and NMR spectra, the aluminides exhibit only one signal due to the crystal structure. DFT calculations were used to calculate Bader charges illustrating the charge transfer in these compounds along with NMR parameters and densities of states. Finally, the bonding situation was assessed by means of ELF calculations rendering these compounds aluminides with positively charged RE^δ+ cations embedded in an [Al₂]^δ- polyanion.

Quantifying the quadrupolar interaction by 45Sc-NMR spectroscopy of single crystals

Single crystals of the compound [ [Formula: see text] ]Cl₄ ⋅ 2H₂O were studied by ⁴⁵Sc-NMR, with the effect of the quadrupolar coupling interaction on the spectra of the spin-7/2 nucleus analysed in the hierarchical framework of perturbation theory. Orientation-dependent spectra acquired at B₀ = 17.6 T showed strong second-order effects due to the comparatively large coupling constant of χ = |14.613 ± 0.006| MHz, with an associated asymmetry parameter of η_Q = 0.540 9 ± 0.000 4. By analysing the splittings of the ±3/2 satellites, which in good approximation are subjected to first-order effects only, the full quadrupolar coupling tensor could be determined. The second-order effects caused by this tensor were calculated according to theoretical predictions for all orientations, and subtracted from both the centres of gravity of the satellites, and the central transitions. This allowed extraction of the full chemical shift tensor, with the eigenvalues being δ₁₁ = (5.6 ± 0.9) ppm, δ₂₂ = (12.4 ± 0.9) ppm, and δ₃₃ = (38.5 ± 0.9) ppm. In spectra acquired at a lower magnetic field of B₀ = 9.4 T, third-order effects could be detected, and similarly quantified using analytical expressions.

Intermetallic phases in the Sc–Ir–In system – synthesis and structure of Sc1.024Ir2In0.976 and Sc3Ir1.467In4

The intermetallic scandium compounds Sc1.024Ir2In0.976 and Sc3Ir1.467In4 were synthesized by reactions of the elements in sealed tantalum ampoules at high temperature followed by annealing for crystal growth. Both structures were refined from single-crystal X-ray diffractometer data: MnCu2Al type, F m 3 ‾ m $Fm‾{3}m$ , a = 639.97(19) pm, wR2 = 0.0376, 41 F 2 values, seven variables for Sc1.024Ir2In0.976 and P 6 ‾ $P‾{6}$ , a = 769.99(5), c = 684.71(4) pm, wR2 = 0.0371, 967 F 2 values, 33 variables for Sc3Ir1.467In4. Sc1.024Ir2In0.976 is a new Heusler phase with a small homogeneity range due to Sc/In and In/Sc mixing. The structure of Sc3Ir1.467In4 is closely related to that of Sc3Rh1.594In4 and belongs to the large family of ZrNiAl superstructures. The striking structural motif is the ordered stacking of empty In6 and filled Ir@In6 prisms with Ir–In distances of 269 pm.

Synthesis of the scandium chloride hydrates ScCl3·3H2O and Sc2Cl4(OH)2·12H2O and their characterisation by X-ray diffraction, 45Sc NMR spectroscopy and DFT calculations

The compounds ScCl3·3H2O (SCTH) and [{Sc(H2O)5(μ-OH)}2]Cl4·2H2O (SCOH), have been synthesised and characterised by single-crystal XRD, 45Sc NMR spectroscopy and DFT calculations, with the crystal structure of SCTH reported here for the first time. From 45Sc NMR measurements under static and MAS conditions, both chemical shift and quadrupolar coupling parameters have been determined. The quadrupolar coupling constants χ for the octahedrally coordinated scandium sites in SCTH are 2.0 ± 0.1 MHz for Sc(1) and 3.81 ± 0.05 MHz for Sc(2). For SCOH, where the hepta-coordination of the single scandium site constitutes a less symmetric electronic environment, 14.68 ± 0.05 MHz was found. DFT calculations for the static SCTH structure consistently overestimate the quadrupolar coupling constants, indicating the possible presence of crystal water dynamics on the NMR time scale.

Rare-earth solid-state NMR spectroscopy of intermetallic compounds: The case of the 175Lu isotope

The feasibility of high-resolution ¹⁷⁵Lu solid-state NMR spectroscopy in intermetallic compounds crystallizing with cubic crystal structures is explored by magic-angle spinning NMR at different magnetic flux densities. The large quadrupole moment of this isotope (3.49 × 10^-28 m²) restricts observation of the NMR signal to nearly perfectly ordered crystalline samples. Signals are successfully detected and analyzed in the binary pnictides LuPn (NaCl-type structure; Pn = P, As, Sb) and the intermetallic compounds LuPtSb and LuAuSn, both crystallizing with the MgAgAs-type structure. Sources of line broadening are discussed based on field-dependent static and MAS-NMR spectra, providing guidance with respect to measurement conditions resulting in reliable results. The results highlight the importance of ionic/covalent bonding effects for the detectability of the signal, which reduce the probability of real structure effects commonly observed in intermetallic compounds. No ¹⁷⁵Lu NMR signals can be observed in various cubic Heusler compounds. This is attributed to mixed site occupancies and other structural defects producing electric field gradients whose interaction with the ¹⁷⁵Lu quadrupole moments broadens the signal beyond detection.

Sc5Pd4Si6- crystal structure and 29Si/45Sc solid state MAS NMR spectroscopic investigations

The silicide Sc₅Pd₄Si₆ was synthesized from the elements by arc-melting. Its structure was refined from single crystal X-ray diffractometer data: Immm, Li₅Cu_3.75P₆ type, a = 397.12(6), b = 945.4(1), c = 1314.4(2) pm, wR₂ = 0.0245, 578 F² values and 29 parameters. The palladium atoms have slightly distorted tetrahedral silicon coordination and a condensation of these PdSi₄ tetrahedra leads to a two-dimensional substructure which is condensed via Si₂ pairs (234 pm Si-Si), forming the [Pd₄Si₆]^δ- polyanionic network. The Sc₅Pd₄Si₆ structure contains three crystallographically independent scandium sites with coordinations Sc1@Pd₄Si₈Sc₆, Sc2@Pd₆Si₆Sc₃ and Sc3@Pd₄Si₆Sc₄. Sc₅Pd₄Si₆ is a Pauli paramagnet with a low susceptibility of 2.9(5) × 10^-5 emu mol^-1 at room temperature. The ²⁹Si MAS-NMR spectrum confirms the presence of two crystallographically distinct sites in a 2 : 1 ratio. Likewise, the three crystallographic scandium sites are well-differentiated by three ⁴⁵Sc MAS NMR signals in the expected 2 : 2 : 1 ratio. Unambiguous assignments could be made based on the comparison of the nuclear electric quadrupolar coupling parameters with predicted values from WIEN2k calculations.

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density Functional Theory Calculations

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K₅Sc₃F₁₄ was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF₄, Li₃ScF₆, KSc₂F₇, and Na₃ScF₆ compounds were studied in detail from solid-state ¹⁹F and ⁴⁵Sc NMR experiments. The ⁴⁵Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The ¹⁹F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the ¹⁹F and ⁴⁵Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state ⁴⁵Sc NMR spectroscopy.

The platinum-rich scandium silicide Sc2Pt9Si3

Single crystals of Sc2Pt9Si3 have been obtained from an arc-melted and inductively annealed sample of the starting composition Sc:4Pt:2Si. The Sc2Pt9Si3 structure (Tb2Pt9Ge3 type, space group C2/c) was refined from single crystal X-ray diffractometer data: a=1303.4(1), b=749.9(1), c=973.5(1), β=116.44(1)°, wR2=0.0731, 1643 F 2 values and 67 variables. The structure contains three basic coordination polyhedra Sc@Pt11, Si1@Pt8 and Si2@Pt8 which show a simple condensation pattern avoiding direct Sc–Si and Si–Si bonding.

Structural Characterization of Intermetallic Compounds by 27Al Solid State NMR Spectroscopy

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(T_0.5T'_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).

29Si,47Ti,49Ti and195Pt solid state MAS NMR spectroscopic investigations of ternary silicides TPtSi, germanides TPtGe (T = Ti, Zr, Hf) and stannide TiPtSn

Silicides TPtSi, germanides TPtGe (T = Ti, Zr, Hf) and stannide TiPtSn show well resolved²⁹Si,⁴⁷Ti,⁴⁹Ti and¹⁹⁵Pt solid state MAS NMR spectra.

Equiatomic intermetallic compounds YTX (T = Ni, Ir; X = Si, Ge, Sn, Pb): a systematic study by 89Y solid state NMR and 119Sn Mössbauer spectroscopy

The equiatomic TiNiSi type tetrelides YTX (space group Pnma) with T = Ni, Ir and X = Si, Ge, Sn, Pb were synthesized from the elements by arc-melting or via high-frequency-melting of the elements in sealed niobium ampoules.

Lightweight hydrogen-storage material Mg(0.65)Sc(0.35)D2 studied with 2H and 2H-{45Sc} MAS NMR exchange spectroscopy

Using double-quantum (2)H MAS NMR with (45)Sc recoupling and Bloch-Siegert compensated (2)H-{(45)Sc} TRAPDOR we have identified the overlapping NMR signals of deuterium with and without scandium neighbors in Mg(0.65)Sc(0.35)D(2), a candidate lightweight material for hydrogen storage. At room temperature we also observe a third type of mobile deuterium. Deuterium mobility among the three NMR-distinct sites has been investigated by means of one-and two-dimensional exchange spectroscopy (Exsy). Complete deuterium exchange within 0.1s is observed, which indicates that the three NMR-distinct sites are close together in the crystal lattice. The weak temperature- and MAS-rate dependences observed in Exsy are indicative for a combination of chemical exchange and spin diffusion.