New Stannide ScAgSn: Determination of the Superstructure via Two-Dimensional 45Sc Solid State NMR

Scandium–copper–indides deriving from the ZrNiAl and MnCu2Al type structures

Phase analytical studies in the Sc–Cu–In system led to samples of the solid solutions ScCu1–x–y In1+x and ScCu2–x In which were studied by X-ray powder diffraction. At room temperature the compounds ScCu1–x–y In1+x crystallize with the ZrNiAl type, space group P 6 ‾ $\overline{6}$ 2m. Exemplarily, the structure of ScCu0.76In1.17 was refined from single crystal X-ray diffractometer data, revealing strong anisotropic displacements for the scandium atoms and a mixed occupied Cu/In site. Superstructure formation is observed at low temperatures. The ScCu0.78In1.14 and ScCu0.76In1.16 structures were refined from diffraction data recorded at 90 K. Both compounds adopt the HfRhSn type, space group P 6 ‾ $\overline{6}$ 2c, a klassengleiche subgroup of index 2; doubling of the subcell c axis. The Cu/In filled trigonal Sc6 prisms are strongly distorted in the superstructure, resulting from pairwise dislocation of the Cu/In atoms from ideal positions within an equidistant chain to shorter (311.0 pm) and longer (392.8 pm) Cu/In–Cu/In distances. Single crystal data of the Heusler phases ScCu1.95In and ScCu1.94In show small degrees of copper vacancies.

Superstructure formation in Sc5Cu2In4

Polycrystalline Sc5Cu2In4 was synthesized by induction melting of the elements and small single crystals were obtained by a slow cooling sequence. Sc5Cu2In4 is the first coinage metal representative in the family of the so-called 5-2-4 intermetallics. The Zr5Ir2In4 type structure of Sc5Cu2In4 was refined from single crystal X-ray diffractometer data: Pnma, a = 1716.75(6), b = 677.94(12), c = 760.69(2) pm, wR2 = 0.0531, 1932 F2 values and 58 variables. Sc5Cu2In4 adopts a superstructure of the Lu5Ni2In4 type (doubling of the b axis and klassengleiche symmetry reduction from Pbam to Pnma), caused by dislocation of the copper atoms (puckering effect). Geometrically, Sc5Cu2In4 is a 4:1 intergrowth structure of distorted AlB2 and CsCl related slabs.

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.

Equiatomic iron-based tetrelides TFeSi and TFeGe (T = Zr, Nb, Hf, Ta) – A 57Fe Mössbauer-spectroscopic study

The equiatomic iron-silicides TFeSi as well as the corresponding germanides TFeGe with the electron-poor 4d and 5d transition metals (T=Zr, Nb, Hf, Ta) have been synthesized from the elements by arc-melting. All samples were characterized through their lattice parameters using powder X-ray diffraction (Guinier technique). Four structures were refined from single-crystal X-ray diffractometer data: a=640.16(3), b=393.45(5), c=718.42(6) pm, Pnma, 390 F 2 values, 20 parameters, wR2=0.0294 for ZrFeSi (TiNiSi type), a=719.63(11), b=1119.27(7), c=649.29(7) pm, Ima2, 1103 F 2 values, 54 parameters, wR2=0.0555 for NbFeGe (TiFeSi type), a=655.96(7), c=372.54(4) pm, P6̅2m, 251 F 2 values, 15 parameters, wR2=0.0260 for HfFeGe (ZrNiAl type) and a=624.10(3), b=378.10(6), c=725.25(7) pm, Pnma, 369 F 2 values, 20 parameters, wR2=0.0513 for TaFeGe (TiNiSi type). The common structural motif of the four different structures is the slightly distorted tetrahedral tetrel (tr) coordination of the iron atoms and a trigonal prismatic coordination of iron by T=Zr, Nb, Hf, Ta. Three compounds were characterized as Pauli-paramagnetic by measuring their susceptibility. The measurement of the electrical resistivity of NbFeSi characterises this compound as a good metal. Furthermore, 57Fe Mössbauer spectra of all compounds could be obtained at room temperature, revealing a clear correlation between the structural distortions and the quadrupole splitting parameters.

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.

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.

Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation

Synthesis of ordered compounds with nano size is of particular interest for tuning the surface properties with enhanced activity and selectivity toward various important industrial catalytic processes. In this work, we synthesized ordered PdCu nanoparticles as highly efficient catalyst for the solvent-free aerobic oxidation of benzylamine. The Pd_xCu_1-x catalysts with different chemical compositions (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) were prepared by polyol method using NaBH₄ as a reducing agent and were well-characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM) energy-dispersive analysis of X-rays, and X-ray absorption fine structure. The effect of different metal concentrations of Pd and Cu on the formation of Pd_xCu_1-x nanoparticles was investigated. The XRD and TEM confirmed the formation of ordered PdCu intermetallic phase with body-centered cubic (BCC) structure for the synthetic composition of Pd/Cu = 1:1. For compositions x = 0, 0.25, 0.75, and 1, Pd_xCu_1-x alloy with face-centered cubic (FCC) structure was observed, whereas mixed phase of BCC and FCC was observed for x = 0.4 and 0.6. The use of strong reducing agent (NaBH₄) was essential to synthesize PdCu ordered phase compared to weak reducing agents such as oleylamine and ascorbic acid. The PdCu nanocatalyst with ordered structure (BCC) showed excellent catalytic activity compared to Pd_xCu_1-x alloy nanoparticles with FCC structure. The atomic ordering in the PdCu intermetallic was the driving force for the enhancement in the catalytic activity with high benzylamine conversion of 94.0% and dibenzylimine selectivity of 92.2% compared to its monometallic and alloy counterparts. Moreover, ordered PdCu alloy showed good recyclability and activity toward the oxidation of different amines.

A ZrNiAl related high-pressure modification of CeRuSn

Monoclinic CeRuSn with its own structure type transforms to a high-pressure modification at 11.5 GPa and 1470 K (1000 t press, Walker type module). The structure of the high-pressure phase was refined from X-ray single crystal diffractometer data at room temperature. The HP-CeRuSn subcell structure adopts the ZrNiAl type: P6[combining macron]2m, a = 751.4(3) and c = 394.6(2) pm, wR2 = 0.0787, 310 F(2) values and 15 variables. The Ru2 atoms within the Sn6 trigonal prisms show a strongly enhanced U33 parameter. Weak satellite reflections indicate a commensurate modulation: (3 + 1)D superspace group P31m(1/3,1/3,γ)000, a = 751.4(3) and c = 394.6(2) pm, γ = -1/3, wR2 = 0.0786, 1584 F(2) values, 32 variables for the main reflections and wR2 = 0.3757 for the satellites of 1(st) order. A description of this new superstructure variant of the ZrNiAl type is possible in a transformed 3D supercell with the space group R3m and Z = 9. The driving force for formation of the modulation is strengthening of Ru-Sn bonding within the comparatively large Ru@Sn6 trigonal prisms. Electronic structure calculations point to an almost depleted Ce 4f shell. This is substantiated by temperature-dependent magnetic susceptibility data. Fitting of the data within the interconfiguration fluctuation model (ICF) resulted in cerium valences of 3.41 at 10 K and 3.31 at 350 K. Temperature dependent specific heat data underline the absence of magnetic ordering.

Syntheses and structures of Sc2Nb(4–x)Sn5, YNb6Sn6, and ErNb6Sn5: exploratory studies in ternary rare-earth niobium stannides

Three new rare-earth (RE) niobium stannides, namely, Sc(2)Nb(4-x)Sn(5) (x = 0.37, 0.52), YNb(6)Sn(6), and ErNb(6)Sn(5), have been obtained by reacting the mixture of corresponding pure elements at high temperature and structurally characterized by single-crystal X-ray diffraction studies. Sc(2)Nb(4-x)Sn(5) crystallizes in the orthorhombic space group Ibam (No. 72) and belongs to the V(6)Si(5) type. Its structure features a three-dimensional (3D) network composed of two-dimensionally (2D) corrugated [Nb(2)Sn(2)] and [Nb(2)Sn(3)] layers interconnected via Nb-Sn bonds, forming one type of one-dimensional (1D) narrow tunnels along the c axis occupied by Sc atoms. YNb(6)Sn(6) crystallizes in the hexagonal space group P6/mmm (No. 191) and adopts the HfFe(6)Ge(6) type, and ErNb(6)Sn(5) crystallizes in the trigonal space group R3m (No. 166) and belongs to the LiFe(6)Ge(5) type. Their structures both feature 3D networks based on 2D [Nb(3)Sn], [Sn(2)], and [RESn(2)] layers (RE = Y, Er). In YNb(6)Sn(6), one type of [Nb(3)Sn] layer is interconnected by [Sn(2)] and [YSn(2)] layers via Nb-Sn bonds to form a 3D network. However, in ErNb(6)Sn(5), two types of [Nb(3)Sn] layers are interlinked by [Sn(2)] and [ErSn(2)] layers via Nb-Sn bonds into a 3D framework. Electronic structure calculations and magnetic property measurements for "Sc(2)Nb(4)Sn(5)" and YNb(6)Sn(6) indicate that both compounds show semimetallic and temperature-independent diamagnetic behavior.

Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in solid-state NMR of spin-7/2 nuclei

We here investigate the sensitivity enhancement of central-transition NMR spectra of quadrupolar nuclei with spin-7/2 in the solid state, generated by fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM). Considerable intensity is gained for the central-transition resonance of single-quantum spectra by means of spin population transfer from the satellite transitions, both under static and magic-angle-spinning (MAS) conditions. It is also shown that incorporation of a SW-FAM train into the excitation part of a 7QMAS sequence improves the efficiency of 7Q coherence generation, resulting in improved signal-to-noise ratio. The application of FAM-type pulse trains may thus facilitate faster spectra acquisition of spin-7/2 systems.