Metallic alloys at the edge of complexity: structural aspects, chemical bonding and physical properties

Extended X-ray Absorption Fine Structure (EXAFS) Measurements on Alkali Metal Superatoms of Ta-Atom-Encapsulated Si16 Cage

The silicon cage nanoclusters encapsulating a tantalum atom, termed Ta@Si16, exhibit characteristics of alkali metal "superatoms (SAs)". Despite this conceptual framework, the precise structures of Ta@Si16 and Ta@Si16+ remain unclear in quantum calculations due to three energetically close structural isomers: C3v, Td, and D4d structures. To identify the geometrical structure of Ta@Si16 SAs, structural analysis was conducted using extended X-ray absorption fine structure (EXAFS) with a high-intensity monochromatic X-ray source, keeping anaerobic conditions. Focusing on "superordered" films, which constitute amorphous thin films composed solely of Ta@Si16 SAs, this analysis preserved locally ordered structures. Spectral comparisons between experimental and simulated Ta L3-edge EXAFS unveil that Ta@Si16 SAs on a substrate adopt a C3v-derived structure, while Si K-edge EXAFS introduces spectral ambiguity in structural identifications, attributed to both intracluster and intercluster scatterings. These findings underscore the significance of locally ordered structure analyses in understanding and characterizing novel nanoscale materials.

Colloidal Synthesis of P-Type Zn3As2 Nanocrystals

Zinc pnictides, particularly Zn3As2, hold significant promise for optoelectronic applications owing to their intrinsic p-type behavior and appropriate bandgaps. However, despite the outstanding properties of colloidal Zn3As2 nanocrystals, research in this area is lacking because of the absence of suitable precursors, occurrence of surface oxidation, and intricacy of the crystal structures. In this study, a novel and facile solution-based synthetic approach is presented for obtaining highly crystalline p-type Zn3As2 nanocrystals with accurate stoichiometry. By carefully controlling the feed ratio and reaction temperature, colloidal Zn3As2 nanocrystals are successfully obtained. Moreover, the mechanism underlying the conversion of As precursors in the initial phases of Zn3As2 synthesis is elucidated. Furthermore, these nanocrystals are employed as active layers in field-effect transistors that exhibit inherent p-type characteristics with native surface ligands. To enhance the charge transport properties, a dual passivation strategy is introduced via phase-transfer ligand exchange, leading to enhanced hole mobilities as high as 0.089 cm2 V-1 s-1. This study not only contributes to the advancement of nanocrystal synthesis, but also opens up new possibilities for previously underexplored p-type nanocrystal research.

From Laves Phases to Quasicrystal Approximants in the Na-Au-Cd System

The exploratory synthesis of gold-based polar intermetallic phases has revealed many new compounds with unprecedented crystal structures, unique bonding arrangements, and interesting electronic features. Here, we further understand the complexity of gold's crystal chemistry by studying the Na-Au-Cd ternary composition space. A nearly continuous structure transformation is observed between the seemingly simple binary NaAu₂-NaCd₂ phases, yielding three new intermetallic compounds with the compositions Na(Au_0.89(5)Cd_0.11(5))₂, Na(Au_0.51(4)Cd_0.49(4))₂, and Na₈Au_3.53(1)Cd_13.47(1). Two compounds adopt different Laves phases, while the third crystallizes in a complex decagonal quasicrystal approximant. All three compounds are related through Friauf-Laves polyhedral building units with the gold/cadmium ratio found to control the transition among the unique crystal structures. Electronic structure calculations subsequently revealed the metallic nature of all three compounds with a combination of polar covalent Na-(Au/Cd) interactions and covalent (Au/Cd)-(Au/Cd) bonding interactions stabilizing each structure. These results highlight the crystal and electronic structure relationship among Laves phases and quasicrystal approximants enabled by the unique chemistry of gold.

Phonon behavior in a random solid solution: a lattice dynamics study on the high-entropy alloy FeCoCrMnNi

High-Entropy Alloys (HEAs) are a new family of crystalline random alloys with four or more elements in a simple unit cell, at the forefront of materials research for their exceptional mechanical properties. Their strong chemical disorder leads to mass and force-constant fluctuations which are expected to strongly reduce phonon lifetime, responsible for thermal transport, similarly to glasses. Still, the long range order would associate HEAs to crystals with a complex disordered unit cell. These two families of materials, however, exhibit very different phonon dynamics, still leading to similar thermal properties. The question arises on the positioning of HEAs in this context. Here we present an exhaustive experimental investigation of the lattice dynamics in a HEA, Fe₂₀Co₂₀Cr₂₀Mn₂₀Ni₂₀, using inelastic neutron and X-ray scattering. We demonstrate that HEAs present unique phonon dynamics at the frontier between fully disordered and ordered materials, characterized by long-propagating acoustic phonons in the whole Brillouin zone.

Structure and selected properties of Al–Cr–Fe alloys with the presence of structurally complex alloy phases

The aim of the study was to supplement the data on the Al₆₅Cr₂₀Fe₁₅ alloy with binary phase structure and the Al₇₁Cr₂₄Fe₅ alloy with multiphase structure prepared with two different cooling rates from the liquid state. The presence of the structurally complex Al₆₅Cr₂₇Fe₈ phase was confirmed by neutron diffraction, scanning electron microscopy with the analysis of chemical composition and transmission electron microscopy. Additionally, the Al₈Cr₅ phase with γ-brass structure was identified for Al₇₁Cr₂₄Fe₅ alloy in both cooling rates from the liquid state. Due to the interesting features of structurally complex alloys, the wear resistance, magnetic properties, and corrosion products after performing electrochemical tests were examined. Based on pin-on-disc measurements, a lower friction coefficient was observed for the Al₆₅Cr₂₀Fe₁₅ alloy (µ ≈ 0.55) compared to the Al₇₁Cr₂₄Fe₅ multiphase alloy (µ ≈ 0.6). The average hardness of the binary phase Al₆₅Cr₂₀Fe₅ alloy (HV_0.1 = 917 ± 30) was higher compared to the multiphase Al₇₁Cr₂₄Fe₅ alloy (HV_0.1 = 728 ± 34) and the single phase Al–Cr–Fe alloys described in the literature. Moreover, the beneficial effect of rapid solidification on hardness was demonstrated. The alloys Al₆₅Cr₂₀Fe₁₅ and Al₇₁Cr₂₄Fe₅ showed paramagnetic behavior, however rapidly solidified Al₇₁Cr₂₄Fe₅ alloy indicated an increase of magnetic properties. The studied alloys were characterized by the presence of passive layers after electrochemical tests. A higher amount of oxides on the surface of the Al₇₁Cr₂₄Fe₅ alloy was recorded due to the positive effect of chromium on the stabilization of the passive layer.

Flux Growth, Crystal Structures, and Electronic Properties of the Ternary Intermetallic Compounds Ca3Pd4Bi8 and Ca3Pt4Bi8

Reaction of the elements yielded Ca₃Pt₄Bi₈ and CaPtBi, which are, to the best of our knowledge, the first reported ternary Ca–Pt–Bi compounds. The compounds crystallize isostructural to the Pd analogs Ca₃Pd₄Bi₈ (own structure type) and CaPdBi (TiNiSi structure type), respectively. Employing a multistep temperature treatment allows for the growth of mm-sized single crystals of Ca₃Pd₄Bi₈ and Ca₃Pt₄Bi₈ from a Bi self-flux. Their crystal structures can be visualized as consisting of a three-dimensional extended polyanion [M₄Bi₈]^6– (M = Pd, Pt), composed of interlinked M–Bi chains propagating along the c direction, and Ca²⁺ cations residing in one-dimensional channels between the chains. First-principles calculations reveal quasi-one-dimensional electronic behavior with reduced effective electron masses along [001]. Bader analysis points to a strong anionic character of the M species (M = Pd, Pt) in Ca₃M₄Bi₈. Thus, it is more appropriate to address the compounds Ca₃Pd₄Bi₈ and Ca₃Pt₄Bi₈ as a palladide and platinide, respectively. Magnetization measurements indicate diamagnetic behavior with no indications for superconductivity down to 2 K. Electrical resistivity data are consistent with metallic behavior and suggest predominant electron–phonon scattering.

Reaction of the elements yielded Ca₃Pt₄Bi₈ and CaPtBi, which are, to the best of our knowledge, the first reported ternary Ca−Pt−Bi compounds.

Four ternary silicides in the La–Ni–Si system: from polyanionic layers to frameworks

Four ternary La–Ni–Si compounds have been synthesized and characterized. They present ordered structures with extended 2D or 3D Ni/Si motifs.

Variants of the X-phase in the Mn-Co-Ge system

We report two new variants of the X-phase (orthorhombic, space group Pnnm) derived from the Mn-Co-Ge system. Two compositionally related crystals were investigated by means of single-crystal X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The Mn14.9Co15.5Ge6.6 and Mn14Co16.2Ge6.8 intermetallic compounds are part of the homogeneity region of the X-phase and adopt the Mn14(Mn0.11Co0.64Si0.25)23 structure type. The composition obtained from refinement of the XRD data is in agreement with the EDS results. In the present study, chemical disorder was only detected on the 8h positions. The ordering is compared with other members of the X-phase family and shows that the degree of disordering depends on the chemical composition. No completely ordered variants of the X-phase have yet been reported.

Binary Intermetallics in the 70 atom % R Region of Two R-Pd Systems (R = Tb and Er): Hidden, Obscured, or Nonexistent?

Although rare-earth-metal–transition-metal (R/T) phase diagrams have been explored extensively, our recent studies have uncovered new previously nonexistent binary intermetallics. These compounds belong to a narrow region between 70 and 71.4 atom % of the rare-earth metal but represent four different structure types. The binaries Tb₇Pd₃ and Er₁₇Pd₇ are compositionally approaching (less than 1 atom % difference) the previously reported R_2.16Pd_0.89 (R = Tb and Er), and apparently form by peritectoid transformation, thus, being hard to detect by fast cooling. Tb₇Pd₃ (1) crystallizes in the Th₇Fe₃ structure type (hP20, P6₃mc, a = 9.8846(4) Å, c = 6.2316(3) Å, Z = 2) while Er₁₇Pd₇ (2) belongs to the Pr₁₇Co₇ type being its second reported representative (cP96, P2₁3, a = 13.365(2) Å, Z = 4). Er₁₇Pd₇ (2) is overlapping with the cubic F-centered Er_2.11Pd_0.89 (3b, Fd3̅m, a = 13.361(1) Å, Z = 32) with practically identical unit cell parameters but a significantly different structure. Electronic structure calculations confirm that heteroatomic R–T bonding strongly dominates in all structures; T–T bonding interactions are individually strong but do not play a significant role in the total bonding.

Polyicosahedral {Pd₄Er₂₂} and {Pd₃Er₂₀} anti-Mackay type clusters with prismatic bridges in the crystal structures of Er₁₇Pd₇ and Er_2.11Pd_0.89.