Turning Gold into “Diamond”: A Family of Hexagonal Diamond-Type Au-Frameworks Interconnected by Triangular Clusters in the Sr–Al–Au System

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4

The new phase Mg₇Pt₄Ge₄ (≡Mg₈□₁Pt₄Ge₄; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg₂PtSi (≡Mg₈Pt₄Si₄), reported in the Li₂CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg₇Pt₄Ge₄. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg₂PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg₂PtGe" reveal potential electronic instabilities at E_F in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg₃Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

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.

Bonding diversity in rock salt-type tellurides: examining the interdependence between chemical bonding and materials properties

Future technologies are in need of solid-state materials showing the desired chemical and physical properties, and designing such materials requires a proper understanding of their electronic structures. In this context, recent research on chalcogenides, which were classified as ‘incipient metals’ and included phase-change data storage materials as well as thermoelectrics, revealed a remarkable electronic behavior and possible state (dubbed ‘metavalency’) proposed for the frontier between entire electron localization and delocalization. Because the members of the family of the polar intermetallics vary widely in their properties as well as electronic structures, one may wonder if the aforementioned electronic characteristics are also achieved for certain polar intermetallics. To answer this question, we have employed quantum-chemical tools to examine the electronic structures of the rock salt-type YTe and SnTe belonging to the families of the polar intermetallics and incipient metals, respectively. To justify these classifications and argue as to why an application of the Zintl–Klemm concept (frequently employed to relate the structural features of tellurides to their electronic structures) could be misleading for YTe and SnTe, the electronic structures of YTe and SnTe were first compared to that of the rock salt-type SrTe. In addition, we carried out a Gedankenexperiment by subsequently modifying the chemical composition from YTe to SnTe, and, by doing so, we shed new light on the interdependence between chemical bonding and materials properties. Gradual changes in the former do not necessarily translate into the latter which may undergo discontinuous modifications.

Future technologies are in need of solid-state materials showing the desired chemical and physical properties, and designing such materials requires a proper understanding of their electronic structures.

Relativistic Effects Stabilize the Planar Wheel-like Structure of Actinide-Doped Gold Clusters: An@Au7 (An = Th to Cm)

Despite the chemistry of actinide-ligand bonding is continuing and of burgeoning interest, investigations of the chemical bonding of bimetallic complexes involving transuranics remain relatively less, and there are rarely studies on the bonding features between actinide and coinage metals (CM). We present a systematic research on the series of An@Au₇ (An = Th to Cm), UCM₇ (CM = Cu, Ag, Au), and WAu₇ clusters to investigate the unique geometries, electronic structures, and chemical bonding between An 5f6d orbitals and CM ns orbitals, and to find their periodicity across the actinides and within the group of transition metals. A unique planar wheel-like structure for An@Au₇ clusters with the help of actinide metals encapsulation via spin-orbit coupling, resulting in An(III). Instead, the transition-metal (TM) element W retains its usual six-gold-coordination structure in WAu₇, thus forcing the seventh Au out of plane. The An-CM interactions, depending on the ion radii, become stronger with the increase of the atomic number of the actinide metals, as well as the CM. These results show that the presence of actinides in clusters can lead to unique electronic and geometrical structures.

The untypical high-pressure Zintl phase SrGe6

The binary strontium germanide SrGe6 was synthesized at high-pressure high-temperature conditions of approximately 10 GPa and typically 1400 K before quenching to ambient conditions. At ambient pressure, SrGe6 decomposes in a monotropic fashion at T = 680(10) K into SrGe2 and Ge, indicating its metastable character. Single-crystal X-ray diffraction data indicate that the compound SrGe6 adopts a new monoclinic structure type comprising a unique three-dimensional framework of germanium atoms with unusual cages hosting the strontium cations. Quantum chemical analysis of the chemical bonding shows that the framework consists of three- and four- bonded germanium atoms yielding the precise electron count Sr[(4bGe0]4[(3b)Ge−]2 in accordance with the 8 − N rule and the Zintl concept. Conflicting with that, a pseudo-gap in the electronic density of states appears clearly below the Fermi level, and elaborate bonding analysis reveals additional Sr–Ge interactions in the concave coordination polyhedron of the strontium atoms.

In-Cage Interactions in the Clathrate Superconductor Sr8 Si46

The clathrate I superconductor Sr₈Si₄₆ is obtained under high‐pressure high‐temperature conditions, at 5 GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heating at T=796(5) K into Si and SrSi₂. The crystal structure of the clathrate is isotypic to that of Na₈Si₄₆. Chemical bonding analysis reveals conventional covalent bonding within the silicon network as well as additional multi‐atomic interactions between Sr and Si within the framework cages. Physical measurements indicate a bulk BCS type II superconducting state below T_c=3.8(3) K.

Modulated vacancy ordering in SrGe6− x (x≈0.45)

The structural properties of modulated SrGe6− x (x≈0.45) were investigated by means of single-crystal and powder X-ray diffraction combined with quantum chemical calculations. The framework compound SrGe6− x adopts a defect variant of the EuGa2Ge4-type crystal structure. Samples of the binary compound with nominal compositions 0≤x≤0.5 were synthesized at pressures from 5 to 6 GPa and a temperature of typically 1400 K. The product reveals diffraction peaks of the EuGa2Ge4-type subcell plus additional reflections indicating an ordered superstructure. Detailed crystal structure analysis evidences the incommensurate nature of the superstructure and a modulation of the vacancy ordering in the germanium network. The computations have shown that the non-stoichiometric composition of the framework with its local defect organization affects the calculated charge of the strontium anions. Although the chemical composition is close to a charge-balanced situation, temperature-dependent resistivity measurements showed metal-type conductivity. At ambient pressure SrGe6− x decomposes exothermally and irreversibly at T=680(10) K into SrGe2 and germanium, indicating its metastable nature at ambient pressure.

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the Crystal Orbital Hamilton Population (COHP) method has been established as an efficient and reliable tool to extract the chemical-bonding information based on electronic-structure calculations of various quantum-chemical types. In this review, we present a brief introduction into the theoretical background of the COHP method and illustrate the latter by diverse applications, in particular by looking at representatives of the class of (polar) intermetallic compounds, usually considered as “black sheep” in the light of valence-electron counting schemes.

AAuAl (A = Ca, Sc, and Ti): Peierls Distortion, Atomic Coloring, and Structural Competition

Using density functional theory, the crystal structure variation of AAuAl (A = Ca, Sc, and Ti) from orthorhombic Co₂Si-type to distorted hexagonal Fe₂P-type and then Ni₂In-type structures is shown to correlate with their electronic structures and valence electron counts, sizes of the active metals A, and site preferences for Au and Al atoms, which are arranged to maximize Au-Al nearest neighbor contacts. An evaluation of chemical pressure imposed by the varying A metals using total energy vs volume calculations indicates that larger unit cell volumes favor the orthorhombic structure, whereas smaller volumes favor the hexagonal structures. The electronic origin of the Mg₂Ga-type crystal structure of ScAuAl, refined as a distorted Fe₂P-type supercell doubled along the c-axis, indicates a Peierls-type distortion mechanism of the Au chains along the c-axis.

Bringing order to large-scale disordered complex metal alloys: Gd2Au15−xSbxand BaAuxGa12−x

New complex metallic alloys, BaAu_xGa_12−xand Gd₂Au_15−xSb_x, display entire planes of disordered atom sites, forming a set planar conformations.

An Icosahedral Quasicrystal and Its 1/0 Crystalline Approximant in the Ca-Au-Al System

A new icosahedral quasicrystalline phase, CaAu_4.5-xAl_1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu_4.4Al_1.6, a_QC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu_3+xAl_1-x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)-9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca-Au-Al system. In the crystalline approximant, eight [Au_3-xAl_1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca_4/4[Au_3-xAl_1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au-Al interactions over Au-Au and Al-Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu_4.5-xAl_1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu_3+xAl_1-x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au_3-xAl_1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.

Alkaline earth-gold-aluminides: synthesis and structure of SrAu3Al2, SrAu2.83Al2.17, BaAu2.89Al2.11 and BaAu7.09Al5.91

New alkaline earth-gold-aluminides were synthesized from the elements in sealed tantalum or quartz ampoules in muffle furnaces at maximum annealing temperatures of 1325 K. The structures were refined from single crystal X-ray diffractometer data. SrAu3Al2 crystallizes in an ordered version of the LT-SrZn5 structure: Pnma, a = 1315.9(3), b = 549.0(1), c = 684.5(3) pm, wR2 = 0.0232, 930 F 2 values, 35 variables. SrAu2.83Al2.17 (a = 1065.0(2), b = 845.0(2), c = 548.1(1) pm, wR2 = 0.0416, 452 F 2 values, 22 variables) and BaAu2.89Al2.11 (a = 1096.1(3), b = 835.7(3), c = 554.0(1) pm, wR2 = 0.0280, 501 F 2 values, 22 variables) both adopt the BaZn5 type, space group Cmcm with Au/Al mixing on the 4c site. The gold and aluminum atoms in both types form three-dimensional networks of condensed tetrahedra with the strontium and barium atoms in large cavities. BaAu7.09Al5.91 is a new member of the NaZn13 type: Fm3̅c, a = 1257.6(2) pm, wR2 = 0.0267, 168 F 2 values, 12 variables. Both the 96i and 8b sites show Au/Al mixing. The crystal chemical details are discussed.