Na2Ga7: A Zintl-Wade Phase Related to "α-Tetragonal Boron".

Na2Ga7 crystallizes with the orthorhombic space group Pnma (no. 62; a = 14.8580(6) Å, b = 8.6766(6) Å, and c = 11.6105(5) Å; Z = 8) and constitutes a filled variant of the Li2B12Si2 structure type. The crystal structure consists of a network of icosahedral Ga12 units with 12 exohedral bonds and four-bonded Ga atoms in which the Na atoms occupy the channels and cavities. The atomic arrangement is consistent with the Zintl [(4b)Ga]- and Wade [(12b)Ga12]2- electron counting approach. The compound forms peritectically from Na7Ga13 and the melt at 501 °C and does not show a homogeneity range. The band structure calculations predict semiconducting behavior consistent with the electron balance [Na+]4[(Ga12)2-][Ga-]2. Magnetic susceptibility measurements show that Na2Ga7 is diamagnetic.

Structural Complexity in the Apparently Simple Crystal Structure of Be2 Ru.

The structural features of the hexagonal layered crystal structure of Be2 Ru (a=5.7508(3) Å, c=3.0044(2) Å, space group P 6 ‾ ${\bar{6}}$ 2m) were investigated by single crystal X-ray diffraction and transmission electron microscopy (TEM). The residual electron density and high-resolution TEM images show that the real structure can be described as an intergrowth of the main hexagonal matrix of the Fe2 P type with minor orthorhombic inclusions of its stacking variants. Such atomic arrangement is stabilized by the charge transfer from Be to Ru and by a system of polar three- and four-atomic bonds involving both components. The calculated electronic density of states (DOS) of Be2 Ru revealed, contrarily to typical intermetallic compounds, a pseudo gap (dip) in the vicinity of the Fermi level. The temperature dependence of the electrical resistivity of Be2 Ru shows metal behaviour in agreement with the non-zero DOS at the Fermi level.

The crystal structure of quaternary (Sn,Pb,Bi)Pt.

Quaternary (Sn,Pb,Bi)Pt was synthesized by melting of the elements in an evacuated silica glass ampoule. The crystal structure was established by single-crystal X-ray diffraction and adopts an atomic arrangement of the NiAs type with additional occupation of the voids. Decisive for the refinement was the composition of the crystals as determined by energy dispersive X-ray spectroscopy (EDXS), resulting in a formula of (Sn0.15Pb0.54Bi0.31)Pt.

Ge32 Co9-x : Creating "Empty" Space by High Pressure.

The compound Ge32 Co9-x (x=0.54(6), a=10.9861(3) Å, space group Im 3 ‾ $\bar 3$ m) prepared under high pressure and at high temperature is metastable under ambient conditions. It crystallizes in a new structure type, Pearson symbol cI82-1.08. The crystal structure represents a slightly distorted cubic primitive arrangement of germanium atoms with part of the Ge cubes filled by cobalt. Analysis of the chemical bonding by real-space methods revealed three-core cluster units Ge16 Co3 and seemingly empty regions comprising either covalent inter-polyhedral Ge-Ge bonds or lone-pairs located at the germanium atoms. The electrical conductivity is metal-like.

The First Alkaline-Earth Azidoaurate(III), Ba[Au(N3 )4 ]2 ⋠4 H2 O.

Transparent, dark orange Ba[Au(N3 )4 ]2 ⋅ 4 H2 O was synthesized by reaction of Ba(N3 )2 and AuCl3 or HAuCl4 in aqueous solution. The novel barium tetraazidoaurate(III) tetrahydrate crystallizes in the monoclinic space group Cc (no. 9) with a=1813.68(17) pm, b=1737.95(11) pm, c=682.04(8) pm and ß=108.849(4)°. The predominant structural features of Ba[Au(N3 )4 ]2 ⋅ 4 H2 O are two crystallographically independent discrete anions [Au(N3 )4 ]- with gold in a tetragonal planar coordination by nitrogen. Vibrational spectra show good agreement with those of other azidoaurates(III). Upon drying, this salt was shown to be a highly explosive material.

Crystal Chemistry and Physics of UCd11.

In the phase diagram U-Cd, only one compound has been identified so far─UCd11 (space group Pm3̅m). Since the discovery of this material, the physical properties of UCd11 have attracted a considerable amount of attention. In particular, its complex magnetic phase diagram─as a result of tuning with magnetic field or pressure─is not well-understood. From a chemical perspective, a range of lattice parameter values have been reported, suggesting a possibility of a considerable homogeneity range, i.e., UCd11-x. In this work, we perform a simultaneous study of crystallographic features coupled with measurements of physical properties. This work sheds light on the delicate relationship between the intrinsic crystal chemistry and magnetic properties of UCd11.

Superconductivity in Crystallographically Disordered LaHg6.4.

The influence of structural disorder on superconductivity is not yet fully understood. A concurrent examination of crystallographic and physical properties of LaHg6.4 reveals that this material enters a superconducting state below Tc = 2.4 K while showing crystallographic disorder in one dimension. Lanthanum mercuride, which crystallizes in a new structure type (space group Cmcm, a = 9.779(2) Å, b = 28.891(4) Å, c = 5.0012(8) Å, Z = 8), has remained out of reach for nearly 50 years. In this crystal structure, strong disorder is present in the channels that propagate along the [001] direction. By implementing a combination of cutting-edge synthesis and characterization techniques, we were able to circumvent the complexity associated with the low formation temperature and chemical reactivity of this substance and study the superconductivity of LaHg6.4 in detail.

Mg29-xPt4+y: Chemical Bonding Inhomogeneity and Structural Complexity.

Mg29-xPt4+y represents the family of complex intermetallic compounds (complex metallic alloys, CMAs). It crystallizes in the cubic non-centrosymmetric space group F4̅3m with a = 20.1068(2) Šand around 400 atoms in a predominantly ordered arrangement. The local disorder around the unit cell origin is experimentally resolved by single-crystal X-ray diffraction in combination with atomic-resolution transmission electron microscopy (TEM, high-angle dark-field scanning TEM) studies. The quantum theory of atoms in molecules-based analysis of atomic charges shows that the unusual mixed Mg/Pt site occupation around the origin results from local charge equilibration in this region of the crystal structure. Chemical bonding analysis reveals for Mg29-xPt4+y─rather unexpected for a crystal structure of this size─space-separated regions of hetero- and homoatomic bonds involving three to six partners (bonding inhomogeneity). Pt-containing 11- and 13-atomic units formed by heteroatomic 3a-, 4a-, and 5a-bonds are condensed via edges and faces to large super-tetrahedrons, which are interlinked by Mg-only 6a-bonds. Spatial separation of the regions with different bonding features is the key difference between the title compound and other CMAs, which are characterized by a predominantly homogeneous distribution of heteroatomic bonds.

A Borosilicide with Clathrate VIII Structure.

The high-pressure phase Na8BxSi46-x (3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na8B4Si42 (space group I43̅m; a = 9.7187(2) Å; Pearson symbol cI54) were obtained at 5-8 GPa and 1200 K. The clathrate I modification exists for the same composition at lower pressure with a larger cell volume (Pm3̅n; a = 9. 977(2) Å; cP54). Profound structural adaptions allow for a higher density of the clathrate VIII type than clathrate I, opening up the perspective of obtaining clathrate VIII type compounds as high-pressure forms of clathrate I.

Be3 Ru: Polar Multiatomic Bonding in the Closest Packing of Atoms.

The new phase Be3 Ru crystallizes with TiCu3 -type structure (space group Pmmn (59), a=3.7062(1) Å, b=4.5353(1) Å, c=4.4170(1) Å), a coloring variant of the hexagonal closest packing (hcp) of spheres. The electronic structure revealed that Be3 Ru has a pseudo-gap close to the Fermi level. A strong charge transfer from Be to Ru was observed from the analysis of electron density within the Quantum Theory of Atoms in Molecules (QTAIM) framework and polar three- and four-atomic Be-Ru bonds were observed from the ELI-D (electron localizability indicator) analysis. This situation is very similar to the recently investigated Be5 Pt and Be21 Pt5 compounds. The unusual crystal chemical feature of Be3 Ru is that different charged species belong to the same closest packing, contrary to typical inorganic compounds, where the cationic components are located in the voids of the closest packing formed by anions. Be3 Ru is a diamagnet displaying metallic electrical resistivity.

Non-innocent cyanido ligands: tetracyanidoferrate(-II) as carbonyl copycat.

While a negative oxidation state occurs rarely for metals in general, this is commonly known for metal carbonyl anions, i.e. carbonyl metalates. Although CO and CN- are isoelectronic, cyanidometalates usually do not exhibit metal centers with negative oxidation states. However, we report on the electron-rich tetrahedral tetracyanidoferrate(-II) anion [Fe(CN)4]6-, which was stabilized in (Sr3N)2[Fe(CN)4] (space group R3c, a = 702.12(2) pm, c = 4155.5(2) pm). Microcrystalline powders were synthesized by a solid-state route, single crystals were obtained from Na metal flux. In comparison to classical cyanidometalates, C-N distances are longer and stretching frequencies are lower as indicated by X-ray diffraction, IR and Raman spectroscopy. Weak C-N, strong Fe-C bonds as well as the anion geometry resemble the isoelectronic tetrahedral carbonyl ferrate [Fe(CO)4]2-. 57Fe Mössbauer spectroscopic measurements reveal a negative isomer shift in agreement with substantially delocalized d electrons due to strong π back-bonding. These results point to a very similar bonding situation of both 18e tetracyanido and tetracarbonyl ferrates including non-innocent redox-active ligands and a d10 closed shell configuration on iron. Hereby, new tetracyanidoferrate(-II) provides a missing link for a more in-depth understanding of the chemical bonding trends of highly-reduced cyanidometalates in the quest for even higher reduced transition metals in this exceptional class of compounds.

Polycation-Polyanion Architecture of the Intermetallic Compound Mg3-xGa1+xIr.

Mg3-xGa1+xIr (x = 0.05) was synthesized by direct reaction of the elements in welded tantalum containers at 1200 °C and subsequent annealing at 500 °C for 30 days. Its crystal structure represents a new prototype and was determined by single-crystal technique as follows: space group P63/mcm, Pearson symbol hP90, Z = 18, a = 14.4970(3) Å, c = 8.8638(3) Å. The composition and atomic arrangement in Mg3GaIr do not follow the 8-N rule due to the lack of valence electrons. Based on chemical bonding analysis in positional space, it was shown that the title compound has a polycationic-polyanionic organization. In comparison with other known intermetallic substances with this kind of bonding pattern, both the polyanion and the polyanion are remarkably complex. Mg3-xGa1+xIr is an example of how the general organization of intermetallic substances (e.g., formation of polyanions and polycations) can be understood by extending the principles of 8-N compounds to electron-deficient materials with multi-atomic bonding.

Mg3Pt2: Anionic Chains in a Eu3Ga2-Type Structure.

The binary phase Mg3Pt2 was prepared by direct reaction between the elements or by spark-plasma synthesis starting with MgH2 and PtCl2. The compound crystallizes in the monoclinic space group C2/c with a = 7.2096(3) Å, b = 7.1912(4) Å, c = 6.8977(3) Å, and ß = 106.072(3)° and is isotypic to Eu3Ga2. Analysis of the electron density within the quantum theory of atoms in molecules shows a significant charge transfer from Mg to Pt in agreement with the electronegativity difference. Further study of the chemical bonding with the electron localizability approach reveals the formation of Pt chains stabilized by a complex system of multicenter interactions involving Mg and Pt species. The metallic character of Mg3Pt2 is confirmed by electronic structure calculations and physical measurements.

Tricyanidoferrates(-IV) and Ruthenates(-IV) with Non-Innocent Cyanido Ligands.

Exceptionally electron-rich, nearly trigonal-planar tricyanidometalate anions [Fe(CN)3 ]7- and [Ru(CN)3 ]7- were stabilized in LiSr3 [Fe(CN)3 ] and AE3.5 [M(CN)3 ] (AE=Sr, Ba; M=Fe, Ru). They are the first examples of group 8 elements with the oxidation state of -IV. Microcrystalline powders were obtained by a solid-state route, single crystals from alkali metal flux. While LiSr3 [Fe(CN)3 ] crystallizes in P63 /m, the polar space group P63 with three-fold cell volume for AE3.5 [M(CN)3 ] is confirmed by second harmonic generation. X-ray diffraction, IR and Raman spectroscopy reveal longer C-N distances (124-128 pm) and much lower stretching frequencies (1484-1634 cm-1 ) than in classical cyanidometalates. Weak C-N bonds in combination with strong M-C π-bonding is a scheme also known for carbonylmetalates. Instead of the formal notation [Fe-IV (CN- )3 ]7- , quantum chemical calculations reveal non-innocent intermediate-valent CN1.67- ligands and a closed-shell d10 configuration for Fe, that is, Fe2- .

"Excess" electrons in LuGe.

The monogermanide LuGe is obtained via high-pressure high-temperature synthesis (5-15 GPa, 1023-1423 K). The crystal structure is solved from single-crystal X-ray diffraction data (structure type FeB, space group Pnma, a=7.660(2) Å, b=3.875(1) Å, and c=5.715(2) Å, RF =0.036 for 206 symmetry independent reflections). The analysis of chemical bonding applying quantum-chemical techniques in position space was performed. It revealed-beside the expected 2c-Ge-Ge bonds in the germanium polyanion-rather unexpected four-atomic bonds between lutetium atoms indicating the formation of a polycation by the excess electrons in the system Lu3+ (2b)Ge2- ×1 e- . Despite the reduced VEC of 3.5, lutetium monogermanide is following the extended 8-N rule with the trend to form lutetium-lutetium bonds utilizing the electrons left after satisfying the bonding needs in the anionic Ge-Ge zigzag chain.

Crystal Structure and Physical Properties of the Cage Compound Hf2B2-2δIr5+δ.

Hf2B2-2δIr5+δ crystallizes with a new type of structure: space group Pbam, a = 5.6300(3) Å, b = 11.2599(5) Å, and c = 3.8328(2) Å. Nearly 5% of the boron pairs are randomly replaced by single iridium atoms (Ir5+δB2-2δ). From an analysis of the chemical bonding, the crystal structure can be understood as a three-dimensional framework stabilized by covalent two-atom B-B and Ir-Ir as well as three-atom Ir-Ir-B and Ir-Ir-Ir interactions. The hafnium atoms center 14-atom cavities and transfer a significant amount of charge to the polyanionic boron-iridium framework. This refractory boride displays moderate hardness and is a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient, and thermal conductivity. The metallic character of this system is also confirmed by electronic structure calculations revealing 5.8 states eV-1 fu-1 at the Fermi level. Zr2B2-2δIr5+δ is found to be isotypic with Hf2B2-2δIr5+δ, and both form a continuous solid solution.

Y4Be33Pt16- a non-centrosymmetric cage superconductor with multi-centre bonding in the framework.

The new ternary compound Y4Be33Pt16 was prepared from elements by arc melting, and its crystal structure was determined from single-crystal X-ray diffraction data (space group I4[combining macron]3d, a = 13.4849(3) Å). The material is the first representative of a new structure type of complex intermetallic compounds and reveals a cage-like crystal structure. Analysis of chemical bonding by means of the electron localizabilty approach indicates ionic interaction of yttrium with the rest of the crystal structure, characteristic for cage compounds, in particular for clathrates. In contrast to the mostly two-centre bonding in the framework of clathrates, the new compound is characterized by a multi-centre interaction within the framework, caused by the demand of the valence electrons in the system. The non-centrosymmetric material enters the superconducting state at TC = 0.9 K.

In-Cage Interactions in the Clathrate Superconductor Sr8 Si46.

The clathrate I superconductor Sr8 Si46 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 Na8 Si46 . 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 Tc =3.8(3) K.

Preparation of cobalt borides by solid-gas reactions.

The reaction of Co with gaseous BBr3 in a temperature range of 700 to 1000 °C was studied using the hot-wire method with an experimental set-up reminiscent of the van Arkel-de Boer method. The borides Co2B und CoB form as layers on the surface of elemental cobalt. The influence of pressure, temperature and time on the reaction rate and on the composition of the borides was investigated. The reaction rate is significantly decreased by small amounts of an inert gas. The adjustment of reaction conditions allows to obtain single-phase and well-crystallized bulk materials of Co2B or CoB.

Interplay of Atomic Interactions in the Intermetallic Semiconductor Be5 Pt.

Semiconducting substances form one of the most important families of functional materials. However, semiconductors containing only metals are very rare. The chemical mechanisms behind their ground-state properties are only partially understood. Our investigations have rather unexpectedly revealed the semiconducting behaviour (band gap of 190 meV) for the intermetallic compound Be5 Pt formed at a very low valence-electron count. Quantum-chemical analysis shows strong charge transfer from Be to Pt and reveals a three-dimensional entity of vertex-condensed empty Be4 tetrahedrons with multi-atomic cluster bonds interpenetrated by the framework of Pt-filled vertex-condensed Be4 tetrahedrons with two-atomic polar Be-Pt bonds. The combination of strong Coulomb interactions with relativistic effects results in a band gap.