On Chemical Bonding in ht-Ga3Rh and Its Effect on Structural Organization and Thermoelectric Behavior.

In the course of systematic studies of intermetallic compounds Ga3TM (TM─transition metal), the compound Ga3Rh is synthesized by direct reaction of the elements at 700 °C. The material obtained is characterized as a high-temperature modification of Ga3Rh. Powder and single-crystal X-ray diffraction analyses reveal tetragonal symmetry (space group P42/mnm, No. 146) with a = 6.4808(2) Å and c = 6.5267(2) Å. Large values and strong anisotropy of the atomic displacement parameters of Ga atoms indicate essential disorder in the crystal structure. A split-position technique is applied to describe the real crystal structure of ht-Ga3Rh. Bonding analysis in ht-Ga3Rh performed on ordered models with the space groups P1̅, P42nm, and P42212 shows, besides the omnipresent heteroatomic Ga-Rh bonds in the rhombic prisms ∞3[Ga8/2Rh2], the formation of homoatomic Ga-Ga bonds bridging the Rh-Rh contacts and the absence of significant Rh-Rh bonding. These features are essential reasons for the experimentally observed disorder in the lattice. In agreement with the calculated electronic density of states, ht-Ga3Rh shows temperature-dependent electrical resistivity of a "bad metal". The very low lattice thermal conductivity of less than 0.5 W m-1 K-1 at 300 K, being lower than those for most other Ga3TM compounds, correlates with the enhanced bonding complexity.

Deciphering the Polymorphism of CaSi2: The Influence of Heat and Composition.

The Zintl phase CaSi2 is a layered compound with stacking variants known as 1P, 3R, and 6R. We extend the series by the 21R polytype formed by rapid cooling of the melt. The crystal structure of 21R-CaSi2 (space group R3̅m) was derived from HRTEM images, and the atomic positions were optimized by using the FPLO code (a = 3.868 Å, c = 107.276 Å). We explore polytype transformations by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and thermal analysis. While 6R-CaSi2 is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi2 as a bulk phase.

Nitridochromate(IV) fluoride - LiCa8[CrN3]2N2F.

LiCa8[CrIVN3]2N2F (Pnnm (#58), a = 17.5230(13) Å, b = 7.3379(5) Å, c = 4.9433(4) Å) is an example of a multinary nitridochromate fluoride, that provides additional information on almost elusive tetravalent nitridochromates. Shorter Cr-N bond lengths compared to those in the previously reported nitridochromates(III), as well as diamagnetic behavior and vibrational spectroscopy data suggest Cr(IV), which is in good agreement with the charge balance and crystal structure refinement. According to band structure calculations, LiCa8[CrIVN3]2N2F is a semiconductor with a band gap of 1.1 eV. The compound features trigonal planar [CrN3]5- units of Cs symmetry, and lithium, calcium, nitrogen and fluorine atoms arranged in a fragment of the rock salt type structure.

Ordering by cation replacement in the system Na2-xLixGa7.

Samples of the pseudo-binary system Na2-xLixGa7 (x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na2Ga7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C (x = 0) to 489(2) °C (x = 1). The boundary compositions Na2Ga7 and Na1Li1Ga7 crystallize with different structure types related by a group-subgroup relation. While the Na-rich compositions (x ≤ 0.5) represent a substitutional solid solution (space group Pnma), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce). The crystal structure of Na1Li1Ga7 (a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal diffraction data, and reveals an anionic framework comprising 12-bonded Ga12 icosahedra and 4-bonded Ga atoms, with alkali-metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa7 a new structure type within the MgB12Si2 structure family. Band structure calculations for the composition NaLiGa7 predict semiconducting behavior consistent with the balance [Na+]2[Li+]2[(Ga12)2-][Ga-]2, considering closo Wade clusters [(12b)Ga12]2- and Zintl anions [(4b)Ga]-. Susceptibility measurements indicate temperature-independent diamagnetic behavior.

Charge Transfer in Be-Ru Compounds.

During the investigation of the binary system Be-Ru two new phases - Be7 Ru4 and Be12 Ru7 - with similar compositions (63.6 at. % Be and 63.2 at. % Be, respectively), are discovered. They both represent new structural prototypes. The phases are located between Be2 Ru (Fe2 P-type structure) and Be3 Ru2 (U3 Si2 -type structure) in the phase diagram. This explains why their crystal structures, solved and refined from single crystal X-ray diffraction data, are described as 2D intergrowth of Fe2 P and U3 Si2 motives. The calculated electronic density of stats (DOS) reveals pronounced minima in the vicinity of the Fermi level for both compounds. Position-space analysis of chemical bonding exhibits the formation of three- and four-atomic polar bonds, involving both, Ru and Be, atoms, and a strong charge transfer from Be to the more electronegative Ru.

Nitridochromate(IV): LiSr2[CrN3].

The quaternary nitridochromate(IV) LiSr2[CrN3] crystallizes in a new structure type with the non-centrosymmetric space group P21 (no. 4) with a = 5.5685(7) Å, b = 5.3828(8) Å, c = 7.5381(1) Å, and ß = 92.291(8)°. Predominant structural features of the compound are slightly nonplanar trigonal units [CrN3]5-, which are connected by three-fold coordinated lithium to form slabs in the (001) plane. Shorter Cr-N bond lengths in comparison with reported nitridochromates(III), as well as diamagnetic behavior and vibrational spectroscopy data indicate Cr(IV), which is in a good agreement with the charge balance. According to electronic structure calculations, the compound is a semiconductor with a band gap of 1.19 eV.

Chemical and Physical Properties of YHg3 and LuHg3.

Amalgams have played an important role in fundamental and applied solid-state chemistry and physics because of the diversity of crystallographic features and properties that they have to offer. Moreover, their peculiar chemical properties can sometimes give rise to unconventional superconducting or magnetic ground states. In the current work, we present an in-depth analysis of single crystals of YHg3 and LuHg3 (Mg3Cd structure type, space group P63/mmc). Both compounds show superconductivity below Tc = 1 ± 0.1 K (YHg3) and Tc = 1.2 ± 0.1 K (LuHg3). Given the high air-sensitivity and toxicity of these compounds, this study was only possible using a number of dedicated experimental techniques.

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.

Polarity-extended 8 - Neff rule for semiconducting main-group compounds with the TiNiSi-type of crystal structure.

Application of chemical bonding analysis in position-space techniques based on combined topological analysis of the electron density and electron-localizability indicator distributions has recently led to the formulation of a polarity-extended 8 - Neff rule for consistent inclusion of quantum chemically obtained polar-covalent bonding data into the classical 8 - N scheme for main-group compounds. Previous application of this scheme to semiconducting main-group compounds of the cubic MgAgAs type of structure with 8 valence electrons per formula unit (8 ve per f.u.) has shown a covalent bonding tendency preferring one zinc blende type partial structure over the other one, which seems to corroborate the classical Lewis picture of maximally four covalent bonds per main-group element. In contrast to the MgAgAs type, the orthorhombic TiNiSi type of structure displays a much higher geometrical flexibility to incorporate different kinds of metal atoms. The analysis of polar-covalent bonding in semiconducting 8 ve per f.u. containing main-group compounds AA'E of this structure type reveals a transition to non-Lewis type bonding scenarios of species E with up to ten polar-covalently bonded metal atoms. This kind of situation is consistently included into the extended 8 - Neff type bonding scheme. A systematic increase of partially covalent bonding from chalcogenides E16 to the tetrelides E14 is found, summing up to as much as 2 covalent bonds E14-A and E14-A', and correspondingly remaining 4 lone pair type electrons on species E14. The familiar notion of this structure type consisting of a '[NiSi]'-type framework with 'Ti'-type atoms filling the voids cannot be supported for the compounds investigated.

Fourier-synthesis approach for static charge-density reconstruction from theoretical structure factors of CaB6.

In a pilot study, electron-density (ED) and ED Laplacian distributions were reconstructed for the challenging case of CaB6 (Pearson symbol cP7) with conceptually fractional B-B bonds from quantum-chemically calculated structure-factor sets with resolutions 0.5 Å-1 ≤ [sin(θ)/λ]max ≤ 5.0 Å-1 by means of Fourier-synthesis techniques. Convergence of norm deviations of the distributions obtained with respect to the reference ones was obtained in the valence region of the unit cell. The QTAIM (quantum theory of atoms in molecules) atomic charges, and the ED and ED Laplacian values at the characteristic critical points of the Fourier-synthesized distributions have been analysed for each resolution and found to display a convergent behaviour with increasing resolution. The presented method(exponent) (ME) type of Fourier-synthesis approach can qualitatively reconstruct all characteristic chemical bonding features of the ED from valence-electron structure-factor sets with resolutions of about 1.2 Å-1 and beyond, and from all-electron structure-factor sets with resolutions of about 2.0 Å-1 and beyond. Application of the ME type of Fourier-synthesis approach for reconstruction of ED and ED Laplacian distributions at experimental resolution is proposed to complement the usual extrapolation to infinite resolution in Hansen-Coppens multipole model derived static ED distributions.

Lithium metal atoms fill vacancies in the germanium network of a type-I clathrate: synthesis and structural characterization of Ba8Li5Ge41.

Clathrate phases with crystal structures exhibiting complex disorder have been the subject of many prior studies. Here we report syntheses, crystal and electronic structure, and chemical bonding analysis of a Li-substituted Ge-based clathrate phase with the refined chemical formula Ba8Li5.0(1)Ge41.0, which is a rare example of ternary clathrate-I where alkali metal atoms substitute framework Ge atoms. Two different synthesis methods to grow single crystals of the new clathrate phase are presented, in addition to the classical approach towards polycrystalline materials by combining pure elements in desired stoichiometric ratios. Structure elucidations for samples from different batches were carried out by single-crystal and powder X-ray diffraction methods. The ternary Ba8Li5.0(1)Ge41.0 phase crystallizes in the cubic type-I clathrate structure (space group Pm3̄n no. 223, a ≈ 10.80 Å), with the unit cell being substantially larger compared to the binary phase Ba8Ge43 (Ba8□3Ge43, a ≈ 10.63 Å). The expansion of the unit cell is the result of the Li atoms filling vacancies and substituting atoms in the Ge framework, with Li and Ge co-occupying one crystallographic (6c) site. As such, the Li atoms are situated in four-fold coordination environment surrounded by equidistant Ge atoms. Analysis of chemical bonding applying the electron density/electron localizability approach reveals ionic interaction of barium with the Li-Ge framework, while the lithium-germanium bonds are strongly polar covalent.

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.

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.

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.

Electronic Structure and Magnetic Properties of a High-Spin MnIII Complex: [Mn(mesacac)3 ] (mesacac=1,3-Bis(2,4,6-trimethylphenyl)-propane-1,3-dionato).

Metal acetylacetonates of the general formula [M(acac)3 ] (MIII =Cr, Mn, Fe, Co) are among the best investigated coordination compounds. Many of these first-row transition metal complexes are known to have unique electronic properties. Independently, photophysical research with different ß-diketonate ligands pointed towards the possibility of a special effect of the 2,4,6-trimethylphenyl substituted acetylacetonate (mesacac) on the electron distribution between ligand and metal (MLCT). We therefore synthesized and fully characterized the previously unknown octahedral title complex. Its solid-state structure shows a Jahn-Teller elongation with two Mn-O bonds of 2.12/2.15 Šand four Mn-O bonds of 1.93 Å. Thermogravimetric data show a thermal stability up to 270 °C. High-resolution mass spectroscopy helped to identify the decomposition pathways. The electronic state and spin configuration of manganese were characterized with a focus on its magnetic properties by measurement of the magnetic susceptibility and triple-zeta density functional theory (DFT) calculations. The high-spin state of manganese was confirmed by the determination of an effective magnetic moment of 4.85 µB for the manganese center.

Chemical Bonding in the Catalytic Platform Material Ga1-x Snx Pd2.

The underlying reasons for the catalytic activity of Ga1-x Snx Pd2 (0 ≤ x ≤ 1) in the semi-hydrogenation of acetylene are analyzed considering electronic structure and chemical bonding. Analysis of the chemical bonding shows pronounced charge transfer from the p elements to palladium and an unusual appearance of the Pd core basins at the surface of the QTAIM (quantum theory of atoms in molecules) atoms. The charge transfer supports the formation of the negatively charged palladium catalytic centers. Gallium-only-coordinated palladium atoms reveal a smaller effective charge in comparison with palladium species having tin in their coordination sphere. Within the empirical tight-binding approach, different influence of the E-Pd distances on the calculation matrix for the energy eigenvalues and the electronic density of states (DOS) leads to an S-like shape of the plot of the energy position of the 4d band center of gravity versus substitution level x. The latter correlates strongly with the catalytic activity and with the varying charge transfer to palladium. The optimal value of negative palladium charge and the closest position of Pd d-states gravity center towards the Fermi level correlates well with the catalytically most active composition x. Combination of all features of the chemical bonding and electronic structure allows more insight into the intrinsic reasons for the catalytic activity variation in the platform material Ga1-x Snx Pd2 (0 ≤ x ≤ 1).

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.

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.

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.

Lone-Pair-Like Interaction and Bonding Inhomogeneity Induce Ultralow Lattice Thermal Conductivity in Filled ß-Manganese-Type Phases.

Finding a way to interlink heat transport with the crystal structure and order/disorder phenomena is crucial for designing materials with ultralow lattice thermal conductivity. Here, we revisit the crystal structure and explore the thermoelectric properties of several compounds from the family of the filled ß-Mn-type phases M 2/n n+Ga6Te10 (M = Pb, Sn, Ca, Na, Na + Ag). The strongly disturbed thermal transport observed in the investigated materials originates from a three-dimensional Te-Ga network with lone-pair-like interactions, which results in large variations of the Ga-Te and M-Te interatomic distances and substantial anharmonic effects. In the particular case of NaAgGa6Te10, the additional presence of different cations leads to bonding inhomogeneity and strong structural disorder, resulting in a dramatically low lattice thermal conductivity (∼0.25 Wm-1 K-1 at 298 K), being the lowest among the reported ß-Mn-type phases. This study offers a way to develop materials with ultralow lattice thermal conductivity by considering bonding inhomogeneity and lone-pair-like interactions.