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.

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.

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.

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.

Beam quality and efficiency of ns-pulsed high-power mid-IR ZGP OPOs compared in linear and non-planar ring resonators.

We compare the performance of ns-pulsed mid-infrared optical parametric oscillators (OPOs) based on ZGP crystals in a simple linear cavity and two non-planar image-rotating resonators. Our 2 µm pump source is a 68 W 10 kHz diffraction limited Ho:LLF MOPA system. In a linear cavity, it allows us to generate up to 38 W of the total average power with M2<2.2 from the ZGP OPO in the 3-5 µm range. Beam quality and stability are significantly improved in both types of non-planar ring resonators (M2<1.8 in RISTRA and M2<1.4 in FIRE), while the total average power is above 21 W. Factors influencing the beam quality such as average power and pump repetition rate are studied in detail.

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.

Polar-Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare-Earth Germanides.

A comparative chemical bonding analysis for the germanides La2 MGe6 (M=Li, Mg, Al, Zn, Cu, Ag, Pd) and Y2 PdGe6 is presented, together with the crystal structure determination for M=Li, Mg, Cu, Ag. The studied compounds adopt the two closely related structure types oS72-Ce2 (Ga0.1 Ge0.9 )7 and mS36-La2 AlGe6 , containing zigzag chains and corrugated layers of Ge atoms bridged by M species, with La/Y atoms located in the biggest cavities. Chemical bonding was studied by means of the quantum chemical position-space techniques QTAIM (quantum theory of atoms in molecules), ELI-D (electron localizability indicator), and their basin intersections. The new penultimate shell correction (PSC0) method was introduced to adapt the ELI-D valence electron count to that expected from the periodic table of the elements. It plays a decisive role to balance the Ge-La polar-covalent interactions against the Ge-M ones. In spite of covalently bonded Ge partial structures formally obeying the Zintl electron count for M=Mg2+ , Zn2+ , all the compounds reveal noticeable deviations from the conceptual 8-N picture due to significant polar-covalent interactions of Ge with La and M ≠ Li, Mg atoms. For M=Li, Mg a formulation as a germanolanthanate M[La2 Ge6 ] is appropriate. Moreover, the relative Laplacian of ELI-D was discovered to reveal a chemically useful fine structure of the ELI-D distribution being related to polyatomic bonding features. With the aid of this new tool, a consistent picture of La/Y-M interactions for the title compounds was extracted.

On Fe-Fe Dumbbells in the Ideal and Real Structures of FeGa3.

The intermetallic phase FeGa3 belongs to the rare examples of substances with transition metals where semiconducting behavior is found. The necessary electron count of 17 ve/fu can be formally derived from eight Fe-Ga and one Fe-Fe two-center-two-electron bond. The situation is reminiscent of the well-known Fe2(CO)9 scenario, where a direct Fe-Fe two-center-two-electron bond was shown to not be present. Fe-Fe interaction in FeGa3 and its substitution variants represents the crucial point for explanation of electronic, thermal transport, and optical properties of this material. Chemical bonding analysis in position space of FeGa3 and Fe2(CO)9 on the basis of the topology of the electron localizability indicator distribution, QTAIM atoms, two- and three-center delocalization indices, domain natural orbitals, IQA analysis, and an evaluation of the Fe-Fe dissociation energy yields a complete picture of the partially compensated Fe-Fe bond, which is nevertheless strong enough to be of decisive importance. Structural reinvestigation of differently synthesized single crystals leads to the composition Fe1+ xGa3 (0 ≤ x ≤ 0.018), where the additional Fe atoms are predicted from DFT/PBE calculations to yield a magnetic moment of about 2 µB/Fe2 atom and metallic in-gap states. Accompanying magnetization and ESR measurements are consistent with this picture.

Hierarchical and chemical space partitioning in new intermetallic borides MNi21B20 (M = In, Sn).

The compounds MNi21B20 (M = In, Sn) have been synthesized and their cubic crystal structure determined (space group Pm3[combining macron]m, lattice parameters a = 7.1730(1) Å and a = 7.1834(1) Å, respectively). The structure can be described as a hierarchical partitioning of space based on a reo-e net formed by Ni3 species with large cubical, cuboctahedral and rhombicuboctahedral voids being filled according to [Ni1@Ni38], [M@Ni312], and [Ni26@B20@Ni324], respectively. The [Ni6@B20] motif inside the rhombicuboctahedral voids features an empty [Ni6] octahedron surrounded by a [B20] cage recently described in E2Ni21B20 (E = Zn, Ga). Position-space bonding analysis using ELI-D and QTAIM space partitioning as well as 2- and 3-center delocalization indices gives strong support to an alternative chemical description of space partitioning based on face-condensed [B@Ni6] trigonal prisms as basic building blocks. The shortest B-B contacts display locally nested 3-center B-B-Ni bonding inside each trigonal prism. This clearly rules out the notion of [Ni6@B20] clusters and leads to the arrangement of 20 face-condensed [B@Ni23Ni33] trigonal prisms resulting in a triple-shell like situation Ni26@B20@Ni324(reo-e), where the shells display comparable intra- and inter-shell bonding. Both compounds are Pauli paramagnets displaying metallic conductivity.

Chemical Bonding Analysis as a Guide for the Preparation of New Compounds: The Case of VIrGe and HfPtGe.

The chemical bonding of transition metal compounds with a MgAgAs-type of crystal structure is analyzed with quantum chemical position-space techniques. The observed trends in QTAIM Madelung energy and nearest neighbor electron sharing explain the occurrence of recently synthesized MgAgAs-type compounds, TiPtGe and TaIrGe, at the boundary to the TiNiSi-type crystal structure. These bonding indicators are used to identify favorable element combinations for new MgAgAs-type compounds. The new phases-the high-temperature VIrGe and the low-temperature HfPtGe-showing this type of crystal structure are prepared and characterized by powder X-ray diffraction and differential thermal analysis.

Interpenetration of a 3D Icosahedral M@Ni12 (M=Al, Ga) Framework with Porphyrin-Reminiscent Boron Layers in MNi9 B8.

Two ternary borides MNi9 B8 (M=Al, Ga) were synthesized by thermal treatment of mixtures of the elements. Single-crystal X-ray diffraction data reveal AlNi9 B8 and GaNi9 B8 crystallizing in a new type of structure within the space group Cmcm and the lattice parameters a=7.0896(3) Å, b=8.1181(3) Å, c=10.6497(4) Šand a=7.0897(5) Å, b=8.1579(4) Å, c=10.6648(7) Å, respectively. The boron atoms build up two-dimensional layers, which consist of puckered [B16 ] rings with two tailing B atoms, whereas the M atoms reside in distorted vertices-condensed [Ni12 ] icosahedra, which form a three-dimensional framework interpenetrated by boron porphyrin-reminiscent layers. An unusual local arrangement resembling a giant metallo-porphyrin entity is formed by the [B16 ] rings, which, due to their large annular size of approximately 8 Å, chelate four of the twelve icosahedral Ni atoms. An analysis of the chemical bonding by means of the electron localizability approach reveals strong covalent B-B interactions and weak Ni-Ni interactions. Multi-center dative B-Ni interaction occurs between the Al-Ni framework and the boron layers. In agreement with the chemical bonding analysis and band structure calculations, AlNi9 B8 is a Pauli-paramagnetic metal.

8 - N rule and chemical bonding in main-group MgAgAs-type compounds.

The chemical bonding of main-group MgAgAs-type compounds is analyzed with quantum chemical direct-space techniques. A new bonding concept is developed that unites the former ionic bonding and polyanionic network models. Polar and nonpolar contributions to the bonding are extracted by the combined analysis of electron density and electron localizability. A direct-space representation of the 8 - N rule is introduced. In this approach, the anions' heteropolar bonds are treated as a superposition of covalent (nonpolar) and lone-pair closed-shell (polar) contributions. The relation between covalent (nonpolar) and lone-pair (polar) character is obtained with the ELI-D/QTAIM basin intersection technique. This ratio depends on the constituting elements. On basis of this approach, MgAgAs-type compounds are compared with Zintl phases, where covalent bonds and lone pairs are spatially separated.

Covalence and ionicity in MgAgAs-type compounds.

MgAgAs-type "half-Heusler" compounds are known to realize two out of three possible atomic arrangements of this structure type. The number of transition metal components typically determines which of the alternatives is favored. On the basis of DFT calculations for all three variants of 20 eight- and eighteen-valence-electron compounds, the experimentally observed structural variant was found to be determined by basically two different bonding patterns. They are quantified by employing two complementary position-space bonding measures. The Madelung energy E((M)(QTAIM)) calculated with the QTAIM effective charges reflects contributions of the ionic interactions to the total energy. The sum of nearest-neighbor delocalization indices ςnn characterizes the covalent interactions through electron sharing. With the aid of these quantities, the energetic sequence of the three atomic arrangements for each compound is rationalized. The resulting systematic is used to predict a scenario in which an untypical atomic arrangement becomes most favorable.

Photoluminescence characterization of KH2PO4 crystal: application to three-dimensional growth-sector identification.

In this work, rapidly grown KH2PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals.

Lanthanoid-transition-metal bonding in bismetallocenes.

Bismetallocenes [Cp2 LuReCp2 ] and [Cp*2 LaReCp2 ] (Cp=cyclopentadienyl; Cp*=pentamethylcyclopentadienyl) were prepared using different synthetic strategies. Salt metathesis-performed in aromatic hydrocarbons to avoid degradation pathways caused by THF-were identified as an attractive alternative to alkane elimination. Although alkane elimination is more attractive in the sense of its less elaborate workup, the rate of the reaction shows a strong dependence on the ionic radius of Ln(3+) (Ln=lanthanide) within a given ligand set. Steric hindrance can cause a dramatic decrease in the reaction rate of alkane elimination. In this case, salt metathesis should be considered the better alternative. Covalent bonding interactions between the Ln and transition-metal (TM) cations has been quantified on the basis of the delocalization index. Its magnitude lies within the range characteristic for bonds between transition metals. Secondary interactions were identified between carbon atoms of the Cp ligand of the transition metal and the Ln cation. Model calculations clearly indicated that the size of these interactions depends on the capability of the TM atom to act as an electron donor (i.e., a Lewis base). The consequences can even be derived from structural details. The observed clear dependency of the LuRu and interfragment LuC bonding on the THF coordination of the Lu atom points to a tunable Lewis acidity at the Ln site, which provides a method of significantly influencing the structure and the interfragment bonding.

Contrasted material responses to nanosecond multiple-pulse laser damage: from statistical behavior to material modification.

This work is dedicated to the study of so-called fatigue effects upon nanosecond laser-induced damage of several crystalline materials and synthetic fused silica irradiated by multiple pulses. The obtained damage probability versus fluence and pulse number data are exploited to determine if the observed fatigue is due to statistics (the more often the material is irradiated, the higher the probability for it to be damaged) or to material modification under irradiation. Whereas 1064 nm irradiation seems to be responsible for statistic behavior, 355 nm irradiation generates material modifications in the case of synthetic fused silica.

Case of a strong antiferromagnetic exchange coupling induced by spin polarization of a Mn-Mn partial single bond.

The large antiferromagnetic coupling in the Mn(IV)-Mn(IV) bond in the Li(6)Ca(2)[Mn(2)N(6)] and Li(6)Sr(2)[Mn(2)N(6)] crystals (J = -739 and -478 cm(-1), respectively, with H = -JS(A)·S(B)) is studied using different theoretical methods: solid-state density functional theory calculations, molecular density functional theory, and post-Hartree-Fock calculations with large embeddings. This magnetic coupling is a challenge for theoretical methods because both correlation and polarization effects are crucial for the correct description of the bond. All methods predict a large antiferromagnetic coupling, but none of the considered methods give a quantitative agreement with the experimental values. The molecular methods, except B3LYP and CASPT2, underestimate the coupling for the calcium compound, while they overestimate it in the strontium compound, within 30%. These methods, on the other hand, strongly underestimate the decrease of the coupling between the two compounds, with the most correlated one predicting the same value for both compounds. The solid-state method overestimates the coupling within 60% but reproduces better their ratio. Analysis of the calculations shows that the magnetic coupling between the local π orbitals is not caused by a direct interaction but by the spin-polarized σ bond.

An iridium-stabilized, formally uncharged Te10 molecule with 3-center-4-electron bonding.

Te for 10: a tricyclic Te(10) molecule is stabilized in an iridium complex. Bonding analysis reveals 3-center-4-electron bonds in the linear Te(3) fragment. The tellurium atoms act as 2-electron donors to the transition-metal atoms.