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 EF 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.

A New Material with a Composite Crystal Structure Causing Ultralow Thermal Conductivity and Outstanding Thermoelectric Properties: Tl2Ag12Te7+δ.

A new state-of-the-art thermoelectric material, Tl2Ag12Te7+δ, which possesses an extremely low thermal conductivity of about 0.25 W m-1 K-1 and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the Zr2Fe12P7 type, but ultimately its structure was refined as a composite structure of a Tl2Ag12Te6 framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P63(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te2 and Te3 fragments in this chain and a refined formula of Tl2Ag11.5Te7.4. The structure consists of a complex network of three-dimensionally connected AgTe4 tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl2Ag12Te7, Tl2Ag12Te7.33, and 2× Tl2Ag12Te7.5, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.

Exo-bonded six-membered heterocycle in the crystal structures of RE7Co2Ge4 (RE = La-Nd).

In this article four new ternary phases of RE7Co2Ge4 (RE = La-Nd) are reported. They were synthesized by a solid-state reaction of the elements at high temperature, and their crystal structures were investigated by single crystal X-ray diffraction data. The isostructural phases crystallize in a new monoclinic structure type, space group P21/c, Z = 4, Pearson code mP52, and Wyckoff sequence e13. They feature planar {Co4Ge6} clusters consisting of a benzene-like six-membered (Co4Ge2) metal heterocycle and four Co-Ge exo-bonds. Band structure calculations were carried out to investigate the electronic structures of La7Co2Ge4, in order to understand the nature of the chemical bonding, and the structure directing factors that determine the atomic ordering and the structure stability. These calculations indicate that the cobalt mid-transition metals are present as (formally) anionic species. They also reveal significant valence electron back-donation from the anionic {Co4Ge6} units to the La-d orbitals through multicenter bonding involving all atoms of the structure, which is key for the structure's stability with respect to incomplete charge transfer. The applicability of the 18 - n rule in rationalizing the Co-Co relative bond strength was also assessed and, the impact of these metal-metal interactions in stabilizing these clusters was described. Similar valence electron back-donation to stabilize further unusual cluster shapes is expected in other rare earth-transition metal-main group phases.

Synthesis, crystal structure, and bonding analysis of the hypoelectronic cubic phase Ca5Pd6Ge6.

The title compound, Ca5Pd6Ge6, was obtained during a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The crystal structure was determined and refined from single-crystal X-ray diffraction data. It crystallizes in a new structure variant of the Y4PdGa12-type structure (Im3̅m, a = 8.7764(4) Å) that features an arrangement of vertex-sharing body-centered cubes of calcium, Ca@Ca8, with a hierarchical bcc network, interpenetrating a second (Pd6Ge6) network consisting of Ge2 dumbbells surrounded by Pd in a strongly flattened octahedron with Pd(µ(2)-η(2),η(4)-Ge2)-like motifs. These octahedra are condensed through the Pd to form a 3D open fcc network. Theoretical band structure calculations suggested that the compound is hypoelectronic with predominantly multicenter-type interatomic interactions involving all three elements and essentially a Hume-Rothery-like regime of electronic stabilization. The similar electronegativity between germanium and palladium atoms has a decisive impact on the bonding picture of the system.

Two-dimensionally stacked heterometallic layers hosting a discrete chair dodecameric ring of water clusters: synthesis and structural study.

The stacked two-dimensional supramolecular compound catena-{Co(amp)3Cr(ox)3·6H2O} (amp = 2-picolylamine, ox = oxalate) has been synthesized from the bimolecular approach using hydrogen bonds. It is built from layers in which both Co(amp)(3+) (D) and Cr(ox)(3-) (A) ions are bonded in a repeating DADADA… pattern along the a and c axes by multiple hydrogen bonds. These layers host a well resolved R12 dodecameric discrete ring of water clusters built by six independent molecules located around the 2c centrosymmetric Wyckoff positions of the P21/n space group in which the compound crystallizes. These clusters are ranged along the [001] direction, occupy 733.5 Å(3) (22.0%) of the unit cell and have a chair conformation via 12 hydrogen bonds. The water molecules of the cluster are linked with stronger hydrogen bonds than those between the cluster and its host, which explains the single continuous step of the dehydration process of the compound.

Valence state driven site preference in the quaternary compound Ca5MgAgGe5: an electron-deficient phase with optimized bonding.

The quaternary phase Ca5Mg0.95Ag1.05(1)Ge5 (3) was synthesized by high-temperature solid-state techniques, and its crystal structure was determined by single-crystal diffraction methods in the orthorhombic space group Pnma-Wyckoff sequence c(12) with a = 23.1481(4) Å, b = 4.4736(1) Å, c = 11.0128(2) Å, V = 1140.43(4) Å(3), Z = 4. The crystal structure can be described as linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaMGe (TiNiSi-type; M = Mg, Ag) structures. Hence, 3 is a hettotype of the hitherto missing n = 3 member of the structure series with the general formula R(2+n)T2X(2+n), previously described with n = 1, 2, and 4. The member with n = 3 was predicted in the space group Cmcm-Wyckoff sequence f(5)c(2). The experimental space group Pnma (in the nonstandard setting Pmcn) corresponds to a klassengleiche symmetry reduction of index two of the predicted space group Cmcm. This transition originates from the switching of one Ge and one Ag position in the TiNiSi-related slab, a process that triggers an uncoupling of each of the five 8f sites in Cmcm into two 4c sites in Pnma. The Mg/Ag site preference was investigated using VASP calculations and revealed a remarkable example of an intermetallic compound for which the electrostatic valency principle is a critical structure-directing force. The compound is deficient by one valence electron according to the Zintl concept, but LMTO electronic structure calculations indicate electronic stabilization and overall bonding optimization in the polyanionic network. Other stability factors beyond the Zintl concept that may account for the electronic stabilization are discussed.

A new heteroleptic oxalate-based compound: poly[[2-(aminomethyl)pyridine]di-µ6-oxalato-chromium(III)potassium(I)].

The title compound, [KCr(C2O2)2(C6H8N2)]n, was obtained from aqueous solution and analyzed with single-crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/c and displays a three-dimensional polymeric architecture built up by bimetallic oxalate-bridged Cr(III)-K helical chains linked through centrosymmetric K2O2 units to yield a sheet-like alternating P/M arrangement which looks like that of the previously described two-dimensional [NaCr(ox)2(pyim)(H2O)]·2H2O [pyim is 2-(pyridin-2-yl)imidazole; Lei et al. (2006). Inorg. Chem. Commun. 9, 486-488]. The Cr(III) ions in each helix have the same chirality. The infinite neutral sheets are eclipsed with respect to each other and are held together by a hydrogen-bonding network involving 2-(aminomethyl)pyridine H atoms and oxalate O atoms. Each sheet gives rise to channels of Cr4K4 octanuclear rings and each resultant hole is occupied by a pair of 2-(aminomethyl)pyridine ligands with partial overlap. The shortest Cr...Cr distance [5.593 (4) Å] is shorter than usually observed in the K-M(III)-oxalate family.

Ca10Pt7Tt3 (Tt = Si, Ge): new platinide phases featuring electron-rich 4c-6e bonded [Pt7Tt3]20- intermetalloid clusters.

Two new phases Ca(10)Pt(7)Tt(3) (with Tt = Si, Ge) were obtained by reacting stoichiometric mixtures of the elements at high temperature. Their structures were refined from single crystal X-ray diffraction data. They are isostructural and crystallize in the Ba(10)Al(3)Ge(7) type structure, space group P6(3)/mcm (No. 193) with a = b = 8.7735(3) Å, c = 13.8260(5) Å, V = 921.66(6) Å(3), Z = 2 for Tt = Si, and a = b = 8.7995(6) Å, c = 13.9217(14) Å, V = 933.56(16) Å(3) for Tt = Ge phase. The most interesting structural features in these phases are the propeller shape {Pt(7)Tt(3)} (Tt = Si, Ge) intermetalloid clusters in a D(3h) local symmetry. LMTO electronic structure calculations and COHP analyses reveal that both Ca(10)Pt(7)Tt(3) (Tt = Si, Ge) phases are charge optimized, which is not predicted by the classical Zintl concept and the octet or Wade-Mingo's rules, but rather by a more complex bonding model based on the unprecedented electron-rich 4c-6e multicenter bonding. The clusters are best described as three-condensed trigonal planar [TtPt(3)](8-) units, resulting in a central Pt atom also with a trigonal planar coordination of three symmetrical equivalent Si/Ge atoms that are further connected to two terminal Pt atoms each. The "trefoil" electron-rich multicenter bonding is proposed here for the first time, and may be viewed as a unique bonding feature with potential relevance for the catalytic properties of the noble metal platinum.

catena-Poly[[[(oxamide dioxime-κ2N,N')copper(II)]-µ-L-tartrato-κ4O1,O2:O3,O4] tetrahydrate]: a chiral nanochannel framework hosting solvent water molecules.

The crystal structure of the title compound, {[Cu(C(4)H(4)O(6))(C(2)H(6)N(4)O(2))]·4H(2)O}(n), contains the central Cu(II) cation in a distorted octahedral coordination, symmetrically chelated by the two imine N atoms of a neutral oxamide dioxime (H(2)oxado) ligand [Cu-N = 1.9829 (16) Å] and unsymmetrically bis-chelated by two halves of the L-(+)-tartrate(2-) (tart) ligands, each half being linked to the Cu(II) cation via the deprotonated carboxylate group and protonated hydroxy group [Cu-O = 1.9356 (14) and 2.4674 (13) Å, respectively]. The extended asymmetric unit is defined by twofold axes, one passing through the Cu(II) cation and the centre of the oxamide dioxime (H(2)oxado) ligand and the another two (symmetry related) bisecting the central C-C bonds of the tartrate ions. The structure is chiral, consisting of enantiomeric linear-chain polymers oriented along [001], with virtual monomeric {Cu(tart(0.5))(2)(H(2)oxado)} repeat units and with the chains interleaved face-to-face into `twin pillars'. Nanochannels exist, running parallel to the c axis and bisecting a and b, which host `double strings' of solvent water molecules. Extensive hydrogen bonding (O-H···O and N-H···O) between the chains and solvent water molecules, together with extended π-σ interactions, consolidate the bulk crystal structure.

Extreme differences in oxidation states: synthesis and structural analysis of the germanide oxometallates A10[Ge9]2[WO4] as well as A(10+x)[Ge9]2[W(1­x)Nb(x)O4] with A = K and Rb containing [Ge9]4- polyanions.

Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A(10+x)[Ge(9)](2)[W(1-x)Nb(x)O(4)] (A = K, Rb; x = 0, 0.35) were obtained by high-temperature solid-state reactions. The crystal structure of the compounds was determined by single-crystal X-ray diffraction experiments. They crystallize in a new structure type (P2(1)/c, Z = 4) with a = 13.908(1) Å, b = 15.909(1) Å, c = 17.383(1) Å, and ß = 90.050(6)° for K(10.35(1))[Ge(9)](2)[W(0.65(1))Nb(0.35(1))O(4)]; a = 14.361(3) Å, b = 16.356(3) Å, c = 17.839(4) Å, and ß = 90.01(3)° for Rb(10.35(1))[Ge(9)](2)[W(0.65(1))Nb(0.35(1))O(4)]; a = 13.8979(2) Å, b = 15.5390(3) Å, c = 17.4007(3) Å, and ß = 90.188(1)° for K(10)[Ge(9)](2)WO(4); and a = 14.3230(7) Å, b = 15.9060(9) Å, c = 17.8634(9) Å, and ß = 90.078(4)° for Rb(10)[Ge(9)](2)WO(4). The compounds contain discrete Ge(9)(4-) Wade's nido clusters and WO(4)(2-) (or NbO(4)(3-)) anions, which are packed according to a hierarchical atom-to-cluster replacement of the Al(2)Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al(2)Cu prototype. The amount of the alkali metal atoms on these diamagnetic compounds corresponds directly to the amount of W substituted by Nb. Thus, the transition metals W and Nb appear with oxidation numbers +6 and +5, respectively, in the vicinity of a [Ge(9)](4-) polyanion. The crystals of the mixed salts were further characterized by Raman spectroscopy. The Raman data are in good agreement with the results from the X-ray structural analyses.

Calcium platinum aluminium, CaPtAl.

A preliminary X-ray study of CaPtAl has been reported previously by Hulliger [J. Alloys Compd (1993), 196, 225-228] based on X-ray powder diffraction data without structure refinement. With the present single-crystal X-ray study, we confirm the assignment of the TiNiSi type for CaPtAl, in a fully ordered inverse structure. All three atoms of the asymmetric unit have .m. site symmetry. The structure features a (∞) (3)[AlPt] open framework with a fourfold coordination of Pt by Al atoms and vice versa. The Ca atoms are located in the large channels of the structure.

Synthesis and characterization of Na5M(2+x)Sn(10-x) (x approximately = 0.5, M = Zn, Hg)--a doped tetrahedral framework structure.

Two homologous and isostructural compounds Na(5)M(2+x)Sn(10-x) (M = Zn, Hg) were obtained by direct reaction of the elements at high temperature. The crystal structures of these novel phases were determined from single-crystal X-ray diffraction data and represent a new structure type in tin chemistry. They crystallize in the space group Pbcn (No. 60, Z = 4) with a = 12.772(1), b = 10.804(1), and c = 12.777(1) A, V = 1763.1(2) A(3) for Na(5)Zn(2.28)Sn(9.72(2)) (I) and a = 12.958(1), b = 10.984(1), and c = 12.960(1) A, V = 1844.5(2) A(3) for Na(5)Hg(2.39)Sn(9.61(1)) (II). The structures consist of an anionic 3D open framework of tetrahedrally coordinated Sn and M atoms interwoven with a cationic 2D array of interconnected {NaNa(4)} tetrahedra. The framework can be partitioned into fragments of realgar-like units {Sn(8-x)M(x)}(2x-) and twice as many {Sn-M}(2-) dimers. Formally, the compounds are charge-balanced Zintl phases for x = 0.5. As the structure refinements lead to x = 0.28 and 0.39 for I and II, respectively, both structures are electron-rich and expected to be metallic. Theoretical investigations at the density functional theory level reveal a deep minimum at the Fermi level for x = 0.5. According to rigid band analyses, the electronic structure of the phases with the experimentally observed compositions corresponds to heavily doped semiconductors, thereby meeting an important requirement of thermoelectric materials.

Non-isovalent substitution in a Zintl phase with the TiNiSi type structure, CaMg(1-x)Ag(x)Ge [x = 0.13†(3)].

Single crystals of the title Ag-substituted calcium magnesium germanide, CaMg(1-x)Ag(x)Ge [x = 0.13 (3)] were obtained from the reaction of the corresponding elements at high temperature. The compound crystallizes with the TiNiSi structure type (Pearson code oP12) and represents an Ag-substituted derivative of the Zintl phase CaMgGe in which a small fraction of the divalent Mg atoms have been replaced by monovalent Ag atoms. All three atoms in the asymmetric unit (Ca, Mg/Ag, Ge) occupy special positions with the same site symmetry (.m.). Although the end member CaAgGe has been reported in an isomorphic superstructure of the same TiNiSi type, higher Ag content in solid solutions could not be achieved due to competitive formation of other, perhaps more stable, phases.

Cation substitution effects in the system Sr2-xBaxBi3 (0 < or = x < or = 1.3): structural distortions induced by chemical pressure.

Crystals of the composition Sr(2-x)Ba(x)Bi(3) (0 < or = x < or = 1.3) have been synthesized from the elements and were characterized by single-crystal and powder X-ray diffraction methods. The compounds crystallize for x = 0, 0.45, 0.86, 1.08, 1.28 in the structure type of the parent compound Sr 2Bi 3 with space group Pnna (No. 52) and Z = 4. Substitution of Sr by Ba leads to a site preference for Ba. The anionic Bi substructures in the pseudoternary system simultaneously distort under remarkable elongation of one distinct Bi-Bi contact. Magnetic measurements for samples with x = 0, 0.45 and 1.08 reveal superconducting transitions at low temperatures. Linear muffin-tin orbital band structure calculations of Sr(2)Bi(3) show strong cation-anion interactions that greatly stabilize the structure. Besides showing characteristics of a typical metal, the band structure plot unveils the co-instantaneous occurrence of flat and steep bands around the Fermi level indicative for superconductivity.

KBi(2-x)Pbx (0 < x

The quasibinary system KBi(2-x)Pbx has been investigated, both experimentally and theoretically. Phases with compositions 0 < or = x < or = 1.2 were synthesized and structurally characterized by X-ray diffraction experiments. For low values of x (0 < or = x < 0.6), KBi(2-x)Pbx adopts the cubic Laves-phase structure MgCu2 (space group Fdm), which contains a rigid framework of corner-condensed symmetry-equivalent tetrahedra formed by randomly distributed Bi and Pb atoms. For compositions x > or = 0.6, these tetrahedra become alternately elongated and contracted. The distortion of the framework lowers the space-group symmetry to F43m (KBi(1.2)Pb(0.8), F43m, Z = 8, a = 9.572(1) A). Magnetometer measurements show that KBi2 (x = 0) is metallic and goes through a superconducting transition below 3.5 K. First principles calculations reveal that the Fd3m --> F43m distortion is largest for KBiPb (x = 1.0), which at the same time turns into a semiconductor. Thus, F43m KBiPb corresponds to a proper charge-balanced Zintl phase, K+[BiPb]-, with separated polyanionic tetrahedra, (Bi2Pb2)2-. However, it was not possible to prepare F43m KBiPb. Syntheses attempting to increase the Pb content in KBi(2-x)Pbx above x = 0.8 yielded additional, not yet characterized, ternary phases.

Nonclassical bonding in the novel structure of Ba2Bi3 and unexpected site preference in the coloring variant Ba2BiSb2.

The new phase Ba(2)Bi(3) crystallizes in the W(2)CoB(2) structure type. Its structure contains rigorously planar anionic layers of (4.6.4.6)(4.6(2))(2) nets with three- and four-bonded Bi, that are separated by Ba atoms. An unexpected site preference is observed in the coloring variant Ba(2)BiSb(2) with Sb occupying only the three-bonded sites. The nonclassical bonding in the anionic network can be rationalized from a reformulation of the Zintl concept as (Ba(2+))(2)[Bi(3)](3)(-)(e(-)). Bonding distances suggest that the extra electron fills Bi-Bi antibonding states. The densities of states obtained from TB-LMTO-ASA calculations show metallic character for both compounds.

Synthesis, characterization, and electronic structure of Ba5In4Bi5: an acentric and one-electron deficient phase.

The new ternary phase Ba(5)In(4)Bi(5) was synthesized by direct reaction of the corresponding elements at high temperature. It crystallizes in a noncentrosymmetric space group and represents a new structure type (tetragonal, P4nc with a=10.620(2) and c=9.009(2) A, Z=2). The structure is built of interconnected heteroatomic clusters of In(4)Bi(5), square pyramids with In(4)-bases and four exo-bonded bismuth atoms (bond to the In atoms). According to Wade's rule the compound is electron-deficient with one electron per cluster, that is, [In(4)Bi(5)](10-) instead of the expected [In(4)Bi(5)](11-) for a closed-shell species. The clusters are discussed also in light of the known heteroatomic deltahedral clusters with the same composition but different charge, [In(4)Bi(5)](3-). Band structure calculations on the new compound suggest substantial participation of barium in the overall bonding of the structure that "accounts" for the electron shortage