| 1. |
- Banenzoue, Charles, et al.
(författare)
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Non-isovalent substitution in a Zintl phase with the TiNiSi type structure, CaMg1-xAgxGe [x=0.13 (3)]
- 2009
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Ingår i: Acta Crystallographica Section E. - 1600-5368. ; 65, s. i90-U56
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Tidskriftsartikel (refereegranskat)abstract
- Single crystals of the title Ag-substituted calcium magnesium germanide, CaMg1-xAgxGe [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an Ag-substituted derivative of the Zintl phase CaMgGe in which a small fraction of the divalent Mg atoms have been replaced by monovalent Ag atoms.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.
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| 2. |
- Doverbratt, Isa, et al.
(författare)
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Ca(10)Pt(7)Tt(3) (Tt = Si, Ge): New Platinide Phases Featuring Electron-Rich 4c-6e Bonded [Pt(7)Tt(3)](20-) Intermetalloid Clusters.
- 2012
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 51:21, s. 11980-11985
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Doverbratt, Isa, et al.
(författare)
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Ca2Pd3Ge, a new fully ordered ternary Laves phase structure
- 2013
-
Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596. ; 197, s. 312-316
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Tidskriftsartikel (refereegranskat)abstract
- The title compound, Ca2Pd3Ge, was prepared as a part of a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The structure was determined and refined from single-crystal X-ray diffraction data. It is a new fully ordered ternary Laves phase with the space group R-3m, Z=3, a=5.6191 (5) angstrom, c=12.1674 (7) angstrom. wR(2)=0.054 (all data) and is isostructural to Mg2Ni3Si (Noreus et al., 1985 [17]) but due to the larger size of all elements in Ca2Pd3Ge, the cell axes are approximately 10% longer. The compound may formally be considered as a Zintl compound, with [Pd3Ge](4-) forming a poly-anionic network and divalent Ca cations located in truncated tetrahedral interstices. The electronic structure and chemical bonding of Ca2Pd3Ge is discussed in terms of LMTO band structure calculations and compared with CaPd2 (MgCu2-type). (C) 2012 Elsevier Inc. All rights reserved.
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| 4. |
- Doverbratt, Isa, et al.
(författare)
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Linear Metal Chains in Ca2M2X (M = Pd, Pt; X = Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability
- 2015
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 27:1, s. 304-315
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Tidskriftsartikel (refereegranskat)abstract
- A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Delta chi) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C-2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd center dot center dot center dot Pd atoms, whereas the metal chains in II-IV are pairwise distorted, resulting in short connected {Pd-2} or {Pt-2} dumbbells that are separated by longer M center dot center dot center dot M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Delta chi between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal-metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M-M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons.
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| 5. |
- Doverbratt, Isa, et al.
(författare)
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Synthesis, Crystal Structure, and Bonding Analysis of the Hypoelectronic Cubic Phase Ca5Pd6Ge6.
- 2015
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 54:18, s. 9098-9104
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Fredrickson, Daniel C., et al.
(författare)
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Bonding schemes for polar intermetallics through molecular orbital models : Ca-supported Pt-Pt Bonds in Ca10Pt7Si3
- 2013
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Ingår i: Crystals. - : MDPI AG. - 2073-4352. ; 3:3, s. 504-516
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Tidskriftsartikel (refereegranskat)abstract
- Exploratory synthesis in the area of polar intermetallics has yielded a rich variety of structures that offer clues into the transition in bonding between Zintl and Hume-Rothery phases. In this article, we present a bonding analysis of one such compound, Ca10Pt7Si3, whose large Ca content offers the potential for negative formal oxidation states on the Pt. The structure can be divided into a sublattice of Ca cations and a Pt-Si polyanionic network built from Pt7Si3 trefoil units linked through Pt-Pt contacts of 3.14 Å. DFT-calibrated Hückel models reveal that the compound adheres well to a Zintl-like electron counting scheme, in which the Pt-Si and Pt-Pt contacts are equated with two-center two-electron bonds. The experimental electron count is in excess of that predicted by 2%, a discrepancy which is attributed to the electron transfer from the Ca to the Pt-Si network being incomplete. For the Pt-Pt contacts, the occupancy of the bonding orbitals is dependent on the participation of the surrounding Ca atoms in bridging interactions. This use of multi-center interactions isolobal to classical two-center two-electron bonds may illustrate one path by which the bonds delocalize as one moves from the Zintl phases toward the Hume-Rothery domain.
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| 7. |
- Hlukhyy, Viktor, et al.
(författare)
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Extreme differences in oxidation states : Synthesis and structural analysis of the germanide oxometallates A 10[Ge 9] 2[WO 4] as well as A 10+x[Ge 9] 2[W 1-xNb xO 4] with A = K and Rb containing [Ge 9] 4- polyanions
- 2012
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 51:7, s. 4058-4065
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Tidskriftsartikel (refereegranskat)abstract
- Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A 10+x[Ge 9] 2[W 1-xNb xO 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 α = 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 α = 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 α = 90.188(1)° for K 10[Ge 9] 2WO 4; and a = 14.3230(7) Å, b = 15.9060(9) Å, c = 17.8634(9) Å, and α = 90.078(4)° for Rb 10[Ge 9] 2WO 4. The compounds contain discrete Ge 9 4- Wades 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 2Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al 2Cu 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.
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| 8. |
- Ponou, Simeon, et al.
(författare)
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alpha-Cd13-xSb10 - The Devil is in the Details
- 2009
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Ingår i: Zeitschrift für Anorganische und Allgemeines Chemie. - : Wiley. - 0044-2313 .- 1521-3749. ; 635:12, s. 1747-1752
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Tidskriftsartikel (refereegranskat)abstract
- A detailed study of the structure of α-Cd13Sb10 was performed using a high quality single crystal obtained from a cadmium-rich melt. The study clearly shows that this sample is cadmium-deficient, and should more properly be labelled α-Cd13–xSb10, but whether this is true in general for this phase, or if it is an effect of the precise conditions of synthesis remains unclear. Like for the similar zinc based compound, small differences in the cadmium (zinc) content may be crucial in determining the electronic transport properties of the material.
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| 9. |
- Ponou, Simeon, et al.
(författare)
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Ca3Ag1+xGe3-x (x=1/3): New Transition Metal Zintl Phase with Intergrowth Structure and Alloying with Aluminum Metal
- 2013
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Ingår i: Zeitschrift für Anorganische und Allgemeine Chemie. - : Wiley. - 0044-2313. ; 639:1, s. 35-40
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Tidskriftsartikel (refereegranskat)abstract
- The ternary Zintl phase Ca3Ag1+xGe3x (x = 1/3) was synthesized by the high-temperature solid-state technique and its crystal structure was refined from single-crystal diffraction data. The compound Ca3Ag1.32Ge2.68(1) adopts the Sc3NiSi3 type structure, crystal data: space group C2/m, a = 10.813(1) angstrom, b = 4.5346(4) angstrom, c = 14.3391(7) angstrom, beta = 110.05(1)degrees and V = 660.48(10) angstrom 3 for Z = 4. Its structure can be interpreted as an intergrowth of fragments cut from the CaGe (CrB-type) and the CaAg1+xGe1x (TiNiSi-type) structures, and it therefore represents an alkaline-earth member of the structure series with the general formula R2+nT2X2+n with n = 4. Unlike the rare-earth homologues that are fully ordered phases, one seventh of the atomic sites in the unit cell of the title compound are mixed occupied (roughly 2/3Ge and 1/3Ag), and this can be explained by the Zintl concept. The alloying of this phase using aluminum metal yielded the isotypic solid solution Ca3(Ag/Al)1+xGe3x, in which the aluminum for silver substitution is strictly localized in the TiNiSi substructure, revealing the very different functionality of the two building blocks.
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| 10. |
- Ponou, Siméon, et al.
(författare)
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Conflict between the Electronic Factors and Structure-Directing Rules in the Intergrowth Structure of Ca4Ag2+xGe4-x with x = 1/2
- 2016
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Ingår i: Crystal Growth and Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 16:10, s. 5946-5953
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Tidskriftsartikel (refereegranskat)abstract
- Combined experimental and theoretical efforts to conceptually understand the structure directing forces in intergrowth structures have led to the discovery of the new ternary phase Ca4Ag2+xGe4-x (x = 0.5), obtained from high-temperature reaction of the elements. It crystallizes in a new structure type according to single-crystal diffraction methods: monoclinic space group C2/m-i10 with a = 10.7516(2) Å, b = 4.5475(1) Å, c = 18.7773(4) Å, β = 93.69(2)°, V = 916.17(3) Å3, Z = 4. The compound corresponds to the n = 2 member of the homologous series Ca2+nAg2+xGe2+n-x, that are built up by linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaAg1+xGe1-x (KHg2 or TiNiSi-type) structures, and may be partitioned in Ag-rich and Ag-free domains. Instead of the predicted Zr2CoSi2-type (C2/m-i5), a simultaneous doubling of the size of the two building blocks is observed with the dimerization of the (Ge2) pairs into Ag-substituted tetramers (AgxGe4-x) due to valence electron shortage. However, the Ag/Ge mixing at one atomic site with roughly one-to-one atomic ratio is therefore unexplained. The electronic band structure calculations and analysis of the chemical bonding provided evidence that the Ag/Ge mixing is rather the result of a direct conflict between the Zintl-Klemm concept and empirically established "structure-directing rules". The implications of these findings for the poorly understood ordered staging structural interfaces, typically observed in secondary Li-ion batteries during charge/discharge process, are briefly discussed.
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| 11. |
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| 12. |
- Ponou, Siméon, et al.
(författare)
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Exo-bonded six-membered heterocycle in the crystal structures of RE7Co2Ge4 (RE = La-Nd)
- 2016
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 45:46, s. 18522-18531
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Ponou, Simeon
(författare)
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On a TiNiSi-Type Superstructure: Synthesis, Crystal and Electronic Structures of CaAgGe and Its Mn-Substituted Derivative
- 2010
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Ingår i: European Journal of Inorganic Chemistry. - : Wiley. - 1099-0682 .- 1434-1948. ; :26, s. 4139-4147
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Tidskriftsartikel (refereegranskat)abstract
- The compound CaAgGe and its Mn-substituted derivative CaMn0.07Ag0.93Ge were synthesized by reaction of the element mixtures at high temperature. Their structures were refined from single-crystal X-ray diffraction data. CaAgGe crystallizes in the isomorphic (i(3)) superstructure of the TiNiSi type (CaCuGe type). The LMTO band structure calculations predicted the CaAgGe phase to be metallic. In addition, it appeared that the valence electron concentration is critical for the atomic ordering and the resulting superstructure. Thus, CaAgGe is one electron short per asymmetric unit, but a drastic narrowing of the electron shortage is achieved through the more complex atomic ordering in the supercell, relative to the simple TiNiSi type subcell. This results in the formation of (Ge-2) dumbbells in the supercell, which ascribe greater electronic flexibility to the structure. Despite the fact that CaAgGe is not a Zintl phase in the strict definition as valence balanced intrinsic semiconductor, its structural aspects and transport properties can still be understood within the Zintl concept. The formal Zintl phase Ca3MnAg2Ge3 could be predicted from the theoretical calculations, but no significant phase width was observed experimentally in the substituted derivative Ca3Mn0.21Ag2.79Ge3.
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| 14. |
- Ponou, Siméon, et al.
(författare)
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Optimization of Chemical Bonding through Defect Formation and Ordering-The Case of Mg7Pt4Ge4
- 2023
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Ingår i: Inorganic Chemistry. - 0020-1669 .- 1520-510X. ; 62:22, s. 8519-8529
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 15. |
- Ponou, Simeon, et al.
(författare)
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Structure and Bonding of an Intergrowth Phase Ca7Ag2+xGe7-x (x approximate to 2/3) Featuring a Zintl-Type Polyanionic Chain
- 2016
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Ingår i: European Journal of Inorganic Chemistry. - : Wiley. - 1099-0682 .- 1434-1948. ; :1, s. 169-176
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Tidskriftsartikel (refereegranskat)abstract
- Single crystals of a new ternary phase, Ca7Ag2+xGe7-x [x = 0.48(3)], were obtained from as side-product of high-temperature solid-state reactions, and its crystal structure determined by X-ray diffraction methods. Following the Zintl concept, the anionic substructure consists of a novel pentameric Zintl anion [Ge-5](12-) with C-2v local symmetry, as well as [AgxGe2-x]((6-3x)-) units accounting for [Ge-2](6-) dimers and isolated Ge4- (and Ag+) species sharing the same atomic sites. DFT-level band structure calculations were carried out on a hypothetical, fully ordered model (x = 0). We found that the electronic structure associated with the planar W-shaped [Ge-5](12-) polyanions is more suited to optimize the structure's stability than the helical configuration of the isoelectronic [Se-5](2-), in the context of incomplete charge transfer from the electropositive metal Ca. Thus, the antibonding states at the Fermi level that are centered on the two [Ge-n]((2n+2)-) oligomers can be depopulated by means of Ge-to-Ca electron back donation, strengthening at the same time the Ge-Ge bonds. These antibonding states also endow the system with substantial electronic flexibility, which may result in some phase width. Finally, plausible local ordering models of Ag/Ge mixing in [AgxGe2-x]((6-3x)-) units for x = 2/3, as expected from the Zintl concept, are briefly discussed within the coloring problem approach.
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| 16. |
- Ponou, Simeon, et al.
(författare)
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Synthesis and Crystal Structure of Ca2Ag1.83Al0.67Ge1.50 : A Solid Solution with the NbCoB-type structure
- 2009
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Ingår i: Zeitschrift für Anorganische und Allgemeines Chemie. - : Wiley. - 0044-2313 .- 1521-3749. ; 635:13-14, s. 2143-2146
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structure of the new phase, Ca2Ag1.83Al0.67Ge1.50 (1) (1) was determined by single-crystal X-ray diffraction, after the compound was obtained by high temperature synthesis technique. 1 crystallizes with the NbCoB type structure in the orthorhombic space group Pmmn (no. 59) with unit cell parameters: a = 20.505(1) angstrom, b = 4.3807(2) angstrom, c = 7.2211(3) angstrom, V = 648.65 angstrom(3) (Z = 5). The structure of the title compound represent a rare intergrowth of the well known KHg2 (or CeCu2) and Fe2P type structures.
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| 17. |
- Ponou, Siméon, et al.
(författare)
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Uncovering new transition metal Zintl phases by cation substitution : the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2-x+zGe2+n-z (n=3, 4)
- 2021
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Ingår i: CrystEngComm. - : Royal Society of Chemistry (RSC). - 1466-8033. ; 23:14, s. 2711-2722
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Tidskriftsartikel (refereegranskat)abstract
- High-temperature solid-state reactions of the respective elements afforded the new transition metal Zintl phases Ca3CuGe3 (Sc3NiSi3 type, monoclinic C2/m - i(7), Pearson code mC28), Ca6MnxAg2-x+zGe6-z (own type, monoclinic P2(1)/m - e(14), Pearson code mP28) and, Ca5MnxAg2-x+zGe5-z (Ca5MgAgGe5 type, orthorhombic Pnma - c(12), Pearson code oP48) as evidenced by single-crystal X-ray diffraction. They are additional representatives of the recently discovered homologous series Ca2+nM2+zGe2+n-z, already reported with M = Ag, Mg. These new phases were rationally prepared, after speculation that Cu and Mn could replace the isovalent Ag and Mg, respectively, to yield isostructural phases. Their crystal chemistry is discussed using established 'structure directing rules'. Their structures are best described according to the Zintl-Klemm formalism as (Ca2+)((2+n))[M2+zGe2+n-z)](2(2+n)-) featuring (poly-)germanide oligomers, [Ge-n]((2n+2)-) with n = 1-5. These Zintl anions interact with the highly polarizing small M (Cu, Ag, Mn) cations through their terminal Ge atoms, while the central Ge atoms are in trigonal prismatic coordination with the active metal Ca. Electronic structure calculations using density functional theory (DFT) were conducted on the idealized fully ordered model of Ca3MGe3 (Sc3NiSi3 type) with M = Cu, Ag for an analysis of the chemical bonding and structure stabilizing factors. Our findings suggest that new transition metal Zintl phases can be obtained through partial to complete replacement of the highly polarizing small s-block cations (Li, Mg) in the Ca-(Li,Mg)-(Ge,Si) system by their isovalent transition metals like Ag, Cu, and Mn. However, due to differences in coordination requirements and possible strong metal-metal bonding between the d-block elements, the resulting transition metal phases may not be isostructural with their Li and Mg counterparts, even when featuring the same type of Zintl anions.
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| 18. |
- Ponou, Simeon, et al.
(författare)
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Valence State Driven Site Preference in the Quaternary Compound Ca5MgAgGe5: An Electron-Deficient Phase with Optimized Bonding.
- 2014
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 53:9, s. 4724-4732
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Tidskriftsartikel (refereegranskat)abstract
- 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 R2+nT2X2+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.
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| 19. |
- Shi, Yixuan, et al.
(författare)
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A New Material with a Composite Crystal Structure Causing Ultralow Thermal Conductivity and Outstanding Thermoelectric Properties : Tl 2 Ag 12 Te 7+δ
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 140:27, s. 8578-8585
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Tidskriftsartikel (refereegranskat)abstract
- A new state-of-the-art thermoelectric material, Tl 2 Ag 12 Te 7+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 Zr 2 Fe 12 P 7 type, but ultimately its structure was refined as a composite structure of a Tl 2 Ag 12 Te 6 framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6 3 (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 Te 2 and Te 3 fragments in this chain and a refined formula of Tl 2 Ag 11.5 Te 7.4 . The structure consists of a complex network of three-dimensionally connected AgTe 4 tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl 2 Ag 12 Te 7 , Tl 2 Ag 12 Te 7.33 , and 2× Tl 2 Ag 12 Te 7.5 , 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.
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| 20. |
- Åkerstedt, Josefin, et al.
(författare)
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Structural Investigation of a Fully Ordered closo-Ge(9)(2-) Cluster in the Compound [K(+)(2,2,2-crypt)](2)Ge(9)(2-)
- 2011
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Ingår i: European Journal of Inorganic Chemistry. - : Wiley. - 1434-1948 .- 1099-1948 .- 1099-0682. ; :26, s. 3999-4005
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Tidskriftsartikel (refereegranskat)abstract
- [K(+)(2,2,2-crypt)] (2)Ge(9)(2-) containing the deltahedral Zintl anion Ge(9)(2-) has been isolated from a solution of K(4)Ge(9) in ethylenediamine in the presence of 2,2,2-crypt, intended as an intermediate in mixed-cluster synthesis. The subsequent crystallographic characterisation shows the closo-Ge(9)(2-) cluster unit to be tricapped trigonal-prismatic with a symmetry very close to D(3h). Each Ge(9)(2-) unit is surrounded by two 2,2,2-crypt units, each with a chelated K(+) cation, viz. [K(+)(2,2,2-crypt)]. The structure characterisation of this ordered Ge(9)(2-) cluster is surprisingly enough the first one reported. A chemical bonding analysis reveals two local minima of D(3h) symmetry, with that of lowest energy being highly similar to that resulting from the crystallographic analysis of low-temperature data. The cluster interaction scheme is based on highly delocalised bonding.
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