Non-isovalent substitution in a Zintl phase with the TiNiSi type structure, CaMg1–xAgxGe [x = 0.13 (3)]

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an Ag-substituted derivative of the Zintl phase CaMgGe in which a small fraction of the divalent Mg atoms have been replaced by monovalent Ag atoms.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

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

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

Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2−x+zGe2+n−z (n = 3, 4)

High-temperature solid-state reactions of the respective elements afforded the new transition metal Zintl phases Ca3CuGe3 (Sc3NiSi3 type, monoclinic C2/m – i7, Pearson code mC28), Ca6MnxAg2−x+zGe6−z (own type, monoclinic P21/m – e14, Pearson code mP28) and, Ca5MnxAg2−x+zGe5−z (Ca5MgAgGe5 type, orthorhombic Pnma – c12, 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, [Gen](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.This article is published as Ponou, Siméon, Gordon J. Miller, and Anja-V. Mudring. "Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca 3 CuGe 3 and Ca 2+ n Mn x Ag 2− x+ z Ge 2+ n− z (n= 3, 4)." CrystEngComm 23, no. 14 (2021): 2711-2722. DOI: 10.1039/D1CE00094B. Copyright 2021 The Royal Society of Chemistry. Attribution 4.0 International (CC BY 4.0). Posted with permission

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

Bonding Schemes for Polar Intermetallics through Molecular Orbital Models: Ca-Supported Pt–Pt Bonds in Ca10Pt7Si3

crystal

Conflict between the Electronic Factors and Structure-Directing Rules in the Intergrowth Structure of Ca4Ag2+xGe4-x with x = 1/2

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

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

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

A new heteroleptic oxalate-based compound: poly[[2-(aminomethyl)pyridine]di-μ 6 -oxalato-chromium(III)potassium(I)]

International audienceThe 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 CrIII-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 CrIII 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-MIII-oxalate family