Polyanionic Network Research Articles

Sr(Hg1–x Sn x )4: variations of the EuIn4-type structure

Abstract Starting from the new compound SrHg2Sn2, which is isoelectronic and also isotypic to the indide SrIn4, the successive substitution of Sn against the electron poor Hg has been investigated in a combined synthetic, crystallographic, and bond-theoretical study. Along the 1:4 section Sr(Hg1–x Sn x )4 a series of compounds with Sn contents x between 0.5 and 0.2 were synthesized from stoichiometric ratios of the elements. Their crystal structures, which represent three different variants of the EuIn4-type structure, have been determined using single crystal X-ray data. The most electron rich compound SrHg2Sn2 crystallizes in the original EuIn4-type [monoclinic, C2/m, a = 1257.9(14), b = 490.1(4), c = 997.8(12) pm, β = 117.60(6)°, Z = 4, R1 = 0.0838], with a fully ordered Hg and Sn distribution. The four atom sites form two different folded ladders with an alternating Hg/Sn distribution. Like in the KHg2-type, the ladders are connected via six-membered rings. In between, double tubes with an internal Sn–Sn bond are connected via further Sn–Sn bonds to form sheets similar to those observed in SiAs. The most electron-poor phase SrHg3.2Sn0.8 crystallizes in a strongly distorted variant of this structure [a = 1172.8(4), b = 497.9(2), c = 1010.0(4) pm, β = 118.860(7)°, Z = 4, R1 = 0.0549]. Herein, additional Hg–Hg bonds are formed, and the open tubes are distorted into rods of edge-sharing rhombohedra resembling the structure motifs of elemental Hg. At an intermediate valence electron (v.e.) number, i.e., in SrHg2.5Sn1.5, an isomorphous tripled superstructure (a = 2704.4(5), b = 493.87(7), c = 1197.1(2) pm, β = 90.838(14)°, Z = 12, R1 = 0.0475) occurs, where the building blocks of the two variants of the EuIn4-type structure alternate in a 1:2 ratio. The bonding situation and the “coloring,” i.e., the Hg/Sn distribution in the polyanionic network, are discussed considering the different sizes of the atoms and the charge distribution (Bader AIM charges), which has been calculated within the framework of the FP-LAPW density functional theory for SrHg2Sn2 and the model compounds “SrHg3Sn” and “SrHg4.”

Vacancy ordering as a driving factor for structural changes in ternary germanides: the new R2Zn(1-x)Ge6 series of polar intermetallics (R = rare-earth metal).

Synthesis and structural characterization of the new compounds R2Zn1-xGe6 (R = La-Nd, Sm, Gd-Ho) is reported. A structural change was revealed along this series by careful analysis of single-crystal X-ray diffraction data. For light rare earths up to Tb the orthorhombic oS72-Ce2(Ga0.1Ge0.9)7 model was established; instead, the Dy compound represents a new structure type (P21/m, mP34, Z = 4, a = 7.9613(3) Å, b = 8.2480(4) Å, c = 10.5309(5) Å, β = 100.861(1)°) being a superstructure of the mS36-La2AlGe6 prototype. The established structural models support the increase of Zn deficiency along the series, suggested by microprobe analysis, and its key role in governing structural changes. The vacancy ordering criterion was applied as a successful approach to find a general scheme including the structures of the ∼R2MGe6 compounds known up to now (R = rare-earth metal, M = transition metal, Mg, Al, Ga) and highlighting the subtle structural differences within this family. According to this scheme, these structures are obtained from a common aristotype (oS20-SmNiGe3) via symmetry reduction based on group-subgroup relations accompanied by ordering of vacancies. This approach was optimized with the help of the ToposPro software and extended to the R2Zn3Ge6 series, enriched with new members (R = Sm, Gd-Ho) during this work. Electronic structure calculations on La2ZnGe6 confirm the presence of infinite covalent germanium zigzag chains and three-bonded corrugated layers connected via Zn atoms to form a polyanionic network stabilized by La atoms.

The Gallium Intermetallics REPdGa3 (RE=La, Ce, Pr, Nd, Sm, Eu) with SrPdGa3-type Structure

Abstract The gallium-rich intermetallic phases REPdGa3 (RE=La, Ce, Pr, Nd, Sm, Eu) were obtained by arc-melting of the elements and subsequent annealing for crystal growth. The samples were studied by X-ray diffraction on powders and single crystals. The structures of three crystals were refined from X-ray diffractometer data: SrPdGa3 type, Cmcm, a=634.3(1), b=1027.2(1), c=593.5(1) pm, wR=0.0621, 380 F2 values, 20 variables for CePd0:80(4)Ga3:20(4), a=635.9(1), b=1027.5(1), c=592.0(1) pm, wR=0.1035, 457 F2 values, 19 variables for CePdGa3, and a=640.7(1), b=1038.2(1), c=593.7(1) pm, wR=0.0854, 489 F2 values, 19 variables for EuPdGa3. The REPdGa3 gallides are orthorhombic superstructure variants of the aristotype ThCr2Si2. The palladium and gallium atoms build up polyanionic [PdGa3]δ- networks with Pd-Ga and Ga-Ga distances of 248 - 254 and 266 - 297 pm, respectively, in EuPdGa3. The rare earth atoms fill cavities within the polyanionic networks. They are coordinated by five palladium and twelve gallium atoms. Taking CePdGa3 as an illustrative representative, the band structure calculations show largely dispersive itinerant s, p bands and little dispersive d (Pd) and f (Ce) bands, the latter crossing the Fermi level at large magnitude leading to magnetic instability in a spin-degenerate state and a subsequent antiferromagnetic ground state with a small moment of ±0.36 μB on Ce. The bonding characteristics indicate a prevailing Ce-Ga bonding versus Pd-Ga and Ce-Pd. Temperature-dependent magnetic susceptibility and 151Eu Mössbauer spectroscopic measurements point to stable trivalent lanthanum, cerium, praseodymium, and neodymium, but divalent europium. SmPdGa3 shows intermediate valence. Antiferromagnetic ordering occurs at TN =5.1(5), 7.0(5), 6.3(5), 11.9(5), and 23.0(5) for RE=Ce, Pr, Nd, Sm, and Eu, respectively.

Silicon‐Tin‐Ordering in RE3Rh9Si2Sn3 (RE = Y, Sm, Gd – Lu)

AbstractThe quaternary intermetallic compounds RE3Rh9Si2Sn3 (RE = Y, Sm, Gd – Lu) were obtained from the elements by arc‐melting and investigated by powder X‐ray diffraction. Needle‐shaped single crystals of Ho3Rh9Si2Sn3 and Lu3Rh9Si2Sn3 were grown in bismuth fluxes and their structures were refined from single‐crystal X‐ray diffractometer data: P$\bar{6}$2m, a = 968.3(2), c = 345.6(1) pm, wR2 = 0.0278, 426 F2 values, 22 variables for Ho3Rh9Si2Sn3, and a = 962.4(2), c = 343.1(2) pm, wR2 = 0.0260, 501 F2 values, 23 variables for Lu3Rh9Si2Sn3. The phases RE3Rh9Si2Sn3 adopt a new simple structure type with complete silicon‐tin ordering. The silicon atoms have trigonal prismatic rhodium coordination (239 pm Si–Rh in Ho3Rh9Si2Sn3) and the tin atoms are located in slightly distorted square prisms of rhodium (268–274 pm Sn–Rh in Ho3Rh9Si2Sn3). Magnetic investigations were performed for RE3Rh9Si2Sn3 (RE = Y, Gd – Yb). Except of Y3Rh9Si2Sn3, all investigated compounds exhibit Curie‐Weiss paramagnetism, but ferromagnetic ordering could only be observed for samples containing Gd – Er as rare earth metal. 119Sn Mössbauer spectroscopic measurements were carried out on Gd3Rh9Si2Sn3. A single resonance at an isomer shift of 1.52(1) mm·s–1 clearly underlines that tin is not part of the polyanionic network and therefore not of a stannide character. Representative 29Si solid state NMR spectroscopy investigations were carried out on the diamagnetic yttrium compound Y3Rh9Si2Sn3. The spectrum indicates the existence of only one distinct Si site consistant with the refined single crystal data.

The Stannides Nd3Pd5Sn5, Gd3Pd4.96(1)Sn5, and GdPdSn

Abstract The stannides Nd3Pd5Sn5 and Gd3Pd4.96(1)Sn5 were obtained by arc‐melting of the elements. Nd3Pd5Sn5 and Gd3Pd4.96(1)Sn5 crystallize with the monoclinic Sm3Pd4.95Sn5 type structure, space group C2/m. Both structures were refined from single crystal diffractometer data: a = 1729.5(4), b = 454.51(8), c = 1428.2(3) pm, β = 99.71(2)°, wR2 = 0.0689, 2220 F2 values, 82 variables for Nd3Pd5Sn5 and a = 1712.2(2), b = 450.36(9) c = 1412.0(3) pm, β = 99.804(2)°, wR2 = 0.0526, 1834 F2 values, 83 variables for Gd3Pd4.96(1)Sn5. The Pd2 site of the gadolinium compound showed site occupancy of only 95.8(7) %. The palladium and tin atoms in these monoclinic stannides build up complex three‐dimensional [Pd5Sn5]δ– polyanionic networks, which leave cavities for the three crystallographically independent rare earth sites: RE1@Pd6Sn8, RE2@Pd7Sn8, and RE3@Pd6Sn9. The networks are stabilized by Pd–Sn, Pd–Pd as well as Sn–Sn bonding. The rare earth atoms bind to the networks via shorter RE–Pd contacts. The Gd3Pd4.96(1)Sn5 crystal showed an adherent second domain, which consisted of GdPdSn: TiNiSi type, Pnma, a = 725.6(3), b = 461.3(3), c = 790.6(3) pm, wR2 = 0.0973, 431 F2 values, 20 variables.

Electronic Structure, Chemical Bonding and Electrochemical Characterization of Li2CuSn2 and Li2AgSn2

Abstract Polycrystalline samples of the stannides Li2CuSn2 and Li2AgSn2 were obtained by high-frequency melting of the elements in sealed niobium ampoules in a water-cooled sample chamber. Both stannides crystallize with the tetragonal Li2AuSn2 type, space group I41/amd. They are characterized by three-dimensional [CuSn2]δ-, respectively [AgSn2]δ- networks which leave large channels for the lithium ions. Electronic structure calculations show extensive filling of the transition metal d bands and residual DOS at the Fermi energy, compatible with metallic character. Calculated Bader charges and the course of the crystal orbital overlap population curves fully support the bonding picture of cationic lithium and a covalently bonded polyanionic network with considerable charge transfer to both, transition metal and tin atoms. Electrochemical investigations have indicated that a reversible insertion and extraction of lithium into the stannides is taking place in the voltage range between 0 and 2:5 V vs. Li=Li+. From CV measurements, the diffusion coefficents of Li2CuSn2 and Li2AgSn2 were estimated to be in the order of 10-14 cm2 s-1

Lithium transition metal pnictides – structural chemistry, electrochemistry, and function

AbstractLithium-transition metal (T)-pnictides (Pn=P, As, Sb, Bi) are an interesting class of materials with greatly differing crystal structures. The transition metal and pnictide atoms build up covalently bonded networks that leave cavities or channels for the lithium atoms. Depending on the bonding of lithium to the polyanionic network, one observes mobility of the lithium atoms. The crystal chemistry, chemical bonding,7Li solid-state NMR, and the electrochemical behavior of the pnictides are reviewed. The structural chemistry is compared with related tetrelides.

Synthesis and crystal chemistry of new ternary pnictides containing lithium--adding structural complexity one step at a time.

This study focuses on the exploration of the ternary systems A-Li-Pn with A = Ca-Ba, Eu and Yb and Pn = As-Bi resulting in the extensions of the ALiPn series (A = Ca, Sr, Eu and Yb; Pn = As-Bi; orthorhombic Pnma) with the MgSrSi structure type and of the A3Li4Pn4 family (A = Ba, Eu; Pn = As-Bi; orthorhombic Immm) with the Zr3Cu4Si4 structure type, as well as the novel compound Eu4Li7Bi6 (monoclinic, C2/m) with its own structure type. The relevant structural relationships are discussed. The building blocks and the topology of the polyanionic networks among the three different structures are compared side-by-side. Electronic band structure calculations for EuLiSb, Eu3Li4Sb4 and Eu4Li7Bi6 are discussed; the magnetic susceptibility and resistivity of single-crystalline Eu3Li4Sb4 are also presented.

Flux growth of Yb(6.6)Ir(6)Sn(16) having mixed-valent ytterbium.

The compound Yb6.6Ir6Sn16 was obtained as single crystals in high yield from the reaction of Yb with Ir and Sn run in excess indium. Single-crystal X-ray diffraction analysis shows that Yb6.6Ir6Sn16 crystallizes in the tetragonal space group P42/nmc with a = b = 9.7105(7) Å and c = 13.7183(11) Å. The crystal structure is composed of a [Ir6Sn16] polyanionic network with cages in which the Yb atoms are embedded. The Yb sublattice features extensive vacancies on one crystallographic site. Magnetic susceptibility measurements on single crystals indicate Curie-Weiss law behavior <100 K with no magnetic ordering down to 2 K. The magnetic moment within the linear region (<100 K) is 3.21 μB/Yb, which is ∼70% of the expected value for a free Yb(3+) ion suggesting the presence of mixed-valent ytterbium atoms. X-ray absorption near edge spectroscopy confirms that Yb6.6Ir6Sn16 exhibits mixed valence. Resistivity and heat capacity measurements for Yb6.6Ir6Sn16 indicate non-Fermi liquid metallic behavior.

Structure and solid-state NMR spectroscopy of the ternary pnictides Li3LaX 2 (X = P, As, Sb, Bi)

The ternary pnictides Li3LaX 2 (X = P, As, Sb, Bi) were synthesized by the reaction of the elements in sealed niobium ampoules in a resistance furnace. The new phase Li3LaAs2 was obtained for the first time and completes this pnictide series. The structure of Li3LaAs2 was investigated by X-ray diffraction on a single crystal: Li3LaSb2 type, $$P \bar{3}$$ m1, a = 435.68(6) pm, c = 710.8(1) pm, wR2 = 0.1033, 162 F 2 values, and ten variables. The structure is a filled derivative of the CaAl2Si2 type. The Li2 atoms have tetrahedral arsenic coordination. These Li2As4 tetrahedra are condensed via common edges forming layers in the ab plane which are separated by lanthanum atoms. The “filling” Li1 atoms are placed within the Li2-As polyanionic network. The chemical shift values of the 7Li solid-state MAS NMR spectra show that the lithium atoms are fully ionized and the compound can be described by the Zintl–Klemm concept as (3Li+)(La3+)(2As3−). The static 7Li solid-state NMR spectra show no lithium mobility.

The Stannides EuPd2Sn2, EuPt2Sn2, EuAu2Sn2, and Eu5.4Sn5.6 – Structure and Magnetic Properties

The europium stannides EuT2Sn2 (T = Pd, Pt, Au) and Eu3Ag5.4Sn5.6 were synthesized by highfrequency melting of the elements in sealed niobium ampoules in a water-cooled sample chamber. All samples were characterized by powder X-ray diffraction. The EuT2Sn2 (T = Pd, Pt, Au) stannides crystallize with the CaBe2Ge2-type structure, space group P4/nmm. The structure of EuPd2Sn2 was refined from single-crystal X-ray diffractometer data: a = 462.44(8), c = 1045.8(3) pm, wR = 0.0402, 237 F2 values and 15 refined variables. The palladium and tin atoms build up a threedimensional [Pd2Sn2] polyanionic network, exclusively with Pd-Sn interactions (261 - 269 pm). The Pd1 and Pd2 atoms have square-pyramidal and tetrahedral tin coordination, respectively. The europium atoms fill large voids within the network. They are coordinated to eight palladium and eight tin atoms. Temperature-dependent magnetic susceptibility studies confirm a stable divalent ground state of the europium atoms. The compounds become ordered antiferromagnetically below 6.3 (EuPd2Sn2), 6.1 (EuPt2Sn2) and 7.7 K (EuAu2Sn2). Eu3Ag5.4Sn5.6 adopts a partially ordered variant of the La3Al11 type, space group Immm, a = 471.33(8), b = 1382.5(4), c = 1032.4(2) pm, wR = 0.0449, 692 F2 values, 30 variables. The three-dimensional [Ag5.4Sn5.6] network shows one silver and one tin site besides two sites with substantial Ag/Sn mixing. The two crystallographically independent europium atoms fill larger and smaller cavities within the [Ag5.4Sn5.6] network. Eu3Ag5.4Sn5.6 also shows divalent europium and antiferromagnetic ordering at TN = 6:9 K. A 151Eu Mössbauer spectrum of Eu3Ag5.4Sn5.6 at 5.2 K shows an isomer shift of δ = −10.61 mms−1, typical for Eu(II) compounds, and a magnetic hyperfine field splitting of BHf = 5.9 T. 119Sn Mössbauer spectra of the four stannides show isomer shifts in the range of δ = 1.78 - 2.20 mms−1, usually observed for tin in intermetallic compounds.

Gold-Tin Ordering in SrAu2Sn2

Samples of the solid solutions SrAuxSn4−x (1:7 ≤ x ≤ 2:2) were obtained by high-frequency melting of the elements in sealed niobium ampoules. Powder and single-crystal X-ray data confirmed the CaBe2Ge2-type structure, space group P4/nmm. The structures of SrAu1.76Sn2.24, SrAu2Sn2, SrAu2.16Sn1.84 (crystal A), SrAu2.16Sn1.84 (crystal B), and SrAu2.22Sn1.78 were refined from singlecrystal diffractometer data. Only the SrAu2Sn2 crystal shows complete Au-Sn ordering while all other crystals show substantial mixed occupancies on the four crystallographically independent sites of the polyanionic networks in which the strontium atoms fill cages of coordination number 16. Temperature-dependent susceptibility measurements have revealed diamagnetism for SrAu2Sn2. 119Sn Mössbauer spectroscopic data of a bulk SrAu2Sn2 sample have resolved the tetrahedral and square-pyramidal tin sites but point to substantial Au-Sn disorder.

The Family of LiCo6P4 Type Compounds – Trends in Electronic Structure and Chemical Bonding

AbstractSelected ternary compounds of the family of AT6X4 (LiCo6P4‐type) intermetallics were studied by ab initio methods with respect to their electronic structures and chemical bonding. The bonding characteristics highlight the role of the three‐dimensional [T6X4] polyanionic networks which, within the hexagonal channels, accommodate loosely bonded A cations. In alkali metal based compounds, when T is replaced by indium (e.g. in KIn6Au4) the reverse occupancy leads to positively charged In (ca. +0.5) and negatively charged Au (ca. –0.9). Peculiar magnetic behavior is traced out whereby a magnetic order is identified only when T is a 3d element, which polarizes and induces a negative small moment on A (U) as in UCr6P4 oppositely to CeRh6X4 for X = Si, Ge.

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.

High-pressure phases of Tb2Ni2Sn and Dy2Ni2Sn

The W2B2Co-type ternary stannides Tb2Ni2Sn and Dy2Ni2Sn (space group Immm) show pressure-induced reconstructive phase transitions. Under high-pressure (8 GPa) and high-temperature (1,470 K) treatment they transform to the tetragonal Mo2B2Fe-type high-pressure (HP) modifications: P4/mbm, a = 732.6(1) pm, c = 371.71(9) pm, wR2 = 0.0319, 225 F 2 values for HP-Tb2Ni2Sn and a = 730.3(2) pm, c = 369.1(2) pm, wR2 = 0.0221, 230 F 2 values for HP-Dy2Ni2Sn, with 12 variables per refinement. At 8 GPa HP-Tb2Ni2Sn and HP-Dy2Ni2Sn started to decompose into the equiatomic stannides TbNiSn and DyNiSn and a yet unknown phase. Single-crystal diffractometry revealed the following data: TiNiSi type, Pnma, a = 714.9(1) pm, b = 445.9(1) pm, c = 766.4(1) pm, wR2 = 0.0558, 575 F 2 values, 20 parameters for TbNiSn and a = 710.5(1) pm, b = 445.31(9) pm, c = 765.7(1) pm for DyNiSn. The cell volumes of HP-Tb2Ni2Sn and HP-Dy2Ni2Sn nicely fit into the Iandelli plot of the whole RE2Ni2Sn (RE = rare earth) series of compounds. The polyanionic networks [NiSn] and [Ni2Sn] of the investigated ternary stannides show covalent Ni–Sn bonding with distorted tetrahedral SnNi4/4 coordination (259−279 pm Ni–Sn) in the RENiSn compounds and a square-planar SnNi4/4 arrangement (291−292 pm Ni–Sn) in HP-Tb2Ni2Sn and HP-Dy2Ni2Sn. The nickel atoms form Ni2 pairs with Ni–Ni distances of 250 pm.

The Stannides Sm6Pd10–x Sn11 and Sm3Pd5–x

Abstract The stannides Sm6Pd10–x Sn11 and Sm3Pd5–x Sn5 were synthesized from the elements by arc-melting. They were studied by powder and single-crystal X-ray data: C2/m, a = 1720.45(10), b = 451.97(2), c = 1420.60(8), β = 99.705(5)°, wR2 = 0.086, 2076 F 2 values, 83 variables for Sm3Pd4.95(1)Sn5 and C2/m, a = 1738.68(7), b = 451.49(1), c = 1692.68(7), β = 121.375(3)°, wR2 = 0.0335, 2164 F 2 values, 85 variables for Sm6Pd9.82(1)Sn11. Both structures show small defects for one palladium site. The palladium and tin atoms in Sm6Pd10–x Sn11 and Sm3Pd5–x Sn5 build up complex three-dimensional polyanionic networks in which the samarium atoms fill cavities within distorted pentagonal channels. In addition to strong covalent Pd–Sn bonding, the polyanions are further stabilized by Pd–Pd and Sn–Sn bonding.

Polyclusters and Substitution Effects in the Na–Au–Ga System: Remarkable Sodium Bonding Characteristics in Polar Intermetallics

A systematic exploration of Na- and Au-poor parts of the Na-Au-Ga system (less than 15 at. % Na or Au) uncovered several compounds with novel structural features that are unusual for the rest of the system. Four ternary compounds Na1.00(3)Au0.18Ga1.82(1) (I), NaAu2Ga4 (II), Na5Au10Ga16 (III), and NaAu4Ga2 (IV) have been synthesized and structurally characterized by single crystal X-ray diffraction: Na1.00(3)Au0.18Ga1.82(1)(I, P6/mmm, a = 15.181(2), c =9.129(2)Å, Z = 30); NaAu2Ga4 (II, Pnma, a = 16.733(3), b = 4.3330(9), c =7.358(3) Å, Z = 4); Na5Au10Ga16 (III, P6(3)/m, a = 10.754(2), c =11.457(2) Å, Z = 2); and NaAu4Ga2 (IV, P2(1)/c, a = 8.292(2), b = 7.361(1), c =9.220(2)Å, β = 116.15(3), Z = 4). Compound I lies between the large family of Bergman-related compounds and Na-poor Zintl-type compounds and exhibits a clathrate-like structure containing icosahedral clusters similar to those in cubic 1/1 approximants, as well as tunnels with highly disordered cation positions and fused Na-centered clusters. Structures II, III, and IV are built of polyanionic networks and clusters that generate novel tunnels in each that contain isolated, ordered Na atoms. Tight-binding electronic structure calculations using linear muffin-tin-orbital (LMTO) methods on II, III, IV and an idealized model of I show that all are metallic with evident pseudogaps at the Fermi levels. The integrated crystal orbital Hamilton populations for II-IV are typically dominated by Au-Ga, Ga-Ga, and Au-Au bonding, although Na-Au and Na-Ga contributions are also significant. Sodium's involvement into such covalency is consistent with that recently reported in Na-Au-M (M = Ga, Ge, Sn, Zn, and Cd) phases.

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

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.

The new Hg-rich barium indium mercurides BaInxHg7−x (x=3.1) and BaInxHg11−x (x=0–2.8)

Abstract The title compounds BaInxHg7−x (x=3.1(1)) and BaInxHg11−x (x=0–2.8) were synthesized from stoichiometric ratios of the elements in Ta crucibles. Their crystal structures have been determined using single crystal X-ray data. BaInxHg7−x (x=3.1(1)) crystallizes in a new structure type (orthorhombic, oC16, space group Cmmm: a=512.02(1), b=1227.68(3), c=668.61(2) pm, Z=2, R1=0.0311). In the structure, the atoms of the three crystallographically different mixed In/Hg positions form planar nets of four-, six- and eight-membered rings. These nets are shifted against each other such that the four-membered rings form empty distorted cubes. The cubes are connected via common edges, corners and folded ladders, which are also found in BaIn2/BaHg2 (KHg2 structure type) and BaIn (α-NaHg type). The Ba atoms are centered in the eight-membered rings and exhibit an overall coordination number of 20. The [BaM20] polyhedra and twice as many distorted [M8] cubes tesselate the space. BaIn2.8Hg8.2 (cubic, cP36, space group Pm 3 ¯ m , a=961.83(1) pm, Z=3, R1=0.0243) is the border compound of the phase width BaInxHg11−x of the rare BaHg11 structure type. In the structure, ideal [M8] cubes (at the corners of the unit cell) and BaM20 polyhedra (at the edges of the unit cell) represent the building blocks comparable to the other new In mercuride. In accordance with the increased In/Hg content, additional M-pure regions appear: the center of the unit cell contains a huge [Hg(1)M(2)12M(3,4)32] polyhedron, a Hg-centered cuboctahedron of In/Hg atoms surrounded by a capped cantellated cube of 32 additional M atoms. For both structure types, the bonding situation and the ‘coloring’, i.e. the In/Hg distribution of the polyanionic network, are discussed considering the different sizes of the atoms and the charge distribution (Bader AIM charges), which have been calculated within the framework of FP-LAPW density functional theory.

Change of the cerium valence with temperature – Structure and chemical bonding of HT-CeRhGe

Polycrystalline CeRhGe was prepared via arc-melting of the elements. Its TiNiSi-related structures (space groups Pnma) were studied by powder diffraction using synchrotron radiation over the temperature range of 315–770 K. CeRhGe shows a first-order structural phase transition at 520 K upon heating. Ab initio elelectronic structure calculations give evidence for the depletion of the cerium 4f band in HT-CeRhGe and in consequence a redistribution of the electron density from the cerium to the rhodium atoms. Purely trivalent cerium atoms in the low-temperature modification (LT) change to intermediate-valent cerium in the high-temperature modification (HT). The integrated crystal orbital Hamilton populations show an enhancement of the Ce–Rh bonding in HT-CeRhGe. The three-dimensional [RhGe] polyanionic network shows drastic puckering of the [Rh3Ge3] hexagons in LT-CeRhGe and a flattening in HT-CeRhGe. The cerium valence change is accompanied by a drastic jump in the lattice parameters: a = 7.42249(8), b = 4.46699(5) and c = 7.1276(1) Å at 315 K vs. a = 7.24579(6), b = 4.47506(4) and c = 7.43579(6) Å at 570 K. Large shifts occur for the x parameter of the rhodium and the z parameter of the cerium atomic positions (Wyckoff sites 4c (x 1/4 z)).