Novel Tin Structure Motives in Superconducting BaSn5 – The Role of Lone Pairs in Intermetallic Compounds [1]

BaSn₅ is the tin richest phase in the system Ba/Sn and is obtained by stoichiometric combination of the elements. The compound peritecticly decomposes under formation of BaSn₃ and a Sn–Ba melt at 430 °C. The structure shows a novel structure motive in tin chemistry. Tin atoms are arranged in graphite‐like layers (honeycombs). Two such layers form hexagonal prisms which are centered by Sn. Consequently the central tin atom has the unusual coordination number 12. The two‐dimensional tin slabs which consist of two 3⁶ and one 6³ nets of Sn atoms are separated by 6³ nets of Ba atoms with Ba above the center of each tin hexagon. The structure of BaSn₅ can be rationalized as a variante of AlB₂ and thus also of the superconducting MgB₂. Temperature dependent magnetic susceptibility measurements show that BaSn₅ is superconducting with T_c = 4.4 K. Reinvestigation of the magnetism of the Ba richer phase BaSn₃ reveals for this compound a T_c of 2.4 K. LMTO band structure and density of states calculations verify the metallic behavior of BaSn₅. The van Hove scenario of high‐temperature cuprate superconductors is discussed for this ‘classical' intermetallic superconductor. An analysis of the electronic structure with the help of fat‐band projections and the electron localization function (ELF) shows that the van Hove singularity in the DOS originates from non‐bonding (lone) electron pairs in the intermetallic phase BaSn₅. The role of lone pairs in intermetallic phases is discussed with respect to superconducting properties.     

Zinc selenium oxochloride, β‐Zn2(SeO3)Cl2, a synthetic polymorph of the mineral sophiite

Dizinc selenium dichloride trioxide, β‐Zn₂(SeO₃)Cl₂, a monoclinic polymorph of the orthorhombic mineral sophiite, has a structure built of distorted ZnO₄Cl₂ octahedra, ZnO₂Cl₂ tetrahedra and SeO₃E tetrahedra (E being the 4s² lone pair of the Se^IV ion), joined through shared edges and corners to form charge‐neutral layers. The Cl atoms and the Se lone pairs protrude from each layer towards adjacent layers. The main structural difference between the mineral and synthetic polymorphs lies in the packing of the layers.     

Neue Zinn‐reiche Stannide der Systeme AII‐Al‐Sn (AII = Ca,Sr, Ba)

New Tin‐rich Stannides of the Systems A^II‐Al‐Sn (A^II = Ca, Sr, Ba) Four new tin‐rich intermetallics of the ternary systems Ca/Sr/Ba‐Al‐Sn were synthesized from stoichiometric amounts of the elements at maximum temperatures of 1200 °C. Their crystal structures, representing two new types, have been determined using single crystal x‐ray diffraction. Close to the 1:1 composition, the structures of the two isotypic compounds A₁₈[Al₄(Al/Sn)₂Sn₄][Sn₄][Sn]₂ (overall composition A₉M₈; A = Sr/Ba, tetragonal, space group P4/mbm, a = 1325.9(1)/1378.6(1), c = 1272.8(2)/1305.4(1) pm, Z = 4, R1 = 0.0430/0.0293) contain three different anionic Sn/Al building units: Isolated Sn atoms (motif I) coordinated by the alkaline earth cations only (comparable to Ca₂Sn), linear Sn chains (II), which are comparable to the anions in trielides related to the W₅Si₃ structure type and finally octahedral clusters [Al₄M₂Sn₄] (III), composed of four Al atoms forming the center plane, two statistically occupied Al/Sn atoms at the apexes and four exohedral Sn attached to Al. Close to the AM₂ composition, two isotypic tin‐rich intermetallics A₉[Al₃Sn₂][(Sn/Al)₄]Sn₆ (overall composition A₉M₁₅; A = Ca/Sr; space group C2/m, a = 2175.2(1)/2231.0(2), b = 1210.8(1)/1247.0(1), c = 1007.4(1)/1042.0(2) pm, β = 103.38(1)/103.42(1)°, Z = 2, R1 = 0.0541/0.0378) are formed. Their structure is best described as a complex three‐dimensional network, that can be considered to consist of the building units of the binary border phases too, i.e. linear zig‐zag chains of Sn (motif I) like in CaSn, ladders of four‐bonded Sn/Al atoms (II) like in SrAl₂ and trigonal‐bipyramidal clusters [Al₃Sn₂] (III) also present in Ba₃Al₅. Despite the complex structures, some statistically occupied Al/Sn positions and the small disorder of one building unit, the bonding in both structure types can be interpreted using the Zintl concept and Wade's electron counting rules when taking partial Sn‐Sn bonds into account.     

Monoclinic superstructure of SrMgF4 with perovskite‐type slabs

Crystals of Ce‐doped SrMgF₄, strontium magnesium tetrafluoride, have been found to have a monoclinic P2₁ structure with doubled a and tripled c cell lengths compared with the orthorhombic Cmcm structure previously reported in the literature. The perovskite‐type slabs, composed of corner‐sharing MgF₆ octahedra and Sr atoms, are stacked along the b axis. The six crystallographically independent MgF₆ octahedra are rotated so as to provide long periodicities along a and c . The coordination numbers and bond distances around the six crystallographically independent Sr atoms are slightly different in each case. In the superstructure, the Sr atoms lie on local mirror planes which are thought to originate at the high‐temperature phase transition.     

A new oxyfluorotellurate(IV), InTe2O5F

A new oxyfluorotellurate(IV), indium fluoridopentaoxidotellurate(IV), InTe₂O₅F, has been synthesized by solid‐state reaction and structurally characterized. The crystal structure consists of a three‐dimensional framework formed by InO₄F₂ octahedra and Te₂O₅ units. The InO₄F₂ octahedra are linked through the F atoms, which lie on twofold axes, giving rise to helical chains. These helical chains are connected via the Te₂O₅ units. The helical chains of indium octahedra surround cavities, into which the lone pairs of electrons of the Te atoms point.     

The channel structure of the mercury(II) selenite(IV) oxide hydrate HgSeO3·HgO·H2O

The main building units of the title compound, dimercury(II) selenite(IV) oxide hydrate, are strongly distorted [Hg1O₆] and [Hg2O₇] polyhedra, and a pyramidal Se^IVO₃ group. Slightly corrugated hexagonal rings made up of six [Hg1O₆] octahedra spread parallel to the ab plane and are connected via [Hg2O₇] polyhedra parallel and perpendicular to this direction, which results in a three‐dimensional arrangement with channels propagating parallel to the c axis. The Se^IVO₃ groups are situated below and above the rings and bridge both types of Hg atoms. The non‐bonding orbitals are stereochemically active and protrude into the channels of the three‐dimensional network. Additional water mol­ecules are located at the centres of the channels and show weak interactions with the Se^IV lone pairs and the O atoms of the Se^IVO₃ groups.     

About SnF2 stannous fluoride. I. Crystallochemistry of α-SnF2

The crystal structure of monoclinic stannous fluoride α-SnF₂ has been refined from single-crystal X-ray data. The unit cell contains four cyclic Sn₄F₈ tetramers. The structure contains two types of Sn atoms: Sn(1) is surrounded tetrahedrally by three fluorine atoms and a lone pair, E, and Sn(2) is surrounded octahedrally by five fluorine atoms and a lone pair. The structure is examined within the framework of Galy's and Brown's models. Topological relationships to rutile are presented.     

Die Kristallstruktur von SrCaCrF7

The Crystal Structure of SrCaCrF₇ By solid state reaction of the component fluorides at elevated temperature single crystals of SrCaCrF₇ were obtained (a = 792.3(2), b = 724.7(2), c = 986.1(2) pm; space group Pnma, Z = 4). The X‐ray structure determination confirmed isotypism with the Ca₂AlF₇ type of structure: Isolated octahedra [CrF₆]^3— (mean Cr‐F: 189.7 pm) are opposed by infinitely netted planar cations [SrFCa_2/2]³⁺ which contain “independent” F atoms 3‐coordinated by alkaline earth atoms only. The Sr atoms prefer (at a level of 80%) the 8‐fold, the Ca atoms the 7‐fold coordinated positions between the octahedra.     

Eine neue Kolorierungsvariante des α‐Mn‐Strukturtyps mit Hauptgruppenelementen in K5Pb24 – Kristallstruktur,Supraleitung und Struktur‐Eigenschafts‐Beziehung

Novel Coloring of the α‐Mn Structure Type with Main Group Elements in K₅Pb₂₄ – Crystal Structure, Superconductivity, and Structure Property Relationship K₅Pb₂₄ was synthesized from the elements in a welded niobium ampoule at 800 °C. The crystal structure was determined from X‐ray single crystal data. Space group I 4 3m, a = 12.358(1) Å, Z = 2, Pearson symbol cI58. The structure of K₅Pb₂₄ shows an ordered atomic distribution on the four crystallographic sites of the α‐Mn structure type. The aristotype is decomposed into cluster units consisting of 48 Pb atoms. The structural subunits are built from four 16‐vertex Frank Kasper polyhedra, which consist of 15 Pb and one K atom (K1). The 16‐vertex polyhedra are centered with another K atom (K2). Four such polyhedra share a common corner (K1) and several edges. 18 shared edges form a truncated tetrahedra of twelve Pb atoms. These atoms form together with four K1 atoms (located in the center of the Frank Kasper polyhedra) a Friauf polyhedra. The result is a ‘supratetrahedra‘ of 48 Pb atoms enclosing five K atoms. The body centered arrangement of this units results in a three‐dimensional framework of Pb atoms. The title compound is the lead‐richest phase of the K/Pb system. Superconducting properties are observed from temperature dependent susceptibility measurements. Field dependent measurements reveal a hard type II superconductor. LMTO and EH band structure calculations verify the metallic behavior. An analysis of the density of states with the help of the electron localization function (ELF) shows the presence of lone pairs in this intermetallic phase. The role of lone pairs is discussed with respect to the superconducting property.     

Die Kristallstruktur von SCl3[Re2Cl9] und ihre Verwandtschaft zum RuBr3‐Typ

The Crystal Structure of SCl₃[Re₂Cl₉] and its Relation to the RuBr₃ Type SCl₃[Re₂Cl₉] was obtained from the reaction of rhenium and SCl₂ at 400 °C. The X‐ray crystal structure determination revealed a monoclinic structure, a = 834.1 pm, b = 1053.3 pm, c = 866.1 pm, β = 91.90°, space group P2₁/m, R₁ = 0.058. The SCl₃⁺ and Re₂Cl₉^– ions have the known structures; the ReRe bond length in the face‐sharing bioctahedron is 272.2 pm. The crystal packing can be derived from the RuBr₃ structure type, which has infinite columns of face‐sharing octahedra; one quarter of the metal atoms are removed and another quarter are replaced by sulfur atoms. The chlorine atoms form a slightly distorted hexagonal closest‐packing. The symmetry relationships are shown in a family tree of group–subgroup relations.     

GeTe2O6, a germanium tellurate(IV) with an open framework

The structure of an already evidenced but still uncharacterized GeTe₂O₆ phase consists of isolated GeO₆ octahedra connected via isolated TeO₃ units. The germanium cations occupy a site with symmetry. The Te and O atoms are in general positions of the P2₁/n space group. This structure corresponds to a new type of tetravalent tellurate and is different from other AB₂X₆ structures in which the B cation presents a stereochemically active electronic lone pair. It derives from the pseudo‐hexagonal MI₂O₆ (M = Mg, Mn, Co and Fe) type by a strong monoclinic distortion caused by the much smaller size of Ge⁴⁺ compared with the divalent M cations.     

Nickel vanadium tellurium oxide,NiV2Te2O10

Single crystals of nickel(II) divanadium(V) ditellurium(IV) decaoxide, NiV₂Te₂O₁₀, were synthesized via a transport reaction in sealed evacuated silica tubes. The compound crystallizes in the triclinic system (space group P). The Ni atoms are positioned in the 1c position on the inversion centre, while the V and Te atoms are in general positions 2i. The crystal structure is layered, the building units within a (010) layer being distorted VO₆ octahedra and NiO₆ octahedra. The metal–oxide layers are connected by distorted TeO₄E square pyramids (E being the 5s² lone electron pair of Te^IV) to form the framework. The structure contains corner‐sharing NiO₆ octahedra, corner‐ and edge‐sharing TeO₄E square pyramids, and corner‐ and edge‐sharing VO₆ octahedra. NiV₂Te₂O₁₀ is the first oxide containing all of the cations Ni^II, V^V and Te^IV.     

Dimorphic ErAgSn and TmAgSn – High‐Pressure and High‐Temperature Driven Phase Transitions

The stannides ErAgSn and TmAgSn have been investigated under high‐temperature (HT) and high‐pressure (HP) conditions in order to investigate their structural chemistry. ErAgSn and TmAgSn are dimorphic: normal‐pressure (NP) ErAgSn and HT‐TmAgSn crystallize into the NdPtSb type structure, P6₃mc, a = 466.3(1), c = 729.0(2) pm for NP‐ErAgSn and a = 465.4(1), c = 726.6(2) pm for HT‐TmAgSn. NP‐ErAgSn was obtained via arc‐melting of the elements and subsequent annealing at 970 K, while HT‐TmAgSn crystallized directly from the melt by rapidly quenching the arc‐melted sample. HT‐TmAgSn transforms to the ZrNiAl type low‐temperature modification upon annealing at 970 K. The high‐pressure (HP) modification of ErAgSn was synthesized under multianvil high‐pressure (11.5 GPa) high‐temperature (1420 K) conditions from NP‐ErAgSn: ZrNiAl type, , a = 728.7(2), c = 445.6(1) pm. The silver and tin atoms in NP‐ErAgSn and HT‐TmAgSn build up two‐dimensional, puckered [Ag₃Sn₃] networks (277 pm intralayer Ag–Sn distance in NP‐ErAgSn) that are charge‐balanced and separated by the erbium and thulium atoms. The fourth neighbor in the adjacent layer has a longer Ag–Sn distance of 298 pm. The [AgSn] network in HP‐ErAgSn is three‐dimensional. Each silver atom has four tin neighbors (281–285 pm Ag–Sn). The [AgSn] network leaves distorted hexagonal channels, which are filled with the erbium atoms. The crystal chemistry of the three phases is discussed.     

MgNB9, a new magnesium nitridoboride

The structure of a new magnesium nitridoboride, MgNB₉, has been refined from single‐crystal X‐ray data. The Mg and N atoms lie on sites with crystallographic 3m symmetry. The structure consists of two layers alternating along the c axis. The NB₆ layer, with B₁₂ icosahedra, has the C₂B₁₃ structure type. Within this layer, boron icosahedra are bonded to N atoms, each coordinating to three boron polyhedra. Another MgB₃ layer, with B₆ octahedra, does not belong to any known structure type. The boron icosahedra and octahedra are connected to each other, thus forming a three‐dimensional boron framework.     

Crystal Structures of MBi2Br7 (M = Rb,Cs) – Filled Variants of AX7 Sphere Packing

The reinvestigation of the pseudo‐binary systems MBr–BiBr₃ (M = Rb, Cs) revealed two new phases with composition MBi₂Br₇. Both compounds are hygroscopic and show brilliant yellow color. The crystal structures were solved from X‐ray single crystal diffraction data. The isostructural compounds adopt a new structure type in the triclinic space group P$\bar{1}$ . The lattice parameters are a = 755.68(3) pm, b = 952.56(3) pm, c = 1044.00(4) pm, α = 76.400(2)°, β = 84.590(2)°, γ = 76.652(2)° for RbBi₂Br₇ and a = 758.71(5) pm, b = 958.23(7) pm, c = 1060.24(7) pm, α = 76.194(3)°, β = 83.844(4)°, γ = 76.338(3)° for CsBi₂Br₇. The crystal structures consist of M⁺ cations in anticuboctahedral coordination by bromide ions and bromidobismuthate(III) layers ²_∞[Bi₂Br₇]^–. The 2D layers comprise pairs of BiBr₆ octahedra sharing a common edge. The Bi₂Br₁₀ double octahedra are further connected by common vertices. The bismuth(III) atoms increase their mutual distance in the double octahedra by off‐centering so that the BiBr₆ octahedra are distorted. The CsBi₂Br₇ type can be interpreted as a common hexagonal close sphere packing of M and Br atoms, in which 1/4 of the octahedral voids are filled by Bi atoms. The structure type was systematically analyzed and compared with alternative types of common packings. The existence of a compound with the suggested composition CsBiBr₄ could not be verified experimentally.     

Tm2.22Co6Sn20 and TmLi2Co6Sn20 stannides as disordered derivatives of the Cr23C6 structure type

A new ternary dithulium hexacobalt icosastannide, Tm_2.22Co₆Sn₂₀, and a new quaternary thulium dilithium hexacobalt icosastannide, TmLi₂Co₆Sn₂₀, crystallize as disordered variants of the binary cubic Cr₂₃C₆ structure type (cF116). 48 Sn atoms occupy sites of m.m2 symmetry, 32 Sn atoms sites of .3m symmetry, 24 Co atoms sites of 4m.m symmetry, eight Li (or Tm in the case of the ternary phase) atoms sites of symmetry and four Tm atoms sites of symmetry. The environment of one Tm atom is an 18‐vertex polyhedron and that of the second Tm (or Li) atom is a 16‐vertex polyhedron. Tetragonal antiprismatic coordination is observed for the Co atoms. Two Sn atoms are enclosed in a heavily deformed bicapped hexagonal prism and a monocapped hexagonal prism, respectively, and the environment of the third Sn atom is a 12‐vertex polyhedron. The electronic structures of both title compounds were calculated using the tight‐binding linear muffin‐tin orbital method in the atomic spheres approximation (TB–LMTO–ASA). Metallic bonding is dominant in these compounds, but the presence of Sn—Sn covalent dumbbells is also observed.     

Die Wechselwirkungen freier Elektronenpaare in Zintl-Phasen: Bandstruktur und Realraumanalyse des cP124 Clathrat-Strukturtyps

Lone Pair Interactions in Zintl Phases: Band Structure and Real Space Analysis of the cP 124 Clathrate Structure Type The compounds Ba₆In₄Ge₂₁, K₆Sn₂₃Bi₂ and K₆Sn₂₅ adopt the cubic clathrate structure type cP124. The only building block of this structure type is a distorted pentagonal dodecahedron of E-atoms (E = In/Ge, Sn/Bi, Sn), which builds up a zeolite-like framework. Each pentagonal dodecahedron is connected to four others through three common faces and one external bond. The polyhedra, as well as resulting cavities and channels of the framework are occupied with electropositive atoms. As a result 17 out of 25 framework atoms are four-connected and in the ternary Zintl-phases Ba₆In₄Ge₂₁ and K₆Sn₂₃Bi₂ lone pairs are localized at all three-connected atoms. Tight binding calculations of the Extended Hückel type show a further band gap in the density of states of the stannides below the Fermi level of K₆Sn₂₃Bi₂, thus giving an explanation for the stability of the electron deficient phase K₆Sn₂₅. Further analyses of the electronic structure using the Electron Localization Function (ELF) and Partial Electron Densities (PED) reveal that the destabilization of bands at the Fermi edge arises from interactions of non-bonding (lone) electron pairs. A remarkable cluster of eight lone pairs is established by the real space representations ELF and PED. The lone pairs are located at the corners inside of a distorted cubic cavity of three-connected framework atoms, each belonging to one pentagonal dodecahedron. The closeness of eight lone pairs leads to the destabilization of one orbital (band). Thus K₆Sn₂₅ can be described as Zintl phase with a two electron deficiency. In Ba₆In₄Ge₂₁ weaker interactions are expected.     

LiBC3: a new borocarbide based on graphene and heterographene networks

Li–B–C alloys have attracted much interest because of their potential use in lithium‐ion batteries and superconducting materials. The formation of the new compound LiBC₃ [lithium boron tricarbide; own structure type, space group P m 2, a = 2.5408 (3) Å and c = 7.5989 (9) Å] has been revealed and belongs to the graphite‐like structure family. The crystal structure of LiBC₃ presents hexagonal graphene carbon networks, lithium layers and heterographene B/C networks, alternating sequentially along the c axis. According to electronic structure calculations using the tight‐binding linear muffin‐tin orbital‐atomic spheres approximations (TB–LMTO–ASA) method, strong covalent B—C and C—C interactions are established. The coordination polyhedra for the B and C atoms are trigonal prisms and for the Li atoms are hexagonal prisms.     

Crystal Structure of NbOBr2

Black, needle shaped crystals of NbOBr₂ were formed by reduction of NbOBr₃ with InBr at 500 °C. It crystallizes isotypically to NbOI₂ (monoclinic, space group C2, a = 13.833(4), b = 3.9079(6), c = 7.023(2) Å, β = 105.026(10)°). The structure consists of NbO₂Br₄ octahedra, which are connected to layers . Nb atoms of edge sharing octahedra form Nb₂ pairs with Nb–Nb distances of 3.144(5) Å. Within corner‐sharing octahedra alternating long and short Nb–O bonds are present, causing the polarity of the structure.