Deciphering the Polymorphism of CaSi2: The Influence of Heat and Composition.

The Zintl phase CaSi2 is a layered compound with stacking variants known as 1P, 3R, and 6R. We extend the series by the 21R polytype formed by rapid cooling of the melt. The crystal structure of 21R-CaSi2 (space group R3̅m) was derived from HRTEM images, and the atomic positions were optimized by using the FPLO code (a = 3.868 Å, c = 107.276 Å). We explore polytype transformations by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and thermal analysis. While 6R-CaSi2 is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi2 as a bulk phase.

Electron Diffraction Tomography on Two-Phase Nanolamellae of Topochemically Synthesized Cu(Sb2S3)Cl.

The dark red semiconductor Cu(Sb2S3)Cl was obtained by leaching the layered precursor Cu(Sb2S3)[AlCl4] in a 0.1 M aqueous HCl solution. The selective extraction of AlCl3 yielded a mica-like lamellar product of poor crystallinity. Misalignment of lamellae down to the nanoscale prevented structure determination by conventional single-crystal X-ray diffraction, but a combination of transmission electron microscopy, selected area electron diffraction, and selected area electron precession diffraction tomography on a nanoscale spot with largely ordered crystalline lamellae revealed the crystal structures of two intergrown modifications. Orthorhombic o-Cu(Sb2S3)Cl and monoclinic m-Cu(Sb2S3)Cl have similar layers to the precursor and differ only in the stacking of the layers. These consist of uncharged Sb2S3 strands, whose sulfide ions, together with chloride ions, coordinate the copper(I) cations. Only one chloride ion remained from the [AlCl4]- group. DFT calculations confirm the structure solution for the orthorhombic form and suggest that the monoclinic structure is metastable against transformation to o-Cu(Sb2S3)Cl.

Ge32 Co9-x : Creating "Empty" Space by High Pressure.

The compound Ge32 Co9-x (x=0.54(6), a=10.9861(3) Å, space group Im 3 ‾ $\bar 3$ m) prepared under high pressure and at high temperature is metastable under ambient conditions. It crystallizes in a new structure type, Pearson symbol cI82-1.08. The crystal structure represents a slightly distorted cubic primitive arrangement of germanium atoms with part of the Ge cubes filled by cobalt. Analysis of the chemical bonding by real-space methods revealed three-core cluster units Ge16 Co3 and seemingly empty regions comprising either covalent inter-polyhedral Ge-Ge bonds or lone-pairs located at the germanium atoms. The electrical conductivity is metal-like.

A Borosilicide with Clathrate VIII Structure.

The high-pressure phase Na8BxSi46-x (3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na8B4Si42 (space group I43̅m; a = 9.7187(2) Å; Pearson symbol cI54) were obtained at 5-8 GPa and 1200 K. The clathrate I modification exists for the same composition at lower pressure with a larger cell volume (Pm3̅n; a = 9. 977(2) Å; cP54). Profound structural adaptions allow for a higher density of the clathrate VIII type than clathrate I, opening up the perspective of obtaining clathrate VIII type compounds as high-pressure forms of clathrate I.

Local magnetism in MnSiPt rules the chemical bond.

Among intermetallic compounds, ternary phases with the simple stoichiometric ratio 1:1:1 form one of the largest families. More than 15 structural patterns have been observed for several hundred compounds constituting this group. This, on first glance unexpected, finding is a consequence of the complex mechanism of chemical bonding in intermetallic structures, allowing for large diversity. Their formation process can be understood based on a hierarchy of energy scales: The main share is contributed by covalent and ionic interactions in accordance with the electronic needs of the participating elements. However, smaller additional atomic interactions may still tip the scales. Here, we demonstrate that the local spin polarization of paramagnetic manganese in the new compound MnSiPt rules the adopted TiNiSi-type crystal structure. Combining a thorough experimental characterization with a theoretical analysis of the energy landscape and the chemical bonding of MnSiPt, we show that the paramagnetism of the Mn atoms suppresses the formation of Mn-Mn bonds, deciding between competing crystal structures.

In-Cage Interactions in the Clathrate Superconductor Sr8 Si46.

The clathrate I superconductor Sr8 Si46 is obtained under high-pressure high-temperature conditions, at 5 GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heating at T=796(5) K into Si and SrSi2 . The crystal structure of the clathrate is isotypic to that of Na8 Si46 . Chemical bonding analysis reveals conventional covalent bonding within the silicon network as well as additional multi-atomic interactions between Sr and Si within the framework cages. Physical measurements indicate a bulk BCS type II superconducting state below Tc =3.8(3) K.

Unconventional Metal-Framework Interaction in MgSi5.

The silicon-rich cage compound MgSi5 was obtained by high-pressure high-temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X-ray diffraction experiments. The crystal structure of MgSi5 (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four-bonded silicon atoms forming Si15 cages enclosing the Mg atoms. Two types of smaller Si8 cages remain empty. The atomic interactions are characterized by two-center two-electron bonds within the silicon framework. In addition, there is evidence for multi-center Mg-Si bonding in the large cavities of the framework and for lone-pair-like interactions in the smaller empty voids.

Classical and nonclassical germanium environments in high-pressure BaGe5.

A new crystalline form of BaGe(5) was obtained at a pressure of 15(2) GPa in the temperature range from 1000(100) to 1200(120) K. Single-crystal electron and powder X-ray diffraction patterns indicate a body-centered orthorhombic structure (space group Imma, Pearson notation oI24) with unit cell parameters a = 8.3421(8) Å, b = 4.8728(5) Å, and c = 13.7202(9) Å. The crystal structure of hp-BaGe(5) consists of four-bonded Ge atoms forming complex layers with Ge-Ge contacts between 2.560(6) and 2.684(3) Å; the Ba atoms are coordinated by 15 Ge neighbors in the range from 3.341(6) to 3.739(4) Å. Analysis of the chemical bonding using quantum chemical techniques in real space reveal charge transfer from the Ba cations to the anionic Ge species. Ge atoms having nearly tetrahedral environments show an electron-localizability-based oxidation number close to 0; the four-bonded Ge atoms with a Ψ-pyramidal environment adopt a value close to 1-. In agreement with the calculated electronic density of states, the compound is a metallic conductor (electrical resistivity of ca. 240 µΩ cm at 300 K), and magnetic susceptibility measurements evidence diamagnetic behavior with χ(0) = -95 × 10(-6) emu mol(-1).

New monoclinic phase at the composition Cu2SnSe3 and its thermoelectric properties.

A new monoclinic phase (m2) of ternary diamond-like compound Cu2SnSe3 was synthesized by reaction of the elements at 850 K. The crystal structure of m2-Cu2SnSe3 was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group Cc, a = 6.9714(2) Å, b = 12.0787(5) Å, c = 13.3935(5) Å, ß = 99.865(5)°, Z = 8). Thermal analysis and annealing experiments suggest that m2-Cu2SnSe3 is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, m2-Cu2SnSe3 is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu-Se and Sn-Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit ZT reaches a maximum value of 0.33 at 650 K.

ZSM-5 zeolite single crystals with b-axis-aligned mesoporous channels as an efficient catalyst for conversion of bulky organic molecules.

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with b-axis-aligned mesopores, synthesized using a designed cationicamphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the b-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The b-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications.

Mimicking the growth of a pathologic biomineral: shape development and structures of calcium oxalate dihydrate in the presence of polyacrylic acid.

The morphogenesis of calcium oxalate hydrates in aqueous solutions was investigated by varying the pH, oxalate concentration, and the concentration of the sodium salt of polyacrylate (PAA). With increasing amounts of PAA in solution, the shape of tetragonal calcium oxalate dihydrate (COD) changes from bipyramidal through elongated bipyramidal prisms to dumbbells and finally reverts to rodlike tetragonal bipyramidal prisms. PAA is incorporated into the prismatic zones of the growing COD crystals, thereby reducing the growth rate of the {100} faces along the <100> direction. Dumbbells start to develop through "non-crystallographic" branching from the prism faces and the formation of "multiple head" crystals. Adsorption of PAA on the rough surfaces of the splitting individuals supports the selection of new subindividuals and leads to the formation of core-shell patterns. The various shapes and structures of the biomimetic COD/PAA crystals and aggregates are closely related to the well-known "pathologic" individuals observed in the urine of patients with urinary disease (including urinary stones).

Homo- and heterovalent substitutions in the new clathrates I Si30P16Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x): synthesis, crystal structure, and thermoelectric properties.

The new cationic clathrates I Si(30)P(16)Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x) were synthesized by the standard ampule technique. The Si(30)P(16)Te(8-x)Se(x) (x = 0-2.3) clathrates crystallize in the cubic space group Pm3̅n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si(30+x)P(16-x)Te(8-x)Br(x) (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (x = 1-3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm3n to Pm3. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10(-6) to 1.0 × 10(-5) K(-1) in the temperature range of 298-1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (ZT = 0.1) was achieved for the Br-bearing clathrate Si(32.1(2))P(13.9(2))Te(6.6(2))Br(1.0(1)) at 750 K.

Superconductivity of structurally disordered Y5Ir6Sn18.

The structural and physical properties of Y5Ir6Sn18 grown from Sn-flux as large single crystals are studied. Y5Ir6Sn18 crystallizes with a unique structure [space group Fm3̄m, a = 13.7706(1) Å], which is characterized by a strong disorder. A transmission electron microscopy (TEM) study indicated that the structural model of Y5Ir6Sn18 obtained from X-ray diffraction methods is an average description of a complex intergrowth of domains with different structural arrangements. The studied stannide is a type-II superconductor with a critical temperature Tc = 2.1 K, a rather weak electron-phonon coupling and conventional s-wave BCS-like mechanisms. Performed theoretical electronic band structure calculations indicated the inconsistency of an idealized structural model earlier reported for Y5Ir6Sn18.

Composition dependent polymorphism and superconductivity in Y3+x{Rh,Ir}4Ge13-x.

Polymorphism is observed in the Y3+xRh4Ge13-x series. The decrease of Y-content leads to the transformation of the primitive cubic Y3.6Rh4Ge12.4 [x = 0.6, space group Pm3̄n, a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb3Rh4Sn13 Remeika prototype, into a body-centred cubic structure [La3Rh4Sn13 structure type, space group I4132, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu3Ir4Ge13 structure type, space group I41/amd, a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric (i.e. x = 0) Y3Rh4Ge13. Analogous symmetry lowering is found within the Y3+xIr4Ge13-x series, where the compound with Y-content x = 0.6 is crystallizing with La3Rh4Sn13 structure type [a = 17.90833(8) Å] and the stoichiometric Y3Ir4Ge13 is isostructural with the Rh-analogue [a = 17.89411(9) Å, a = 17.9353(1) Å]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y3+xRh4Ge13-x series are found to be weakly-coupled BCS-like superconductors with Tc = 1.25, 0.43 and 0.6, for x = 0.6, 0.4 and 0, respectively. They also reveal low thermal conductivity (<1.5 W K-1 m-1 in the temperature range 1.8-350 K) and small Seebeck coefficients. The latter are common for metallic systems. Y3Rh4Ge13 undergoes a first-order phase transition at Tf = 177 K, with signatures compatible to a charge density wave scenario. The electronic structure calculations confirm the instability of the idealized Yb3Rh4Sn13-like structural arrangements for Y3Rh4Ge13 and Y3Ir4Ge13.

The Intermetallic Semiconductor ht-IrGa3: a Material in the in-Transformation State.

The compound IrGa3 was synthesized by direct reaction of the elements. It is formed as a high-temperature phase in the Ir-Ga system. Single-crystal X-ray diffraction analysis confirms the tetragonal symmetry (space group P42 /mnm, No. 136) with a = 6.4623(1) Å and c = 6.5688(2) Å and reveals strong disorder in the crystal structure, reflected in the huge values and anisotropy of the atomic displacement parameters. A model for the real crystal structure of ht-IrGa3 is derived by the split-position approach from the single-crystal X-ray diffraction data and confirmed by an atomic-resolution transmission electron microscopy study. Temperature-dependent electrical resistivity measurements evidence semiconductor behavior with a band gap of 30 meV. A thermoelectric characterization was performed for ht-IrGa3 and for the solid solution IrGa3-x Zn x .

Synthesis and electron holography studies of single crystalline nanostructures of clathrate-II phases K(x)Ge136 and Na(x)Si136.

Crystalline nanosized particles of clathrate-II phases K(x)Ge(136) and Na(x)Si(136) were obtained from a dispersion of alkali metal tetrelides in ionic liquids based on DTAC/AlCl(3), which were slowly heated to 120-180 °C. The nanoparticles are bullet-shaped with typical dimensions of about 40 nm in width and 140-200 nm in length. Detailed structure investigations using high-resolution transmission electron microscopy (HRTEM) and electron holography reveal the crystallinity and dense morphology of the clathrate nanorods.

BaGe(5): a new type of intermetallic clathrate.

BaGe(5) constitutes a new type of intermetallic clathrate obtained by decomposition of clathrate-I Ba(8)Ge(43)(3) at low temperatures. The crystal structure consists of characteristic layers interconnected by covalent bonds. BaGe(5) is a semiconducting Zintl phase.

Crystal structure, phase transition and properties of indium(III) sulfide.

Poly- and single-crystalline samples of In0.67□0.33In2S4 thiospinel were obtained by various powder metallurgical and chemical vapor transport methods, respectively. All synthesized samples contained ß-In0.67□0.33In2S4 modification only, independent of the synthesis procedure. High-resolution powder X-ray diffraction (PXRD) experiments at 80 K enabled the observation of split tetragonal reflections (completely overlapped at room temperature), which prove the correctness of the crystal structure model accepted for the ß-polymorph. Combining single-crystal XRD, transmission electron microscopy and selected-area electron diffraction studies, the presence of three twin domains in the as-grown crystals was confirmed. A high temperature PXRD study revealed both abrupt (in full widths at half maxima of main reflections and in unit-cell volume) and gradual (in intensity of satellites and c/a ratio) changes in the vicinity of the α-ß phase transition. These observations, together with a clear endothermic peak in the heat capacity, the magnitude of enthalpy/entropy change and the temperature dependence of electrical resistivity (associated with hysteresis), hinted towards the 1st order type of transition. Three scenarios, based on Rietveld refinement analysis, were considered for the description of the crystal structure evolution from ß- to α-modification, including the (3+3)D-modulated cubic structure at 693 K as an intermediate state during the ß-α transformation. The Seebeck coefficient, electrical resistivity and thermal conductivity were not only influenced by phase transition, but also by annealing conditions (S-poor or S-rich atmosphere). Density functional theory calculations predicted semiconducting behavior of In0.67□0.33In2S4, as well as instability of the fictitious InIn2S4 thiospinel.

Indium thiospinel In1-x□xIn2S4- structural characterization and thermoelectric properties.

A detailed study of polycrystalline indium-based In1-x□xIn2S4 (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented (□- vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase α- and ß-In1-x□xIn2S4 were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In2S3), respectively. In contrast, it is shown that In0.72□0.28In2S4 contains both α- and ß-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and ß-In1-x□xIn2S4 is conducted for the first time. α-In1-x□xIn2S4 (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|µV K-1) and moderate charge carrier mobility on the level of ∼20 cm2 V-1 s-1 at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m-1 K-1 at 760 K) for larger In-content are observed. Although ß-In0.67□0.33In2S4 (ß-In2S3) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼104 cm2 V-1 s-1 at RT). These findings indicate that structural disorder in the α-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZTmax = 0.2 at 760 K is nearly the same for both α- and ß-In1-x□xIn2S4.

Compositional evolution of the NaZn13 structure motif in the systems La-Ni-Ga and Ce-Ni-Ga.

Phase relationship and structural behaviour in the substitutional series LaNi13-xGax and CeNi13-xGax have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi13-xGax resulting in five varieties of the NaZn13 structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi8.5Si4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi7In6 structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn13, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi13-xGax shows similar behaviour. The corresponding tI56-I ↔oI56 ↔oF112 ↔tI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi13-xGax samples show paramagnetic behavior, whereas the CeNi13-xGax series exhibits Curie-Weiss paramagnetism.