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  1. Chemically driven superstructural ordering leading to giant unit cells in unconventional clathrates Cs 8 Zn 18 Sb 28 and Cs 8 Cd 18 Sb 28

    The unconventional clathrates, Cs8Zn18Sb28 and Cs8Cd18Sb28, were synthesized and reinvestigated. These clathrates exhibit unique and extensive superstructural ordering of the clathrate-I structure that was not initially reported. Cs8Cd18Sb28 orders in the Iad space group (no. 230) with 8 times larger volume of the unit cell in which most framework atoms segregate into distinct Cd and Sb sites. The structure of Cs8Zn18Sb28 is much more complicated, with an 18-fold increase of unit cell volume accompanied by significant reduction of symmetry down to P2 (no. 3) monoclinic space group. This structure was revealed by a combination of synchrotron X-ray diffraction and electronmore » microscopy techniques. A full solid solution, Cs8Zn18-xCdxSb28, was also synthesized and characterized. These compounds follow Vegard's law in regard to their primitive unit cell sizes and melting points. Variable temperature in situ synchrotron powder X-ray diffraction was used to study the formation and melting of Cs8Zn18Sb28. Due to the heavy elements comprising clathrate framework and the complex structural ordering, the synthesized clathrates exhibit ultralow thermal conductivities, all under 0.8 W m-1 K-1 at room temperature. Cs8Zn9Cd9Sb28 and Cs8Zn4.5Cd13.5Sb28 both have total thermal conductivities of 0.49 W m-1 K-1 at room temperature, among the lowest reported for any clathrate. Cs8Zn18Sb28 has typical p-type semiconducting charge transport properties, while the remaining clathrates show unusual n–p transitions or sharp increases of thermopower at low temperatures. Estimations of the bandgaps as activation energy for resistivity dependences show an anomalous widening and then shrinking of the bandgap with increasing Cd-content.« less
  2. Tuning Fe–Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl Anions

    A one-dimensional (1D) chain compound (en = ethylenediamine), featuring tetrahedral FeSe2 chains separated by [Fe(en)3]2+ cations and Cl anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe–Se backbone is similar to previously reported compounds with isolated 1D FeSe2 chains. 57Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)3]3(FeSe2)4Cl2 with Fe3+ centers in the [FeSe2]- chains and Fe2+ centers in the [Fe(en)3]2+ complexes. SQUID magnetometry indicates that is paramagnetic with a reduced average effective magnetic moment, μeff = 9.51 μB per formula unit, and a negative Weiss constant, θmore » = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe2]- chains. Furthermore, weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)3]2+ and Cl ions is also capable of stabilizing mixed-valent 2D Fe–Se puckered layers in the crystal structure of [Fe(en)3]4(Fe14Se21)Cl2, where Fe14Se21 layers have a unique topology with large open pores. Property measurements of [Fe(en)3]4(Fe14Se21)Cl2 could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.« less
  3. Clathrate BaNi2P4: An Interplay of Heat and Charge Transport Due to Strong Host–Guest Interactions

    Heat and charge transport properties of the metallic unconventional clathrate BaNi2P4, hosting Ba cations in oversized Ni8P16 cages, are investigated. A novel method of single-crystal growth was developed, yielding 2–3 mm sized crystals of BaNi2P4. We also developed a setup to accurately measure thermal conductivity and electrical resistivity of the synthesized single crystals in a wide temperature range avoiding crystal remounting. BaNi2P4 has a metallic temperature dependence of its electrical resistivity (decreasing with decreasing temperature) and manifests an unconventional T2 power law for 50 K < T < 300 K; below 50 K, the power-law exponent increases gradually such thatmore » below 10 K the power law is T5, a predicted but extremely rarely experimentally observed dependence for peculiar electron–phonon interactions. Electronic band structure calculations, consistent with measurements of de Haas–van Alphen oscillations, show large band dispersions with significant contributions of Ba orbitals to states near the Fermi level, which is atypical for clathrates. The thermal properties of BaNi2P4 were probed using a combination of variable-temperature single-crystal X-ray diffraction experiments, heat capacity measurements, first-principles phonon dispersion calculations, and inelastic neutron scattering measurements. BaNi2P4 exhibits significant hybridization of the Ba-guest and Ni–P-framework vibrational modes, which may be enhanced via the detected split of the Ba position, which results in strong Ba-framework interactions.« less
  4. Crystallographic facet selective HER catalysis: exemplified in FeP and NiP 2 single crystals

    How the crystal structures of ordered transition-metal phosphide catalysts affect the hydrogen-evolution reaction (HER) is investigated by measuring the anisotropic catalytic activities of selected crystallographic facets on large (mm-sized) single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP2). We find that different crystallographic facets exhibit distinct HER activities, in contrast to a commonly held assumption of severe surface restructuring during catalytic activity. Moreover, density-functional-theory-based computational studies show that the observed facet activity correlates well with the H-binding energy to P atoms on specific surface terminations. Direction dependent catalytic properties of two different phosphides with different transition metals, crystal structures, andmore » electronic properties (FeP is a metal, while m-NiP2 is a semiconductor) suggests that the anisotropy of catalytic properties is a common trend for HER phosphide catalysts. This realization opens an additional rational design for highly efficient HER phosphide catalysts, through the growth of nanocrystals with specific exposed facets. Furthermore, the agreement between theory and experimental trends indicates that screening using DFT methods can accelerate the identification of desirable facets, especially for ternary or multinary compounds. The large single-crystal nature of the phosphide electrodes with well-defined surfaces allows for determination of the catalytically important double-layer capacitance of a flat surface, Cdl = 39(2) μF cm-2 for FeP, useful for an accurate calculation of the turnover frequency (TOF). X-ray photoelectron spectroscopy (XPS) studies of the catalytic crystals that were used show the formation of a thin oxide/phosphate overlayer, presumably ex situ due to air-exposure. This layer is easily removed for FeP, revealing a surface of pristine metal phosphide.« less
  5. Synthesis, structure, and transport properties of Ba8Cu16 – xAuxP30 clathrate solid solution

    In this work, a new clathrate solid solution Ba8Cu16 – xAuxP30 (x = 4, 8, 12) was synthesized by a high-temperature solid-state annealing method. The crystal structures of Ba8Cu16 – xAuxP30 were determined by single crystal x-ray diffraction. This clathrate solid solution crystallizes in the orthorhombic superstructure of clathrate-I type with 23 crystallographically independent framework sites, eight of them are occupied by Au/Cu and 15 are exclusively occupied by P atoms. The distribution of Au and Cu atoms over these eight framework sites is not random with a clear preference for Au to occupy the largest (Au/Cu)P4 tetrahedra in themore » framework. The thermal stability and thermoelectric properties of the Ba8Cu16 – xAuxP30 solid solution were evaluated. Low thermal conductivity was achieved for Ba8Cu16 – xAuxP30 due to the combination of the host–guest crystal structure with rattling Ba atoms with the presence of heavy Au atoms and substitutional Cu/Au disorder in the clathrate framework.« less
  6. Synthesis, Crystal Growth, and Transport Properties of van der Waals Tetrel Pnictide GeAs2

    Here, bulk GeAs2 was synthesized utilizing a vapor transport reaction with iodine. Thermal stability tests under dynamic conditions show that GeAs2 decomposes below 700 K, in contrast to the reported congruent melting at 1029 K measured at saturated As vapor pressure. GeAs2 is a p-type narrow bandgap (~0.4 eV) semiconductor. From a thermoelectric standpoint, GeAs2 outperforms previous computational predictions in thermopower and thermal conductivity. Yet, electrical resistivity is significantly higher than predicted values, resulting in the low overall thermoelectric figure of merit. Aliovalent doping strategies for GeAs2 should be developed to achieve reasonable thermoelectric performance.
  7. LiSi 3 As 6 and Li 2 SiAs 2 with flexible SiAs 2 polyanions: synthesis, structure, bonding, and ionic conductivity

    Two novel ternary phases, LiSi3As6 and Li2SiAs2, have been synthesized and characterized. Both phases have an identical Si : As ratio of 1 : 2 providing insight on how layers of the parent phase SiAs2 accommodate excess electrons from Li cations to form Si–As anionic frameworks. LiSi3As6 exhibits a variety of bonding schemes involving Si–Si and As–As bonds, as well as corner-sharing SiAs4 tetrahedra, while Li2SiAs2 is isostructural to the previously reported Li2SiP2, with adamantane-like Si4As10 units connected into 3D framework. LiSi3As6 and Li2SiAs2 are predicted to be indirect semiconductors which was experimentally confirmed by optical properties characterization. Li2SiAs2 exhibitsmore » low thermal conductivity of 1.20 W m–1 K–1 at 300 K in combination with a room temperature ionic conductivity of 7 × 10–6 S cm–1, an order of magnitude greater than that of the phosphide and nitride analogues, indicating its potential as a solid-state Li-ion conductor.« less
  8. Chemical Flexibility of Mg in Pnictide Materials: Structure and Properties Diversity

    Magnesium, element no. 12 on the periodic table, is the second member of the alkaline-earth metal family. Often, Mg is considered as an electropositive metal like its heavier congeners, Ca, Sr, and Ba. In this review, another important aspect of Mg chemistry, namely, the ability to form covalent bonds to more electronegative elements, is considered with a focus on pnictides. Magnesium’s flexible coordination numbers and bond distances are similar to those of main group elements (Al) or late- and post-transition metals (Mn, Cu, Zn, Cd). In this work, selected Mg-pnictides are discussed to emphasize the chemical and structural diversity ofmore » Mg which results in a variety of physical properties. Thermoelectric, Mg-ion battery, and nonlinear optical applications of select Mg-containing compounds are summarized, providing examples on the exploitation of Mg chemical bonding flexibility for the design of novel functional materials.« less
  9. Directing Boron–Phosphorus Bonds in Crystalline Solid: Oxidative Polymerization of P=B=P Monomers into 1D Chains

    Over 20 years after the last inorganic ternary B–P compound was reported, Na2BP2, a new compound containing one-dimensional B–P polyanionic chains has been synthesized. Common high-temperature synthetic methods required for the direct reaction of boron and phosphorus negate the possible formation of metastable or low temperature phases. In this study, oxidative elimination was used to successfully condense 0D BP23– anionic monomers found in a Na3BP2 precursor into unique 1D BP22– chains consisting of five-member B2P3 rings connected by bridging P atoms in the crystal structure of Na2BP2. In conclusion, the synthesis was guided by in situ X-ray powder diffraction studies,more » which revealed the metastable nature of the products of oxidative elimination reactions. Na2BP2 is predicted to be an electron balanced semiconductor which was confirmed by UV–vis spectroscopy with the experimentally determined band gap of 1.1 eV.« less
  10. Chemical and Electrochemical Lithiation of van der Waals Tetrel-Arsenides

    Lithiation of van der Waals tetrel–arsenides, GeAs and SiAs, has been researched. Electrochemical lithiation demonstrated large initial capacities of over 950 mAh g-1 accompanied by rapid fading over successive cycling in the voltage range 0.01–2 V. Limiting the voltage range to 0.5–2 V achieved more stable cycling, which was attributed to the intercalation process with lower capacities. Ex situ powder X–ray diffraction confirmed complete amorphization of the samples after lithiation, as well as recrystallization of the binary tetrel–arsenide phases after full delithiation in the voltage range 0.5–2 V. Solid–state synthetic methods produce layered phases, in which Si–As or Ge–As layersmore » are separated by Li cations. The first layered compounds in the corresponding ternary systems were discovered, Li0.9Ge2.9As3.1 and Li3Si7As8, which crystallize in the Pbam (No. 55) and P2/m (No. 10) space groups, respectively. Semiconducting layered GeAs and SiAs accommodate the extra charge from Li cations through structural rearrangement in the Si–As or Ge–As layers and eventually by replacement of the tetrel dumbbells with sets of Li atoms. Ge and Si monoarsenides reflected high structural flexibility and a mild ability for reversible lithiation.« less
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