Direct Band Gap Semiconductors with Two- and Three-Dimensional Triel-Phosphide Frameworks (Triel=Al, Ga, In).

Recently, several ternary phosphidotrielates and -tetrelates have been investigated with respect to their very good ionic conductivity, while less focus was pointed towards their electronic structures. Here, we report on a novel series of compounds, in which several members possess direct band gaps. We investigated the known compounds Li3AlP2, Li3GaP2, Li3InP2, and Na3InP2 and describe the synthesis and the crystal structure of novel Na3In2P3. For all mentioned phosphidotrielates reflectance UV-Vis measurements reveal direct band gaps in the visible light region with decreasing band gaps in the series: Li3AlP2 (2.45 eV), Li3GaP2 (2.18 eV), Li3InP2 (1.99 eV), Na3InP2 (1.37 eV), and Na3In2P3 (1.27 eV). All direct band gaps are confirmed by quantum chemical calculations. The unexpected property occurs despite different structure types. As a common feature all compounds contain EP4 tetrahedra, which share exclusively vertices for E=In and vertices as well as edges for E=Al and Ga. The structure of the novel Na3In2P3 is built up by a polyanionic framework of six-membered rings of corner-sharing InP4 tetrahedra. As a result, the newly designed semiconductors with direct band gaps are suitable for optoelectronic applications, and they can provide significant guidance for the design of new functional semiconductors.

Further Insights into the Chemical Synthesis of F2 and on Drying moist HF.

It is well established that solid K2MnF6 reacts with excess SbF5 forming elemental F2. However, if the reaction is carried out in anhydrous HF (aHF) as a solvent, this is surprisingly not the case. Instead, the green Mn(IV) compound K3[(MnIVF)(SbF6)5]F is obtained. The reductive elimination of F2 was not observed under the applied conditions. The compound was characterized by its crystal structure, by Raman spectroscopy, and by quantum-chemical solid-state calculations. It crystallizes in the monoclinic space group P21/c, mP164, with the lattice parameters a = 12.2393(13), b = 12.167(2), c = 20.115(5) Å, ß = 110.805(8)°, V = 2800.1(9) Å3, Z = 4 at T = 200 K. As the use of strictly anhydrous HF is crucial in this and other similar reactions, methods for drying moist HF are discussed.

Enhancing electrocatalytic activity in metallic thin films through surface segregation of carbon.

Thin layers of commonly used adhesion metals i.e., Cr and Ti were annealed to investigate and estimate their impact on the electrochemical properties of the carbon nanomaterials grown on top of them. The microstructure, surface chemistry, and electrochemical activities of these materials were evaluated and compared with those of as-deposited thin films. The results from X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, grazing incidence X-ray diffraction (GIXRD), time-of-flight elastic recoil detection analysis (TOF-ERDA), and conductive atomic force microscopy (C-AFM) indicated the formation of a catalytic graphite layer on Cr following annealing, while no such layer was formed on Ti. This is attributed to the formation of the Cr2O3 layer on annealed Cr, which acts as a barrier to carbon diffusion into the underlying Cr. Conversely, Ti exhibits a high solubility for both carbon and oxygen, preventing the formation of the graphite layer. Cyclic voltammetry results showed that annealed Cr electrodes are electrochemically active towards both dopamine (DA) and ascorbic acid (AA) while no electrochemical activity is exhibited by annealed Ti. Quantum chemical calculations suggested that the presence of carbon as graphene or an amorphous form is critical for the oxidation reaction of probes. These results are significant for comprehending how the distinct solubilities of typical interstitial solutes influence the microstructure of adhesion metal layers and consequently yield diverse electrochemical properties.

Ba12 [BN2 ]6.67 H4 : A Disordered Anti-Skutterudite filled with Nitridoborate Anions.

Skutterudites are of high interest in current research due to their diversity of structures comprising empty, partially filled and filled variants, mostly based on metallic compounds. We herein present Ba12 [BN2 ]6.67 H4 , forming a non-metallic filled anti-skutterudite. It is accessed in a solid-state ampoule reaction from barium subnitride, boron nitride and barium hydride at 750 °C. Single-crystal X-ray and neutron powder diffraction data allowed to elucidate the structure in the cubic space group Im 3 ‾ ${\bar{3}}$ (no. 204). The barium and hydride atoms form a three-dimensional network consisting of corner-sharing HBa6 octahedra and Ba12 icosahedra. Slightly bent [BN2 ]3- units are located in the icosahedra and the voids in-between. 1 H and 11 B magic angle spinning (MAS) NMR experiments and vibrational spectroscopy further support the structure model. Quantum chemical calculations coincide well with experimental results and provide information about the electronic structure of Ba12 [BN2 ]6.67 H4 .

[(µ3 -F)(BrF5 )3 ]- - An Unprecedented Molecular Fluoridobromate(V) Anion in Cs[Br3 F16 ].

The reaction of Cs[BrF6 ] with BrF5 gave the compound Cs[Br3 F16 ] with the unprecedented propeller-shaped, C3 -symmetric [(µ3 -F)(BrF5 )3 ]- anion. All other currently known fluoridobromates(V) contain only octahedral [BrF6 ]- anions, which, unlike the related [IF6 ]- anions, never exhibited stereochemical activity of the lone pair on the Br atoms. Despite the same coordination number of six for the Br atom in the [BrF6 ]- and [(µ3 -F)(BrF5 )3 ]- anions, the longer µ3 -F-Br bonds provide additional space, allowing the lone pairs on the Br atoms to become stereochemically active. Cs[Br3 F16 ] was characterized by single-crystal X-ray diffraction, Raman spectroscopy, and quantum-chemical calculations for both the solid-state compound and the isolated anion at 0 K. Intrinsic bond orbital calculations show that the µ3 -F-Br bond is essentially ionic in nature and also underpin the stereochemical activity of the lone pairs of the Br(V) atoms.

A Computational Study on Closed-Shell Molecular Hexafluorides MF6 (M=S, Se, Te, Po, Xe, Rn, Cr, Mo, W, U) - Molecular Structure, Anharmonic Frequency Calculations, and Prediction of the NdF6 Molecule.

Quantum chemical methods were used to study the molecular structure and anharmonic IR spectra of the experimentally known closed-shell molecular hexafluorides MF6 (M=S, Se, Te, Xe, Mo, W, U). First, the molecular structures and harmonic frequencies were investigated using Density Functional Theory (DFT) with all-electron basis sets and explicitly considering the influence of spin-orbit coupling. Second, anharmonic frequencies and IR intensities were calculated with the CCSD(T) coupled cluster method and compared, where available, with IR spectra recorded by us. These comparisons showed satisfactory results. The anharmonic IR spectra provide means for identifying experimentally too little studied or unknown MF6 molecules with M=Cr, Po, Rn. To the best of our knowledge, we predict the NdF6 molecule for the first time and show it to be a true local minimum on the potential energy surface. We used intrinsic bond orbital (IBO) analyses to characterize the bonding situation in comparison with the UF6 molecule.

Discrete Mono-, Di-, and Trinuclear Anions [MoOF5]-, [MoVOF5]2-, [MoO2F4]2-, [Mo2O2F9]-, [Mo3O3F13]-, and the Infinite Chain Anion [MoO2F3]- Obtained from Reactions of MoOF4: Synthesis and Analysis of the Structure-Chemical Relations of the Compounds.

The herein-reported oxyfluoridometallate salts were synthesized and structurally characterized during the studies of the Lewis acidity of MOF4 (M = Mo, W) with various fluoride ion donors (RbF, CsF, TlF, AgF, SrF2, BaF2, PbF2) in different solvents (aqHF 48%, aHF, BrF3, ClF3). Phase-pure MoOF4 was either synthesized by hydrolysis of MoF6 with SiO2 in anhydrous HF (aHF) or by reactions of BrF3 with MoO2 or MoO3, respectively. The compound was characterized by infrared and Raman spectroscopy, solid-state quantum-chemical calculations, as well as powder and single-crystal X-ray diffraction. MoOF4 reacted with PbF2 in aHF forming Pb[MoOF5]2, while under comparable conditions, WOF4 formed Pb3[WOF5]4F2, containing the [WOF5]- anion. Salts containing such [MoOF5]- anions were also directly obtained from reactions of BrF3, MoO3, and AF2 (A = Sr, Ba), while with AgF, the compound Ag[Mo2O2F9] was observed. ClF3 reacted with MoO3 to form [ClOF2][Mo3O3F13]. Carrying out similar reactions in aqueous HF (aqHF) in autoclaves under hydrofluorothermal conditions leads to O-richer compounds with the composition A[MoO2F4] (A = Sr, Ba). With RbF or Tl2(CO3), the compounds A[MoO2F3] (A = Rb, Tl) were obtained. With CsF reduction to Mo(V) occurred as Cs2[MoVOF5] was formed. We report on similarities and differences within the respective anions and within the crystal structures of these compounds.

Experimental and computational study of the role of defects and secondary phases on the thermoelectric properties of TiNi1+xSn (0 ≤x≤ 0.12) half Heusler compounds.

The half Heusler TiNiSn compound is a model system for understanding the relationship among structural, electronic, microstructural and thermoelectric properties. However, the role of defects that deviate from the ideal crystal structure is far from being fully described. In this work, TiNi1+xSn alloys (x= 0, 0.03, 0.06, 0.12) were synthesized by arc melting elemental metals and annealed to achieve equilibrium conditions. Experimental values of the Seebeck coefficient and electrical resistivity, obtained from this work and from the literature, scale with the measured carrier concentration, due to different amounts of secondary phases and interstitial nickel. Density functional theory calculations showed that the presence of both interstitial Ni defects and composition conserving defects narrows the band gap with respect to the defect free structure, affecting the transport properties. Accordingly, results of experimental investigations have been explained confirming that interstitial Ni defects, as well as secondary phases, promote a metallic behavior, raising the electrical conductivity and lowering the absolute values of the Seebeck coefficient.

Mechanical Properties of Polypropylene-Cellulose Biocomposites: Molecular Dynamics Simulations Combined with Constant Strain Method.

The use of biocomposites is increasing due to their recyclability, biodegradability, and decreased CO2 emission levels compared to pure polyolefin plastics. Furthermore, suitably engineered biocomposites can provide, for example, superior mechanical properties for various applications. However, the correlations between the atomic-level structure and mechanical properties of most biocomposites are not yet understood. Atomistic molecular dynamics (MD) simulations provide a powerful way to examine the atomic-level structure and mechanical properties of biocomposites. In this study, polypropylene-cellulose biocomposites were examined using maleic anhydride grafted polypropylene (PP-MAH) as a coupling agent. The biocomposites were studied with the Materials Studio program package and COMPASSII force field, using the constant strain approach for mechanical properties. The results were comparable to the experimental literature values, showing that that MD can be applied to study the atomic-level structure-property correlations of polypropylene-cellulose biocomposites.

Evolutionary Algorithm-Based Crystal Structure Prediction of CuxZnyOz Ternary Oxides.

Binary zinc(II) oxide (ZnO) and copper(II) oxide (CuO) are used in a number of applications, including optoelectronic and semiconductor applications. However, no crystal structures have been reported for ternary Cu-Zn-O oxides. In that context, we investigated the structural characteristics and thermodynamics of CuxZnyOz ternary oxides to map their experimental feasibility. We combined evolutionary crystal structure prediction and quantum chemical methods to investigate potential CuxZnyOz ternary oxides. The USPEX algorithm and density functional theory were used to screen over 4000 crystal structures with different stoichiometries. When comparing compositions with non-magnetic CuI ions, magnetic CuII ions, and mixed CuI-CuII compositions, the magnetic Cu2Zn2O4 system is thermodynamically the most favorable. At ambient pressures, the thermodynamically most favorable ternary crystal structure is still 2.8 kJ/mol per atom higher in Gibbs free energy compared to experimentally known binary phases. The results suggest that thermodynamics of the hypothetical CuxZnyOz ternary oxides should also be evaluated at high pressures. The predicted ternary materials are indirect band gap semiconductors.

Probing Charge-Transfer Processes in a Covalently Linked [Ge9 ]-Cluster Imine Dyad.

C60 donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge9 ] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge9 ] cluster linked to an extended π-electron system. [Ge9 {Si(TMS)3 }2 {CH3 C=N}-DAB(II)Dipp ]- (1- ) is formed upon the reaction of [Ge9 {Si(TMS)3 }2 ]2- with bromo-diazaborole DAB(II)Dipp -Br in CH3 CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1- yields the deep green, zwitterionic cluster [Ge9 {Si(TMS)3 }2 {CH3 C=N(H)}-DAB(II)Dipp ] (1-H) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.

Lithium-ion Mobility in Li6 B18 (Li3 N) and Li Vacancy Tuning in the Solid Solution Li6 B18 (Li3 N)1-x (Li2 O)x.

All-solid-state batteries are promising candidates for safe energy-storage systems due to non-flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open-framework boron structure. The host-guest structure Li6 B18 (Li3 N) comprises large hexagonal pores filled with ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ Li7 N] strands that represent a perfect cutout from the structure of α-Li3 N. Variable-temperature 7 Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol-1 and thus much lower than pristine Li3 N. The formation of the solid solution of Li6 B18 (Li3 N) and Li6 B18 (Li2 O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li3 N and Li2 O.

Fast and Tunable Phosphorescence from Organic Ionic Crystals.

Crystalline diphosphonium iodides [MeR2 P-spacer-R2 Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 µs at 297 K. Radiative rate constants kr as high as 2.8×105  s-1 deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Bromine Pentafluoride BrF5 , the Formation of [BrF6 ]- Salts, and the Stereochemical (In)activity of the Bromine Lone Pairs.

BrF5 can be prepared by treating BrF3 with fluorine under UV light in the region of 300 to 400 nm at room temperature. It was analyzed by UV-Vis, NMR, IR and Raman spectroscopy. Its crystal structure was redetermined by X-ray diffraction, and its space group was corrected to Pnma. Quantum-chemical calculations were performed for the band assignment of the vibrational spectra. A monoclinic polymorph of BrF5 was quantum chemically predicted and then observed as its low-temperature modification in space group P21 /c by single crystal X-ray diffraction. BrF5 reacts with the alkali metal fluorides AF (A=K, Rb) to form alkali metal hexafluoridobromates(V), A[BrF6 ] the crystal structures of which have been determined. Both compounds crystallize in the K[AsF6 ] structure type (R 3 ‾ ${\bar 3}$ , no. 148, hR24). For the species [BrF6 ]+ , BrF5 , [BrF6 ]- , and [IF6 ]- , the chemical bonds and lone pairs on the heavy atoms were investigated by means of intrinsic bond orbital analysis.

Hybrid Inorganic-Organic Complexes of Zn, Cd, and Pb with a Cationic Phenanthro-diimine Ligand.

The phosphonium-decorated phenanthro-imidazolyl pyridine ligand, LP+Br, readily reacts with zinc(II) and cadmium(II) bromides to give inorganic-organic zero-dimensional compounds [LP+ZnBr2]2[ZnBr4] (1) and [(LP+)2Cd2Br4][CdBr4] (2), respectively, upon crystallization. These salts are moderately fluorescent in the solid state under ambient conditions (λem = 458 nm, Φem = 0.11 for 1; λem = 460 nm, Φem = 0.13 for 2). Their emission results from spin-allowed electronic transitions localized on the organic component with the negligible effect of [MBr4]2- and MBr2 units. Contrary to ionic species 1 and 2, lead(II) bromide affords a neutral and water-stable complex [(LP+)2Pb3Br8] (3), showing weak room-temperature phosphorescence arising from spin-orbit coupling due to the heavy atom effect. The emission, which is substantially enhanced for the amorphous sample of 3 (λem = 575 nm, Φem = 0.06), is assigned to the intraligand triplet excited state, which is a rare phenomenon among Pb(II) molecular materials.

Thermodynamic Properties of Crystalline Cellulose Allomorphs Studied with Dispersion-Corrected Density Functional Methods.

The phonon properties and thermodynamics of four crystalline cellulose allomorphs, Iα, Iß, II, and III1, have been investigated using dispersion-corrected density functional theory (DFT). In line with experimental findings, the free energy differences between the studied cellulose allomorphs are small, less than 1 kJ/mol per atom. The calculated specific heat at constant volume (Cv) has been compared with the available experimental data in the temperature range 10-300 K. Quasiharmonic approximation has been employed to study thermodynamics and specific heat at constant pressure (Cp). For the studied temperature range of 10-400 K, the specific heat of all cellulose allomorphs shows very similar behavior. The calculated and experimental specific heat agree well at low temperatures below 100 K, but the deviation between theory and experiment increases with temperature. This may be due to increasing phonon anharmonicity as the temperature increases.

Chemi-Inspired Silicon Allotropes-Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles.

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4- clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si-Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4- based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Structural Properties and Magnetic Ground States of 100 Binary d-Metal Oxides Studied by Hybrid Density Functional Methods.

d-metal oxides play a crucial role in numerous technological applications and show a great variety of magnetic properties. We have systematically investigated the structural properties, magnetic ground states, and fundamental electronic properties of 100 binary d-metal oxides using hybrid density functional methods and localized basis sets composed of Gaussian-type functions. The calculated properties are compared with experimental information in all cases where experimental data are available. The used PBE0 hybrid density functional method describes the structural properties of the studied d-metal oxides well, except in the case of molecular oxides with weak intermolecular forces between the molecular units. Empirical D3 dispersion correction does not improve the structural description of the molecular oxides. We provide a database of optimized geometries and magnetic ground states to facilitate future studies on the more complex properties of the binary d-metal oxides.

Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration.

In the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor-acceptor phosphonium dyes (D-π-)n A+ [X- ] (π=-(C6 H4 )x -). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion-paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration-induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π-spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor-acceptor arms.

Photochemistry with Chlorine Trifluoride: Syntheses and Characterization of Difluorooxychloronium(V) Hexafluorido(non)metallates(V), [ClOF2 ][MF6 ] (M=V, Nb, Ta, Ru, Os, Ir, P, Sb).

A photochemical route to salts consisting of difluorooxychloronium(V) cations, [ClOF2 ]+ , and hexafluorido(non)metallate(V) anions, [MF6 ]- (M=V, Nb, Ta, Ru, Os, Ir, P, Sb) is presented. As starting materials, either metals, oxygen and ClF3 or oxides and ClF3 are used. The prepared compounds were characterized by single-crystal X-ray diffraction and Raman spectroscopy. The crystal structures of [ClOF2 ][MF6 ] (M=V, Ru, Os, Ir, P, Sb) are layer structures that are isotypic with the previously reported compound [ClOF2 ][AsF6 ], whereas for M=Nb and Ta, similar crystal structures with a different stacking variant of the layers are observed. Additionally, partial or full O/F disorder within the [ClOF2 ]+ cations of the Nb and Ta compounds occurs. In all compounds reported here, a trigonal pyramidal [ClOF2 ]+ cation with three additional Cl⋅⋅⋅F contacts to neighboring [MF6 ]- anions is observed, resulting in a pseudo-octahedral coordination sphere around the Cl atom. The Cl-F and Cl-O bond lengths of the [ClOF2 ]+ cations seem to correlate with the effective ionic radii of the MV ions. Quantum-chemical, solid-state calculations well reproduce the experimental Raman spectra and show, as do quantum-chemical gas phase calculations, that the secondary Cl⋅⋅⋅F interactions are ionic in nature. However, both solid-state and gas-phase quantum-chemical calculations fail to reproduce the increases in the Cl-O bond lengths with increasing effective ionic radius of M in [MF6 ]- and the Cl-O Raman shifts also do not generally follow this trend.