Pressure-Assisted Synthesis of Highly Crystalline 1T''-Lix MoS2.

Lix MoS2 is not only a lithium battery material, but is also an important precursor for the synthesis of MoS2 nanomaterials. Current syntheses of MoS2 , such as in n-butyllithium/LiBH4 or electrochemically, are not satisfying in terms of defined stoichiometry and crystallinity, so an accurate experimental crystal structure determination of this important and widely used material has been long awaited. A high-pressure/high-temperature synthesis yielded highly crystalline 1T''-Lix MoS2 (x=1, 1.333). 1T''-LiMoS2 crystallizes in the space group P 1 ‾ $\bar 1$ with a=6.2482(3) Å, b=6.6336(3) Å, c=6.7480(3) Å, α=119.321(2)°, ß=90.010(2)° and γ=90.077(2)°. The arrangement of Mo atoms within the b-c-plane confirmed a predicted Peierls distortion. A similar atom distribution pattern to that of Mo is also observed for the lithium atoms. Calculation of bond valence site energies gave an activation barrier of 1.244 eV for 2D lithium-ion migration. For x=1.333, a phase-pure synthesis was achieved.

Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2.

Building on studies of monoclinic Li3SiNO2, a polymorph, ß-Li3SiNO2, with a previously unknown structure type was synthesized. The ß-phase crystallizes in the orthorhombic space group Pbca (no. 61) with lattice parameters of a = 18.736(2), b = 11.1267(5), c = 5.0897(3) Å, and a cell volume of V = 1057.2(1) Å3. Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSi2N3 according to Rietveld analysis) containing exclusively one or the other polymorph, depending solely on the cooling rate. In contrast to the monoclinic phase, orthorhombic ß-Li3SiNO2 additionally contains a third layer and shows a layer-sequence of the type ABCB. Doped with the activator ion Eu2+, the new polymorph exhibits an intense yellow emission (λmax = 586 nm, fwhm = 89 nm, 0.33 eV, 2650 cm-1) under irradiation with UV to blue light. Hence, the structural difference between the two polymorphs goes along with a significant blue-shift of 16 nm. The results from single-crystal diffraction and single-grain luminescence measurements were confirmed by Rietveld analysis of bulk samples and powder luminescence data.

Theoretical and 27Al NMR Spectroscopic Investigations of Binary Intermetallic Alkaline-Earth Aluminides.

The binary alkaline-earth aluminides AEAl2 (AE = Ca and Sr) and AEAl4 (AE = Ca-Ba) have been synthesized from the elements and investigated via powder X-ray diffraction experiments. CaAl2 adopts the cubic MgCu2-type structure (Fd3̅m), while SrAl2 crystallizes in the orthorhombic KHg2-type (Imma). LT-CaAl4 crystallizes with the monoclinic CaGa4-type (C2/m), while HT-CaAl4, SrAl4, and BaAl4 adopt the tetragonal BaAl4-type structure (I4/mmm). The close structural relation of the two CaAl4 polymorphs was established using a group-subgroup relation in the Bärnighausen formalism. In addition to the room-temperature and normal pressure phase of SrAl2, a high-pressure/high-temperature phase has been prepared using multianvil techniques, and its structural and spectroscopic parameters were determined. Elemental analysis by inductively coupled plasma mass spectrometry showed that no significant impurities with other elements besides the weighed ones are present and the chemical compositions match the synthesized ones. The title compounds have been furthermore investigated by 27Al solid-state magic angle spinning NMR experiments to validate the crystal structure and to gain information about the influence of the composition on the electron transfer and the NMR characteristics. This has also been investigated from a quantum chemical point of view using Bader charges, while the stabilities of the binary compounds in the three phase diagrams (Ca-Al, Sr-Al and Ba-Al) have been studied by calculations of formation energies per atom.

First ternary tungsten tellurate(IV) WTe2O7 with unique crystal structure type.

At multianvil high-pressure/high-temperature conditions of 10 GPa and 1273 K, the first ternary tungsten tellurate WTe2O7 is formed, starting from a stoichiometric mixture of WO3 and TeO2. The compound crystallizes triclinic in a hitherto unknown crystal structure type with the space group P1̄; (no. 2), and was refined from single-crystal X-ray diffractometer data: a = 538.3(1), b = 687.5(1), c = 802.3(1) pm, α = 72.4(1)°, ß = 85.7(1)°, γ = 68.1(1)°, wR2 = 0.0323, GooF = 1.048, 3157 F2 values, and 106 variables. The main motifs of the crystal structure are pairs of edge-linked [WO6]6- octahedra and fourfold oxygen-coordinated Te4+ atoms. The oxidation state of W6+ and Te4+ was further verified by measuring the characteristic binding energy values for the W 4f and the Te 3d core levels via X-ray photoelectron spectroscopy (XPS). In addition, DFT calculations of the structure, the associated electron localisation functions (ELF) and vibrational spectra have been carried out. The theoretical data clearly demonstrates the impact of the residual electron density located at the Te4+ ions, which can be directly interpreted as the presence of lone electron pairs within the solid structure.

The crystal structure and luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2.

The compound Li3SiNO2:Eu2+ was synthesized in high temperature solid-state reactions in weld shut tantalum ampules and the crystal structure of Li3SiNO2 has been determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 1049.01(3), b = 1103.42(3), c = 511.86(2) pm, ß = 116.14(1)°, and a volume of V = 0.53187(2) nm3. This compound is built up from two different layers, which are arranged alternately along the crystallographic a-axis. The results from single-crystal diffraction were confirmed by the Rietveld analysis of bulk samples. Moreover, Li3SiNO2 could be successfully doped with the activator ion Eu2+ and the luminescence spectroscopy of single-crystals revealed broad band emission at λmax = 601 nm (fwhm = 90 nm).

Coexistence of Two Different Distorted Octahedral [MnF6 ]3- Sites in K3 [MnF6 ]: Manifestation in Spectroscopy and Magnetism.

As a consequence of the static Jahn-Teller effect of the 5 E ground state of MnIII in cubic structures with octahedral parent geometries, their octahedral coordination spheres become distorted. In the case of six fluorido ligands, [MnF6 ]3- anions with two longer and four shorter Mn-F bonds making elongated octahedra are usually observed. Herein, we report the synthesis of the compound K3 [MnF6 ] through a high-temperature approach and its crystallization by a high-pressure/high-temperature route. The main structural motifs are two quasi-isolated, octahedron-like [MnF6 ]3- anions of quite different nature compared to that met in ideal octahedral MnIII Jahn-Teller systems. Owing to the internal electric field of Ci symmetry dominated by the next-neighbour K+ ions acting on the MnIII sites, both sites, the pseudo-rhombic (site 1) and the pseudo-tetragonally elongated (site 2) [MnF6 ]3- anions are present in K3 [MnF6 ]. The compound was characterized by single-crystal and powder X-ray diffraction, and magnetometry as well as by FTIR, Raman, and ligand field spectroscopy. A theoretical interpretation of the electronic structure and molecular geometry of the two Mn sites in the lattice is given by using a vibronic coupling model with parameters adjusted from multireference ab-initio cluster calculations.

La3 B6 O13 (OH): The First Acentric High-Pressure Borate Displaying Edge-Sharing BO4 Tetrahedra.

La3 B6 O13 (OH) was obtained by a high-pressure/high-temperature experiment at 6 GPa and 1673 K. The compound crystallizes in the space group P21 (no. 4) with the lattice parameters a=4.785(2), b=12.880(4), c=7.433(3) Å, and ß=90.36(10)°, and is built up of corner- as well as edge-sharing BO4 tetrahedra. It represents the first acentric high-pressure borate containing these B2 O6 entities. The compound develops borate layers of "sechser"-rings with the La3+ cations positioned between the layers. Single-crystal and powder X-ray diffraction, vibrational and MAS NMR spectroscopy, second-harmonic generation (SHG) and thermoanalytical measurements, as well as computational methods were used to affirm the proposed structure and the B2 O6 entities.

Ferroelastic lattice rotation and band-gap engineering in quasi 2D layered-structure PdSe2 under uniaxial stress.

Transition metal dichalcogenides (TMDCs) have attracted extensive attention in recent years for their novel physical and chemical properties as well as promising applications in the future. In the present paper, based on first-principles simulations, we focused on the bulk of the TMDC material PdSe2 and provided new insights into its unique structural properties and electronic structures under uniaxial stress. For the first time, we revealed that this orthorhombic PdSe2 is an intrinsic ferroelastic material with stress-driven 90° lattice rotation in the layer stacking direction. Strikingly, the ferroelastic phase transition originated from the bond reconstructions in the unusual square-planar (PdSe4)2- structural units. Specifically, low switching barriers and strong ferroelastic signals rendered room-temperature shape memory accessible. Moreover, the ferroelastic phase transition was accompanied with semiconductor-to-metal-to-semiconductor transitions under uniaxial compressive stress, which could be applied in electronic switching devices. In addition, the band gap was closely associated to the interlayer spacing, which could be engineered by the uniaxial tensile stress. These outstanding stress-engineered properties suggest that orthorhombic PdSe2 is a promising material for potential applications in microelectromechanical and nanoelectronic devices.

UF4 and the High-Pressure Polymorph HP-UF4.

A laboratory-scale synthesis of UF4 is presented that utilizes the reduction of UF6 with sulfur in anhydrous hydrogen fluoride. An excess of sulfur can be removed by vacuum sublimation, yielding pure UF4 , as shown by powder X-ray diffraction, micro X-ray fluorescence analysis, infrared and Raman spectroscopy, as well as magnetic measurements. Furthermore, a single-crystalline, high-pressure modification of UF4 was obtained in a multi-anvil press at elevated temperatures. The high-pressure polymorph HP-UF4 was characterized by means of single-crystal and powder X-ray diffraction, as well as by magnetic measurements, and presents a novel crystal structure type. Quantum-chemical calculations show the HP-modification to be 10 kJ mol-1 per formula unit higher in energy compared to UF4 .

Li3Co1.06(1)TeO6: synthesis, single-crystal structure and physical properties of a new tellurate compound with CoII/CoIII mixed valence and orthogonally oriented Li-ion channels.

A tellurate compound with CoII/CoIII mixed valence states and lithium ions within orthogonally oriented channels was realized in Li3Co1.06(1)TeO6. The single-crystal structure determination revealed two independent and interpenetrating Li/O and (Co,Te)/O substructures with octahedral oxygen coordination of the metal atoms. In contrast to other mixed oxides, a honeycomb-like ordering of CoO6 and TeO6 octahedra was not observed. Li3Co1.06(1)TeO6 crystallizes orthorhombically with the following unit cell parameters and refinement results: Fddd, a = 588.6(2), b = 856.7(2), c = 1781.5(4) pm, R1 = 0.0174, wR2 = 0.0462, 608 F2 values, and 33 variables. Additional electron density in tetrahedral voids in combination with neighboring face-linked and under-occupied octahedral lithium sites offers an excellent possible diffusion pathway for lithium ions. According to the symmetry of the crystal structure the diffusion pathways in Li3Co1.06(1)TeO6 were found in two orthogonal orientations. The CoII/CoIII mixed valence was investigated via X-ray photoelectron spectroscopy (XPS), revealing a composition comparable to that derived from single-crystal X-ray diffractometry. Magnetic susceptibility measurements underlined the coexistence of CoII and CoIII, the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of Li3Co1.06(1)TeO6 was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10-6 S cm-1 at 573 K.

Verbeekite, the Long-Unknown Crystal Structure of Monoclinic PdSe2.

Verbeekite, a monoclinic polymorph of PdSe2, was reported for the first time in 2002 by Roberts et al. The mineral has been discovered in the Musonoi Cu-Co-Mn-U mine, Democratic Republic of Congo, and was named after Dr. Théodore Verbeek, the first geoscientist who studied the palladium mineralization there (1955-1967). Until today, the crystal structure of this very rare mineral has been unknown. By syntheses via multianvil high-pressure/high-temperature methods at 11.5 GPa and 1300 °C, synthetic verbeekite could be obtained in a high degree of purity and comparatively good crystal quality, which made it possible to determine the full crystal structure for PdSe2 verbeekite from single-crystal X-ray diffractometer data: I2/a, a = 671.0(2) pm, b = 415.42(8) pm, c = 891.4(2) pm, ß = 92.42(3)°, V = 248.24(4) Å3, R1 = 0.0368, wR2 = 0.0907 (all data). In contrast to layered PdS2-type PdSe2, verbeekite exhibits a novel crystal structure type of dichalcogenides of the platinum-group metals with (Se2)2- dimer anions connecting the layers. The possibility of different arrangements of the characteristic (Se2)2- dumbbells is the reason for the various polymorphs of the dichalcogenides, with now five known PdSe2 representatives.

A ZrNiAl related high-pressure modification of CeRuSn.

Monoclinic CeRuSn with its own structure type transforms to a high-pressure modification at 11.5 GPa and 1470 K (1000 t press, Walker type module). The structure of the high-pressure phase was refined from X-ray single crystal diffractometer data at room temperature. The HP-CeRuSn subcell structure adopts the ZrNiAl type: P6[combining macron]2m, a = 751.4(3) and c = 394.6(2) pm, wR2 = 0.0787, 310 F(2) values and 15 variables. The Ru2 atoms within the Sn6 trigonal prisms show a strongly enhanced U33 parameter. Weak satellite reflections indicate a commensurate modulation: (3 + 1)D superspace group P31m(1/3,1/3,γ)000, a = 751.4(3) and c = 394.6(2) pm, γ = -1/3, wR2 = 0.0786, 1584 F(2) values, 32 variables for the main reflections and wR2 = 0.3757 for the satellites of 1(st) order. A description of this new superstructure variant of the ZrNiAl type is possible in a transformed 3D supercell with the space group R3m and Z = 9. The driving force for formation of the modulation is strengthening of Ru-Sn bonding within the comparatively large Ru@Sn6 trigonal prisms. Electronic structure calculations point to an almost depleted Ce 4f shell. This is substantiated by temperature-dependent magnetic susceptibility data. Fitting of the data within the interconfiguration fluctuation model (ICF) resulted in cerium valences of 3.41 at 10 K and 3.31 at 350 K. Temperature dependent specific heat data underline the absence of magnetic ordering.

Synthesis and characterization of the new strontium borogermanate Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32).

The strontium borogermanate Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) was synthesized by high-temperature solid-state reaction of SrO, GeO2, and H3BO3 in a NaF/KF flux system using platinum crucibles. The structure determination revealed that Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) crystallizes in the trigonal space group P321 (No. 150) with the parameters a = 800.7(2) and c = 488.8(2) pm, with R1 = 0.0281, wR2 = 0.0671 (all data), and Z = 1. The crystal structure of Sr(3-x/2)B(2-x)Ge(4+x)O14 (x = 0.32) consists of distorted SrO8 cubes, GeO6 octahedra, GeO4 tetrahedra, and BO4 tetrahedra. In addition to the structural investigations, Raman and IR spectroscopic investigations were carried out.

Oxonium ions substituting cesium ions in the structure of the new high-pressure borate HP-Cs(1-x)(H(3)O)(x)B(3)O(5) (x=0.5-0.7).

The new high-pressure borate HP-Cs1-x (H3 O)x B3 O5 (x=0.5-0.7) was synthesized under high-pressure/high-temperature conditions of 6 GPa/900 °C in a Walker-type multianvil apparatus. The compound crystallizes in the monoclinic space group C2/c (Z=8) with the parameters a=1000.6(2), b=887.8(2), c=926.3(2) pm, ß=103.1(1)°, V=0.8016(3) nm(3) , R1=0.0452, and wR2=0.0721 (all data). The boron-oxygen network is analogous to those of the compounds HP-MB3 O5 , (M=K, Rb) and exhibits all three structural motifs of borates-BO3 groups, corner-sharing BO4 tetrahedra, and edge-sharing BO4 tetrahedra-at the same time. Channels inside the boron-oxygen framework contain the cesium and oxonium ions, which are disordered on a specific site. Estimating the amount of hydrogen by solid-state NMR spectroscopy and X-ray diffraction led to the composition HP-Cs1-x (H3 O)x B3 O5 (x=0.5-0.7), which implies a nonzero phase width.

High-pressure synthesis and characterization of new actinide borates, AnB4O8 (An=Th, U).

New actinide borates ThB4O8 and UB4O8 were synthesized under high-pressure, high-temperature conditions (5.5 GPa/1100 °C for thorium borate, 10.5 GPa/1100 °C for the isotypic uranium borate) in a Walker-type multianvil apparatus from their corresponding actinide oxide and boron oxide. The crystal structure was determined on basis of single-crystal X-ray diffraction data that were collected at room temperature. Both compounds crystallized in the monoclinic space group C2/c (Z=4). Lattice parameters for ThB4O8: a=1611.3(3), b=419.86(8), c=730.6(2) pm; ß=114.70(3)°; V=449.0(2) Å(3); R1=0.0255, wR2=0.0653 (all data). Lattice parameters for UB4O8: a=1589.7(3), b=422.14(8), c=723.4(2) pm; ß=114.13(3)°; V=443.1(2) Å(3); R1=0.0227, wR2=0.0372 (all data). The new AnB4O8 (An=Th, U) structure type is constructed from corner-sharing BO4 tetrahedra, which form layers in the bc plane. One of the four independent oxygen atoms is threefold-coordinated. The actinide cations are located between the boron-oxygen layers. In addition to Raman spectroscopic investigations, DFT calculations were performed to support the assignment of the vibrational bands.

Ce4Ag3Ge4O(0.5)--chains of oxygen-centered [OCe2Ce(2/2)] tetrahedra embedded in a [CeAg3Ge4] intermetallic matrix.

The oxidation of an intermetallic phase under high-pressure/high-temperature conditions led to the synthesis of Ce4Ag3Ge4O(0.5) exhibiting [OCe2Ce(2/2] tetrahedral chains, in which the oxygen atoms statistically occupy the tetrahedral centres. Starting from a 1:1:1 CeAgGe precursor (NdPtSb type), a multianvil high-pressure/high-temperature experiment at 11.5 GPa and 1250-1300 °C revealed Ce4Ag3Ge4O(0.5), crystallizing in the space group Pnma with the following lattice parameters: a = 2087.3(4), b = 439.9(1), and c = 1113.8(2) pm. Magnetic measurements showed Curie-Weiss behavior above 100 K with an experimental magnetic moment of 2.42 µB per Ce atom, close to the value for the free Ce(3+) ion, clearly indicating trivalent cerium in Ce4Ag3Ge4O(0.5). Full potential GGA+U band structure calculations resulted in metallic properties and a magnetic ground state with one unpaired 4f-electron per cerium in agreement with the experiments.

Effects of gigapascal level pressure on protein structure and function.

Information on very high pressure (VHP) effects on proteins is limited and therefore effects of VHP on chemistry, structure and function of two model proteins in medical use were studied. VHP (8 GPa) application to l-asparaginase (L-ASNase) resulted in faster mobility on clear native gels. VHP induced generation of lower-MW forms of L-ASNase but VHP treatment did not deteriorate asparaginase activity. Electrophoretic patterns in native and denaturing gels were comparable for untreated and pressurized recombinant human growth hormone (rhGH). rhGH function, however, was deteriorated as shown by a bioassay. In L-ASNase and rhGH a series of protein modifications and amino acid exchanges (indicating cleavage of covalent bonds) were revealed that may probably lead to functional and conformational changes. The findings have implications in protein chemistry, structure, and function and are useful for designing biotechnological applications of protein products.

Crystal structures, phase-transition, and photoluminescence of rare earth carbodiimides.

Rare earth carbodiimides with the general formula RE 2(CN 2) 3 crystallize with two modifications. A monoclinic( C2/m) modification is obtained for RE = Y, Ce-Tm and a rhombohedral ( R3 c) modification for RE = Tm-Lu. The space group R3 c is confirmed by single-crystal structure determination on Lu 2(CN 2) 3 and indexed powder patterns of RE = Tm, Yb and Lu. The use of diverse chemical syntheses conditions for Tm 2(CN 2) 3 revealed the dimorphic character of this compound. In addition, pressure experiments on Tm 2(CN 2) 3 have induced a phase-transition from rhombohedral to monoclinic. This transformation comprises an increase of the coordination number of Tm from 6 to 7, and a unit-cell volume reduction in the order of 20 %. The photoluminescence behavior of lanthanide doped Gd 2(CN 2) 3:Ln samples is presented with different activators (Ln = Ce, Tb) revealing a broad band emission of Gd 2(CN 2) 3:Ce, quite similar to that of the well-known YAG:Ce.

Microstrain sensitivity of orbital and electronic phase separation in SrCrO3.

An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.