Electron-deficient and polycenter bonds in the high-pressure γ-B28 phase of boron

The peculiar bonding situation in γ boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B(12) clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B(12) dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.     

Diamond anvils with a spherical support designed for X-ray and neutron diffraction experiments in DAC

Here, we present new diamond anvils with a spherical support designed for applications in diamond anvil cell (DAC) technique. The main feature of the anvils is the diamond crown of a spherical shape. The assembly of the spherical diamond fixed within a spherical support of a seat made of tungsten carbide or hard metals provides enhanced stability, simple alignment, and large optical and X-ray aperture that makes it very useful for broad applications in DAC technique, particularly for single crystal X-ray and powder neutron diffraction. The anvils were tested in various experiments conducted in a wide pressure–temperature range and showed a very good performance.     

Boron Phosphorus Nitride at Extremes: PN6 Octahedra in the High‐Pressure Polymorph β‐BP3N6

The high‐pressure behavior of non‐metal nitrides is of special interest for inorganic and theoretical chemistry as well as materials science, as these compounds feature intriguing elastic properties. The double nitride α‐BP₃N₆ was investigated by in situ single‐crystal X‐ray diffraction (XRD) upon cold compression to a maximum pressure of about 42 GPa, and its isothermal bulk modulus at ambient conditions was determined to be 146(6) GPa. At maximum pressure the sample was laser‐heated, which resulted in the formation of an unprecedented high‐pressure polymorph, β‐BP₃N₆. Its structure was elucidated by single‐crystal XRD, and can be described as a decoration of a distorted hexagonal close packing of N with B in tetrahedral and P in octahedral voids. Hence, β‐BP₃N₆ is the first nitride to contain PN₆ octahedra, representing the much sought‐after proof of principle for sixfold N‐coordinated P that has been predicted for numerous high‐pressure phases of nitrides.     

Cotunnite-Structured Titanium Dioxide

Combined theoretical and experimental studies led to discovery of a new polymorph modification of titanium dioxide with nine-coordinated titanium. Among polycrystalline materials, it is one of the least compressible (with a bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa).     

High Pressure Mössbauer Studies on FCC Fe53Ni47 Alloy

Perturbed angular correlation measurements of the hyperfine interaction of ¹¹¹In in sapphire show, that after implantation and annealing at 1000°C, the fraction of undisturbed probe atoms exhibiting a unique quadrupole interaction with ν _Q = 219(1) MHz (η = 0) varies between 50% at 4 K, 5% at 100 K and 80% at 973 K in a reversible manner. A possible explanation for this surprising behaviour is the influence of so-called ‘after effects’ following the EC-decay of ¹¹¹In to ¹¹¹Cd. Immediately after the decay the ¹¹¹Cd is in an ionized state, then collects electrons from its surroundings and reaches the ground state. The different electronic configurations that arise during this relaxation process affect the amplitude (f _u) and the damping (δ _u) of the unique quadrupole interaction.     

In situ Raman spectroscopic study of the pressure induced structural changes in ammonia borane

The effect of static compression up to 65 GPa at ambient temperature on ammonia borane, BH(3)NH(3), has been investigated using in situ Raman spectroscopy in a diamond anvil cells. Two phase transitions were observed at approximately 12 GPa and previously not reported transition at 27 GPa. It was demonstrated that ammonia borane behaves differently under compression at quasi-hydrostatic and non-hydrostatic conditions. The ability of BH(3)NH(3) to generate second harmonic of the laser light observed up to 130 GPa suggests that the non-centrosymmetric point group symmetry is preserved in the material up to very high pressures.     

In-situ combined X-ray diffraction and electrical resistance measurements at high pressures and temperatures in diamond anvil cells

A method of simultaneous X-ray diffraction and electrical resistance measurements of materials in a diamond anvil cell has been developed. The experimental arrangement for the electrical measurements uses a gasket as a part of the electrode scheme. Application of the new methodology is demonstrated on the examples of the study of high-pressure high-temperature behavior of Pr and Fe–Ni alloys. Simultaneously obtained diffraction and resistance data provide the different, from earlier studies, interpretations of the topology of the dhcp–fcc phase transition in the phase diagram of Pr. Namely, the shape of a hysteretic region is mostly defined by kinetics of the dhcp phase growth rather than by thermodynamic driving force at the fcc to dhcp phase transformation. The results of combined X-ray diffraction and resistance measurements of Fe0.₈Ni0.₂ alloy up to 18 GPa and 425 K are reported along with the complementary resistance measurements of Fe0.₉Ni0.₁ alloy.     

Ground-state properties of boron-doped diamond

Boron-doped diamond undergoes an insulator-metal or even a superconducting transition at some critical value of the dopant concentration. We study the equilibrium lattice parameter and bulk modulus of boron-doped diamond experimentally and in the framework of the density functional method for different levels of boron doping. We theoretically consider the possibility for the boron atoms to occupy both substitutional and interstitial positions and investigate their influence on the electronic structure of the material. The data suggest that boron softens the lattice, but softening due to substitutions of carbon with boron is much weaker than due to incorporation of boron into interstitial positions. Theoretical results obtained for substitution of carbon are in very good agreement with our experiment. We present a concentration dependence of the lattice parameter in boron-doped diamond, which can be used for to identify the levels of boron doping in future experiments. The text was submitted by the authors in English.     

High-Pressure Synthesis of Metal–Inorganic Frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 with Polymeric Nitrogen Linkers

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal–inorganic frameworks Hf₄N₂₀⋅N₂, WN₈⋅N₂, and Os₅N₂₈⋅3 N₂ via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf₄N₂₀, WN₈, and Os₅N₂₈) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N₂ also leads to a non-centrosymmetric polynitride Hf₂N₁₁ that features double-helix catena-poly[tetraz-1-ene-1,4-diyl] nitrogen chains [−N−N−N=N−]_∞.