γ-BaB2O4: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO4 Tetrahedra.

The α- and ß-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB2O4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB2O4, crystallizes in a centrosymmetrical group of monoclinic syngony (P21/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, ß = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB2O4 structure is the presence of edge-sharing tetrahedra [B2O6] which form infinite double chains ∞[B4O4O8/2] stretching along the a axis. The number of known structural types with the [B2O6] group is limited. Phase γ-BaB2O4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B2O6] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB2O4 modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB2O4 and ß-BaB2O4 modifications. In the Raman spectra of γ-BaB2O4, the most intense band at a frequency of 853 cm-1 was found to correspond to the symmetric bending mode of the B-O-B-O ring in edge-sharing tetrahedra.

Experimental and Ab Initio Studies of Intrinsic Defects in "Antizeolite" Borates with a Ba12(BO3)66+ Framework and Their Influence on Properties.

The porous Ba12(BO3)66+ framework of the so-called "antizeolite" borates with channels along the c axis is capable of accommodating various guest anionic groups, e.g. [BO3]3-, [F2]2-, [F4]4-, and [(Li,Na)F4]3-. Taking as an example the Ba12(BO3)6[BO3][LiF4] crystal, we put forward the argument that the optical properties of "antizeolite" borates are strongly influenced by the degree of channel packing with anionic groups and, correspondingly, by the conjugated intrinsic defects. With the use of optical, electron-spin resonance, Raman spectroscopy, and ab initio calculations, it was shown that intrinsic defects largely impact the absorption of light in the visible and UV regions (the color of the bulk crystals can change from colorless to dark brown), absorption-edge position, dichroism, and other optical properties. The change in the optical absorption in the visible range is caused by the appearance of new states in the electronic structure inside the band gap, which are associated mainly with the presence of single and double F centers-fluorine vacancies that capture electrons-in [□F4]4-, [F2]2-, and [LiF4]3- groups. The formation of F centers in the [F2]2- group is the most energetically favorable. It has been found that Ba12(BO3)6[BO3][LiF4] crystals are optically active gyrotropic with an isotropic point at 499 nm at 300 K and are of interest for practical application as narrow-band light filters.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons at ambient and high pressures.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons (PAHs) was found at 500-773 K and ambient and high (3.5 GPa) pressures. The most intensive oligomerization at 1 bar and 3.5 GPa occurs at 740-823 K. PAH carbonization at high pressure is the final stage of oligomerization and occurs as a result of sequential oligomerization and polymerization of the starting material, caused by overlapping of π-orbitals, a decrease of intermolecular distances, and finally the dehydrogenation and polycondensation of benzene rings. Being important for building blocks of life, PAHs and their oligomers can be formed in the interior of the terrestrial planets with radii less than 2270 km.

Effect of cation substitution on bridgmanite elasticity: A key to interpret seismic anomalies in the lower mantle.

Seismological observations show that, in some regions of the lower mantle, an increase in bulk sound velocity, interestingly, occurs in the same volume where there is a decrease in shear velocity. We show that this anti-correlated behavior occurs on cation substitution in bridgmanite by making single crystal elasticity measurements of MgSiO3 and (Mg,Fe,Al)(Si,Al)O3 using inelastic x-ray scattering in the ambient conditions. Cation substitution of ferrous iron and aluminum may explain large low shear velocity provinces in the lower mantle.

Boron-doped diamond heater and its application to large-volume, high-pressure, and high-temperature experiments.

A temperature of 3500 degrees C was generated using a diamond resistance heater in a large-volume Kawai-type high-pressure apparatus. Re and LaCrO(3) have conventionally been used for heaters in high-pressure studies but they cannot generate temperatures higher than 2900 degrees C and make in situ x-ray observations difficult due to their high x-ray absorption. Using a boron-doped diamond heater overcomes these problems and achieves stable temperature generation for pressure over 10 GPa. The heater starting material is a cold-compressed mixture of graphite with boron used to avoid the manufacturing difficulties due to the extreme hardness of diamond. The diamond heater was synthesized in situ from the boron-graphite mixture at temperature of 1600+/-100 degrees C and pressure of 20 GPa. By using the proposed technique, we have employed the diamond heater for high-temperature generation in a large-volume high-pressure apparatus. Achievement of temperatures above 3000 degrees C allows us to measure the melting points of the important constituents in earth's mantle (MgSiO(3), SiO(2), and Al(2)O(3)) and core (Fe and Ni) at extremely high pressures.