Boron: a Hunt for Superhard Polymorphs

Boron is a unique element, being the only element, all known polymorphs of which are superhard, and all of its crystal structures are distinct from any other element. The electron-deficient bonding in boron explains its remarkable sensitivity to even small concentrations of impurity atoms and allows boron to form peculiar chemical compounds with very different elements. These complications made the study of boron a great challenge, creating also a unique and instructive chapter in the history of science. Strange though it may sound, the discovery of boron in 1808 was ambiguous, with pure boron polymorphs established only starting from the 1950s-1970s, and only in 2007 was the stable phase at ambient conditions determined. The history of boron research from its discovery to the latest discoveries pertaining to the phase diagram of this element, the structure and stability of beta-boron, and establishment of a new high-pressure polymorph, gamma-boron, is reviewed

Thermoelastic Equation of State of Boron Suboxide B6O up to 6 GPa and 2700 K: Simplified Anderson-Gr\"uneisen Model and Thermodynamic Consistency

p-V-T equation of state of superhard boron suboxide B6O has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Gr\"uneisen parameter {\delta}T. The model includes bulk modulus B0 =181 GPa and its first pressure derivative B0' = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e. a = 1.4x10-5 K-1 and b = 5x10-9 K-2, as well as {\delta}T = 6. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to volume at standard conditions, V0. The fitted thermal expansion at 0.1 MPa is well consistent with the experimental data, as well as with ambient-pressure heat capacity cp, bulk modulus B0 and {\delta}T describing its evolution with volume and temperature. The fitted value of Gr\"uneisen parameter {\gamma} = 0.85 is in agreement with previous empiric estimations for B6O and experimental values for other boron-rich solids

On melting of boron phosphide under pressure

Melting of cubic boron phosphide, BP has been studied at pressures to 9 GPa using synchrotron X-ray diffraction and electrical resistivity measurements. It has been found that above 2.6 GPa BP melts congruently, and the melting curve exhibits negative slope -60(7) K/GPa, which is indicative of a higher density of the melt as compared to the solid phase.Comment: 3 pages, 1 figur

Crystal structure of dense pseudo-cubic boron allotrope, pc-B52, by powder X-ray diffraction

During past years, a number of reports have been published on synthesis of tetragonal allotrope of boron, t-B52 phase. However, no unambiguous characterization of the crystal structure has been performed to the present time, while remarkable variation of the a/c lattice-parameter ratio raises strong doubts about its uniqueness. Here the Rietveld refinement of the crystal structure of the high pressure - high temperature boron phase synthesized by a direct solid-state transformation of rhombohedral beta-B106 at 20 GPa and 2500 K has been reported for the first time. Although this boron allotrope belongs to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c ratio of sqr(2)

Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.Comment: Review on superhard B-C-N-O phase

High pressure synthesis of FeO-ZnO solid solutions with rock salt structure: in situ X-ray diffraction studies

X-ray diffraction with synchrotron radiation has been used for the first time to study chemical interaction in the FeO-ZnO system at 4.8 GPa and temperatures up to 1300 K. Above 750 K, the chemical reaction between FeO and ZnO has been observed that resulted in the formation of rock salt (rs) Fe1-xZnxO solid solutions (0.3 \leq x \leq 0.85). The lattice parameters of these solid solutions have been in situ measured as a function of temperature under pressure, and corresponding thermal expansion coefficients have been calculated.Comment: 9 pages, 2 figures, 1 tabl