Phase Diagram of the B−B₂O₃ System at 5 GPa: Experimental and Theoretical Studies

Abstract

X-ray diffraction with synchrotron radiation has been used to study in situ the chemical interaction of beta-rhombohedral boron with boron (III) oxide and phase relations in the B-B2O3 system at pressures up to 6 GPa in the temperature range from 300 to 2800 K. The B-B2O3 system has been thermodynamically analyzed, and its equilibrium phase diagram at 5 GPa has been constructed. Only one thermodynamically stable boron suboxide, B6O, exists in the system. It forms eutectic equilibria with boron and B2O3.

First and second‐order Raman scattering of B6O

“…[5,10] We have established that at such low pressure the formation of B 6 O in amounts large enough for the X-ray diffraction is observed at temperatures higher than 1800 K (15-min heating), while the complete transformation to B 6 O occurs at temperatures above 2000 K (5-min heating).…”

High-pressure route to superhard boron-rich solids

“…Even the phase diagram of elemental boron is still open to question, i.e. the theoretical diagram proposed by Shirai et al [13] contradicts not only the metastability of the rhombohedral α-B 12 phase at ambient pressure [14][15][16], but also the slope of the boron melting curve established in the recovery [17] and in situ [18] experiments.…”

Integrated form of the Anderson-Grüneisen equation of state forp-V-Tdata fit: Application to the compounds of boron, carbon, silicon and some metals.

“…We need to explore the HPHT thermodynamics for understanding the syntheses of these new materials for new challenging applications as superhard [5,6] [8]. Although a part of the lacking data can be replaced by fitted parameters of common models [26][27][28] or with ab initio calculations [24], the reliable p-V-T equations of state (EOS) data are crucial for that.…”

“…We need to explore the HPHT thermodynamics for understanding the syntheses of these new materials for new challenging applications as superhard [5,6], advanced electronic [7] and photovoltaic [8], superconductors [9] as well as clathrate thermoelectric materials [10,11]: (1) boron allotropes [12] (orthorhombic γ-B 28 [13][14][15][16], pseudo-cubic t'-B 52 [17]) and boron-rich compounds (subnitride B 13 N 2 [18,19]), (2) superhard compounds with diamond structure (nanostructured cBN [20] and c-BC 5 [21,22]); (3) covalent clathrates of new stoichiometries (Na 4-x Si 24 [8,10] & Na 24+x Si 136 [10,11]) and even (4) new unexpected semiconductors, like antifluorite Mg 2 C [23,24], dense Mg 2 C 3 [25] and pure silicon allotrope with quasi-direct bandgap, Si 24 [8]. Although a part of the lacking data can be replaced by fitted parameters of common models [26][27][28] or with ab initio calculations [24], the reliable p-V-T equations of state (EOS) data are crucial for that.…”

“…We need to explore the HPHT thermodynamics for understanding the syntheses of these new materials for new challenging applications as superhard [5,6] [8]. Although a part of the lacking data can be replaced by fitted parameters of common models [26][27][28] or with ab initio calculations [24], the reliable p-V-T equations of state (EOS) data are crucial for that.In the present paper we describe the method of construction of such equations of state using integrated form of the Anderson-Grüneisen equation [29,30]. The method is efficient even in the case of small number of experimental data [31] and may be easily combined with ab initio, semiempirical and even empirical modeling [32].…”