High-pressure route to superhard boron-rich solids

Abstract

International audienceNovel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids

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

“…At the same time, the thermal expansion parameters are known just for B 6 O [39,40] and β-B 106 [41]. For both dense allotropes, γ-B 28 and t'-B 52 , we propose to take, in the first approximation, values of β-B 106 [41], while for boron subnitride B 13 N 2 one can take a value of suboxide B 6 O having similar crystal structure .…”

“…Figure shows that the parameters well agree with in situ experimental observations for B 13 N 2 . Better fit may be obtained with δ T = B 0 ' = 4 (just like in the case of B 6 O, δ T = B 0 ' = 6) or by adjusting the a and b thermal expansion parameters, or even by suggestion of a pressure drop from 5 to 4 GPa. So, the lack of experimental data does not allow making a choice, and so far we suggest a value of δ T = 5.5.…”

“…For both dense allotropes, γ-B 28 and t'-B 52 , we propose to take, in the first approximation, values of β-B 106 [41], while for boron subnitride B 13 N 2 one can take a value of suboxide B 6 O having similar crystal structure . Except for B 6 O, the δ T parameter -linking 300-K p-V data with 0.1-MPa thermal expansion data -was fixed to 5.5. Figure shows that the parameters well agree with in situ experimental observations for B 13 N 2 .…”

“…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.…”