The crystal structures and properties of boron-silicon (B-Si) compounds under pressure have been systematically explored using particle swarm optimization structure prediction method in combination with first-principles calculations. Three new stoichiometries, B 2 Si, BSi, and BSi 2 , are predicted to be stable gradually under pressure, where increasing pressure favors the formation of silicon rich B-Si compounds. In the boron-rich compounds, the network of boron atoms changes from B 12 icosahedron in the ambient phases to the similar buckled graphenelike layers in the high-pressure phases, which crystalize in the same P3m1 symmetry but with different numbers of boron layers between adjacent silicon layers. Phonon calculations show that these structures might be retained to ambient conditions as metastable phases. Further electron-phonon coupling calculations indicate that the high-pressure phases of boron-rich compounds might superconduct at 1 atm, with the highest T c value of 21 K from the Allen-Dynes equation in P3m1 B 2 Si, which is much higher than the one observed in boron doped diamond-type silicon. Moreover, further fully anisotropic Migdal-Eliashberg calculations indicate that B 2 Si is a two-gap anisotropic superconductor and the estimated T c might reach up to 30 K at 1 atm. On the silicon-rich side, BSi 2 is predicted to be stable in the CuAl 2 -type structure. Our current results significantly enrich the phase diagram of the B-Si system and will stimulate further experimental study.

The study of superconductivity in compressed hydrides is of great interest due to measurements of high critical temperatures (T c ) in the vicinity of room temperature, beginning with the observations of LaH 10 at 170-190 GPa. However, the pressures required for synthesis of these high-T c superconducting hydrides currently remain extremely high. Here we show the investigation of crystal structures and superconductivity in the La-B-H system under pressure with particle-swarm intelligence structure-searches methods in combination with first-principles calculations. Structures with seven stoichiometries, LaBH, LaBH 4 , LaBH 6 , LaBH 7 , LaBH 8 , La(BH) 3 , and La(BH 4 ) 3 were predicted to become stable under pressure. Remarkably, the hydrogen atoms in LaBH 8 were found to bond with B atoms in a manner that is similar to that in H 3 S. Lattice dynamics calculations indicate that LaBH 7 and LaBH 8 become dynamically stable at pressures as low as 109 and 48 GPa, respectively. Moreover, the two phases were predicted to be superconducting with a critical temperature T c of 93 K and 156 K at 110 GPa and 55 GPa, respectively (μ * = 0.1). The present results provide guidance for future experiments targeting hydride superconductors with both low synthesis pressures and high T c .

We have performed a systematic study on the crystal structures and electronic properties of two ternary hydrides, YSH 6 and LaSH 6 , under pressure, using the particle swarm optimization method and first-principles calculations. As a result of extensive structure searches, metallic YSH 6 and LaSH 6 are thermodynamically stable between 195-237 and 170-300 GPa, respectively. Interestingly, in YSH 6 eight neighboring hydrogen atoms form octagons, and the octagons in different layers are connected by four sulfur atoms, forming a cagelike structure with a Y atom at the center, while those octagons in the same layer form polyphenylene-like chains via one shared side. In LaSH 6 , however, hydrogen atoms form both curved "H 5 " chains or straight chains when bonded to sulfur atoms. Furthermore, electron-phonon coupling calculations indicate that YSH 6 and LaSH 6 are promising superconductors with estimated T c values of 91 and 35 K at 210 and 300 GPa, respectively. These results provide guidance for future experimental studies and stimulate more exploration on ternary hydrides.

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