The covalent frameworks found in certain compounds, such as the S–H skeleton in H3S and the H cage in LaH10, play an essential role in their superconductivity. These compounds have the feature of bonding unsaturation (a deficiency of electrons in their covalent bonding) in common. Developing an understanding of the relationship between superconductivity and bonding unsaturation in these materials can provide new ideas for the design of superconducting materials. In this work, we explored the high-pressure phase diagram of binary P–S compounds using first-principles swarm structural calculations. In addition to the previously reported P2S and P3S structures, we identified that P5S, P8S, and P11S also have a common structural character of six-coordinated octahedral networks; however, their bonding unsaturation are distinct due to the different valence electron numbers and unequal ratios of P and S atoms. These features provide an ideal model for exploring the bonding-unsaturation dependence of superconductivity. We estimated the average bonding unsaturation of these P-rich compounds based on the valence electron numbers and the coordination numbers of the central P/S atoms. Interestingly, the resultant average bonding unsaturation was found to be proportional to the predicted superconducting transition temperature. This finding was also verified in MH9 (M = Y, Th, and Pr) and doped H3S (Si, C, and P) compounds. Our work provides an opportunity to gain a deeper understanding of bonding-unsaturation-dependent superconductivity.

Novel structural building blocks in compounds could induce interesting physical and chemical properties. Although phosphorus tends to form very different motifs, the existence of lone pair electrons has always prevented the formation of graphenelike structures. Here, the application of first-principles swarm structural calculations has allowed us to predict the stability of pressure-induced hexagonal LaP 2 H 2 containing graphenelike phosphorus, which derives from the trigonal bipyramid configuration of P atoms regulated by symmetric hydrogen bonds. LaP 2 in LaP 2 H 2 has the same configuration as MgB 2 , and P and H atoms form a three-dimensional framework as H 3 S. Interestingly, LaP 2 H 2 shows a superconductivity dominated by the graphenelike phosphorus layer and its coupling with La atoms. On the other hand, LaP 2 H 2 is not only superconducting at a lower pressure than the H-rich LaPH 6 , but it also shows a superconducting transition temperature three times higher. Our work provides an example which extends the landscape of conventional superconductors at lower pressures.

Ternary hydrides are ideal candidates for high-temperature superconductivity. However, their electronic properties correlate strongly with the charge transfer between their consistent elements. In this work we propose nine Na-B-H compounds, which combines charge modulation between Na and B/H atoms with pressure, with unusual structural motifs and interesting electronic properties. In the metallic Pmma NaBH 3 , with both ionic and covalent frameworks, formed by H atoms bonded to Na/B atoms, a low-frequency phonon dominated superconductivity is predicted with a T c of 86.8 K, distinct from the high-frequency H-derived superconductivity associated with covalent H-based skeletons or H cages/sheets in superhydrides. Furthermore, in nonmetallic NaBH n (n = 1, 2, and 6) and R-3m NaBH 3 , the B atom exhibits an sp 3 -hybridized feature analogous to the C atom, resulting in the formation of several alkanelike B-H motifs, such as a graphanelike layer and an ethane-shaped molecule. Our work presents an important step toward the understanding of metal borohydrides.

Pressure, as a controllable thermodynamic parameter, can substantially modulate the structural topologies, e.g., phase transition, and electronic properties of the known compounds. Herein, we found that the compression of H-rich...