Coexistence of Superconductivity and Superhardness in Beryllium Hexaboride Driven by Inherent Multicenter Bonding

Discovery of superconductivity in technetium borides at moderate pressures

Advances in theoretical calculations have boosted the search for high-temperature superconductors, such as sulfur hydrides and rare-earth polyhydrides. However, the required extremely high pressures for stabilizing these superconductors has handicapped further implementation. Based upon thorough structural searches, we identified a series of unprecedented superconducting technetium borides at moderate pressures, including TcB (P63/mmc) with a superconducting transition temperature of Tc = 20.2 K at ambient pressure and TcB2 (P6/mmm) with Tc = 23.1 K at 20 GPa. Superconductivity in these technetium borides mainly originates from the coupling between the low-frequency vibrations of technetium atoms and the dominant technetium-4d electrons at the Fermi level. Our work therefore presents a fresh group in the family of superconducting borides, whose diversified crystal structures suggest rich possibilities in the discovery of other superconducting transition-metal borides.

Superconductivity of cubic MB6 (M = Na, K, Rb, Cs)

Previous studies have shown that NaB6, KB6, and RbB6 adopting Pm3̄m are superconductors with a relatively high Tc under ambient conditions. In this paper, we conducted systematic structural and related properties research on CsB6 through a genetic evolution algorithm and total energy calculations based on density functional theory between 0 and 20 GPa. Our results reveal a cubic Pm3̄m CsB6, which is dynamically stable under the pressures we studied. We systematically calculated the formation enthalpies, electronic properties, and superconducting properties of Pm3̄m MB6 (M = Na, K, Rb, Cs). They all exhibit metallic features, and boron has high contributions to band structures, density of states, and electron-phonon coupling (EPC). The calculated results about the Helmholtz free energy difference of Pm3̄m CsB6 at 0, 10, and 20 GPa indicate that it is stable upon chemical decomposition (decomposition to simple substances Cs and B) from 0 to 400 K. The phonon density of states indicates that boron atoms occupy the high frequency area. The EPC results show that Pm3̄m CsB6 is a superconductor with Tc = 11.7 K at 0 GPa, close to NaB6 (13.1 K), KB6 (11.7 K), and RbB6 (11.3 K) at 0 GPa in our work, which indicates that boron atoms play an essential role in superconductivity: vibrations of B6 regular octagons lead to the high Tc of Pm3̄m MB6. Our work about Pm3̄m hexaborides provides a supplementary study on the borides of the group IA elements (without Fr and Li) and has an important guiding significance for the experimental synthesis of CsB6.

Hardness and superconductivity in tetragonal LiB4 and NaB4

Boron-based compounds have triggered substantial attention due to their multifunctional properties, incorporating excellent hardness and superconductivity. While tetragonal metal borides LiB4 and NaB4 with BaAl4-type structure and striking clathrate boron motif have been induced under compression, there is still a lack of deep understanding of their potential properties at ambient pressure. We herein conduct a comprehensive study on I4/mmm-structured LiB4 and NaB4 under ambient pressure via first-principles calculations. Remarkably, both LiB4 and NaB4 are found to possess high Vickers hardness of 39 GPa, which is ascribed to the robust boron framework with strong covalency. Furthermore, their high hardness values together with distinguished stability make them highly potential superhard materials. Meanwhile, electron-phonon coupling analysis reveals that both LiB4 and NaB4 are conventional phonon-mediated superconductors, with critical temperatures of 6 and 8 K at 1 atmosphere pressure (atm), respectively, mainly arising from the coupling of B 2p electronic states and the low-frequency phonon modes associated with Li-, Na-, and B-derived vibrations. This work provides valuable insights into the mechanical and superconducting behaviors of metal borides and will boost further studies of emergent borides with multiple functionalities.

Pressure-induced evolution of structures and promising superconductivity of ScB6

Unique multicenter bonding in boron-rich materials leads to the formation of complicated structures and intriguing properties. ScB₆, as a sister compound, possibly possesses high hardness and superconducting critical temperature in this family under ambient pressure. Here, phase transitions, chemical bonding states and electronic properties of ScB₆ at high pressure are uncovered using particle swarm optimization (PSO) combined with first-principles calculations. The phase sequence of P2₁/m → C2/m → Cmcm for ScB₆ has been identified under high pressure. Interestingly, the evolution of a boron framework is from a graphene-like layer to a planar B₄ ring, B₆ and B₇ cycle, and non-planar B₈ cycle, which interconnect a graphene-like network. These phases of ScB₆ are expected to be hard materials due to the excellent mechanical behaviors by the mechanical property calculations. Although the metallic features of the three phases reduce their hardness, the further electron-phonon coupling calculations indicate that the three phases of ScB₆ are superconducting phases under high pressures.

Pressure-Induced Evolution of Crystal and Electronic Structure of Ammonia Borane

Ammonia borane (NH₃BH₃) has long attracted considerable interest for its high hydrogen content and easy dehydrogenation conditions which make it a promising hydrogen storage material. Here, we report on a computational study of the structural stability and phase transition sequence of NH₃BH₃ and associated lattice dynamics and electronic properties in a wide pressure range up to 300 GPa. The results confirm previously reported structures, including the experimentally observed orthorhombic Pmn2₁ structure at low temperature and ambient pressure, and predict the phase transition sequence Pmn2₁ → Pc → P2₁ → P1̅ for NH₃BH₃. Our calculations also reveal systematic trends of monotonically decreasing band gap with rising pressure in the three high-pressure NH₃BH₃ phases, which nevertheless all remain nonconducting up to the highest pressure of 300 GPa examined in this work. The present findings elucidate structural and electronic properties of NH₃BH₃ over an extensive pressure range, providing knowledge essential to further study of NH₃BH₃ in an expanded pressure-temperature phase space.

Pressure induced semiconductor-semimetal-superconductor transition of magnesium hexaborides

A thorough structural exploration was performed for MgB₆ combining the global structure searching method with first-principles calculations. Besides the known Cmcm phase, new phases, i.e. I4/mmm, C2/m-I, C2/m-II and P2₁/m, were predicted to be stable in the pressure range of 18-100 GPa. Unexpectedly, Cmcm-MgB₆ was found to be a semiconductor with an indirect band gap of 0.38 eV with the HSE06 functional, in good agreement with the experimental finding. I4/mmm-MgB₆ stabilized above 18 GPa exhibits semimetallic behaviour with a topological node-line near the Fermi level. Consequently, C2/m-I MgB₆ with a sandwich structure similar to MgB₂ is predicted to be a superconductor with a critical temperature (T_c) of 9.5 K. By analysing the electronic structure, the intriguing semiconductor-semimetal-superconductor transition may be ascribed to the delocalization of more B-p electrons in the boron sublattice. The novel functions uncovered for MgB₆ may inspire more efforts to discover materials with intriguing properties.

Predicted superhard phases of Zr-B compounds under pressure

Boron-rich zirconium borides are potential candidates for superhard or multifunctional materials with excellent physical properties. Using first-principle methods with structure searching, various stoichiometric zirconium-boron compounds have been investigated under pressure. Four unexpected phases of Pmmm-ZrB, C2/m-ZrB3, Cmcm-ZrB6, Amm2-ZrB6 are uncovered. Structurally, the B-B bonding patterns evolve from zig-zag chains to triple graphite-like layers with the increase in B content. The three-dimensional covalent bonding networks of Zr-B and B-B were unraveled due to the observation of charge localization between B-B and B-Zr by electronic localization function analysis and crystal orbital Hamilton population. Interestingly, the predicted Cmcm and Amm2 phases for ZrB6 can be experimentally synthesized at moderate pressures and quenching can recover these products to ambient conditions as potential superhard materials due to their Vickers hardness beyond 40 GPa. Our work provides a key perspective toward the understanding of novel chemical bonding in B-rich transition metals compounds and gives direction for the experimental synthesis of superhard materials.

Superconductivity with high hardness in Mo3C2

This work synthesized a high hardness and superconductive polycrystalline Mo₃C₂ material by the HPHT method. Mo₃C₂ exhibits superconductivity below 8.2 K and its hardness is far higher than that of the traditionally used superconductive materials.