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• A novel hexagonal structure h-Na 2 B 2 H was proposed at ambient condition. • h-Na 2 B 2 H may be synthesized by utilizing HTHP technology. • h-Na 2 B 2 H displays ductile manner at ambient pressure. • h-Na 2 B 2 H holds two-dimensional conductivity along B atom layers. • h-Na 2 B 2 H has excellent superconductivity at ambient pressure. Alkaline metal and alkaline earth metal borohydrides may provide ideal candidates for high-temperature superconductors. By replacing the metal element of the layered hexagonal structure Li 2 B 2 H in our earlier work, a novel energetically metastable hexagonal Na 2 B 2 H, designated as h-Na 2 B 2 H, was proposed at ambient condition. First-principles calculations suggest that h-Na 2 B 2 H may be prepared from Na 3 B 20 , Na, and H 2 or from the pure elements by utilizing high-temperature and high-pressure technology. The h-Na 2 B 2 H exhibits the highest tensile stress of 24.2 GPa at a strain of 0.245 along the [1–10] direction among the four directions [100], [001], [110], and [1–10]. Electronic property calculations reveal that h-Na 2 B 2 H holds two-dimensional conductivity along B atom layers. Electron–phonon coupling calculation indicates h-Na 2 B 2 H has a T c of 42.4 K with λ = 1.73. The superconductivity in h-Na 2 B 2 H primarily depends on the Na and B atomic vibrations. Thus, h-Na 2 B 2 H can be a potential superconducting material with 2D conductivity at atmosphere pressure. [ABSTRACT FROM AUTHOR]

By using the particle swarm optimization (PSO) algorithm implemented in the CALYPSO code, two novel metastable LiB 2 structures, namely, P 42/ mmc -LiB 2 and I 41/ amd -LiB 2 , were proposed at ambient pressure. Compared with the Cm -LiB 2 and AlB 2 -type P 6/ mmm -LiB 2 , the four predicted LiB 2 structures are potentially hard materials as indicated by the empirical model formula at atmospheric pressure. The electronic band structure and density of state calculations showed that the conductivities of P 42/ mmc- , I 41/ amd -, and P 6/ mmm -LiB 2 were strong, whereas that of Cm -LiB 2 was weak. Electron–phonon coupling calculations indicated that the T c values of P 42/ mmc- , I 41/ amd -, and P 6/ mmm -LiB 2 structures were approximately 20.1 K, 12.6 K, and 31.4 K at ambient pressure, respectively. The present results may serve as a reference in the experimental and theoretical study of LiB 2 compounds. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Three novel Li 2 B phases are proposed. • These Li 2 B phases are more energy stable than Cmcm -Li 2 B at ambient pressure. • oC12–Li 2 B may be experimentally synthesized under ambient pressure. • hP6–Li 2 B has potential superconductivity. Abstract Few experimental and theoretical superconductivity works have focused on Li–B systems under the Li-rich regime. Here, we utilized the recently developed particle-swarm optimization method for structural search to propose three novel Li 2 B phases. These phases include the layered sandwich structure hP6–Li 2 B (P-3m1 , 164) and the two chain structures mS12–Li 2 B (C2/m , 12) and oC12–Li 2 B (Cmcm , 63). The structural stability of the three novel Li 2 B phases was confirmed on the basis of elastic constant and phonon dispersion calculations. Under ambient pressure, the three Li 2 B structures are notably more thermodynamically stable than the previously proposed Cmcm -Li 2 B structure and oC12–Li 2 B may be experimentally synthesized by α -LiB and Li. Electronic band structure and partial density of states analyses showed that our three proposed phases were metallic. The quasi-2D character in the band structure of hP6–Li 2 B was consistent with that of MgB 2 and indicated that hP6–Li 2 B has potential superconductive properties. Our electron–phonon coupling calculations illustrated that mS12– and oC12–Li 2 B exhibited superconductivity at 0.03 K under 10 Gpa and 0.14 K under 0 Gpa, respectively. Moreover, we found that hP6–Li 2 B has low superconductivity with a predicted T c of 0.09 K at 20 GPa, and its λ and T c values increased within the pressure range of 20–50 GPa and then decreased under pressures exceeding 60 GPa. The maximum λ and T c values under 60 GPa were 0.33 and 0.65 K, respectively. [ABSTRACT FROM AUTHOR]

• A high–pressure orthorhombic LiBH 2 phase is proposed by using CALYPSO. • oS16-LiBH 2 may be an intermediate phase involved in dehydriding Pnma -LiBH 4. • oS16-LiBH 2 turns its semiconductor into a conducting property from 150 to 200 GPa. • oS16-LiBH 2 has a potential superconductivity under ultra-high pressure. • Pressure–induced phonon softening has a great impact on the T c of oS16-LiBH 2. Hydrogen–rich materials are expected to become high–temperature superconductors after their metallization at pressures considerably lower than pure hydrogen. Combined with light elements Li and B, a high–pressure orthorhombic stable LiBH 2 phase at 150 GPa is proposed by using CALYPSO and ab initio calculations. oS16-LiBH 2 may be a highly plausible intermediate phase involved in dehydriding Pnma -LiBH 4. Meanwhile, oS16-LiBH 2 turns its semiconductor into a conducting property within the pressure range of 150 to 200 GPa and exhibits stronger conductivity along with an increasing pressure. Our electron–phonon coupling calculations reveal the softening of transverse acoustic mode B 3 u enhances the superconductivity of oS16-LiBH 2 above 300 GPa, and its λ and T c values increase to 1.15 and 10 K under 600 GPa, respectively. Furthermore, when the pressure exceeds 700 GPa, the B 3 u phonon softening makes oS16-LiBH 2 dynamically unstable. Pressure–induced phonon softening may effectively improve the superconducting properties of hydrides within a certain pressure range. [ABSTRACT FROM AUTHOR]