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1. Novel magnesium borides and their superconductivity

   https://doi.org/10.1039/c7cp00840f  

   Davari Esfahani, M. Mahdi; Zhu, Qiang; Dong, Huafeng; Oganov, Artem R.; Wang, Shengnan; Rakitin, Maksim S.; Zhou, Xiang-Feng (January 2017, Physical Chemistry Chemical Physics)

   With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB 3 and Mg 3 B 10 . Experimentally known MgB 2 is stable in the entire pressure range 0–200 GPa, while MgB 7 and MgB 12 are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB 4 , which becomes unstable against decomposition into MgB 2 and MgB 7 at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB 2 -type structure. However, with the exception of MgB 2 , all other magnesium borides have low electron–phonon coupling constants λ of 0.32–0.39 and are predicted to have T c below 3 K. 

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2. A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure

   https://doi.org/10.3390/cryst9090441  

   Geng, Nisha; Bi, Tiange; Zarifi, Niloofar; Yan, Yan; Zurek, Eva (September 2019, Crystals)

   Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na2S3 with C 2 / c and Imm2 symmetry, C 2 -Na2S5 and C 2 -Na2S8. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa, P 4 / m m m -Na2S8 was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the β -Po phase of elemental sulfur. The structures of the most stable M3S and M4S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K. 

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3. Experimental and Computational Studies on Superhard Material Rhenium Diboride under Ultrahigh Pressures

   https://doi.org/10.3390/ma13071657  

   Burrage, Kaleb C.; Lin, Chia-Min; Chen, Wei-Chih; Chen, Cheng-Chien; Vohra, Yogesh K. (April 2020, Materials)

   An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 ± 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures. 

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4. Machine learning and evolutionary prediction of superhard B-C-N compounds

   https://doi.org/10.1038/s41524-021-00585-7  

   Chen, Wei-Chih; Schmidt, Joanna N.; Yan, Da; Vohra, Yogesh K.; Chen, Cheng-Chien (July 2021, npj Computational Materials)

   Abstract We build random forests models to predict elastic properties and mechanical hardness of a compound, using only its chemical formula as input. The model training uses over 10,000 target compounds and 60 features based on stoichiometric attributes, elemental properties, orbital occupations, and ionic bonding levels. Using the models, we construct triangular graphs for B-C-N compounds to map out their bulk and shear moduli, as well as hardness values. The graphs indicate that a 1:1 B-N ratio can lead to various superhard compositions. We also validate the machine learning results by evolutionary structure prediction and density functional theory. Our study shows that BC₁₀N, B₄C₅N₃, and B₂C₃N exhibit dynamically stable phases with hardness values >40 GPa, which are superhard materials that potentially could be synthesized by low-temperature plasma methods. 

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5. Design of high-temperature superconductors at moderate pressures by alloying AlH3 or GaH3

   https://doi.org/10.1063/5.0159590  

   Liang, Xiaowei; Wei, Xudong; Zurek, Eva; Bergara, Aitor; Li, Peifang; Gao, Guoying; Tian, Yongjun (January 2024, Matter and Radiation at Extremes)

   Since the discovery of hydride superconductors, a significant challenge has been to reduce the pressure required for their stabilization. In this context, we propose that alloying could be an effective strategy to achieve this. We focus on a series of alloyed hydrides with the AMH6 composition, which can be made via alloying A15 AH3 (A = Al or Ga) with M (M = a group ⅢB or IVB metal), and study their behavior under pressure. Seven of them are predicted to maintain the A15-type structure, similar to AH3 under pressure, providing a platform for studying the effects of alloying on the stability and superconductivity of AH3. Among these, the A15-type phases of AlZrH6 and AlHfH6 are found to be thermodynamically stable in the pressure ranges of 40–150 and 30–181 GPa, respectively. Furthermore, they remain dynamically stable at even lower pressures, as low as 13 GPa for AlZrH6 and 6 GPa for AlHfH6. These pressures are significantly lower than that required for stabilizing A15 AlH3. Additionally, the introduction of Zr or Hf increases the electronic density of states at the Fermi level compared with AlH3. This enhancement leads to higher critical temperatures (Tc) of 75 and 76 K for AlZrH6 and AlHfH6 at 20 and 10 GPa, respectively. In the case of GaMH6 alloys, where M represents Sc, Ti, Zr, or Hf, these metals reinforce the stability of the A15-type structure and reduce the lowest thermodynamically stable pressure for GaH3 from 160 GPa to 116, 95, 80, and 85 GPa, respectively. Particularly noteworthy are the A15-type GaMH6 alloys, which remain dynamically stable at low pressures of 97, 28, 5, and 6 GPa, simultaneously exhibiting high Tc of 88, 39, 70, and 49 K at 100, 35, 10, and 10 GPa, respectively. Overall, these findings enrich the family of A15-type superconductors and provide insights for the future exploration of high-temperature hydride superconductors that can be stabilized at lower pressures. 

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