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1. Creating superconductivity in WB2 through pressure-induced metastable planar defects

   https://doi.org/10.1038/s41467-022-35191-8  

   Lim, J.; Hire, A. C.; Quan, Y.; Kim, J. S.; Xie, S. R.; Sinha, S.; Kumar, R. S.; Popov, D.; Park, C.; Hemley, R. J.; et al (December 2022, Nature Communications)

   Abstract High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB 2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, T c of 17 K at 91 GPa. Upon further compression up to 187 GPa, the T c gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB 2 (hP3, space group 191, prototype AlB 2 ). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB 2 ) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials. 

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2. Pressure-induced superconductivity in a novel germanium allotrope

   https://doi.org/10.1016/j.mtphys.2024.101338  

   Deng, Liangzi; Zhang, Jianbo; Sakai, Yuki; Tang, Zhongjia; Adnani, Moein; Dahal, Rabin; Litvinchuk, Alexander P.; Chelikowsky, James R.; Cohen, Marvin L.; Hemley, Russell J.; et al (February 2024, Materials Today Physics)

   - (Ed.)

   High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for α-Ge. Raman measurements revealed structural phase transitions and possible electronic topological transitions under pressure up to 58 GPa. Based on pressure-dependent resistivity measurements, superconductivity was induced above 2 GPa and the maximum Tc of 6.8 K was observed under 4.6 GPa. Interestingly, a superconductivity enhancement was discovered during decompression, indicating the possibility of maintaining pressure-induced superconductivity at ambient pressure with better superconducting performance. Density functional theory analysis further suggested that the electronic structure of Ge (oP32) is sensitive to its detailed geometry and revealed that disorder in the β-tin structure leads to a higher Tc in comparison to the perfect β-tin Ge. 

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3. Polymorphism in the Ruddlesden-Popper Nickelate La(3)Ni(2)O(7): Discovery of a Hidden Phase with Distinctive Layer Stacking

   https://doi.org/10.18126/tnn7-en21  

   Chen, J; Thind, AS; Sharma, S; LaBollita, H; Peterson, G; Zheng, H; Phelan, DP; Botana, AS; Klie, RF; Mitchell, JF (January 2024, Materials Data Facility)

   We report the discovery of a novel form of Ruddlesden−Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa. 

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4. Polymorphism in the Ruddlesden–Popper Nickelate La3Ni2O7: Discovery of a Hidden Phase with Distinctive Layer Stacking

   https://doi.org/10.1021/jacs.3c14052  

   Chen, Xinglong; Zhang, Junjie; Thind, Arashdeep S; Sharma, Shekhar; LaBollita, Harrison; Peterson, Gordon; Zheng, Hong; Phelan, Daniel P; Botana, Antia S; Klie, Robert F; et al (February 2024, Journal of the American Chemical Society)

   We report the discovery of a novel form of Ruddlesden–Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa. 

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5. Road to Room-Temperature Superconductivity: Tc above 260 K in Lanthanum Superhydride under Pressure

   Russell J. Hemley, Muhtar Ahart (January 2019, Proceedings of the International Symposium Superconductivity and Pressure: A Fruitful Relationship on the Road to Room Temperature Superconductivity. May, 21-22 - 2018. Madrid - Spain)

   The use of high pressure to realize superconductivity in the vicinity of room temperature has a long history, much of it focused on achieving this in hydrogen-rich materials. This paper provides a brief overview of the work presented at this May 2018 conference, together with background on motivation and techniques, the theoretical predictions of superconductivity in lanthanum hydride, and the subsequent experimental confirmation. Theoretical calculations using density-functional based structure-search methods combined with BCS-type models predicted a new class of dense, hydrogen-rich materials – superhydrides (MHx, with x > 6 and M selected rare earth elements) – with superconducting critical temperatures (Tc) in the vicinity of room-temperature at and above 200 GPa pressures. The existence of a series of these phases in the La-H system was subsequently confirmed experimentally, and techniques were developed for their syntheses and characterization, including measurements of structural and transport properties, at megabar pressures. Four-probe electrical transport measurements of a cubic phase identified as LaH10 display signatures of superconductivity at temperatures above 260 K near 200 GPa. The results are supported by pseudo-four probe conductivity measurements, critical current determinations, low-temperature x-ray diffraction, and magnetic susceptibility measurements. The measured high Tc is in excellent agreement with the original calculations. The experiments also reveal additional superconducting phases with Tc between 150 K and above 260 K. This effort highlights the novel physics in hydrogen-rich materials at high densities, the success of ‘materials by design’ in the discovery and creation of new materials, and the possibility of new classes of superconductors Tc‘s at and above room temperature. 

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