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The crystal structures of Sb 2 Te 3- y Se y ( y = 0.6 and y = 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb 2 Te 3- y Se y used in this study was determined to be Sb 2 Te 2.19(9) Se 0.7(2) for y = 0.6 and Sb 2 Te 1.7(1) Se 1.3(3) for y = 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb 2 Te 2.19(9) Se 0.7(2) showed a structural phase transition from a rhombohedral structure (space group No. 166, R 3¯ m ) (phase I) to a monoclinic structure (space group No. 12, C 2/ m ) (phase II), with increasing pressure up to ∼9 GPa. A new structural phase (phase II') emerged at 17.7 GPa, a monoclinic structure with the space group C 2/ c (No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12, C 2/ m ) (phase III) was observed at 21.8 GPa. In contrast, the sample of Sb 2 Te 1.7(1) Se 1.3(3) provided only phase I (space group No. 166, R 3¯ m ) and phase II (space group No. 12, C 2/ m ), showing one structural phase transition from 0-19.5 GPa. These samples were not superconductors at ambient pressure, but superconductivity suddenly appeared with increasing pressure. Superconductivity with superconducting transition temperatures ( T c 's) of 2 and 4 K was observed above 6 GPa in phase I of Sb 2 Te 2.19(9) Se 0.7(2) . In this sample, the T c values of 6 and 9 K were observed in phase II and phase II' or III of Sb 2 Te 2.19(9) Se 0.7(2) , respectively. Superconductivity with T c 's of 4 and 5 K suddenly emerged in Sb 2 Te 1.7(1) Se 1.3(3) at 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. A T c value of 9 K was finally found above 15 GPa. The magnetic field dependence of T c in phase II of Sb 2 Te 2.19(9) Se 0.7(2) and Sb 2 Te 1.7(1) Se 1.3(3) followed a p -wave polar model, suggesting topologically nontrivial superconductivity., (© 2021 IOP Publishing Ltd.)

The topological insulator PdBi 2 exhibits two different crystal phases at ambient pressure, i.e., 'α-PdBi 2 ' and ' -PdBi 2 '. The pressure dependence of crystal structure and superconductivity of α-PdBi 2 has been fully elucidated thus far. However, the physical properties of β-PdBi 2 crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of β-PdBi 2 under pressure based on synchrotron x-ray diffraction (XRD) patterns. The temperature dependence of β-PdBi 2 indicates its superconductivity with a superconducting transition temperature ( T c ) as high as 4.10 K, and its crystal structure is tetragonal [space group of I 4/ mmm (no. 139)]. The XRD patterns at 0-22.0 GPa indicate no structural phase transitions, and the unit cell volume shrinks monotonically with pressure, unlike the behavior of α-PdBi 2 . Furthermore, α-PdBi 2 transformed to β-PdBi 2 under pressure. This suggests that β-PdBi 2 is stable under pressure. The superconductivity is clearly observed at 0-11.8 GPa, and the value of T c is almost constant at ∼4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicates that the superconductivity is not attributed to a simple s -wave dirty limit but an s -wave clean or p -wave polar model. This is the first systematic study of superconductivity of topological insulator β-PdBi 2 under pressure., (© 2021 IOP Publishing Ltd.)

The crystal structure of BiSbTeSe 2 (Bi 2- x Sb x Te 3- y Se y ( x = 1.0 and y = 2.0)) at 0-29 GPa is investigated through synchrotron x-ray diffraction (XRD) and two structural phase transitions are discovered. The stoichiometry of BiSbTeSe 2 employed in this study is Bi 1.19(4) Sb 0.81(4) Te 0.83(4) Se 2.17(4) , as determined from energy-dispersive x-ray spectroscopy. The sample demonstrated structural transitions, from a rhombohedral structure (space group no 166, R 3¯ m ) (phase I) to a monoclinic structure (space group no 12, C 2/ m ) (phase II), and from phase II to a 9/10-fold monoclinic structure (space group no 12, C 2/ m ) (phase III). The temperature dependence of resistance ( R - T plot) exhibited a semiconducting behavior in a low pressure range and changed from semiconducting to metallic behavior with increasing pressure. Pressure-driven superconductivity is observed above 9.1 GPa in Bi 1.19(4) Sb 0.81(4) Te 0.83(4) Se 2.17(4) . The pressure phase corresponds to phase II. The superconducting transition temperature, T c , increased with pressure. The maximum T c value is 8.3 K at 19.1 GPa. The magnetic field dependence of T c in phase II of Bi 1.19(4) Sb 0.81(4) Te 0.83(4) Se 2.17(4) is proceeded by a p -wave polar model, indicating topologically nontrivial superconductivity. In addition, the emergence of superconductivity and the change in superconducting behavior are closely associated with the structural transitions., (© 2020 IOP Publishing Ltd.)

We synthesized the alkaline-earth metal-doped FeSe compounds (NH 3 ) y AE x FeSe (AE: Ca, Sr and Ba), using the liquid NH 3 technique, to determine their superconducting properties and crystal structures. Multiple superconducting phases were obtained in each sample of (NH 3 ) y Ca x FeSe and (NH 3 ) y Ba x FeSe, which showed two superconducting transition temperatures ( T c 's) as high as 37-39 K and 47-48 K at ambient pressure, hereinafter referred to as the 'low- T c phase' and 'high- T c phase', respectively. The high- T c phases in (NH 3 ) y Ca x FeSe and (NH 3 ) y Ba x FeSe were metastable, and rapidly converted to their low- T c phases. However, T c values of 38.4 K and 35.6 K were recorded for (NH 3 ) y Sr x FeSe, which displayed different behavior than (NH 3 ) y Ca x FeSe and (NH 3 ) y Ba x FeSe. The Le Bail fitting of x-ray diffraction (XRD) patterns provided lattice constants of c = 16.899(1) Å and c = 16.8630(8) Å for the low- T c phases of (NH 3 ) y Ca x FeSe and (NH 3 ) y Ba x FeSe, respectively. The lattice constants of their high- T c phases could not be determined due to the disappearance of the high T c phase within a few days. The XRD pattern for (NH 3 ) y Sr x FeSe indicated the coexistence of two phases with c = 16.899(3) Å and c = 15.895(4) Å. The former value of c in (NH 3 ) y Sr x FeSe is almost the same as those of the low- T c phases in (NH 3 ) y Ca x FeSe and (NH 3 ) y Ba x FeSe. Therefore, the phase with c = 16.899(3) Å in (NH 3 ) y Sr x FeSe must correspond to the superconducting phase with the T c of 38.4 K, while the superconducting phase with T c = 35.6 K is assigned to the crystal phase with c = 15.895(4) Å. For (NH 3 ) y Sr x FeSe, a high- T c phase with T c = 47-48 K has not yet been obtained, but a new phase showing the T c value of 35.6 K was clearly obtained. This is the first systematic study of the preparation, crystal structure, and superconductivity of alkaline-earth metal-doped FeSe, (NH 3 ) y AE x FeSe., (© 2020 IOP Publishing Ltd.)

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr 2 , that exhibits a superconducting transition temperature, T c , as high as 6.6 K at ambient pressure, in order to complete the T c -pressure (p ) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T c value decreased with an increase in pressure, but the onset superconducting transition temperature, [Formula: see text], increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R  -  T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr 2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr 2 (M: alkali-earth metal atom), under pressure.