Pressure-induced structural transition and huge enhancement of superconducting properties of single-crystal Fe 0.99 Ni 0.01 Se 0.5 Te 0.5 unconventional superconductor

We report high-pressure structural studies (52 GPa) at room temperature combined with magnetic [( M(T ):1GPa] and electrical resistivity [( ρ(T) :0-21GPa)] measurements down to 2 K on Fe 0.99 Ni 0.01 Se 0.5 Te 0.5 superconductor using designer diamond anvils (D-DAC) pressure cell. The M(T ) data show huge enhancement of superconducting transition temperature ( T c ) from 8.62 to 14.8 K (1 GPa) and ρ(T) reveal maximum enhancement of T c ~ 30.5 K at 3 GPa (d T c /d P = ~ 7.19 K/GPa) followed by moderate decrease of T c up to 19 K at 7.5 GPa, and further increasing pressure T c gets vanished at 10.6 GPa. The reduction of T c due to the occurrence of structural transition that is likely associated with possible reduction of charge carriers in the density of states in Fermi surface. The high-pressure XRD measurement shows tetragonal phase exists up to 7 GPa, followed by mixed phase which is visible between 7.5 GPa and 14.5 GPa. The structural transformation occurs at 15 GPa from tetragonal ( P 4 /nmm ) to NiAs -type hexagonal phase ( P6 3 /mmc ) and it is stable up to 52 GPa, confirmed from the equation of state (EOS) and it can be correlated with variation of T c under pressure for Fe 0.99 Ni 0.01 Se 0.5 Te 0.5 chalcogenide superconductors.