The crystal structure and superconductivity of strontium, an alkaline earth metal, at low temperature and high pressure

Focusing on alkaline earth metals that exhibit high superconducting transition temperature (Tc) among elements whose Tc increases with pressure, we performed powder X-ray diffraction and electrical resistivity measurements of strontium pressurized at low temperature. In the case of the compression at room temperature, strontium undergoes a transformation to an incommensurate phase (host–guest phase, Sr-V) with a complex atomic arrangement. By compression at low temperature, strontium shows two high-pressure phases, Sr-VI and Sr-VII having the different structure from the host–guest phase (Sr-V). Simultaneous measurements of X-ray diffraction and electrical resistivity revealed that these high-pressure phases exhibit superconductivity, and that the Tc in the Sr-VI phase rises with increasing pressure, reaching a maximum at 14.3 K (150 GPa). This is higher than the previously reported maximum Tc value of 8 K (58 GPa) in the host–guest phase (Sr-V). This result provides a sequence of structural phase transitions and superconductivity common to alkaline earth metals.