  Pressure dependence of the electronic and crystal structures of
K$_{x}$Fe$_{2-y}$Se$_{2}$, which has pressure-induced two superconducting domes
of SC I and SC II, was investigated by x-ray emission spectroscopy and
diffraction. X-ray diffraction data show that compressibility along the c-axis
changes around 12 GPa, where a new superconducting phase of SC II appears. This
suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray
emission spectroscopy data also shows the change in the electronic structure
around 12 GPa. These results can be explained by the scenario that the two SC
domes under pressure originate from the change of Fermi surface topology.
Present results here show that the nesting condition plays a key role in
stabilizing the superconducting state helping to address outstanding
fundamental question as to why the SC II appears under pressure.
