  A new BiS2-based superconductor Bi2(O,F)S2 was discovered. This is a layered
compound consisting of alternate stacking structure of rock-salt-type BiS2
superconducting layer and fluorite-type Bi(O,F) blocking layer. Bi2(O,F)S2 was
obtained as the main phase by topotactic fluorination of undoped Bi2OS2 using
XeF2, which is the first topotactic synthesis of an electron-doped
superconductor via reductive fluorination. With increasing F-content, a- and
c-axis length increased and decreased, respectively, and Tc increased up to 5.1
K.
