Crystallographic phase transition within the magnetically ordered state of Ce₂Fe₁₇

X-ray diffraction experiments were performed on polycrystalline and single-crystal specimens of Ce₂Fe₁₇ at temperatures between 10 and 300 K. Below T_t=118±2 K, additional weak superstructure reflections were observed in the antiferromagnetically ordered state. The superstructure can be described by a doubling of the chemical unit cell along the c direction in hexagonal notation with the same space group R3̅ m as the room-temperature structure. The additional antiferromagnetic satellite reflections observed in earlier neutron diffraction experiments can be conclusively related to the appearance of this superstructure.