Crystal-lattice modulation and phonon anomaly associated with strong p-f mixing effect of CeSb

4f-electron states in unusual magnetically ordered phases of the low-carrier system CeSb have been investigated by X-ray diffraction and inelastic neutron scattering. Satellite peaks of the X-ray diffraction were found below T_N = 17 K. The observed pattern is explained by a model based on the periodic stacking of the ordered Ce-ion planes with the Γ₈-like p-f mixing state and the paramagnetic Γ₇ planes. Thus, the 4f-electron orbital order is evidenced. Inelastic neutron scattering experiments show a new excitation with a phonon character appearing below T_N. This excitation is thought to be a vibration mode changing the interatomic distance between the neighboring Ce and Sb ions forming the p-f mixing orbital.