X-ray diffraction studies at low temperatures have been performed in order
to investigate thermal properties of crystal lattices in the low-carrier-de
nsity systems cerium monopnictides. CeP and CeAs show large crystal-lattice
contractions with increasing temperatures up to about 120 K and 90 K, resp
ectively. Due to the strong p-f mixing, the excited crystal field Gamma(s)-
like state is expected to produce a shorter interatomic bond length between
cerium ions and neighboring pnictogens than that realized by the Gamma(7)
ground state. The experimental results of the lattice contractions at low t
emperatures are explained by the thermal average of these crystal field sta
tes.