  We report the investigation of the structural stability of
Co$_{(1-x)}$Ni$_x$Si monosilicides for $0<x<1$. As CoSi crystallizes in the
FeSi-type structure (B20) and NiSi is stable in the MnP-type structure (B31), a
complete set of samples has been synthesized and a systematic study of phase
formation under different annealing conditions were carried out in order to
understand the reason of such a structural transition when x goes from 0 to 1.
This study has revealed a limit in the solubility of Ni in CoSi B20 structure
of about 17.5 at.% and of Co in NiSi B31 phase of about 13 at.%. For
$0.35<x<0.74$ both B20 and B31 phases are present in the sample at there
respective limits of solubility. The temperature dependence of the magnetic
susceptibility has also been measured revealing diamagnetic behaviors. Optimal
structural parameters and phase stability of the solid solution have been
investigated using self-consistent full-potential linearized augmented plane
wave method (FP-LAPW) based on the density functional theory (DFT). This
calculation well predicts the structural instability observed experimentally.
