  High pressure behavior of potassium chlorate (KClO$_3$) has been investigated
from 0-10 GPa by means of first principles density functional theory (DFT)
calculations. The calculated ground state parameters, transition pressure and
phonon frequencies using semiempirical dispersion correction scheme are in
excellent agreement with experiment. It is found that KClO$_3$ undergoes a
pressure induced first order phase transition with an associated volume
collapse of 6.4$\%$ from monoclinic (\emph{P2$_1$/m}) $\rightarrow$
rhombohedral (\emph{R3m}) structure at 2.26 GPa, which is in good accord with
experimental observation. However, the transition pressure was found to
underestimate (0.11 GPa) and overestimate (3.57 GPa) using LDA and GGA
functionals, respectively. Mechanical stability of both the phases are
explained from the calculated single crystal elastic constants. In addition,
the zone center phonon frequencies have been calculated using density
functional perturbation theory at ambient as well as at high pressure and the
lattice modes are found to soften under pressure between 0.6 to 1.2 GPa. The
present study reveals that the observed structural phase transition leads to
changes in the decomposition mechanism of KClO$_3$ which corroborates with the
experimental results.
