  We computed the optical properties and the folded and unfolded band structure
of Cu-doped KCl crystals. The calculations use the plane-wave pseudo-potential
approach implemented in the ABINIT electronic structure package within the
first-principles density-functional theory framework. Cu substitution into
pristine KCl crystals requires calculation by the supercell (SC) method from a
theoretical perspective. This procedure shrinks the Brillouin zone, resulting
in a folded band structure that is difficult to interpret. To solve this
problem and gain insight into the effect of cuprous ion (Cu+) on electronic
properties; We unfolded the band structure of SC KCl:Cu to directly compare
with the band structure of the primitive cell (PC) of pristine KCl. To
understand the effect of Cu substitution on optical absorption, we calculated
the imaginary part of the dielectric function of KCl:Cu through a
sum-over-states formalism and broke it down into different band contributions
by partially making an iterated cumulative sum (ICS) of selected valence and
conduction bands. As a result, we identified those interband transitions that
give rise to the absorption peaks due to the Cu ion. These transitions include
valence and conduction bands formed by the Cu-3d and Cu-4s electronic states.
To investigate the effects of doping position, we consider different doping
positions, where the Cu dopant occupies all the substitutional sites replacing
host K cations. Our results indicate that the doping position's effects give
rise to two octahedral shapes in the geometric structure. The distorted-twisted
octahedral square bipyramidal geometric-shape induces a difference in the
crystal field splitting energy compared to that of the perfect octahedral
square bipyramidal geometric-shape.
