Effect of cation-vacancy superstructure on the phonon dynamics in KNi2Se2

Density functional theory investigations of effects arising from ordered structure of cation vacancies in ${\mathrm{KNi}}₂{\mathrm{Se}}₂$ are reported. The simulated cation-deficient ${\mathrm{K}}_x{\mathrm{Ni}}_{2\ensuremath{-}y}{\mathrm{Se}}₂$ phases with $x=0.8$, $y=0.0$, and $y=0.4$ lie within the stoichiometry range of experimental samples produced by the self-flux method or oxidative deintercalation of a vacancy-free system. Results of the present studies indicate pronounced impact of cation vacancy superstructure on the structural, electronic, and vibrational properties of ${\mathrm{KNi}}₂{\mathrm{Se}}₂$. Revealed modifications of the local structure, atomic bond lengths, electronic, and phonon bands, which are especially noticeable in the system with both potassium and nickel deficiencies, are reflected in the simulated neutron pair-distribution functions, the phonon and Raman spectra, which are provided to facilitate both experimental verification of the predicted effects and analysis of the phase composition of a multiphase K-Ni-Se material.