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 KNi2Se2 are reported. The simulated cation-deficient KxNi2-ySe2 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 KNi2Se2. 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.