Ab initio phonon dynamics in the layered ternary diselenide KNi2Se2

• Lattice dynamics of KNi2Se2 is investigated from first-principles.
• Some Raman-active modes carry low intensities.
• Inelastic neutron scattering spectra reflect dynamics of the Ni-sublattice.
• Significant enhancement of conduction-band effective mass is found.

Dynamical properties of KNi2Se2 lattice are investigated within density functional theory and the approximation of harmonic phonons. Partial phonon densities of states indicate that Ni and Se vibrations span the entire spectral range extending to 27 meV, whereas K vibrations are limited to 11–18 meV. It is shown that inelastic neutron scattering spectra reflect the dynamics of Ni atoms within [Ni2Se2] blocks and the high-energy EgEg mode is hardly observed in the Raman spectra due to its low intensity. The electronic contribution to the heat capacity of 47 mJmol−1K−2 is estimated at very low temperatures.