Ab initio study of the lattice dynamics of CsNiF3.

Lattice dynamics of the quasi-one-dimensional ferromagnetic chain-like structure of CsNiF(3) has been studied using density functional theory and the direct method. Investigations were limited to the harmonic approximation. Antiferromagnetic and ferromagnetic spin orderings on Ni atoms were considered. It is found that phonons remain practically insensitive to the type of magnetic arrangement. The difference in the calculated Helmholtz free energies between antiferro- and ferromagnetic ordering is too small to provide sufficient information on the preference of the type of magnetic ordering in CsNiF(3). Calculated acoustic phonons agree very well with the inelastic neutron scattering experiments, while the optical phonons remain in an acceptable agreement with Raman and infrared measurements. Comparison of the experimental heat capacity and the calculated lattice contribution to the heat capacity shows that the magnetic contribution is negligible above 20-30 K. Thermal motions of particular atoms in CsNiF(3) crystals are highly anisotropic with surprisingly high mean-squared vibrations of Cs ions which exceed thermal vibrations of very light F ions. Such a behavior could be explained by the difference of the force constants between the Cs and F sites which overcomes the effect associated with the difference between masses of Cs and F ions. Nickel cations reveal very high on-site force constants, i.e. very low amplitudes of thermal vibrations, as they form some kind of rigid rods in the CsNiF(3) lattice. Calculated elastic constants indicate CsNiF(3) to be rather a soft material.