Theoretical Study of the Phonon Energy and Specific Heat of Ion-Doped LiCsSO 4 -Bulk and Nanoparticles.

Using a microscopic model, the temperature dependence of two phonon modes, ω0 = 32 cm -1 and 72 cm -1 , and their damping of the ferroelastic LiCsSO 4 compound, are calculated within Green's function technique. It is observed that the first mode increases whereas the second one decreases with increasing temperature T . This different behavior is explained with different sign of the anharmonic spin-phonon interaction constant. At the ferroelastic phase transition temperature TC, there is a kink in both modes due to the spin-phonon interaction. The phonon damping increases with T , and again shows an anomaly at TC. The contributions of the spin-phonon and phonon-phonon interactions are discussed. TC is reduced by decreasing the nanoparticle size, and can be enhanced by doping with K, Rb and NH 4 ions at the Cs site. TC decreases by doping with Na, K or Rb on the Li site. The specific heat Cp also shows a kink at TC. Cp decreases with decreasing nanoparticle size and the peak disappears, whereas Cp increases with increasing K ion doping concentration.