Theoretical Study of the Phonon Energy and Specific Heat of Ion-Doped LiCsSO4-Bulk and Nanoparticles

Using a microscopic model, the temperature dependence of two phonon modes, omega 0 = 32 cm(-1) and 72 cm(-1), and their damping of the ferroelastic LiCsSO4 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 NH4 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.