Browsing by Author "Yurtseven, Hamit"

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Damping Constant (Linewidth) and the Relaxation Time of the Brillouin LA Mode for the Ferroelectric-Paraelectric Transition in PbZr1-xTixO3
(IEEE-INST Electrical Electronics Engineers INC, 2016) Kiracı, Ali; Kiracı, Ali; 42475
The damping constant (linewidth) of the longitudinal acoustic (LA) mode is calculated as a function of temperature using the observed Brillouin frequencies of this mode from the literature for the ferroelectric-paraelectric transition (T-C = 657 K) in PbZr1-xTixO3 (x = 0.45). For this calculation of the damping constant, the pseudospin-phonon coupled model and the energy fluctuation model are used by fitting to the observed data for the Brillouin frequencies of the LA mode in the ferroelectric (T < T-C) and paraelectric (T > T-C) phases of this compound (x = 0.45). Values of the activation energy are deduced for both ferroelectric and paraelectric phases. The relaxation time is also obtained by means of fitting to the observed data from the literature for the inverse relaxation time at various temperatures in the paraelectric phase of PbZr1-xTixO3. The temperature dependences of the damping constant and of the relaxation time with the values of the activation energy that we have calculated indicate that the pseudospin-phonon coupled model and the energy fluctuation model are capable of describing the ferroelectric-paraelectric transition (T-C = 657 K) in PbZr1-xTixO3 (x = 0.45) adequately.
Order-disorder transition in the ferroelectric LiTaO3
(2019) Kiracı, Ali; Yurtseven, Hamit; 42475
The temperature dependences of the damping constant and the relaxation time are calculated by using the Raman frequencies of a* and the lowest A(1) (TO) phonons in the ferroelectric phase close to the ferroelectric-paraelectric transition in LiTaO3 (T-C = 963 K). Both calculations are performed by considering the frequency as an order parameter for the pseudospin-phonon (PS) and the energy fluctuation (EF) models using the observed data from the literature. Values of the activation energies of this crystal are also deduced by using both models in this crystal. Our results show that the PS and EF models can describe the observed behavior adequately for the order-disorder transition in LiTaO3.
Phenomenological approaches on the Nd3+ doped ferroelectric LaBGeO5
(2021) Kiracı, Ali; Kiracı, Ali; Yurtseven, Hamit
Two phenomenological models, namely the compressible Ising model and Landau model, have been used to analyze the specific heat and the dielectric constant data, respectively for the pure and Nd3+ doped LaBGeO5 (LBG) crystals. The critical exponent of the specific heat was extracted in both ferroelectric and paraelectric phases of the crystals studied here within the temperature intervals of (Formula presented.) The extracted values of the critical exponent were then used to predict some thermodynamic quantities such as the enthalpy, entropy, and the Gibbs free energy. Regarding the analysis of dielectric constant data within the framework of the Landau theory, the observed values of the birefringence were associated with the order parameter below the transition temperature Tc. The Landau coefficients (Formula presented.) and (Formula presented.) were determined. Our results indicate a second-order phase transition mechanism.
Pressure dependence of the Raman frequency calculated from the volume data close to the ferroelectric-paraelectric transition in PbTiO3
(Taylor&Francis, 2017) Kiracı, Ali; Kiracı, Ali; 42475
We calculate the pressure dependence of the Raman frequencies of some Raman modes by using the observed volume data through the mode Gruneisen parameters for the ferroelectri-paraelectric transition in PbTiO3. The mode Gruneisen parameters which we have determined using the observed Raman frequencies for the soft modes, increase considerably with increasing pressure toward the transition pressure (PC similar to 11 GPa) from the tetragonal (ferroelectric) to the cubic (paraelectric) phase in PbTiO3. Variation of the mode Gruneisen parameter with the pressure is rather smooth for the other Raman modes studied as compared to the drastic change at PC for the soft modes in this ferroelectric material. Raman frequencies (energy shifts) of the modes which we have calculated, decrease from the ferroelectric to the paraelectric phase with the exception of the optical modes of E(3LO) and E(3TO) whose Raman frequencies increase with increasing pressure. This decrease and/or increase in the energy shifts for the Raman modes which we have calculated as also observed experimentally, shows a continuous manner, indicating a second order character rather that the first order transition from the tetragonal to the cubic phase in PbTiO3.
Pressure-dependent Raman modes near the cubic-tetragonal transition in strontium titanate
(Wiley, 2018) Kiracı, Ali; Yurtseven, Hamit; 42475
The pressure dependence of the Raman frequency shifts of various Raman modes is calculated at room temperature using the volume data from the literature for the cubic-tetragonal transition in SrTiO3. The isothermal mode Gruneisen parameters of those Raman modes are obtained, which decrease with increasing pressure for this molecular crystal. Calculated Raman frequencies are then used to predict the damping constant and the inverse relaxation time of those Raman modes as a function of pressure by means of the pseudospin-phonon (PS) coupled model and the energy fluctuation (EF) model to describe the cubic-tetragonal transition in SrTiO3. Also, the values of the activation energy are extracted for the Raman modes studied using both models (PS and EF). Our predicted damping constant and the inverse relaxation time for the Raman modes, can be compared with the experimental measurements close to the cubic-tetragonal transition in SrTiO3.