Fizik Bilim Dalı

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A phenomenological study on ferroelectric β-glycine
(2021) Kiracı, A; 42475
The anomalous behavior of the specific heat for (Formula presented.) -glycine was analyzed in terms of the compressible Ising model approximately 5 K below and above the ferrelectric-paraelectric phase transition temperature of TC = 252 K. The obtained value of the critical exponent (Formula presented.) = 0.12 in the ferroelectric phase (T < TC) was consistent with that predicted from the 3-d Ising model ((Formula presented.) = 0.13), while the obtained value of (Formula presented.) = 0.32 in the paraelectric phase (T > TC) was consistent with that predicted from the 2-d potts model ((Formula presented.) = 0.30). Some thermodynamic quantities such as the internal energy, the entropy and the free energy of (Formula presented.) -glycine were then predicted in terms of these extracted values of the critical exponents close to the phase transition temperature of TC. Our calculated values of the thermodynamic quantities are in good agreement with the observed data.
Damping constant and the inverse relaxation time calculated as a function of pressure using the X-ray diffraction data close to the cubic-tetragonal phase transition in SrTiO3
(2019) Kiracı, Ali; Kiracı, Ali; 42475
The damping constant is calculated as a function of pressure at room temperature using the normalized intensity as an order parameter near the cubic-tetragonal phase transition in SrTiO3. The observed X-ray diffraction data are used for the normalized intensities to calculate the damping constant () from the pseudospin-phonon (PS) coupled model and the energy fluctuation (EF) model, which is fitted to the observed FWHM data from the literature for comparison. Using the calculated values, the pressure dependence of the inverse relaxation time () is predicted close to the cubic-tetragonal phase transition in SrTiO3. Our calculated damping constant from both models explains the observed FWHM satisfactorily and our prediction of the inverse relaxation time can also be compared with the experimental measurements when they are available in the literature.
Investigation of the anomalous behavior of the linewidth (damping constant) for the Raman Ag modes in SrSnO3 ceramic
(2022) Kiracı, Ali; 42475
The anomalous behavior of the linewidth (damping constant) for the Raman Ag modes of 223 cm−1 and 260 cm−1 that contribute to the phase transition mechanism of SrSnO3 ceramic close to the phase transition temperatures of TC1= 650 K and TC2= 530 K, respectively, was calculated as function of temperature from the pseudospin-phonon coupled (PS) and the energy fluctuation (EF) models below (T < TC) and above (T > TC) the phase transition temperatures of TC1 and TC2. For this calculation, the frequency shift of these modes were associated as the order parameter and disorder parameter below and above TC, respectively. Our results are in good agreement with the observed linewidth of these 223 cm−1 and 260 cm−1 Raman Ag modes. In addition, these two models (PS and EF) were used to deduce the values of the activation energy for SrSnO3 ceramic below and above the phase transition temperatures of TC1 and TC2.
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.

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