Ortak Dersler Bölümü

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Citation - WoS: 1
Citation - Scopus: 1
Effect of Estimation Under Nonnormality on the Phase Ii Performance of Linear Profile Monitoring Approaches
(Wiley, 2019) Aytacoglu, Burcu; Bayrak, Ozlem Turker
The number of studies about control charts proposed to monitor profiles, where the quality of a process/product is expressed as function of response and explanatory variable(s), has been increasing in recent years. However, most authors assume that the in-control parameter values are known in phase II analysis and the error terms are normally distributed. These assumptions are rarely satisfied in practice. In this study, the performance of EWMA-R, EWMA-3, and EWMA-3(d(2)) methods for monitoring simple linear profiles is examined via simulation where the in-control parameters are estimated and innovations have a Student's t distribution or gamma distribution. Instead of the average run length (ARL) and the standard deviation of run length, we used average and standard deviation of the ARL as performance measures in order to capture the sampling variation among different practitioners. It is seen that the estimation effect becomes more severe when the number of phase I profiles used in estimation decreases, as expected, and as the distribution deviates from normality to a greater extent. Besides, although the average ARL values get closer to the desired values as the amount of phase I data increases, their standard deviations remain far away from the acceptable level indicating a high practitioner-to-practitioner variability.
Citation - WoS: 2
Citation - Scopus: 2
Thermoluminescence Characterization of (Ga2se3)0.25 - (Ga2s3)0.75 Single Crystal Compounds
(Elsevier Sci Ltd, 2020) Guler, I; Gasanly, N. M.; Isik, M.
Ga2Se3 and Ga2S3 compounds take attention due to their potential applications in photovoltaics. Defects and impurities may affect the quality of optoelectronic devices. Therefore, it is worthwhile to determine the parameters (activation energy, order of kinetics, frequency factor) of traps associated with the defects and/or impurities. The aim of the present paper is to investigate the trapping parameters of (Ga2Se3)(0.25) - (Ga2S3)(0.75) single crystal which is one of the member of (Ga2Se3)(x) - (Ga2S3)(1-x) mixed crystals. For this purpose, thermoluminescence (TL) experiments were performed on (Ga2Se3)(0.25) - (Ga2S3)(0.75) single crystals in the 10-300 K region. TL spectra were also recorded using various heating rates in between 0.2 and 1.0 K/s and stopping temperatures from 30 to 60 K to get the detailed information about the characteristics of the trapping parameters. TL glow curves exhibited the overlapped peaks. The stopping temperature experimental data indicated that traps present quasi-continuous distribution within the band gap. Initial rise method analyses were applied to get the activation energies of quasi-continuously distributed revealed traps. Thermal activation energies of distributed traps were found as increasing from 108 to 246 meV as stopping temperature was increased from 30 to 60 K. The structural characteristics (lattice constants and atomic compositions of constituent elements) of used compound were also reported in the present study.
Citation - WoS: 7
Citation - Scopus: 7
Exploring the Linear and Nonlinear Optical Behavior of (Tlins2)0.75 Insights From Ellipsometry Measurements
(Elsevier, 2023) Guler, I.; Gasanly, N.; Isik, M.
The search for layered structured new semiconductor materials with remarkable optical properties has become a driving force, especially for materials science. Tl2In2S3Se [(TlInS2)0.75(TlInSe2)0.25], a fascinating compound, holds great promise for advanced photonic and optoelectronic applications. In the present study, the linear and nonlinear optical properties of Tl2In2S3Se layered single crystals were studied by ellipsometry measurements. The variation of refractive index, extinction coefficient, absorption coefficient and skin depth with energy were investigated. Applying the derivative analysis technique to the absorption spectrum, indirect bandgap was found as 2.19 eV. The refractive index data was analyzed considering single-effective-oscillator model. The lattice dielectric constant, plasma frequency, carrier density to the effective mass ratio and zero-frequency refractive index were found. Moreover, the change in optical conductivity with energy yielded to determine the direct bandgap as 2.40 eV. The optical parameters of nonlinear refractive index, first-and third-order nonlinear susceptibilities were also reported.
Citation - WoS: 4
Citation - Scopus: 4
Impaired Redox Homeostasis in the Heart Left Ventricles of Aged Rats Experiencing Fast-Developing Severe Hypobaric Hypoxia
(Springer, 2019) Agascioglu, Eda; Colak, Ridvan; Demirel, Haydar; Cakatay, Ufuk
Despite its rare occurrence, humans and animals have been prone to getting fast developing severe hypobaric hypoxia. Understanding the redox homeostasis related response of an aging heart to this type of hypoxia are crucially important, since the metabolism of myocardial tissue depends on the redox status of proteins. Rodents can tolerate hypoxic stress better than human subjects. This study was aimed at investigating the effects of fast developing severe hypobaric hypoxia on redox status biomarkers; such as, advanced oxidation protein products (AOPP), lipid hydroperoxides (LHPs), protein carbonyl groups (PCO), protein thiol groups (P-SH), and total thiol groups (T-SH) on the myocardial left ventricles of young and aged Wistar rats. The rats were gradually ascended and exposed to an 8000-meter hypobaric hypoxia. While AOPP levels showed no difference, the TSH and PSH concentrations decreased, and the PCO and LHP increased in both of the hypoxic groups than the controls. The TSH and PSH were lower, and AOPP, PCO and LHP were found to be higher in the elderly hypoxic groups than in the young ones. The significant outcome of the study represents that an 8000-meter hypobaric hypoxia could be considered as a severe hypoxic stress, but not life-treating for the rats and would affect both the young and aged left ventricles similarly in respect to impaired redox status. However, if the percentage increases are taken into consideration, it seems that the higher rate of protein oxidation occurs in young hearts; meanwhile aged hearts are more prone to T-SH oxidation.
Citation - WoS: 4
Citation - Scopus: 6
Structural and Optical Properties of (Tlins2)0.75 Thin Films Deposited by Thermal Evaporation
(Springer, 2023) Guler, I.; Isik, M.; Gasanly, N.
Layered semiconductor materials have become a serious research topic in recent years, thanks to their effective optical properties. In this article, the thin-film structure of Tl2In2S3Se [(TlInS2)(0.75)(TlInSe2)(0.25)] material with layered structure was grown by thermal evaporation method. The structural, morphological, and optical properties of the deposited thin films were examined. X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and atomic force microscopy (AFM) techniques were used to get information about structural and morphological properties of the thin films. XRD pattern presented well-defined peaks associated with monoclinic crystalline structure. The crystallite size, dislocation density, and lattice strain of the films were also obtained from the analyses of XRD pattern. EDS analysis showed that atomic compositional ratios of the Tl, In, S, and Se elements are consistent with chemical formula of Tl2In2S3Se. The optical characterization of thin film was performed using transmission and Raman spectroscopy techniques. Raman spectrum offered information about the vibrational modes of the thin film. The analyses of the transmission spectrum presented the indirect and direct band gap energies of the Tl2In2S3Se thin film as 2.23 and 2.52 eV, respectively. The further analyses on the absorption coefficient resulted in Urbach energy of 0.58 eV.
Raman Wavenumbers Calculated as a Function of Pressure From the Mode Gruneisen Parameter of Pzt (X = 0.48) Ceramic Close To the Monoclinic-Cubic Transition
(World Scientific Publ Co Pte Ltd, 2019) Kiraci, A.
The isothermal mode Gruneisen parameter gamma(T)(P) of some Raman modes in PbZr1-xTixO3 (PZT, x = 0.48) were calculated as a function of pressure by means of the observed pressure-dependent volume data of PZT (x = 0.48) crystal from the literature at room temperature of 298 K. Those calculated values of gamma(T)(P) were then used to compute the pressure dependence of the Raman modes in PZT (x = 0.48) ceramic studied here. The observed and calculated values of the Raman wavenumbers in PZT were in good agreement, which indicates that the isothermal mode Gruneisen parameter can also be used to predict the pressure-dependent wavenumbers of some other perovskite-type crystals. Additionally, the pressure dependence of the thermodynamic quantities such as isothermal compressibility kappa(T), thermal expansion alpha(P) and the specific heat C-P - C-V of PZT (x = 0.48) ceramic were predicted at constant temperature of 298 K. Here, the experimentally measurable thermodynamic quantities calculated for PZT (x = 0.48) ceramics provide theoretically a significant opportunity for testing.
Citation - WoS: 4
Citation - Scopus: 4
Growth and Characterization of Nabi(Mo0.5w0.5o4)2 Single Crystal: a Promising Material for Optoelectronic Applications
(Elsevier Sci Ltd, 2023) Isik, M.; Guler, I.; Gasanly, N. M.
The structural and optical characteristics of NaBi(Mo0.5W0.5O4)2 single crystals grown by Czochralski method were investigated. X-ray diffraction (XRD) pattern exhibited four well-defined peaks related to tetragonal crystalline structure with a space group I41/a. Raman and infrared transmittance spectra were recorded to investigate vibrational properties of the compound. Room temperature transmission spectrum was measured to reveal band gap energy of the crystal. The derivative spectral and absorption coefficient analyses resulted in direct band gap energy of 3.19 and 3.18 eV, respectively. Urbach energy of the crystal was also determined as 0.17 eV from photon energy dependency of absorption coefficient. The structural and optical parameters ob-tained for NaBi(Mo0.5W0.5O4)2 were compared with the parameters of the NaBi(XO4)2 (X: Mo,W) compounds to understand the effect of the composition on the studied properties. The reported characteristics of NaBi (Mo0.5W0.5O4)2 point out that the compound has significant potential to be used in optoelectronic devices.
Citation - Scopus: 1
Linear Contrasts in One-Way Classification Ar(1) Model With Gamma Innovations
(Hacettepe Univ, Fac Sci, 2016) Senoglu, Birdal; Bayrak, Ozlem Turker
In this study, the explicit estimators of the model parameters in oneway classification AR(1) model with gamma innovations are derived by using modified maximum likelihood (MML) methodology. We also propose a new test statistic for testing linear contrasts. Monte Carlo simulation results show that the MML estimators have higher efficiencies than the traditional least squares (LS) estimators and the proposed test has much better power and robustness properties than the normal theory test.
Citation - WoS: 5
Citation - Scopus: 5
Growth and Temperature Tuned Band Gap Characteristics of Nabi(Moo4)2 Single Crystal
(Iop Publishing Ltd, 2023) Isik, M.; Guler, I; Gasanly, N. M.
Structural and optical properties of double sodium-bismuth molybdate NaBi(MoO4)(2) semiconductor compound was investigated by x-ray diffraction, Raman and transmission experiments. From the x-ray diffraction experiments, the crystal that has tetragonal structure was obtained. Vibrational modes of the crystal were found from the Raman experiments. Transmission experiments were performed in the temperature range of 10-300 K. Derivative spectroscopy analysis and absorption spectrum analysis were performed to get information about the change in band gap energy of the crystal with temperature. It was observed that the band gap energies of the crystal at different temperatures obtained from these techniques are well consisted with each other. By the help of absorption spectrum which was obtained from transmission measurements performed at varying temperatures, absolute zero value of the band gap and average phonon energy as 3.03 +/- 0.02 eV and Eph = 24 +/- 0.2 meV, respectively. Moreover, based on absorption spectrum analysis the Urbach energy of the crystal was obtained as 0.10 eV.
Citation - WoS: 7
Citation - Scopus: 8
Damping Constant and the Relaxation Time Calculated for the Lowest-Frequency Soft Mode in the Ferroelectric Phase of Cd2nb2o7
(Elsevier Gmbh, Urban & Fischer verlag, 2016) Yurtseven, H.; Kiraci, A.
The temperature dependence of the phonon frequency omega(ph) and of the damping constant Gamma(sp) due to pseudospin-phonon coupling of the lowest-frequency soft mode is calculated in the ferroelectric phase near the transition temperature (T-C =196 K) in Cd2Nb2O7. Raman frequency of the soft mode is used as an order parameter which is calculated from the molecular field theory. On that basis, the damping constant is calculated by fitting the expressions from the pseudospin-phonon coupled model and the energy fluctuation model to the observed linewidth from the literature below T-C in Cd2Nb2O7. From our analysis, we find that the molecular field theory is adequate for the soft mode behaviour and that both models are also satisfactory for the divergence behaviour of the damping constant as T-C is approached from the ferroelectric phase in Cd2Nb2O7. Values of the activation energy U are extracted from the temperature dependence of the damping constant (HWHM) of the soft mode in the ferroelectric phase of this crystal. Also, the inverse relaxation time is predicted using the Raman frequency and damping constant close to the T-C in the ferroelectric phase of Cd2Nb2O7, which increases considerably as T-C is approached from the ferroelectric phase, as observed experimentally. (C) 2016 Elsevier GmbH. All rights reserved.
Citation - WoS: 4
Citation - Scopus: 4
Optical Characterization of (Tlins2)0.5 Crystal by Ellipsometry: Linear and Optical Constants for Optoelectronic Devices
(Springer, 2023) Guler, I.; Isik, M.; Gasanly, N.
TlInSSe [(TlInS2)(0.5)(TlInSe2)(0.5)] crystals have garnered significant attention as promising candidates for optoelectronic applications due to their exceptional optoelectrical characteristics. This study focused on investigating the linear and nonlinear optical properties of TlInSSe layered single crystals through ellipsometry measurements. The X-ray diffraction analysis revealed the presence of four distinct peaks corresponding to a monoclinic crystalline structure. In-depth analysis was conducted to examine the variations of refractive index, extinction coefficient, and complex dielectric function within the energy range of 1.25-6.15 eV. By employing derivative analysis of the absorption coefficient and utilizing the Tauc relation, the indirect and direct bandgap energies of TlInSSe crystals were determined to be 2.09 and 2.26 eV, respectively. Furthermore, this research paper presents findings on oscillator energy, dispersion energy, Urbach energy, zero and high frequency dielectric constants, plasma frequency, carrier density to effective mass ratio, nonlinear refractive index, and first-order and third-order nonlinear susceptibilities of TlInSSe crystals.
Citation - WoS: 6
Citation - Scopus: 6
A Phenomenological Study on Ferroelectric Pyridinium Tetrafluoroborate (C5nh6) Bf4
(Elsevier, 2019) Kiraci, A.
The temperature dependence of the specific heat C-V of (C5NH6)BF4 was analyzed according to a power law formula with a critical exponent alpha deduced from the compressible Ising model in the vicinity of the phase transition temperatures of T-C1 = 238 K and T-C2 = 204 K. The extracted values of the critical exponent alpha within the temperature intervals of vertical bar T - T-C1 vertical bar < 6 K and also T - T-C2 < 6 K were consistent with that predicted from the 3d-Ising model (alpha = 0.10) while obtained values of alpha within the temperature interval of T-C2 - T < 6 K were consistent with that predicted from 2-d potts model (alpha = 0.30). In addition, the thermodynamic quantities: the internal energy (U), the entropy (S) and the Helmholtz free energy (F) of this compound were calculated on the basis of the extracted values of the critical exponent a below and above the phase transition temperatures of T-C1 and T-C2.
Citation - WoS: 14
Citation - Scopus: 15
Structural and Optical Properties of Ga2se3 Crystals by Spectroscopic Ellipsometry
(Springer, 2019) Isik, M.; Gasanly, N. M.; Gasanova, L. G.; Babayeva, R. F.; Guler, I.
Optical and crystalline structure properties of Ga2Se3 crystals were analyzed utilizing ellipsometry and x-ray diffraction (XRD) experiments, respectively. Components of the complex dielectric function (epsilon=epsilon(1)+i epsilon(2)) and refractive index (N=n+ik) of Ga2Se3 crystals were spectrally plotted from ellipsometric measurements conducted from 1.2eV to 6.2eV at 300K. From the analyses of second-energy derivatives of epsilon(1) and epsilon(2), interband transition energies (critical points) were determined. Absorption coefficient-photon energy dependency allowed us to achieve a band gap energy of 2.02eV. Wemple and DiDomenico single effective oscillator and Spitzer-Fan models were accomplished and various optical parameters of the crystal were reported in the present work.
Citation - WoS: 5
Citation - Scopus: 5
Temperature Dependence of the Polarization, Dielectric Constant, Damping Constant and the Relaxation Time Close To the Ferroelectric-Paraelectric Phase Transition in Linbo3
(Elsevier Gmbh, Urban & Fischer verlag, 2017) Yurtseven, H.; Kiraci, A.
We calculate the order parameter (spontaneous polarization) and the inverse dielectric susceptibility at various temperatures in the ferroelectric phase of LiNbO3 for its ferroelectric-paraelectric phase transition (T-C =1260 K) using the Landau phenomenological model. For this calculation, the Raman frequencies of the soft optic mode (TO1) are used as the order parameter and the fitting procedure is employed for both the order parameter and the inverse dielectric susceptibility by means of the observed data from the literature. The temperature dependences of the damping constant and the inverse relaxation time are also computed using the pseudospin-phonon coupled model and the energy fluctuation model for the ferroelectric phase of LiNbO3. The activation energy is deduced from the damping constant for both models studied and compared with the k(B)T(C) value of LiNbO3. We find that the order parameter (Raman frequency of the TO1 mode) and the inverse dielectric susceptibility decrease with increasing temperature, as expected from the mean field model. We also find that the damping constant and the inverse relaxation time of this soft mode increases and decreases, respectively, with increasing temperature on the basis of the two models studied in the ferroelectric phase of LiNbO3. This indicates that our method of calculation is satisfactory to describe the observed behaviour of the ferroelectric-paraelectric phase transition in LiNbO3. (C) 2016 Elsevier GmbH. All rights reserved.
Citation - WoS: 10
Citation - Scopus: 11
Temperature Dependence of the Damping Constant and the Relaxation Time Close To the Tetragonal-Cubic Phase Transition in Srzro3
(Elsevier Science Bv, 2017) Kiraci, A.; Yurtseven, H.
The damping constant Gamma(sp) due to the pseudospin-phonon coupling is calculated as a function of temperature using the pseudospin-phonon coupled model and the energy fluctuation model close to the tetragonal-cubic transition (T-C = 1443 K) in SrZrO3. Using the observed Raman frequencies and the linewidth (FWHM) of the soft modes (E-g and A(1g)) from the literature, predictions of both models studied, are examined for the tetragonal-cubic transition in this crystalline system. Values of the activation energy U are extracted and also the inverse relaxation time is predicted as a function of temperature close to the phase transition studied in SrZrO3. Divergence behaviour of the damping constant (FWHM) of the soft modes is predicted from both models as also observed experimentally when T-C is approached from the tetragonal to the cubic phase in SrZrO3. The relaxation time also diverges close to the T-C in this crystal. It is indicated that the tetragonal cubic transition is of a second order as predicted from both models studied here, as also observed experimentally in SrZrO3. (C) 2016 Elsevier B.V. All rights reserved.
Citation - WoS: 8
Citation - Scopus: 8
Thermoluminescence in Gallium Sesquisulfide Single Crystals: Usual and Unusual Heating Rate Dependencies
(Elsevier Gmbh, 2018) Isik, M.; Gasanova, L.; Mahammadov, A.; Gasanly, N.; Guler, I
Thermoluminescence (TL) experiments were conducted for Ga2S3 crystals to obtain information about trapping parameters. TL measurements were performed from 10 to 300 K with varying heating rates in the range of 0.2-0.8 K/s. Two TL glow peaks centered at 44K (peak A) and 91 K (peak B) were observed at heating rate of beta=0.5 K/s. For peak A, TL intensity decreased whereas that for peak B increased with elevating the heating rates that means anomalous heating rate occurred for peak B. TL glow curves were analyzed using initial rise method to find activation energies of traps. Distribution of trap centers was investigated using T-max - T-stop method. Quasi-continuous distributions with increasing activation energies from 40 to 135 meV and 193 to 460 meV were attributed to trap centers A and B, respectively. (C) 2018 Elsevier GmbH. All rights reserved.
Citation - WoS: 4
Citation - Scopus: 8
Creating Infographics Based on the Bridge21 Model for Team-Based and Technology-Mediated Learning
(informing Science inst, 2019) Sullivan, Kevin; Akkoyunlu, Buket; Nuhoglu-Kibar, Pinar
Aim/Purpose The main aim of this study was modeling a collaborative process for knowledge visualization, via the creation of infographics. Background As an effective method for visualizing complex information, creating infographics requires learners to generate and cultivate a deep knowledge of content and enables them to concisely visualize and share this knowledge. This study investigates creating infographics as a knowledge visualization process for collaborative learning situations by integrating the infographic design model into the team-based and technology-mediated Bridge21 learning model. Methodology This study was carried out using an educational design perspective by conducting three main cycles comprised of three micro cycles: analysis and exploration; design and construction; evaluation and reflection. The process and the scaffolding were developed and enhanced from cycle to cycle based on both qualitative and quantitative methods by using the infographic design rubric and researcher observations acquired during implementation. Respectively, twenty-three, twenty-four, and twenty-four secondary school students participated in the infographic creation process cycles. Contribution This research proposes an extensive step-by-step process model for creating infographics as a method of visualization for learning. It is particularly relevant for working with complex information, in that it enables collaborative knowledge construction and sharing of condensed knowledge. Findings Creating infographics can be an effective method for collaborative learning situations by enabling knowledge construction, visualization and sharing. The Bridge21 activity model constituted the spine of the infographic creation process. The content generation, draft generation, and visual and digital design generation components of the infographic design model matched with the investigate, plan and create phases of the Bridge21 activity model respectively. Improvements on infographic design results from cycle to cycle suggest that the revisions on the process model succeeded in their aims. The rise in each category was found to be significant, but the advance in visual design generation was particularly large. Recommendations for Practitioners The effectiveness of the creation process and the quality of the results can be boosted by using relevant activities based on learner prior knowledge and skills. While infographic creation can lead to a focus on visual elements, the importance of wording must be emphasized. Being a multidimensional process, groups need guidance to ensure effective collaboration. Recommendation for Researchers The proposed collaborative infographic creation process could be structured and evaluated for online learning environments, which will improve interaction and achievement by enhancing collaborative knowledge creation. Impact on Society In order to be knowledge constructors, innovative designers, creative communicators and global collaborators, learners need to be surrounded by adequate learning environments. The infographic creation process offers them a multidimensional learning situation. They must understand the problem, find an effective way to collect information, investigate their data, develop creative and innovative perspectives for visual design and be comfortable for using digital creation tools. Future Research The infographic creation process could be investigated in terms of required learner prior knowledge and skills, and could be enhanced by developing pre-practices and scaffolding.
Citation - WoS: 5
Citation - Scopus: 6
Ring Annulation Versus Alkylation of Pyrrole With Α-Phosphoryl Ketones
(Pergamon-elsevier Science Ltd, 2016) Tasgin, Dilek Isik; Unaleroglu, Canan
In this study; novel aryl, heteroaryl, pyrrolyl and phosphoryl groups containing pyrrolizines were synthesized by the ring annulation reaction of aryl or heteroaryl substituted alpha-phosphoryl-alpha,beta-unsaturated ketones with pyrrole under mild reaction conditions. This domino reaction involves scandium triflate catalyzed addition of pyrrole to the double bond of alpha-phosphoryl-alpha,beta-unsaturated ketones, carbonyl group and finally ring annulation sequences. The presented work provided a convenient way for the synthesis of novel bispyrrolic compounds. (C) 2016 Elsevier Ltd. All rights reserved.
Citation - WoS: 3
Citation - Scopus: 3
A Phenomenological Study on Ferroelastic Kh3(Seo3)2 and Kd3(Seo3)2
(Pergamon-elsevier Science Ltd, 2021) Kiraci, A.
The anomalous behavior of the Brillouin frequency shift omega of the soft acoustic mode for KH3(SeO3)(2) and KD3(SeO3)(2), and also the volume thermal expansion proportional to(V) and the specific heat C of KH3(SeO3)(2) have been analyzed for various temperature intervals close to the phase transition temperatures TC at around 211 K and 302 K, respectively. Our extracted values of the critical exponent from C and proportional to(V) of KH3(SeO3)(2) within the temperature intervals of vertical bar T -T-C vertical bar < 7 K are nearly zero, while the critical exponent extracted from the Brillouin frequency shift data of both KH3(SeO3)(2) and KD3(SeO3)(2) within the same temperature interval of vertical bar T -T-C vertical bar < 7 K decrease very rapidly toward to zero as the phase transition temperatures are approached. Our results indicate that the critical behavior of omega, proportional to(V) and C of both KH3(SeO3)(2) and KD3(SeO3)(2) can be described with the same critical exponent within the temperature interval of vertical bar T -T-C vertical bar < 2 K.
Citation - WoS: 4
Citation - Scopus: 5
Calculation of the Raman Frequency, Damping Constant (Linewidth) and the Relaxation Time Near the Tetragonal-Cubic Transition in Pbtio3
(Elsevier Gmbh, 2017) Yurtseven, H.; Kiraci, A.
Frequencies, damping constants and the relaxation times of some Raman modes including the two soft modes are calculated as a function of pressure near the tetragonal-cubic transition in PbTiO3. Calculation of the Raman frequencies is performed using the observed volume data from the literature by means of the mode Gruneisen parameter at various pressures. Pressure dependence of the damping constant and the relaxation time is predicted using the pseudospin-phonon coupled model and the energy fluctuation model by considering that the Raman frequency can be taken as the order parameter (spontaneous polarization) for the tetragonal-cubic transition in PbTiO3. Expressions from both models for the damping constants are fitted to the observed Raman linewidths of the two soft modes and for the other Raman modes the damping constant and the relaxation time are predicted close to the transition. We find that damping constants diverge and the inverse relaxation time decreases for the soft modes with increasing pressure near the critical pressure (P-c similar to 11 GPa). The other Raman modes exhibit unusual critical behavior. Our results indicate that the observed behavior of the Raman frequencies can be predicted from the volume data through the mode Gruneisen parameter for the tetragonal-cubic transition in PbTiO3. The damping constant and the relaxation time for the Raman modes can also be predicted adequately using the pseudospin-phonon coupled model and the energy fluctuation model to explain the mechanism of the phase transition between the tetragonal and cubic phases in PbTiO3. (C) 2017 Elsevier GmbH. All rights reserved.

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