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Citation - WoS: 4
Citation - Scopus: 2
Elastic and Anelastic Behavior Associated With Structural Transitions in CsPbBr3
(Amer Chemical Soc, 2025) Luo, Pingjing; He, Zhengwang; Yang, Dexin; Aktas, Oktay; Ding, Xiangdong; Zhang, Xuefeng
Strain coupling and relaxation dynamics critically influence the photovoltaic and photoluminescent performances of metal halide perovskites. Here, resonant ultrasound spectroscopy is employed to study the elastic and anelastic properties associated with the octahedral tilting transitions in the optoelectronic semiconductor CsPbBr3 over the temperature range 303-468 K. The cubic-to-tetragonal transition near 405 K is marked by pronounced elastic softening accompanied by a sharp increase in acoustic loss. High anelastic loss below this transition reveals the presence of mobile ferroelastic twin walls that become pinned by lead vacancies at a temperature interval near 380 K in the tetragonal phase. The elastic softening in the cubic phase is strongly correlated to dynamic effects such as the local polar fluctuations. This local disordered effect is further verified by the anomalously high attenuation in the orthorhombic structure, in which the ferroelastic twin walls might become mobile.
The Temperature Dependence of Elastic Constants and Zener Anisotropy in Single Crystal KTaO3 Below Room Temperature
(Pleiades Publishing Ltd, 2025) Li, C. H.; Ding, X. D.; Carpenter, M. A.; Aktas, O.
We present the first experimental report of the complete elastic tensor of incipient ferroelectric KTaO3 between 9.6 and 307 K, using resonant ultrasound spectroscopy (RUS). The elastic constants C-11 and C-44, as well as the bulk and shear moduli increase roughly linearly upon cooling. Deviations from such dependence are a result of impurities. Zener anisotropy factor is 0.697 at room temperature and decreases to 0.627 at 9.6 K, indicating increasing elastic anisotropy. The temperature dependence of C-12 is anomalous with no stiffening on cooling, which could be a result of different defect species influencing C-12 but not the other constants, possibly due to defect anisotropy. The values of elastic constants and anisotropy are compared at room temperature and below 10 K with the literature reporting experimental results and density functional theory calculations.
Citation - WoS: 2
Citation - Scopus: 1
Phase Transition Behavior in Ferroelectric BaTi0.8Zr0.2O3: Evidence of Polar Cluster Reorientation Above Curie Temperature
(Aip Publishing, 2025) Aktas, Oktay; Romero, Francisco Javier; He, Zhengwang; Linyu, Gan; Ding, Xiangdong; Martin-Olalla, Jose-Maria; Lookman, Turab
We study the phase transition behavior of the ferroelectric BaTi0.8Zr0.2O3 in the paraelectric region above the Curie temperature. The investigation of the phase transition using caloric, dielectric, and elastic measurements indicates that the ferroelectric transition at T-c = 292 K is continuous and displays weakly relaxor characteristics. The nonlinear scaling of entropy and polarization, as well as the temperature dependencies of dielectric and elastic properties, indicates the presence of local structures in the paraelectric phase. The non-zero remnant polarization is measured up to a characteristic temperature T* similar to 350 K. This temperature coincides with the temperature where the dielectric constant deviates from the Curie-Weiss law and is identified as the coherence temperature T*, associated with the formation of static polar nanostructures. Finally, direct current field cooling in the paraelectric phase using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, attributed to the re-orientation of polar nanostructures. The observed remnant effect, along with the temperature dependence of the piezoelectric effect and its time dependence below and above T*, is consistent with increased coherence and slower dynamics of these structures on cooling, leading to symmetry-disallowed remnant piezoelectricity due to glassy behavior below T*.

Journal	Count
Journal of Applied Physics	1
Physics of the Solid State	1
The Journal of Physical Chemistry Letters	1

Aktaş, Oktay

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3

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3

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3

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