Hiroyuki Takenaka

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Organometal halide perovskites have been intensely studied in the past 5 years, inspired by their certified high photovoltaic power conversion efficiency. Some of these materials are room-temperature ferroelectrics. The presence of switchable ferroelectric domains in methylammonium lead triiodide, CH3NH3PbI3, has recently been observed via piezoresponse(More)
Relaxor ferroelectrics have been a focus of intense attention due to their anomalous properties, and understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics. We investigate the local structure and dynamics in 75%PbMg(1/3)Nb(2/3)O(3)-25%PbTiO(3) using molecular dynamics simulations and the(More)
We present a modified bond-valence model of PbTiO3 based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and(More)
Relaxor ferroelectrics have been used in a variety of applications, such as piezoelectrics, capacitors, and transducers. Relaxors exhibit broadened permittivity peaks with dispersion of the frequencydependent dielectric constant. In particular, T ;max, the temperature maximum of the ð!Þ, follows the Vogel Fulcher relationship described by ! 1⁄4 !0e Ua=kBðT(More)
Hybrid halide perovskites exhibit nearly 20% power conversion efficiency, but the origin of their high efficiency is still unknown. Here, we compute the shift current, a dominant mechanism of the bulk photovoltaic (PV) effect for ferroelectric photovoltaics, in CH₃NH₃PbI₃ and CH₃NH₃PbI(3-x)Cl(x) from first-principles. We find that these materials give(More)
Despite more than 50 years of investigation, it is still unclear how the underlying structure of relaxor ferroelectrics gives rise to their defining properties, such as ultrahigh piezoelectric coefficients, high permittivity over a broad temperature range, diffuse phase transitions, strong frequency dependence in dielectric response, and phonon anomalies.(More)
Relaxor ferroelectrics have been a focus of intense attention due to their fascinating physical properties. Their diffuse phase transitions have been explained by the polar nanoregion model. Nevertheless, fundamental characterization of structure and dynamics in relaxors is still a long-standing challenge. Better scientific understanding of the microscopic(More)
Density functional theory study of hypothetical PbTiO3-based oxysulfides John A. Brehm,* Hiroyuki Takenaka,† Chan-Woo Lee,‡ Ilya Grinberg,§ Joseph W. Bennett,‖ Michael Rutenberg Schoenberg,¶ and Andrew M. Rappe# The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, 231 S. 34th Street, Philadelphia, Pennsylvania(More)
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