Browsing by Author "Tian, R"
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- ItemAnomalous structural evolution and glassy lattice in mixed‐halide hybrid perovskites(Wiley, 2022-05) Shahrokhi, S; Dubajic, M; Dai, ZZ; Bhattacharyya, S; Mole, RA; Rule, KC; Bhadbhade, MM; Tian, R; Mussakhanuly, N; Guan, X; Yin, Y; Nielsen, MP; Hu, L; Lin, CH; Chang, SLY; Wang, DY; Kabakova, IV; Conibeer, G; Bremner, S; Li, XG; Cazorla, C; Wu, THybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed‐halide hybrid perovskite single crystals of MAPbI3‐xBrx (MA = CH3NH3+ and x = 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed‐halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed‐halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties. © 2022 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
- ItemPhononic structure engineering: the realization of einstein rattling in calcium cobaltate for the suppression of thermal conductivity(Springer Nature, 2016-07-26) Tian, R; Kearley, GJ; Yu, DH; Ling, CD; Pham, AN; Embs, JP; Shoko, E; Li, SPhonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9. © The Author(s) 2016 - CC-BY - This work is licensed under a Creative Commons Attribution 4.0 International License.