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Lead-free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature-derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs2AgBiBr6 double perovskites is demonstrated. Along with the thermochromism, temperature-dependent changes in the bond lengths of Ag Symbol of the Klingon Empire Br (R-Ag Symbol of the Klingon Empire Br) and Bi Symbol of the Klingon Empire Br (R-Bi Symbol of the Klingon Empire Br) are observed. The first-principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R-Ag Symbol of the Klingon Empire Br and R-Bi Symbol of the Klingon Empire Br at high temperatures. The synergy of anharmonic fluctuations and associated electron-phonon coupling, and the peculiar spin-orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs2AgBiBr6 shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead-free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation-induced bandgap narrowing of Cs2AgBiBr6 provide new insights for the further development of optoelectronic devices based on the lead-free halide double perovskites., Funding Agencies|Knut and Alice Wallenberg Foundation; National Natural Science Foundation of China [61704078, 61722403]; Natural Science Foundation of Jiangsu Province of China [BK20160990]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]; European Commission Marie Skodowska-Curie actions [691210]; European Commission SOLAR-ERA.NET; Swedish Energy Agency (Energimyndigheten); Swedish Research Council (FORMAS); China Scholarship Council; Recruitment Program of Global Youth Experts in China; National Key Research and Development Program of China [2016YFB0201204]; Program for JLU Science and Technology Innovative Research Team; Swedish Research Council (VR) [2014-4750]; Ministry of Education and Science of the Russian Federation [074-02-2018-327]

Continual progress has been achieved in information technology through unrelenting miniaturisation of the single memory bit in integrated ferromagnetic, ferroelectric, optical, and related circuits. However, as miniaturisation approaches its theoretical limit, new memory materials are being sought. Herein, we report a unique material exhibiting single‐molecule electric polarisation switching that can operate above room temperature. The phenomenon occurs in a Preyssler‐type polyoxometalate (POM) cluster we call a single‐molecule electret (SME). It exhibits all the characteristics of ferroelectricity but without long‐range dipole ordering. The SME affords bi‐stability as a result of the two potential positions of localisation of a Tb3+ ion trapped in the POM, resulting in extremely slow relaxation of the polarisation and electric hysteresis with high spontaneous polarisation and coercive electric fields. Our findings suggest that SMEs can potentially be applied to ultrahigh‐density memory and other molecular‐level electronic devices operating above room temperature., This study was supported by JSPS KAKENHI Grant Numbers JP16H04223, JP16K14101, JP24350095, and JP25220803;the Joint Research Project between JSPS and the National Science Foundation of China (NSFC);and JSPS Core-to-Core Program, A. Advanced Research Networks. This work was also performed under the aegis of the Canon Science Promotion Foundation, Murata Science Foundation, Casio Science Promotion Foundation, and the Cooperative Research Program of “Network Joint Research Centre for Materials and Devices”. M.K. is also funded by CNRS, France., アクセプト後にキーワード・アブストラクト等変更あり。