Starting with a theoretical picture of the Coulomb Floppy Network, previously developed for empty perovskite materials, e.g. ScF3, we demonstrate a prominent role played by the entropic effects in a broader context of the entire perovskite family. In the case of ScF3, those effects are essential for understanding a number of anomalous properties: negative thermal expansion, entropic elasticity, thermal stabilization of high symmetry cubic phase, strong anharmonic and colossal pressure-induced softening. A conceptually similar theoretical approach is applicable to regular perovskites, as well. In particular, we show how intrinsically unstable cubic structure gets stabilized at finite temperature thanks to a combination of thermal fluctuations and steric constraints. The same entropic effect also manifests itself in the strong anharmonic of soft phonon modes.
*This work at Brookhaven National Laboratory was supported by Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, U.S. Department of Energy (DOE), under contract DE-SC0012704. Work at BNL's Center for Functional Nanomaterials (CFN) was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, under the same contract.
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Publication:D Wendt, E Bozin, J Neuefeind, K Page, W Ku, L Wang, B Fultz, A V Tkachenko, I A Zaliznyak, "Entropic elasticity and negative thermal expansion in a simple cubic crystal", Science advances 5 (11), eaay2748 AV Tkachenko, IA Zaliznyak, "Empty perovskites as Coulomb floppy networks: Entropic elasticity and negative thermal expansion", Physical Review B 103 (13), 134106 IA Zaliznyak, E Bozin, AV Tkachenko, Comment on "Colossal Pressure-Induced Softening in Scandium Fluoride", Physical Review Letters 126 (17), 179601
