CAREER: Manipulating Barocaloric Effects in Two-Dimensional Perovskites

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Non-technical summary:Phase transitions (i.e., going from a solid to a liquid) enable large changes in the properties of a material to be triggered by a small change in an external stimulus, and thus provide a versatile mechanism for the design of advanced, responsive materials. Despite the tremendous importance of phase transitions, creating materials that transition between two phases—each with a particular set of desired properties—is a challenge. Through this award, funded by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, Prof. Jarad Mason aims to advance the basic science of phase transitions through a fundamental study of the structural and chemical factors that control transitions between ordered and disordered states within hybrid (i.e., organic and inorganic) materials, as well as how these transitions are affected by the application of pressure. Such phase transitions offer exciting opportunities for addressing many critical societal challenges, including how to reversibly store high densities of thermal energy and how to develop more sustainable cooling technologies that do not rely on environmentally harmful volatile refrigerants. In addition, this project aims to broaden participation in scientific research and to facilitate public engagement in basic science and technological innovations through curriculum development, mentorship of high school students, and outreach to K-12 students, high school teachers, and the general public. Technical summary:Barocaloric effects are thermal changes in a material that result from the application or removal of hydrostatic pressure. These effects, which can be used to drive solid-state cooling, heat pump, and thermal energy storage cycles, are strongest when a material experiences a large change in volume and entropy over a narrow temperature range, such as during a sharp order–disorder phase transition. Although critical to realizing the full potential of barocaloric effects, it remains difficult to predictably manipulate order-disorder phase transitions in the solid state, and much remains to be understood about the specific structural and chemical factors that contribute to barocaloric effects at a molecular level. With this CAREER project, Prof. Mason will address these challenges through a systematic investigation of barocaloric effects associated with chain-melting phase transitions in two-dimensional hybrid perovskites. Owing to their synthetic tunability, two-dimensional perovskites serve as a powerful platform to establish fundamental structure–property relationships that advance the development of barocaloric materials. Specifically, the principal hypothesis guiding this research is that the organic bilayers and inorganic sheets in two-dimensional metal–halide perovskites can be synthetically tuned to control the entropy changes, enthalpy changes, volume changes, hysteresis, and kinetics of chain-melting transitions and, consequently, their barocaloric properties. In this project, new materials synthesis and in-depth characterization by X-ray diffraction, calorimetry, neutron scattering, and infrared and solid-state NMR spectroscopies are utilized to investigate the thermodynamics, kinetics, and reversibility of order–disorder transitions in two-dimensional perovskites.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.