Chemistry at high pressure: Tuning functional materials properties
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Introduction Functional materials possess peculiar native properties such as magnetism, energy storage, superconductivity, piezoelectricity, and ferroelectricity, among others. These can be exploited in several areas of materials science. Modulation of properties is achieved through several routes, such as chemical doping(s), defect creation/control, nanostructuring, and thinfilm fabrication, with the aim of enhancing or tuning specific characteristics. Hydrostatic pressure is an effective tool to control the fundamental properties of a functional material, and its effects can possibly be reproduced through chemical substitution within the lattice (resulting in chemical pressure and strain in thin films). Among the different structural families of functional materials, perovskites are ubiquitous, being the basic motif of several superconductors, magnets, energy-storage, ferroelectric, and piezoelectric materials. They are intriguing materials from a structural point of view, characterized by subtle structural phase transitions (induced by temperature and chemical doping), strongly affecting their physical properties, and by a relatively soft lattice (easily deformable). These two characteristics make them ideal candidates for high-pressure investigations. To illustrate the properties of functional materials manipulated by the application of an external pressure, we present selected results of high-pressure studies on two highly
representative examples of perovskite-based materials— manganites and organic–inorganic halide perovskites. While possessing the same crystal structure, these two classes of materials show significantly different features and are suited to different potential applications, in particular, colossal magnetoresistance (manganites) and photovoltaics (hybrid perovskites). In both cases, through high-pressure studies, we obtain a deeper understanding of the underlying microscopic mechanisms related to magnetoresistant or photovoltaic responses.
Manganites Mixed-valence manganites, A1-xA′xMnO3 (where A is a trivalent rare-earth and A′ a divalent alkaline-earth metal), show rich phase diagrams with a variety of magnetic, structural, and electronic phases upon varying x and temperature T.1–3 Several hole-doped manganites (x < 0.5) show colossal magnetoresistance (CMR), an anomalously large variation of resistance on applying a magnetic field, which results from a complex interplay between structural, magnetic, and electronic degrees of freedom driven by internal (x, chemical composition) or external (T, P, magnetic field) variables. Experimental and theoretical investigations concur that Jahn–Teller (JT) distortion of MnO6 octahedra is mainly responsible for CMR and the related insulator-tometal transition (IMT). (The Jahn–Teller distortion refers
Paolo Postorino, Department of Physics, Sapienza Università di Roma, Italy; [email protected] Lorenzo Malavasi, Department of Chemistry, University of Pavia, Italy; [email protected] doi:10.1557/mrs.2017.214
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• VOLUME 42 • OCTOBER 2
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