SuperHydra

Superhydrides, i.e. hydrides with a ultra-high hydrogen content, have been predicted at planetary pressures, and recently observed in several binary systems, such as Li-H, Al-H, Na- H, etc. In 2014, one of these compounds, SH3, broke the record for superconducting critical temperatures (Tc) previously held by the cuprates, setting the bar at 203 K. High-Tc superconductivity in metallic superhydrides is expected on the basis of conventional Migdal-Eliashberg theory, due to the high phonon frequencies involved in the superconducting pairing. At the same time, superhydrides are also attractive for hydrogen storage applications, because they permit to realize relative hydrogen densities much higher than current hydrogen storage materials. The high pressure needed to stabilize these phases is an obvious limitation to their practical use. However, a recent report of superconductivity in PdH with Tc of 61 K at ambient pressure upon cold compression, suggests that some superhydrides phases could be quenched down to ambient conditions, making their study attractive also for prospective technological applications. In this project we employ ab-initio methods for crystal structure prediction, thermodynamics and superconductivity, to study the high-pressure phase diagrams up to the multi-Megabar range of three classes hydrides:metal hydrides, covalent (molecular) hydrides, and covalent- ionic hydrides, commonly used for hydrogen-storage applications. Our main goal is to identify new high-Tc superconducting phases at high pressures and metastable H-rich phases which could be quenched down to ambient pressure for superconductivity or hydrogen-storage applications. The project will have a big impact on fundamental research, as the behavior of H-rich solids at high pressures is currently object of intense investigation. Furthermore, three types of predictions, which may be verified within the proposed time-frame, would have a transformative impact: (i) a high-Tc, superconductor at ambient pressure; (ii) a new superhydride phase, with performances comparable to the best current hydrogen-storage materials; (iii) a room temperature superconductor at high pressures.

The aim of the "Superhydra" project is to design new superhydride superconductors using a combination of advanced computational methods. Superconductors are materials which, below a characteristic temperature, called critical temperature, conduct electricity without dissipation (zero resistivity) and expel magnetic fields (perfect diamagnetism). This makes them suitable for a variety of technological applications, ranging from electrical grid components, to biomedical sensing and magnetic levitation vehicles. The biggest limiting factor to the actual use and commercialisation of superconductors is represented by their extremely low operating temperatures: most materials known nowadays have critical temperatures well below nitrogen boiling point (-196 C). A few months before the start of our project, the group of Mikhail Eremets, in Mainz, had just reported the discovery of the first member of a completely new class of superconductors, called superhydrides, formed compressing mixtures of molecular hydrogen and other elements to pressures as high as million of atmospheres (Megabar) in a diamond anvil cell. Superhydrides exhibited unprecedentedly high critical temperatures ("only" a few tens degrees below zero), which made them extremely attractive for fundamental research. The primary goal of our project was to understand the nature and of origin of superconductivity in existing superhydrides, and design new ones. Computer-based methods for the quantum mechanical simulation of material properties have reached an accuracy that permits to simulate on a computer all the aspects of the extremely complex experiments needed to synthesise these fascinating materials and measure their superconducting properties. Armed with these methods, our group has gradually acquired a well recognised international standing, following, and in some cases, anticipating the main developments in this fast-evolving field. The main highlights of our project include the first fully microscopic calculation of superconducting properties of the newly discovered sodalite-clathrate hydrides, the first calculation of full ternary phase diagrams of ternary hydrides under pressure, and the proposal of a new mechanism to obtain high-temperature superhydrides close to room temperature, exploiting chemical precompression in ternary hydrides. In addition to material discoveries, in our project we have developed a set of computational tools and workflows to generate and process large databases of computational data for material discovery, which can be applied to several problems besides superconductivity.