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Compression of hydrogen-rich hydrides has been proposed as an alternative way to attain the atomic metallic hydrogen state or high-temperature superconductors. However, it remains a challenge to get access to these states by synthesizing novel polyhydrides with unusually high hydrogen-to-metal ratios. Here we synthesize a series of cerium (Ce) polyhydrides by a direct reaction of Ce and H 2 at high pressures. We discover that cerium polyhydride CeH 9 , formed above 100 GPa, presents a three-dimensional hydrogen network composed of clathrate H 29 cages. The electron localization function together with band structure calculations elucidate the weak electron localization between H-H atoms and confirm its metallic character. By means of Ce atom doping, metallic hydrogen structure can be realized via the existence of CeH 9 . Particularly, Ce atoms play a positive role to stabilize the sublattice of hydrogen cages similar to the recently discovered near-room-temperature lanthanum hydride superconductors. Obtainment of hydrogen-rich metal hydrides that are high-temperature superconductors has been demonstrated under very high pressure, but is still largely unexplored. Here the authors synthesize CeH 9 , with a structure related to solid metallic hydrogen, at relatively low pressure and without need for heating.

Motivated by the recent report of room-temperature superconductivity at near-ambient pressure in N-doped lutetium hydride, we performed a comprehensive, detailed study of the phase diagram of the Lu–N–H system, looking for superconducting phases. We combined ab initio crystal structure prediction with ephemeral data-derived interatomic potentials to sample over 200,000 different structures. Out of the more than 150 structures predicted to be metastable within ~50 meV from the convex hull we identify 52 viable candidates for conventional superconductivity, for which we computed their superconducting properties from Density Functional Perturbation Theory. Although for some of these structures we do predict a finite superconducting T c , none is even remotely compatible with room-temperature superconductivity as reported by Dasenbrock et al. Our work joins the broader community effort that has followed the report of near-ambient superconductivity, confirming beyond reasonable doubt that no conventional mechanism can explain the reported T c in Lu–N–H. Superconductivity was recently reported experimentally in nitrogen-doped lutetium hydride with T c = 294 K at a pressure of 1 GPa. Here, via theoretical calculations, the authors find no structures capable of supporting conventional superconductivity in the Lu-N-H system at ambient pressure.

The solar system’s outer planets, and many of their moons, are dominated by matter from the H–C–N–O chemical space, based on solar system abundances of hydrogen and the planetary ices H₂O, CH₄, and NH₃. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle–core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, groundstate density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets’ interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H–C–N–O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H–C–N–O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO—isoelectronic to diamond—emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures.

Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure-property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program.

Ice is one of the most extensively studied condensed matter systems. Yet, both experimentally and theoretically several new phases have been discovered over the last years. Here we report a large-scale density-functional-theory study of the configuration space of water ice. We geometry optimise 74,963 ice structures, which are selected and constructed from over five million tetrahedral networks listed in the databases of Treacy, Deem, and the International Zeolite Association. All prior knowledge of ice is set aside and we introduce “generalised convex hulls” to identify configurations stabilised by appropriate thermodynamic constraints. We thereby rediscover all known phases (I–XVII, i, 0 and the quartz phase) except the metastable ice IV. Crucially, we also find promising candidates for ices XVIII through LI. Using the “sketch-map” dimensionality-reduction algorithm we construct an a priori, navigable map of configuration space, which reproduces similarity relations between structures and highlights the novel candidates. By relating the known phases to the tractably small, yet structurally diverse set of synthesisable candidate structures, we provide an excellent starting point for identifying formation pathways. Ice is one of the most well-studied condensed matter systems, yet new phases are still being discovered. Here the authors report a large-scale computational study of the configuration space of water ice, creating a navigable “sketch-map” including new predicted phases as well as relationships between different structures.

We provide evidence that citrate anions bridge between mineral platelets in bone and hypothesize that their presence acts to maintain separate platelets with disordered regions between them rather than gradual transformations into larger, more ordered blocks of mineral. To assess this hypothesis, we take as a model for a citrate bridging between layers of calcium phosphate mineral a double salt octacalcium phosphate citrate (OCP-citrate). We use a combination of multinuclear solid-state NMR spectroscopy, powder X-ray diffraction, and first principles electronic structure calculations to propose a quantitative structure for this material, in which citrate anions reside in a hydrated layer, bridging between apatitic layers. To assess the relevance of such a structure in native bone mineral, we present for the first time, to our knowledge, ¹⁷O NMR data on bone and compare them with ¹⁷O NMR data for OCP-citrate and other calcium phosphate minerals relevant to bone. The proposed structural model that we deduce from this work for bone mineral is a layered structure with thin apatitic platelets sandwiched between OCP-citrate–like hydrated layers. Such a structure can explain a number of known structural features of bone mineral: the thin, plate-like morphology of mature bone mineral crystals, the presence of significant quantities of strongly bound water molecules, and the relatively high concentration of hydrogen phosphate as well as the maintenance of a disordered region between mineral platelets.

Magnesium Oxide (MgO) and water (H 2 O) are abundant in the interior of planets. Their properties, and in particular their interaction, significantly affect the planet interior structure and thermal evolution. Here, using crystal structure predictions and ab initio molecular dynamics simulations, we find that MgO and H 2 O can react again at ultrahigh pressure, although Mg(OH) 2 decomposes at low pressure. The reemergent MgO-H 2 O compounds are: Mg 2 O 3 H 2 above 400 GPa, MgO 3 H 4 above 600 GPa, and MgO 4 H 6 in the pressure range of 270–600 GPa. Importantly, MgO 4 H 6 contains 57.3 wt % of water, which is a much higher water content than any reported hydrous mineral. Our results suggest that a substantial amount of water can be stored in MgO rock in the deep interiors of Earth to Neptune mass planets. Based on molecular dynamics simulations we show that these three compounds exhibit superionic behavior at the pressure-temperature conditions as in the interiors of Uranus and Neptune. Moreover, the water-rich compound MgO 4 H 6 could be stable inside the early Earth and therefore may serve as a possible early Earth water reservoir. Our findings, in the poorly explored megabar pressure regime, provide constraints for interior and evolution models of wet planets in our solar system and beyond. Magnesium Oxide and water are abundant in the interior of planets. Here, the authors predict three new MgO-H 2 O compounds: Mg 2 O 3 H 2 , MgO 3 H 4 and MgO 4 H 6 , and they exhibit superionic behavior in planetary interior conditions.

Nitrogen-doped lutetium hydride has recently been proposed as a near-ambient-conditions superconductor. Interestingly, the sample transforms from blue to pink to red as a function of pressure, but only the pink phase is claimed to be superconducting. Subsequent experimental studies have failed to reproduce the superconductivity, but have observed pressure-driven colour changes including blue, pink, red, violet, and orange. However, discrepancies exist among these experiments regarding the sequence and pressure at which these colour changes occur. Given the claimed relationship between colour and superconductivity, understanding colour changes in nitrogen-doped lutetium hydride may hold the key to clarifying the possible superconductivity in this compound. Here, we present a full microscopic theory of colour in lutetium hydride, revealing that hydrogen-deficient LuH 2 is the only phase which exhibits colour changes under pressure consistent with experimental reports, with a sequence blue-violet-pink-red-orange. The concentration of hydrogen vacancies controls the precise sequence and pressure of colour changes, rationalising seemingly contradictory experiments. Nitrogen doping also modifies the colour of LuH 2 but it plays a secondary role compared to hydrogen vacancies. Therefore, we propose hydrogen-deficient LuH 2 as the key phase for exploring the superconductivity claim in the lutetium-hydrogen system. Finally, we find no phonon-mediated superconductivity near room temperature in the pink phase. Nitrogen-doped lutetium hydride, recently proposed as a superconductor at near-ambient conditions, features distinct color changes from blue to pink to red as a function of pressure. Using theoretical calculations, the authors identify the pink phase as hydrogen-deficient LuH 2 and find that this phase is not a phonon-mediated superconductor near room temperature. Further, the color is controlled by the concentration of hydrogen vacancies.

Energy landscapes provide a conceptual framework for structure prediction, and a detailed understanding of their topological features is necessary to develop efficient methods for their exploration. The ability to visualize these surfaces is essential, but the high dimensionality of the corresponding configuration spaces makes this visualization difficult. Here, we present stochastic hyperspace embedding and projection (SHEAP), a method for energy landscape visualization inspired by state-of-the-art algorithms for dimensionality reduction through manifold learning, such ast-SNE and UMAP. The performance of SHEAP is demonstrated through its application to the energy landscapes of Lennard-Jones clusters, solid-state carbon, and the quaternary systemC+H+N+O. It produces meaningful and interpretable low-dimensional representations of these landscapes, reproducing well-known topological features such as funnels and providing fresh insight into their layouts. In particular, an intrinsic low dimensionality in the distribution of local minima across configuration space is revealed.

Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases. Experimental studies of hydrogen at high pressure are challenging, so theory is central to understanding its phase behaviour; however, computed phase diagrams do not agree with previous measurements. Here, the authors use a quantum Monte Carlo method and present results in qualitative agreement with experiment.