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Controlling the formation and stoichiometric content of the desired phases of materials has become of central interest for a variety of fields. The possibility of accessing metastable states by initiating reactions by X-ray-triggered mechanisms over ultrashort time scales has been enabled by the development of X-ray free electron lasers (XFELs). Utilizing the exceptionally high-brilliance X-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with nonstoichiometric changes in hydrogen content as probed at a repetition rate of 4.5 MHz using time-resolved X-ray diffraction. Exploiting non-equilibrium pathways, we synthesize and characterize a hydride in a Weaire-Phelan structure type at pressures as low as 125 GPa, predicted using a crystal structure search, with a hydrogen content of 4.0-5.75 hydrogens per cation, that is enthalpically metastable on the convex hull.

Data-driven methods have transformed the prospects of the computational chemical sciences, with machine-learned interatomic potentials (MLIPs) speeding up calculations by several orders of magnitude. I reflect on theory-driven, as opposed to data-driven, discovery based on ab initio random structure searching (AIRSS), and then introduce two new methods that exploit machine-learning acceleration. I show how long high-throughput anneals, between direct structural relaxation, enabled by ephemeral data-derived potentials (EDDPs), can be incorporated into AIRSS to bias the sampling of challenging systems towards low-energy configurations. Hot AIRSS (hot-AIRSS) preserves the parallel advantage of random search, while allowing much more complex systems to be tackled. This is demonstrated through searches for complex boron structures in large unit cells. I then show how low-energy carbon structures can be directly generated from a single, experimentally determined, diamond structure. An extension to the generation of random sensible structures, candidates are stochastically generated and then optimised to minimise the difference between the EDDP environment vector and that of the reference diamond structure. The distance-based cost function is captured in an actively learned EDDP. Graphite, small nanotubes and caged, fullerene-like, structures emerge from searches using this potential, along with a rich variety of tetrahedral framework structures. Using the same approach, the pyrope, Mg 3 Al 2 (SiO 4 ) 3 , garnet structure is recovered from a low-energy AIRSS structure generated in a smaller unit cell with a different chemical composition. The relationship of this approach to modern diffusion-model-based generative methods is discussed.

A seventh blind test of crystal structure prediction was organized by the Cambridge Crystallographic Data Centre featuring seven target systems of varying complexity: a silicon and iodine-containing molecule, a copper coordination complex, a near-rigid molecule, a cocrystal, a polymorphic small agrochemical, a highly flexible polymorphic drug candidate, and a polymorphic morpholine salt. In this first of two parts focusing on structure generation methods, many crystal structure prediction (CSP) methods performed well for the small but flexible agrochemical compound, successfully reproducing the experimentally observed crystal structures, while few groups were successful for the systems of higher complexity. A powder X-ray diffraction (PXRD) assisted exercise demonstrated the use of CSP in successfully determining a crystal structure from a low-quality PXRD pattern. The use of CSP in the prediction of likely cocrystal stoichiometry was also explored, demonstrating multiple possible approaches. Crystallographic disorder emerged as an important theme throughout the test as both a challenge for analysis and a major achievement where two groups blindly predicted the existence of disorder for the first time. Additionally, large-scale comparisons of the sets of predicted crystal structures also showed that some methods yield sets that largely contain the same crystal structures., (open access.)

A key challenge in materials discovery is to find high-temperature superconductors. Hydrogen and hydride materials have long been considered promising materials displaying conventional phonon-mediated superconductivity. However, the high pressures required to stabilize these materials have restricted their application. Here, we present results from high-throughput computation, considering a wide range of high-symmetry ternary hydrides from across the periodic table at ambient pressure. This large composition space is then reduced by considering thermodynamic, dynamic, and magnetic stability before direct estimations of the superconducting critical temperature. This approach has revealed a metastable ambient-pressure hydride superconductor, Mg_{2}IrH_{6}, with a predicted critical temperature of 160 K, comparable to the highest temperature superconducting cuprates. We propose a synthesis route via a structurally related insulator, Mg_{2}IrH_{7}, which is thermodynamically stable above 15 GPa, and discuss the potential challenges in doing so.

Using first-principles calculations and crystal structure search methods, we found that many covalently bonded molecules such as H 2 , N 2 , CO 2 , NH 3 , H 2 O and CH 4 may react with NaCl, a prototype ionic solid, and form stable compounds under pressure while retaining their molecular structure. These molecules, despite whether they are homonuclear or heteronuclear, polar or non-polar, small or large, do not show strong chemical interactions with surrounding Na and Cl ions. In contrast, the most stable molecule among all examples, N 2 , is found to transform into cyclo-N 5 - anions while reacting with NaCl under high pressures. It provides a new route to synthesize pentazolates, which are promising green energy materials with high energy density. Our work demonstrates a unique and universal hybridization propensity of covalently bonded molecules and solid compounds under pressure. This surprising miscibility suggests possible mixing regions between the molecular and rock layers in the interiors of large planets., (© The Author(s) 2024. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.)

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., (© 2023. The Author(s).)

Machine-learned interatomic potentials are fast becoming an indispensable tool in computational materials science. One approach is the ephemeral data-derived potential (EDDP), which was designed to accelerate atomistic structure prediction. The EDDP is simple and cost-efficient. It relies on training data generated in small unit cells and is fit using a lightweight neural network, leading to smooth interactions which exhibit the robust transferability essential for structure prediction. Here, we present a variety of applications of EDDPs, enabled by recent developments of the open-source EDDP software. New features include interfaces to phonon and molecular dynamics codes, as well as deployment of the ensemble deviation for estimating the confidence in EDDP predictions. Through case studies ranging from elemental carbon and lead to the binary scandium hydride and the ternary zinc cyanide, we demonstrate that EDDPs can be trained to cover wide ranges of pressures and stoichiometries, and used to evaluate phonons, phase diagrams, superionicity, and thermal expansion. These developments complement continued success in accelerated structure prediction., (© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).)

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., (© 2023. Springer Nature Limited.)

Differentiable programming has facilitated numerous methodological advances in scientific computing. Physics engines supporting automatic differentiation have simpler code, accelerating the development process and reducing the maintenance burden. Furthermore, fully differentiable simulation tools enable direct evaluation of challenging derivatives-including those directly related to properties measurable by experiment-that are conventionally computed with finite difference methods. Here, we investigate automatic differentiation in the context of orbital-free density functional theory (OFDFT) simulations of materials, introducing PROFESS-AD. Its automatic evaluation of properties derived from first derivatives, including functional potentials, forces, and stresses, facilitates the development and testing of new density functionals, while its direct evaluation of properties requiring higher-order derivatives, such as bulk moduli, elastic constants, and force constants, offers more concise implementations than conventional finite difference methods. For these reasons, PROFESS-AD serves as an excellent prototyping tool and provides new opportunities for OFDFT.

The discovery of 250-kelvin superconducting lanthanum polyhydride under high pressure marked a significant advance toward the realization of a room-temperature superconductor. X-ray diffraction (XRD) studies reveal a nonstoichiometric LaH 9.6 or LaH 10±δ polyhydride responsible for the superconductivity, which in the literature is commonly treated as LaH 10 without accounting for stoichiometric defects. Here, we discover significant nuclear quantum effects (NQE) in this polyhydride, and demonstrate that a minor amount of stoichiometric defects will cause quantum proton diffusion in the otherwise rigid lanthanum lattice in the ground state. The diffusion coefficient reaches ~10 -7 cm 2 /s in LaH 9.63 at 150 gigapascals and 240 kelvin, approaching the upper bound value of interstitial hydrides at comparable temperatures. A puzzling phenomenon observed in previous experiments, the positive pressure dependence of the superconducting critical temperature T c below 150 gigapascals, is explained by a modulation of the electronic structure due to a premature distortion of the hydrogen lattice in this quantum fluxional structure upon decompression, and resulting changes of the electron-phonon coupling. This finding suggests the coexistence of the quantum proton fluxion and hydrogen-induced superconductivity in this lanthanum polyhydride, and leads to an understanding of the structural nature and superconductivity of nonstoichiomectric hydrogen-rich materials., (© 2023. The Author(s).)

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., (© 2023. The Author(s).)

Most metals adopt simple structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) structures in specific groupings across the periodic table, and many undergo transitions to surprisingly complex structures on compression, not expected from conventional free-electron-based theories of metals. First-principles calculations have been able to reproduce many observed structures and transitions, but a unified, predictive theory that underlies this behavior is not yet in hand. Discovered by analyzing the electronic properties of metals in various lattices over a broad range of sizes and geometries, a remarkably simple theory shows that the stability of metal structures is governed by electrons occupying local interstitial orbitals and their strong chemical interactions. The theory provides a basis for understanding and predicting structures in solid compounds and alloys over a broad range of conditions.

Carbon dioxide capture is essential to achieve net-zero emissions. A hurdle to the design of improved capture materials is the lack of adequate tools to characterise how CO 2 adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a promising probe of CO 2 capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that 17 O NMR is a powerful probe of CO 2 capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by 17 O NMR experiments on a series of CO 2 -loaded frameworks that clearly identify ammonium carbamate chain formation and provide evidence for a mixed carbamic acid - ammonium carbamate adsorption mode. We further find that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Our work paves the way for investigations of carbon capture chemistry that can enable materials design., (© 2022. The Author(s).)

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the H 2 δ - units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the H 2 δ - bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to H 2 δ - and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH 4 , where the precompression of H 2 δ - units at 50 GPa results in bond lengths comparable to that of pure H 2 above 275 GPa.

Water in nanoscale cavities is ubiquitous and of central importance to everyday phenomena in geology and biology. However, the properties of nanoscale water can be substantially different from those of bulk water, as shown, for example, by the anomalously low dielectric constant of water in nanochannels 1 , near frictionless water flow 2 or the possible existence of a square ice phase 3 . Such properties suggest that nanoconfined water could be engineered for technological applications in nanofluidics 4 , electrolyte materials 5 and water desalination 6 . Unfortunately, challenges in experimentally characterizing water at the nanoscale and the high cost of first-principles simulations have prevented the molecular-level understanding required to control the behaviour of water. Here we combine a range of computational approaches to enable a first-principles-level investigation of a single layer of water within a graphene-like channel. We find that monolayer water exhibits surprisingly rich and diverse phase behaviour that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400 kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials. Notably, this suggests that nanoconfinement could be a promising route towards superionic behaviour under easily accessible conditions., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Aluminum, as the most abundant metallic elemental content in the Earth's crust, usually exists in the form of alumina (Al 2 O 3 ). However, the oxidation state of aluminum and the crystal structures of aluminum oxides in the pressure range of planetary interiors are not well established. Here, we predicted two aluminum suboxides (Al 2 O, AlO) and two superoxides (Al 4 O 7 , AlO 3 ) with uncommon stoichiometries at high pressures using first-principle calculations and crystal structure prediction methods. We find that the P4/nmm Al 2 O becomes stable above ~765 GPa and may survive in the deep mantles or cores of giant planets such as Neptune. Interestingly, the Al 2 O and AlO are metallic and have electride features, in which some electrons are localized in the interstitials between atoms. We find that Al 2 O has an electrical conductivity one order of magnitude higher than that of iron under the same pressure-temperature conditions, which may influence the total conductivity of giant planets. Our findings enrich the high-pressure phase diagram of aluminum oxides and improve our understanding of the interior structure of giant planets., Competing Interests: CJP is an author of the CASTEP code and receives royalty payments from its commercial sales by Dassault Systemes., (Copyright © 2022 Tianheng Huang et al.)

KSiH 3 exhibits 4.1 wt% experimental hydrogen storage capacity and shows reversibility under moderate conditions, which provides fresh impetus to the search for other complex hydrides in the K-Si-H system. Here, we reproduce the stable Fm 3̄ m phase of K 2 SiH 6 and uncover two denser phases, space groups P 3̄ m 1 and P 6 3 mc at ambient pressure, by means of first-principles structure searches. We note that P 3̄ m 1-K 2 SiH 6 has a high hydrogen content of 5.4 wt% and a volumetric density of 88.3 g L -1 . Further calculations suggest a favorable dehydrogenation temperature T des of -20.1/55.8 °C with decomposition into KSi + K + H 2 . The higher hydrogen density and appropriate dehydrogenation temperature indicate that K 2 SiH 6 is a promising hydrogen storage material, and our results provide helpful and clear guidance for further experimental studies. We found three further potential hydrogen storage materials stable at high pressure: K 2 SiH 8 , KSiH 7 and KSiH 8 . These results suggest the need for further investigations into hydrogen storage materials among such ternary hydrides at high pressure.

We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic 13 C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state 13 C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms. In memoriam, to Neil Ashcroft, who inspired us all. , (Creative Commons Attribution license.)

Titanium dioxide has been extensively studied in the rutile or anatase phase, while its high-pressure phases are less well-understood, despite that many are thought to have interesting optical, mechanical, and electrochemical properties. First-principles methods, such as density functional theory (DFT), are often used to compute the enthalpies of TiO 2 phases at 0 K, but they are expensive and, thus, impractical for long time scale and large system-size simulations at finite temperatures. On the other hand, cheap empirical potentials fail to capture the relative stabilities of various polymorphs. To model the thermodynamic behaviors of ambient and high-pressure phases of TiO 2 , we design an empirical model as a baseline and then train a machine learning potential based on the difference between the DFT data and the empirical model. This so-called Δ-learning potential contains long-range electrostatic interactions and predicts the 0 K enthalpies of stable TiO 2 phases that are in good agreement with DFT. We construct a pressure-temperature phase diagram of TiO 2 in the range 0 < P < 70 GPa and 100 < T < 1500 K. We then simulate dynamic phase transition processes by compressing anatase at different temperatures. At 300 K, we predominantly observe an anatase-to-baddeleyite transformation at about 20 GPa via a martensitic two-step mechanism with a highly ordered and collective atomic motion. At 2000 K, anatase can transform into cotunnite around 45-55 GPa in a thermally activated and probabilistic manner, accompanied by diffusive movement of oxygen atoms. The pressures computed for these transitions show good agreement with experiments. Our results shed light on how to synthesize and stabilize high-pressure TiO 2 phases, and our method is generally applicable to other functional materials with multiple polymorphs.

Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor made in a laboratory, can reach a critical temperature (T&#95;{c}) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low pressures by alloying small-radius elements and hydrogen to form ternary H-based superconductors with alloy backbones. We identify a "fluorite-type" backbone in compositions of the form AXH&#95;{8}, which exhibit high-temperature superconductivity at moderate pressures compared with other reported hydrogen-based superconductors. The Fm3[over ¯]m phase of LaBeH&#95;{8}, with a fluorite-type H-Be alloy backbone, is predicted to be thermodynamically stable above 98 GPa, and dynamically stable down to 20 GPa with a high T&#95;{c}∼185  K. This is substantially lower than the synthesis pressure required by the geometrically similar clathrate hydride LaH&#95;{10} (170 GPa). Our approach paves the way for finding high-T&#95;{c} ternary H-based superconductors at conditions close to ambient pressures.

Competing Interests: Conflict of interest The authors declare that they have no conflict of interest.

Superionic states simultaneously exhibit properties of a fluid and a solid. Proton (H + ) superionicity in ice, H 3 O, He-H 2 O, and He-NH 3 compounds is well-studied. However, hydride (H - ) superionicity in H-rich compounds is rare, being associated with instability and strongly reducing conditions. Silicon, sodium, and hydrogen are abundant elements in many astrophysical bodies. Here, we use first-principles calculations to show that, at high pressure, Na, Si, and H can form several hypervalent compounds. A previously unreported superionic state of Na 2 SiH 6 results from unconstrained H - in the hypervalent [SiH 6 ] 2- unit. Na 2 SiH 6 is dynamically stable at low pressure (3 GPa), becoming superionic at 5 GPa, and re-entering solid/fluid states at about 25 GPa. Our observation of H - transport opens up a new field of H - conductors. It also has implications for the formation of conducting layers at depth in exotic carbon exoplanets, potentially enhancing the habitability of such planets.

All elements that form diatomic molecules, such as H&#95;{2}, N&#95;{2}, O&#95;{2}, Cl&#95;{2}, Br&#95;{2}, and I&#95;{2}, are destined to become atomic solids under sufficiently high pressure. However, as revealed by many experimental and theoretical studies, these elements show very different propensity and transition paths due to the balance of reduced volume, lone pair electrons, and interatomic bonds. The study of F under pressure can illuminate this intricate behavior since F, owing to its unique position on the periodic table, can be compared with H, with N and O, and also with other halogens. Nevertheless, F remains the only element whose solid structure evolution under pressure has not been thoroughly studied. Using a large-scale crystal structure search method based on first principles calculations, we find that, before reaching an atomic phase, F solid transforms first into a structure consisting of F&#95;{2} molecules and F polymer chains and then into a structure consisting of F polymer chains and F atoms, a distinctive evolution with pressure that has not been seen in any other elements. Both intermediate structures are found to be metallic and become superconducting, a result that adds F to the elemental superconductors.

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF 3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF 2 and CuF 2 . Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F - sublattices and that of LiF is established. Initial lithiation of FeF 3 forms FeF 2 on the particle's surface, along with a cation-ordered and stacking-disordered phase, A-Li x Fe y F 3 , which is structurally related to α-/β-LiMn 2+ Fe 3+ F 6 and which topotactically transforms to B- and then C-Li x Fe y F 3 , before forming LiF and Fe. Lithiation of FeF 2 and CuF 2 results in a buffer phase between FeF 2 /CuF 2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.