Room-temperature superconductivity is arguably the greatest challenge in condensed matter physics, with significant practical and commercial implications if it can be solved. There are no physical laws preventing this from occurring; indeed, superconductivity has been observed in so many different materials under so many different conditions that it is almost a "generic" property of nonmagnetic metals. This guides our viewpoint that high-temperature superconductivity is possible, if difficult to realize. Here, we lay out two grand challenges facing the field, titled the Prediction Challenge and the Engineering Challenge, and put forward a programmatic approach for overcoming them. The Prediction Challenge addresses the fact that our ability to predict new conventional superconductors has dramatically advanced in recent years, but most predicted materials are not experimentally synthesizable. To address this challenge, we propose a shift from modeling the superconducting critical temperature and dynamic stability toward high-throughput ab initio and predictive thermodynamics/synthesis modeling. The Engineering Challenge describes how we can control superconductivity with various "knobs," including pressure, nanostructuring, and light. However, our ability to predict how a specific knob will modify a given superconductor is limited, making it difficult to fully exploit them. We describe the current status and identify areas where additional work is needed to fully exploit six of the most common knobs. Progress in both of these grand challenges, while closely integrating theory and experiment into a continuous feedback loop and incorporating insights from fields beyond physics and materials science, could unlock the underlying keys to room-temperature superconductivity.

The packing and geometry of compressed small molecule precursors largely dictate the kinetically controlled formation processes of carbon nanothread materials. Structural ordering and chemical homogeneity of nanothread products may deteriorate through competing reaction pathways, and molecular phase transitions can disrupt precursor stacking geometries. Here, we report the formation of well-ordered, double-core nanothreads from compressed α-furil via a unique polymorphic transition pathway that serves to optimize molecular geometry for pressure-induced reaction. At ~1.6 GPa, α-furil transforms to the photoactive trans-planar conformation, which was previously theorized but not observed. Crystalline packing of the trans-planar structure provides closely overlapping molecular stacks that result in topochemical-like Diels-Alder cycloaddition reactions between furan rings upon further compression. The controlled reaction pathways on both sides of the molecule produce two linked “cores” of chemically homogenous nanothreads, and successive nucleophilic addition reactions crosslink a large fraction of the diketone bridge between monomers.

Clathrates are guest/host framework compounds composed of polyhedral cages, yet despite their prevalence among tetrahedral network formers, clathrates with a carbon host lattice remain unrealized synthetic targets. Here, we report a type-I carbon-based framework-a ubiquitous clathrate structure type found throughout compounds containing tetrahedral building blocks. Following a boron-stabilization scheme based on first-principles predictions in the Ca-B-C system at high pressure, type-I Ca8BxC46-x (x ≈ 9) was synthesized in the archetypal Pm[Formula: see text]n lattice with stability derived from substitutionally disordered boron atoms on hexagonal ring framework positions. The synthesized clathrate, which is recoverable to ambient conditions, expands topological network similarity across tetrahedral systems and opens possibilities for a broad family of diamond-like, carbon-based compounds with tunable properties based on the wide potential for guest/host-atom substitutions and framework versatility.

The low-pressure stabilization of superconducting hydrides with high critical temperatures (T_cs) remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition-metal complex hydrides (hydrido complexes)-typically regarded as hydrogen storage materials-exhibit a large range of compounds stabilized at low pressure with recent predictions for high-T_c superconductivity. Motivated by this class of materials, we investigated complex hydride formation in the Mg-Pt-H system, which has no known ternary hydride compounds. Guided by ab initio structural predictions, we successfully synthesized a novel complex transition-metal hydride, Mg_4Pt_3H_6, using laser-heated diamond anvil cells. The compound forms in a body-centered cubic structural prototype at moderate pressures between 8-25 GPa. Unlike the majority of known hydrido complexes, Mg_4Pt_3H_6 is metallic, with formal charge described as 4[Mg]^2+.3[PtH_2]^2-. X-ray diffraction (XRD) measurements obtained during decompression reveal that Mg_4Pt_3H_6 remains stable upon quenching to ambient conditions. Magnetic-field and temperature-dependent electrical transport measurements indicate ambient-pressure superconductivity with T_c (50 reasonable agreement with theoretical calculations. These findings clarify the phase behavior in the Mg-Pt-H system and provide valuable insights for transition-metal complex hydrides as a new class of hydrogen-rich superconductors.

We demonstrate the crystallization of a polar octafluoronaphthalene (OFN, C10F8)-phthalazine (Phth, C8H6N2) cocrystal formed in a 1:2 ratio by slow evaporation. The crystal structure and vibrational properties of the cocrystal were determined using powder/single-crystal X-ray diffraction (XRD) and Fourier-Transform Infrared (FTIR) spectroscopy, and confirmed with density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. The molecular pi-stacking of aromatic rings is unconventional compared with that of other arene-perfluoroarene cocrystals. Phth molecules are offset and misaligned with respect to the major axis of OFN due to combined pi-pi and dipole-dipole interactions, enabling overall electric polarization attributed to the dipole moment of Phth. Our calculations show that OFN:2Phth is an insulator with a band gap >2.4 eV. The electric polarization was calculated to be 7.1 mu C cm(-2), while the shear piezoelectric coefficient (d(34)) may be as large as 11.4 pC N-1.

The recent discovery of type-VII boron-carbon clathrates with calculated superconducting transition temperatures approaching similar to 100 K has sparked interest in exploring new conventional superconductors that may be stabilized at ambient pressure. The electronic structure of the clathrate is highly tunable based on the ability to substitute different metal atoms within the cages, which may also be large enough to host small molecules. Here we introduce molecular hydrogen (H-2) within the clathrate cages and investigate its impact on electron-phonon coupling interactions and the superconducting transition temperature (T-c). Our approach involves combining molecular hydrogen with the new diamond-like covalent framework, resulting in a hydrogen-encapsulated clathrate, (H-2)B3C3. A notable characteristic of (H-2)B3C3 is the dynamic behavior of the H-2 molecules, which exhibit nearly free rotations within the B-C cages, resulting in a dynamic structure that remains cubic on average. The static structure of (H-2)B3C3 (a snapshot in its dynamic trajectory) is calculated to be dynamically stable at ambient and low pressures. Topological analysis of the electron density reveals weak van der Waals interactions between molecular hydrogen and the B-C cages, marginally influencing the electronic structure of the material. The electron count and electronic structure calculations indicate that (H-2)B3C3 is a hole conductor, in which H-2 molecules donate a portion of their valence electron density to the metallic cage framework. Electron-phonon coupling calculation using the Migdal-Eliashberg theory predicts that (H-2)B3C3 possesses a T-c of 46 K under ambient pressure. These results indicate potential for additional light-element substitutions within the type-VII clathrate framework and suggest the possibility of molecular hydrogen as a new approach to optimizing the electronic structures of this new class of superconducting materials.

Solid-state synthesis represents an alternative to solution-phase chemistry that can provide routes to materials typically unobtainable by conventional methods. However, multiple competing reaction pathways under high-pressure conditions makes the targeted synthesis of chemically homogeneous systems a challenge. Nanothreads, one-dimensional diamondoid polymers formed through the compression of aromatic hydrocarbons present a unique opportunity to carry out high pressure reactions in a controlled and predictable manner. We hypothesize that through careful consideration of molecular stacking and intermolecular forces (e.g., H-bonding), it is possible to form chemically homogeneous nanothreads that retain precisely located chemical functionality. Herein, we report the scalable solid-state polymerization of 2,5-furandicarboxylic acid through sequential [4 + 2] Diels Alder cycloaddition reactions. The resulting nanothread product is decorated with a high density of pendant carboxylate groups, presenting new opportunities for post-synthetic processing and functional applications. Transition metal coordination is demonstrated for the functionalized threads, representing proof-of-concept for the utilization of nanothreads as independent synthons and the possibility for novel, extended multidimensional networks.

The germanosilicide Na4-xGeySi16-y (0.4 ≤ x ≤ 1.1, 4.7 ≤ y ≤ 9.3) was synthesized under high-pressure, high-temperature conditions. The novel guest-host compound comprises a unique tetrel framework with dual channels housing sodium and smaller, empty (Si,Ge)9 units. The arrangement represents a new structure type with an overall structural topology that is closely related to a hypothetical carbon allotrope. Topological analysis of the structure revealed that the guest environment space cannot be tiled with singular polyhedra as in cage compounds (e.g., clathrates). The analysis of natural tilings provides a convenient method to unambiguously compare related tetrel-rich structures and can help elucidate new possible structural arrangements of intermetallic compounds.

Understanding new mechanisms for phase transformation in carbon is of considerable interest. This study investigates on the compression conditions required to create recoverable diamond during room-temperature high-pressure compression of glassy carbon. Under non-hydrostatic compression conditions when shear is present, glassy carbon transforms into an oriented graphitic structure at ∼45 GPa, and then forms mixed diamond and lonsdaleite nanocrystals when the pressure is higher than ∼80 GPa. In contrast, during hydrostatic compression no significant changes in the microstructure was observed, highlighting glassy carbon’s resilience under compression. Molecular dynamics modelling supports the proposed model that shear drives the phase transition mechanism and causes a temperature spike that drives crystallisation. Our work demonstrates that shear is key to high-pressure diamond formation in the absence of heating.

Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-T-c phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg2IrH6 may help overcome this challenge with calculations of high-T-c superconductivity (65K < T-c < 170K) in a material that is stable at atmospheric pressure. In this paper, the synthesis of Mg2IrH6 was targeted over a broad range of P-T conditions, and the resulting products were characterized using x-ray diffraction (XRD) and vibrational spectroscopy, in concert with first-principles calculations. The results indicate that the charge-balanced complex hydride Mg2IrH5 is more stable over all conditions tested up to approximately 28 GPa. The resulting hydride is isostructural with the predicted superconducting Mg2IrH6 phase except for a single hydrogen vacancy, which shows a favorable replacement barrier upon insertion of hydrogen into the lattice. Bulk Mg2IrH5 is readily accessible at mild P-T conditions and may thus represent a convenient platform to access superconducting Mg2IrH6 via nonequilibrium processing methods. Finally, the critical factors influencing the calculated range of superconducting transition temperatures for this material are discussed.

Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-T_c phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg_2IrH_6 may help overcome this challenge with calculations of high-T_c superconductivity (65 K< T_c < 170 K) in a material that is stable at atmospheric pressure. In this work, the synthesis of Mg_2IrH_6 was targeted over a broad range of P-T conditions, and the resulting products were characterized using X-ray diffraction (XRD) and vibrational spectroscopy, in concert with first-principles calculations. The results indicate that the charge-balanced complex hydride Mg_2IrH_5 is more stable over all conditions tested up to ca 28 GPa. The resulting hydride is isostructural with the predicted superconducting Mg_2IrH_6 phase except for a single hydrogen vacancy, which shows a favorable replacement barrier upon insertion of hydrogen into the lattice. Bulk Mg_2IrH_5 is readily accessible at mild P-T conditions and may thus represent a convenient platform to access superconducting Mg_2IrH_6 via non-equilibrium processing methods.

Compression of small molecules can induce solid-state reactions with products that are difficult or impossible to obtain through solution-phase synthesis. Of particular interest is the topochemical-like reaction of arenes to produce polymeric nanomaterials rich in sp3 carbon. However, high reaction onset pressures and poor control over high-pressure reaction selectivity remain significant challenges to be addressed. Herein, the incorporation of electron withdrawing/donating groups into π-stacked arenes is proposed as a strategy to reduce reaction barriers and onset pressures. Charge transfer cocrystals represent systems with optimal π-stacking and reduced energy barriers for intermolecular cycloaddition reactions, however, competing side-chain reactions between functional groups must also be considered. For the case of a diaminobenzene:tetracyanobenzene cocrystal, amidine formation between side groups is the first reaction to occur with an onset pressure near 9 GPa, as characterized using vibrational spectroscopy, X-ray diffraction, and computational studies. High-pressure reactivity is system-dependent and while functionalized arenes are predicted to exhibit reduced-barrier energy cycloaddition pathways, directed reactions between side groups can be used as a novel strategy for the formation unique polymeric materials.

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.

Carbon nanothreads form a novel class of hydrogenated, diamond-like materials synthesized at high pressure from simple aromatic substances, theoretically predicted to exhibit unique mechanical properties, some of which may also exhibit optical and transport properties of potential technological interest, depending on the particular aromatic precursor. Our study focuses on cocrystals of two very similar aromatic molecules: diphenylacetylene and stilbene, both comprised of two rings connected by two-carbon units featuring triple and double bonds, respectively. We prepared the cocrystals by recrystallization from solution, producing six different compositions between the two end-member values of 100% diphenylacetylene and 100% stilbene. These samples were then compressed to final pressures of similar to 30 GPa, in diamond anvil cells, at room temperature. The compression induced copolymerization results in the formation of double-core carbon nanothreads. These nanothreads are comprised of two one-dimensional diamond-like cores connected through cis-polyacetylene-like backbones of variable length, produced from the topochemical polymerization of the acetylene moieties of diphenylacetylene. The resulting materials were characterized via optical absorption spectroscopy and X-ray diffraction. Very interestingly, the recovered materials exhibited variable optical absorption in the visible and near-infrared spectral region, resembling the low-energy edges of HOMO-LUMO band gaps in dielectric materials. Particularly, the absorption edge of our materials shifts to lower energies with increasing the diphenylacetylene content within the cocrystal precursor and, consequently, with increasing the lengths of the conjugated carbon chains. The materials properties range from semiconductor behavior to wide band gap insulating behavior at the two extremes of 100% and null diphenylacetylene content. Pressure-induced copolymerization thus represents a methodology for synthesizing novel carbon nanothreads with finely variable optical properties.

We predict theoretically a carbon-based clathrate in the bipartite sodalite structure, SrB3C3, that is thermodynamically stable at high pressure. This clathrate is predicted to be a dynamically stable superconductor with an estimated Tc of 42 K at ambient pressure. Calculated stress-strain relations for SrB3C3 clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Boron substitution aids in the stabilization of SrB3C3 clathrate, and offers valuable insights into design guidelines for various carbon-based materials.

Inspired by the synthesis of XB3C3 (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB6C6 binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with Tc being highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliashberg Tc of 88 K, is predicted to possess the highest Tc among the studied Pm3̅n XB3C3 or Pm3̅ XYB6C6 phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.

The phase diagram of the Mg-C system has been constructed up to 20 GPa and similar to 4000 K based on complementary Thermo-Calc simulations and experimental data obtained in both ex situ and in situ experiments using X-ray diffraction with synchrotron radiation. Three high-pressure magnesium carbides, namely, beta-Mg2C3, its high-temperature form gamma-Mg2C3, and antifluorite Mg2C, have p-T domains of thermodynamic stability. At the same time, the carbides accessible by ambient-pressure synthesis, alpha-Mg2C3 and MgC2, are either metastable or unstable, depending on the temperature, at least up to 20 GPa. Experimental observations show that at ambient conditions, all carbides are metastable and remain unchanged at least for years.

Stishovite is a key mineral for understanding the deep Earth water cycle because of its potential as a main carrier for water into the transition zone and lower mantle. During subduction-related metamorphism of basaltic oceanic crust, stishovite stabilizes at 8–9 GPa and comprises 10–25 vol% of the bulk mineralogy, with some experimental studies indicating that stishovite can accommodate 3.5 wt% H 2 O or more in the transition zone and upper lower mantle. This large water solubility has been explained by a hydrogarnet substitution mechanism (1Si 4+ ↔ 4H + ) and/or the incorporation of interstitial molecular water. To investigate water speciation and hydrogen isotope behavior, we synthesized partially deuterated hydrous stishovite at 9 GPa and 450 °C in a multi-anvil press (MA). The hydrous stishovite contains on average 1.69 ± 0.05 wt% water, which is consistent with earlier MA studies but is significantly lower than the 3.5 wt% reported from in situ diamond anvil cell (DAC) studies made at higher pressures and temperatures. 1 H MAS NMR spinning sideband characteristics suggest a high abundance of interstitial molecular water in hydrous stishovite, while the presence of a hydrogarnet defect cannot be ruled out. Unit-cell volumes and deuterium enrichment in the quenched hydrous stishovite indicate that ~ 45% of the water is lost from the stishovite upon quenching and decompression of the experiment, consistent with a higher solubility. This implies that the pristine water contents of a P – T – f O 2 equilibrated hydrous stishovite cannot be quenched to 1 atm and room temperature from classical MA experiments. We further present a capillary-based recovery method for fluid from experimental capsules, allowing direct determination of the D / H ratio of the experimental fluid and indirect determination of the hydrous stishovite. Using Rayleigh modeling to account for the quench-related water loss, we find that, at 450 °C and 9 GPa, deuterium is 3.5–4.5 times enriched in hydrous stishovite relative to coexisting aqueous fluid. This is opposite of what is commonly observed for mineral–fluid pairs above 300 °C, rendering hydrous stishovite a potential sink for deuterium and decreasing the D / H ratio of coexisting aqueous fluids. Partial decomposition (30–60%) of hydrous stishovite during mantle upwelling and production of primary basaltic melts could be accompanied by high-temperature D / H fractionation, decreasing the hydrogen isotope composition of such melts towards “mantle-like” δD values between −75 and −220‰.

Boron substitution represents a promising approach to stabilize carbon clathrate structures, but no thermodynamically stable substitution schemes have been identified for frameworks other than the type-VII (sodalite) structure type. To investigate the possibility for additional tetrahedral carbon-based clathrate networks, more than 5000 unique boron decoration schemes were investigated computationally for type-I and type-II carbon clathrates with a range of guest elements including Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. Density functional theory calculations were performed at 10 and 50 GPa, and the stability and impact of boron substitution were evaluated. The results indicate that the boron-substituted carbon clathrates are stabilized under high-pressure conditions. Full cage occupancies of intermediate-sized guest atoms (e.g., Na, Ca, and Sr) are the most favorable energetically. Clathrate stability is maximized when the boron atoms are substituted within the hexagonal rings of the large [5(12)6(2)]/[5(12)6(4)] cages. Several structures with favorable formation enthalpies <-200 meV/atom were predicted, and type-I Ca8B16C30 is on the convex hull at 50 GPa. This structure represents the first thermodynamically stable type-I clathrate identified and suggests that boron-substituted carbon clathrates may represent a large family of diamond-like framework materials with a range of structure types and guest/framework substitutions.

Compared with conventional, solution-phase approaches, solid-state reaction methods can provide unique access to novel synthetic targets. Nanothreads-one-dimensional diamondoid polymers formed through the compression of small molecules-represent a new class of materials produced via solid-state reactions, however, the formation of chemically homogeneous products with targeted functionalization represents a persistent challenge. Through careful consideration of molecular precursor stacking geometry and functionalization, we report here the scalable synthesis of chemically homogeneous, functionalized nanothreads through the solid-state polymerization of 2,5-furandicarboxylic acid. The resulting product possesses high-density, pendant carboxyl functionalization along both sides of the backbone, enabling new opportunities for the post-synthetic processing and chemical modification of nanothread materials applicable to a broad range of potential applications.