The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios. Computational methods have been developed to guide synthesis by predicting structures at specific compositions and predicting compositions for known crystal structures, with notable successes. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them.
Alkali metal intercalation into polyaromatic hydrocarbons (PAHs) has been studied intensely following reports of superconductivity in a number of potassium-and rubidium-intercalated materials. There are however no reported crystal structures to inform understanding of the chemistry and physics because of the complex reactivity of PAHs with strong reducing agents at high temperature. Here we present the synthesis of crystalline K2Pentacene and K2Picene by a solid-solid insertion protocol that uses potassium hydride as a redox-controlled reducing agent to access the PAH dianions, enabling determination of their crystal structures. In both cases, the inserted cations expand the parent herringbone packings by reorienting the molecular anions to create multiple potassium sites within initially dense molecular layers, and thus interact with the PAH anion π-systems. The synthetic and crystal chemistry of alkali metal intercalation into PAHs differs from that into fullerenes and graphite, where the cation sites are pre-defined by the host structure.Reaction of alkali and alkaline earth metals with carbon-based molecular solids has been extensively studied in the search for novel magnetic and electronic properties, with a particular focus on superconductivity. [1][2][3][4][5][6] For example, alkali metal intercalation into solid C60 produces A3C60 superconductors, with Tc as high as 38 K for Cs3C60 at 7 kbar. 6 In these materials, cations occupy the interstitial voids that already exist in the host e.g., in the fcc lattice of C60 (Figure 1). 7 Reaction of potassium with picene (C22H14), a phenacene composed of five fused benzene rings, has been reported to afford superconductivity at 18 K. 8 Superconductivity in other alkali-metal polyaromatic hydrocarbons (PAHs) was subsequently reported in phenanthrene-, dibenzopentacene-and coronene-based materials, with the highest reported Tc of 33 K claimed 2 in potassium-doped 1,2:8,9-dibenzopentacene. [9][10][11] Despite the significant interest in these materials, to date no crystal structure has been determined for any of the alkali-metal PAH systems. This structural information is a prerequisite for materials design and for understanding of both physical properties and reaction chemistry.Picene, like many other PAHs (including molecules such as phenanthrene 12 which is reported to afford superconductivity on cation insertion), crystallises in the herringbone structure, 13 with layers consisting of two parallel one-dimensional chains of molecules with opposing inclinations defined by an intermolecular angle, ω, of 57.89(7)° (Supplementary Figure 1). 14 The largest voids in the structure are located between the picene layers, adjacent to the saturated C-H bonds and far from the electron density of the PAH π-systems ( Figure 1a). This contrasts with C60, where the octahedral and tetrahedral voids in the fcc lattice are adjacent to the conjugated π-electron system (Figure 1b Pentacene is the linear isomer of picene, and also adopts the herringbone structure, 16 with ω = 52...
SUMMARYQuantitative morphological data obtained from Xanthoria parietina by light and electron microscopy is related to measurements of photosynthesis rate and estimates of biotrophic ribitol transfer. It is concluded (a) that diffusive CO2 flux into the alga is greatly reduced by the resistance of the fungal cortex, (b) that diffusion can easily account for the mass transfer of ribitol from alga to fungus, and (c) that haustoria have no role in the biotrophic transfer of carbohydrate in lichens.
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