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Zeolitic imidazolate frameworks are widely thought of as being analogous to inorganic AB2 phases. We test the validity of this assumption by comparing simplified and fully atomistic machine-learning models for local environments in ZIFs. Our work addresses the central question to what extent chemical information can be "coarse-grained" in hybrid framework materials. [ABSTRACT FROM AUTHOR]

The scandium (Sc) – alloyed Sb2Te3 phase-change alloy has recently been found to enable ultrafast crystal nucleation due to the formation of Sc-stabilized octahedral motifs in the amorphous phase, rendering cache-type phase-change memory feasible. When yttrium (Y) is added, however, non-octahedral bonding patterns form in the amorphous Sb2Te3-based network even though Y has a valence electron configuration similar to that of Sc and also forms perfect octahedral bonding environments with tellurium in the YTe crystal. Here we elucidate the origin of this difference between Sc and Y, by carrying out thorough ab initio simulations and orbital-based bonding analyses on amorphous Y–Sb–Te and Sc–Sb–Te compounds. We also demonstrate how the smooth overlap of atomic positions (SOAP) similarity kernel can be used to quantify the structural similarity of local motifs in the amorphous phase with respect to various crystalline yttrium and scandium tellurides, both in the nearest-neighbor shell and beyond. We find that the bonding contrast of Y- and Sc-centered structural motifs in amorphous Sb2Te3 stems from their parent crystals at high Te concentrations. The larger atomic radius of Y and the weaker charge transfer when bonded with Te is found to allow more Te neighbors and cause a more open bonding environment, leading to higher coordination numbers and non-octahedral environments. We discuss the implications of the different local environments for practical applications in memory devices. [ABSTRACT FROM AUTHOR]

The resistance drift phenomenon observed in amorphous chalcogenide phase-change materials (PCMs) hinders the development of PCM-based neuro-inspired computing devices. It has been observed that the drift in electrical resistance can be effectively reduced by substituting Ge with Sn in the prototype PCM GeTe, forming amorphous (Ge1−xSnx)Te solids. However, the atomistic and chemical origin of such drift suppression phenomenon remains unclear. In this work, we carry out thorough ab initio simulations and chemical bonding analyses of amorphous Ge–Sn–Te materials. We show that the two critical driving forces for glass relaxation in PCMs, i.e. the amount of tetrahedral motifs and the degree of Peierls distortion, are gradually reduced as Sn content increases. Such trend can be explained by the increased ionicity brought about by the Ge → Sn substitution. Our work suggests that an optimal Sn-rich GeSnTe composition could be reached for PCM-based neuro-inspired computing with ultralow resistance drift. [ABSTRACT FROM AUTHOR]

Graphite and non-graphitising ("hard") carbons are important anode materials for battery technologies. The electrochemical intercalation of alkali metals in graphite has been widely studied by first-principles density-functional theory (DFT). However, similar investigations of disordered "hard" and nanoporous carbons have been challenging due to the structural complexity involved. Here, we combine DFT with machine-learning (ML) methods to study the intercalation of alkali metal (Li, Na, K) atoms in model carbon systems over a range of densities and degrees of disorder. We use a stochastic approach to compute voltage–filling profiles, studying the three metal species side-by-side, and we analyse the ionic charges of metal atoms as a function of filling. Our study provides atomic-scale insight into the intercalation of all three alkali metals that are relevant to batteries, and it thereby makes a key step towards the DFT/ML-driven modelling of energy materials. [ABSTRACT FROM AUTHOR]

Disordered nanoporous and “hard” carbons are widely used in batteries and supercapacitors, but their atomic structures are poorly determined. Here, we combine machine learning and DFT to obtain new atomistic insight into carbonaceous energy materials. We study structural models of porous and graphitic carbons, and Na intercalation as relevant for sodium-ion batteries. [ABSTRACT FROM AUTHOR]

Crystal structure prediction algorithms, including ab initio random structure searching (AIRSS), are intrinsically limited by the huge computational cost of the underlying quantum-mechanical methods. We have recently shown that a novel class of machine learning (ML) based interatomic potentials can provide a way out: by performing a high-dimensional fit to the ab initio energy landscape, these potentials reach comparable accuracy but are orders of magnitude faster. In this paper, we develop our approach, dubbed Gaussian approximation potential-based random structure searching (GAP-RSS), towards a more general tool for exploring configuration spaces and predicting structures. We present a GAP-RSS interatomic potential model for elemental phosphorus, which identifies and correctly “learns” the orthorhombic black phosphorus (A17) structure without prior knowledge of any crystalline allotropes. Using the tubular structure of fibrous phosphorus as an example, we then discuss the limits of free searching, and discuss a possible way forward that combines a recently proposed fragment analysis with GAP-RSS. Examples of possible tubular (1D) and extended (3D) hypothetical allotropes of phosphorus as found by GAP-RSS are discussed. We believe that in the future, ML potentials could become versatile and routine computational tools for materials discovery and design. [ABSTRACT FROM AUTHOR]

The thermal motion of atoms in crystals is quantified by anisotropic displacement parameters (ADPs). Here we show that dispersion-corrected periodic density-functional theory can be used to compute accurate ADPs for transition metal carbonyls, which serve as model systems for crystalline organometallic and coordination compounds. [ABSTRACT FROM AUTHOR]

In chemical crystallography, the thermal motion of scattering centres is commonly described by anisotropic displacement parameters (ADPs). Very recently, it has been shown that ADPs are not only accessible by diffraction experiments but also via theory: this emerging approach seems promising but must be thoroughly tested. In this study, we have performed specifically tailored X-ray diffraction (XRD) experiments in fine steps between 100 and 300 K which allow detailed comparison to ab initio data from dispersion-corrected density functional theory (DFT) combined with periodic lattice-dynamics. The compound chosen for this study, crystalline pentachloropyridine (C5NCl5), is well suited for this purpose: it represents a molecular crystal without H atoms, thus posing no challenge to XRD; its solid-state structure is controlled by dispersion and halogen-bonding interactions; and the ADPs associated with the peripheral Cl atoms show strong temperature dependence. Quality criteria in direct and in reciprocal space prove that ADPs are predicted with high confidence for the temperature range between 100 and 200 K, and that several economic dispersion corrections to DFT can be reliably employed for this purpose. Within the limits we have explored here, the ab initio prediction of ADPs appears to be a facile and complementary tool, especially in those cases where diffraction data cannot provide a straightforward model for thermal motion. [ABSTRACT FROM AUTHOR]

We identify a similar feature in the chemical-bonding nature of seemingly different phase-change materials (PCMs) for data storage. This affords new insight into the “next-generation” material In3SbTe2, establishes a hitherto missing link to the more ubiquitous Ge–Sb–Te alloys, and encourages the search for new PCMs beyond established electron-counting schemes. [ABSTRACT FROM AUTHOR]

The thermal motion of atoms and functional groups is a key characteristic of any molecular crystal, and such motion derived from scattering experiments is conveniently visualised by means of thermal ellipsoids (the famous "ORTEP" drawings). Unfortunately, it is often impossible to obtain the underlying anisotropic displacement parameters (ADPs) for hydrogen atoms, due to their low X-ray scattering power, and sometimes no ADPs can be refined at all even for heavier atoms. In these cases, it would seem advantageous to estimate ADPs by first-principles techniques, and indeed such ORTEP plots have become available ab initio very recently. Here, we test this young method for a representative set of hydrogen-bonded molecular crystals: first, we study urea (CON2H4) as a well-known benchmark, then, its all-nitrogen analogue guanidine (CN3H5); finally, we move on to rubidium guanidinate (RbCN3H4) as a specimen with pronounced ionic interactions. For all three systems, ADPs have been obtained from density-functional theory (DFT) based phonon computations using the PHONOPY software. The results are compared with neutron-diffraction data as the experimental "benchmark" in this regard, and a critical discussion of experimental aspects is given. We observe excellent agreement between experiment and theory for the hydrogen-bonded systems urea and guanidine at low temperature, whereas high-temperature data for guanidine deviate visibly, and the more salt-like RbCN3H4 may suffer from a less-than-ideal description even at 12 K. Both are discussed in depth as there are possible solutions and directions for further research. Generally, the present results shine a favourable light on a future, more routine application of combined experimental/ theoretical approaches in chemical crystallography. [ABSTRACT FROM AUTHOR]

Mutually perpendicular hydrogen bonds, halogen bonds and stacking interactions are investigated in the seemingly simple crystal structure of bromomalonic aldehyde (C3H3O2Br). This combined experimental and theoretical study sheds new light on the role of electrostatic interactions in molecular crystalline networks and poses a caveat to look beyond what may be termed "chemical intuition". [ABSTRACT FROM AUTHOR]

A family of alkali-metal guanidinates (M = Na–Cs) has been reported recently, representing all-nitrogen analogues of alkali bicarbonates and containing the elusive guanidinate monoanion CN3H4−. Here, we describe the synthesis and characterisation of LiCN3H4 as the last representative of alkali guanidinates. Single crystals of LiCN3H4 were obtained using a solvothermal route in liquid ammonia, and the crystal structure was determined at 100 K using single-crystal X-ray diffraction (monoclinic, P21/c, Z = 4, a = 7.251(2) Å, b = 4.532(2) Å, c = 9.051(2) Å, β = 103.315(3)°). The structure of LiCN3H4 is reminiscent of the previously reported NaCN3H4 type in that both guanidinates contain tetrahedrally nitrogen-coordinated alkali-metal cations. The linking of these tetrahedra differs, however: they share corners in NaCN3H4 to form one-dimensionally infinite chains running through the crystal, whereas isolated, edge-sharing tetrahedra (Li2N6 motif) are found in the LiCN3H4 structure described here. Periodic density-functional theory (DFT) computations at the dispersion-corrected PBE + D2 level not only correctly reproduce the structural preferences, but also indicate that a NaCN3H4 polymorph should be viable adopting the lithium guanidinate type but much less easily so vice versa. [ABSTRACT FROM AUTHOR]

The covalent nature of short hydrogen bonds has been under debate for long. Here we show that the crystal orbital Hamilton population (COHP) bonding indicator gives new, complementary evidence of covalent hydrogen- - acceptor interactions in the molecular solid state. [ABSTRACT FROM AUTHOR]

Phosphide-based materials have been investigated as promising candidates for solid electrolytes, among which the recently reported Li 9 AlP 4 displays an ionic conductivity of 3 mS cm -1 . While the phases Li-Al-P and Li-Ga-P have already been investigated, no ternary indium-based phosphide has been reported up to now. Here, we describe the synthesis and characterization of the first lithium phosphidoindate Li 3 InP 2 , which is easily accessible via ball milling of the elements and subsequent annealing. Li 3 InP 2 crystallizes in the tetragonal space group I 4 1 / acd with lattice parameters of a = 12.0007(2) and c = 23.917(5) Å, featuring a supertetrahedral polyanionic framework of interconnected InP 4 tetrahedra. All lithium atoms occupy tetrahedral voids with no partial occupation. Remarkably, Li 3 InP 2 is not isotypic to the previously reported homologues Li 3 AlP 2 and Li 3 GaP 2 , which both crystallize in the space group Cmce and feature 2D layers of connected tetrahedra but no supertetrahedral framework. DFT computations support the observed stability of Li 3 InP 2 . A detailed geometrical analysis leads to a more general insight into the structural factors governing lithium ion mobility in phosphide-based materials: in the non-ionic conducting Li 3 InP 2 the Li ions exclusively occupy tetrahedral voids in the distorted close packing of P atoms, whereas partially filled octahedral voids are present in the moderate ionic conductors Li 2 SiP 2 and Li 2 GeP 2 ., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

Antimony selenide (antimonselite, Sb 2 Se 3 ) is a versatile functional material with emerging applications in solar cells. It also provides an intriguing prototype to study different modes of bonding in solid chalcogenides, all within one crystal structure. In this study, we unravel the complex bonding nature of crystalline Sb 2 Se 3 by using an orbital-based descriptor (the crystal orbital Hamilton population, COHP) and by analysing phonon properties and interatomic force constants. We find particularly interesting behaviour for the medium-range Sb···Se contacts, which still contribute significant stabilisation but are much softer than the "traditional" covalent bonds. These results have implications for the assembly of Sb 2 Se 3 nanostructures, and bond-projected force constants appear as a useful microscopic descriptor for investigating a larger number of chalcogenide functional materials in the future.

A family of alkali-metal guanidinates (M = Na-Cs) has been reported recently, representing all-nitrogen analogues of alkali bicarbonates and containing the elusive guanidinate monoanion CN3H4(-). Here, we describe the synthesis and characterisation of LiCN3H4 as the last representative of alkali guanidinates. Single crystals of LiCN3H4 were obtained using a solvothermal route in liquid ammonia, and the crystal structure was determined at 100 K using single-crystal X-ray diffraction (monoclinic, P2(1)/c, Z = 4, a = 7.251(2) Å, b = 4.532(2) Å, c = 9.051(2) Å, β = 103.315(3)°). The structure of LiCN3H4 is reminiscent of the previously reported NaCN3H4 type in that both guanidinates contain tetrahedrally nitrogen-coordinated alkali-metal cations. The linking of these tetrahedra differs, however: they share corners in NaCN3H4 to form one-dimensionally infinite chains running through the crystal, whereas isolated, edge-sharing tetrahedra (Li2N6 motif) are found in the LiCN3H4 structure described here. Periodic density-functional theory (DFT) computations at the dispersion-corrected PBE + D2 level not only correctly reproduce the structural preferences, but also indicate that a NaCN3H4 polymorph should be viable adopting the lithium guanidinate type but much less easily so vice versa.