Thermal expansion properties of organic crystals: a CSD study

The thermal expansion properties of crystalline organic compounds are investigated by data mining of the Cambridge Structural Database (CSD). The mean volumetric thermal expansion coefficient is 168.8 × 10-6 K-1 and the mean uniaxial thermal expansion coefficient is 71.4 × 10-6 K-1, based on 745 and 1129 different observations, respectively. Normal and anomalous coefficients can be identified using these values and the associated standard deviations. The anisotropy of the thermal expansion is also evaluated and found to have a very broad distribution. 4719 different structures, comprising 4093 different molecular compounds and 626 additional polymorphs have been analyzed on their thermal expansion properties. Approximately 34% of these structures may have at least one orthogonal axis with negative thermal expansion, much more than generally believed. Moreover 127 structures have been identified which could have negative volumetric thermal expansion. Experimental validation using a robust protocol with data collected at more than 2 different temperatures is required to validate these cases.

Fullerene rotational dynamics generate disordered configurations that suppress thermal conductivity in superatomic crystals

The thermal conductivity of fullerene-based superatomic crystals (SACs) is investigated using molecular dynamics simulations. The temperature-dependent predictions agree with the trends of previous measurements. The thermal conductivity behavior emerges as a result of the C₆₀ molecule rotational dynamics and orientation, which are quantified using the root mean square displacements of the carbon atoms and the relative orientations of the C₆₀s. At low temperatures, the C₆₀s exhibit small rotations around equilibrium positions (i.e., librations). When the librating C₆₀s are orientationally-ordered, as in the [C₆₀] and [Co₆Se₈(PEt₃)₆][C₆₀]₂ SACs, thermal conductivity decreases with increasing temperature, as is typical for a crystal. When the librating C₆₀s are orientationally-disordered, however, as in the [Co₆Te₈(PEt₃)₆][C₆₀]₂ SAC, thermal conductivity is lower and temperature independent, as is typical for an amorphous solid. At higher temperatures, where the C₆₀s in all three SACs freely-rotate and are thus dynamically disordered, thermal conductivity is temperature independent. The abrupt changes driven by the C₆₀ dynamics suggest that fullerene-based SACs can be designed to be thermal conductivity switches based on a variety of external stimuli.

Designing disorder into crystalline materials

Crystals are a state of matter characterized by periodic order. Yet, crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies or magnetic structure. Disorder can sometimes be random but, more usually, it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this Review, we survey the core design principles that guide targeted control over correlated disorder. We show how these principles - often informed by long-studied statistical mechanical models - can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics and metal-organic frameworks. We conclude with a forward-looking discussion of the exciting link between disorder and function in responsive media, thermoelectrics and topological phases.

Structural manifestation of partial proton ordering and defect mobility in ice Ih

High precision lattice-parameter measurements provide a potential roadmap to producing partially-ordered states of water ice.

Nanoscale Imaging of Charge Carrier and Exciton Trapping at Structural Defects in Organic Semiconductors

Charge carrier and exciton trapping in organic semiconductors crucially determine the performance of organic (opto-)electronic devices such as organic field-effect transistors, light-emitting diodes, or solar cells. However, the microscopic origin of the relevant traps generally remains unclear, as most spectroscopic techniques are unable to simultaneously probe the electronic and morphological structure of individual traps. Here, we employ low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) as well as tight-binding calculations derived from ab initio calculations to image the localized electronic states arising at structural defects in thin C60 films (<10 ML). The spatially and spectrally resolved STM-induced luminescence at these states reveals an enhanced radiative decay of excitons, which is interpreted in terms of the local symmetry lowering and the trapping of excitons by an X-trap. The combined mapping of the STM-induced luminescence, electronic structure, and morphology thus provides new insights into the origin and characteristics of individual exciton traps in organic semiconductors and offers new avenues to study charge carrier and exciton dynamics on molecular scales.

Use of intensity quotients and differences in absolute structure refinement

Several methods for absolute structure refinement were tested using single-crystal X-ray diffraction data collected using Cu Kα radiation for 23 crystals with no element heavier than oxygen: conventional refinement using an inversion twin model, estimation using intensity quotients in SHELXL2012, estimation using Bayesian methods in PLATON, estimation using restraints consisting of numerical intensity differences in CRYSTALS and estimation using differences and quotients in TOPAS-Academic where both quantities were coded in terms of other structural parameters and implemented as restraints. The conventional refinement approach yielded accurate values of the Flack parameter, but with standard uncertainties ranging from 0.15 to 0.77. The other methods also yielded accurate values of the Flack parameter, but with much higher precision. Absolute structure was established in all cases, even for a hydrocarbon. The procedures in which restraints are coded explicitly in terms of other structural parameters enable the Flack parameter to correlate with these other parameters, so that it is determined along with those parameters during refinement.

Heteroepitaxial growth of C60 on tetracene single crystal

We have developed a method for epitaxial growth of C60 thin films on tetracene single crystals. The crystal orientation of the C60 film was examined by reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD). In-situ observation by RHEED revealed that the C60 crystallizes from the very initial stage of the deposition (0.1 nm). A 6-fold symmetric pattern, which was observed in a XRD polar scan, can be taken as direct evidence for the epitaxial growth of C60 commensurate with the tetracene (001) surface lattice.

PASCal: a principal axis strain calculator for thermal expansion and compressibility determination

This article describes a web-based tool (PASCal; principal axis strain calculator; http://pascal.chem.ox.ac.uk) designed to simplify the determination of principal coefficients of thermal expansion and compressibilities from variable-temperature and variable-pressure lattice parameter data. In a series of three case studies,PASCalis used to reanalyse previously published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two-dimensional metal–organic framework [Cu2(OH)(C8H3O7S)(H2O)]·2H2O is found to exhibit the strongest area negative thermal expansion (NTE) effect yet observed; second, the widely used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and third, the high-pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility effect that arises from correlated tilting of SnO6and SiO4coordination polyhedra.

AB INITIO CALCULATIONS OF INTERMOLECULAR INTERACTION POTENTIALS OF FULLERENE-FRAGMENTS SYSTEMS

We have performed the ab initio molecular orbital calculations for combinations of the fullerene-fragments as the models of the nonbonding interaction of C 60 dimer at the preferred configurations in the simple cubic phase. The intermolecular interaction potentials have been calculated using several basis sets with MP2 level of the electron correlation energy and the basis set superposition error correction. The strong dispersion attractions that is dominant in the van der Waals interaction has been found for the combinations of the fullerene-fragments. The equilibrium intermolecular distances obtained are in good agreement with the corresponding experimental value. The repulsive region of the intermolecular interaction calculated by ab initio method is found to be express by an atom–atom interaction potential with an anisotropic exponential type repulsive term, which is less steep than the conventional Lennard–Jones type potential.

Pathological features of rat lung following inhalation and intratracheal instillation of C(60) fullerene

We evaluated the pulmonary pathological features of rats that received a single intratracheal instillation and a 4-week inhalation of a fullerene. We used fullerene C(60) (nanom purple; Frontier Carbon Co. Ltd, Japan) in this study. Male Wistar rats received intratracheal dose of 0.1, 0.2, or 1 mg of C(60), and were sacrificed at 3 days, 1 week, 1 month, 3 months, 6 months, and 12 months. In the inhalation study, Wistar rats received C(60) or nickel oxide by whole-body inhalation for 6 h/day, 5 days/week, 4 weeks, and were sacrificed at 3 days, 1 month, and 3 months after the end of exposure. During the observation period, no tumors or granulomas were observed in either study. Histopathological evaluation by the point counting method (PCM) showed that a high dose of C(60) (1 mg) instillation led to a significant increase of areas of inflammation in the early phase (until 1 week). In the inhalation study of the C(60)-exposed group, PCM evaluation showed significant changes in the C(60)-exposed group only at 3 days after exposure; after 1 month, no significant changes were observed. The present study demonstrated that the pulmonary inflammation pattern after exposure to well-characterized C(60) via both intratracheal and inhalation instillation was slight and transient. These results support our previous studies that showed C(60) has no significant adverse effects in intratracheal and inhalation instillation studies.

Real-space calculation of powder diffraction patterns on graphics processing units

A new software for calculating the powder diffraction pattern of nano-sized objects has been developed to run on graphics processing units (GPUs). This solution is well suited to the inherently parallel structure of the Debye function, which is the core of the computation algorithm. Advantages and perspectives in view of the improving performance of GPUs are illustrated by several representative case studies.

Solvent engineering for shape-shifter pure fullerene (C60)

As a highly anticipated technique for bottom-up nanotechnology, i.e., shape control of pure functional molecules, we here report controlled formation of two-dimensional (2D) objects such as hexagons and rhombi and their selective shape shifting into one-dimensional (1D) rods through solvent-dependent changes of crystal lattice, all from pure C(60). Uniformly shaped rhombi and hexagons were obtained at tert-butyl alcohol/toluene and i-propyl alcohol/CCl(4) interfaces, respectively. In addition, exposure of these 2D nanosheets to water induced selective transformation into 1D nanorods. Nanorhombi were converted to short nanorods upon exposure to water. This shape shift is accompanied by changes in crystalline structures from a mixed fcc/hexagonal to pure fcc lattice, the latter of which is almost identical with morphologically similar C(60) nanowhiskers. Metastable nanorhombi which possess a strained mixed crystalline structure metamorphosize into the more stable short nanowhisker (nanorods). In contrast, the stable nanohexagon of a single lattice (and so less strain) does not undergo shape shifting. These results clearly demonstrate controlled formation of 2D nanosheets with various shapes (hexagons, rhombi, etc.) and selective shape shifting to nanorods (short nanowhiskers) all from pure C(60) molecules by very simple solvent treatments.

Mechanical properties of solid C(60) studied with density functional tight binding method augmented by an empirical dispersion term

The bulk modulus of solid fcc C(60) was calculated with a density functional tight binding method, augmented by an empirical van der Waals force. The predicted modulus of 9.1 GPa is in good agreement with the experimental measurements. We found that the geometric structures of C(60) molecules in the solid fcc phase are considerably changed under strong compression, due to variations in the type of hybridization of carbon atoms. We also observed a reduction of the HOMO-LUMO gap under compression which is attributed to the overlap of π orbitals from neighboring C(60) molecules. Finally, we showed that the dispersion energy correction in the adopted scheme plays an essential role for the quantitative description of the weakly bound C(60) solid.

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.

Two-stage rotational disordering of a molecular crystal surface: C60

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C60 fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2x2) state, and a high-temperature (1x1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.