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DL_MULTI has been developed to extend the Molecular Dynamics simulation program DL_POLY to model rigid molecules whose intermolecular interactions include a distributed multipole model for the electrostatic interactions. The adaptations use anisotropic atom–atom potentials, corresponding to atomic multipoles up to hexadecapole. The lattice sums of these multipoles are evaluated using the Ewald method, using a technique utilizing Stone's S functions which describe the multipoles in a molecule fixed reference frame. An algorithm for determining suitable cutoffs is described and errors in the direct space part of the Ewald summation discussed. Thus DL_MULTI provides a general purpose MD program for studying polar rigid organic molecules in their liquid and crystalline states with a realistic intermolecular potential suitable for studying polymorphism. Example applications to uracil and 5-azauracil show that, with the new summation method, a realistic electrostatic model can be used without excessive computer...

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.

Molecular dynamics (MD) simulations are capable of giving considerable insight into the polymorphism of organic molecules, a problem of major concern to the pharmaceutical and other speciality chemicals industries. We illustrate some of the challenges involved in small organic systems, which have complex solid-state phase behaviour, including characterizing rotationally disordered phases, modelling polymorphs with very different hydrogen bonding motifs and explaining the solvent dependence of a polymorphic system. Simulating the dynamics within the organic solid state can be very demanding of the model for the weak forces between the molecules. This has led to the development of DL_MULTI so that a distributed multipole electrostatic model can be used to describe the orientation dependence of hydrogen bonding and π–π stacking more realistically. Once a simulation is correctly reproducing the known crystal structures, there are also considerable system-specific challenges in extracting novel insights from t...

The DL_POLY package provides a set of classical molecular dynamics programs that have application over a wide range of atomic and molecular systems. Written for parallel computers they offer capabilities stretching from small systems consisting of a few hundred atoms running on a single processor, up to systems of several million atoms running on massively parallel computers with thousands of processors. In this article we describe the structure of the programs and some applications.

Molecular dynamics simulations have been performed on crystalline imidazole at 100 K and 5-azauracil at 310 K with a model intermolecular potential that includes a distributed multipole representation of the molecular charge distribution using the program DL_MULTI. The anisotropic atom–atom electrostatic model enabled the experimental crystal structures to be reproduced well in a constant pressure simulation and the simulations showed a physically reasonable thermal expansion relative to the minimum in the static lattice energy. The rigid-body molecular motions in a subsequent constant volume simulation were analysed to obtain the k = 0 frequencies corresponding to different symmetry representations, via the translational and rotational velocity autocorrelation functions. These frequencies were contrasted with the corresponding harmonic lattice modes calculated with the same molecular model and intermolecular potential. The agreement was good, with most, but not all, modes decreasing in frequency in the f...

Rigid-body, k = 0 phonon frequencies have been calculated within the crystal structure modeling program DMAREL, enabling the use of anisotropic atom−atom model potentials. Five organic crystals (hexamethylenetetramine, naphthalene, pyrazine, imidazole, and α-glycine) were chosen to sample a range of intermolecular interactions for determining the sensitivity of the calculated frequencies to changes in the empirical repulsion-dispersion parameters and the electrostatic model. A carefully parameterized simple exp-6 model can describe vibrations in simple van der Waals crystals and some hydrogen bonded crystals reasonably well. However, for weaker polar interactions, an accurate model of the electrostatics is needed. Bending of weak polar interactions and shearing of close contacts with delocalized π-systems are particularly sensitive to the description of electrostatic interactions. Point charge models generally underestimate the resistance to deforming hydrogen bonds, and a distributed multipole model stab...

A series of nonempirical intermolecular potentials has been developed for the cyanuric chloride dimer, using the overlap model to determine the anisotropy of the repulsive wall around each atom. Calibration against intermolecular perturbation theory calculations enables the penetration and charge-transfer energy to be explicitly included with the exchange-repulsion to give a simple repulsion model in an anisotropic atom−atom form. These model repulsion potentials are used in conjunction with an atomic multipole electrostatic model and an atom−atom dispersion model to give nonempirical potential models, which are tested for their ability to reproduce the crystal structure of cyanuric chloride. The best nonempirical potential is successfully used to construct a simpler transferable model for closely related azaaromatic chlorides. The nonempirical potential reproduces the experimental space group of cyanuric chloride, unlike some empirically fitted repulsion−dispersion potentials. This first nonempirical rep...

Elastic constants of a set of molecular organic crystals have been calculated within the crystal modeling program DMAREL, which was developed to allow the use of highly accurate, anisotropic atom−atom potentials. A set of six molecular crystals (durene, m-dinitrobenzene, the β form of resorcinol, pentaerythritol, urea, and hexamethylenetetramine) has been chosen to sample a range of intermolecular interactions and crystal symmetries. The sensitivity of such calculations to variations in empirical repulsion−dispersion parameters and the electrostatic model is determined and discussed relative to the other errors in the theoretical model and typical experimental uncertainties. We find that model potentials whose functional form has been simplified often reproduce crystal structures and lattice energies adequately but perform poorly when used to calculate elastic constants. This is because more theoretically justified potentials are required to satisfactorily model the curvature at the equilibrium geometries...

Molecular dynamics (MD) simulation is used to calculate the elastic constants and their temperature and pressure derivatives, and the T-P-V equation of state of MgO. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In addition, to account for the observed large Cauchy violation of the elastic constants of MgO, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal. Quantum correction to the MD pressure is made using the Wigner-Kirkwood expansion of the free energy. Required energy parameters, including oxygen breathing parameters, were derived empirically to reproduce the observed molar volume and elastic constants of MgO, and their measured temperature and pressure derivatives as accurately as possible. The MD simulation with BSM is found to be very successful in reproducing accurately the measured molar volumes and individual elastic constants of MgO over a wide temperature and pressure range. The errors in the simulated molar volumes are within 0.3% over the temperature range between 300 and 3000 K at 0 GPa, and within 0.1% over the pressure range from 0 up to 50 GPa at 300 K. The simulated bulk modulus is found to be correct to within 0.7% between 300 and 1800 K at 0 GPa. Here we present the MD simulated T-P-V equation of state of MgO as an accurate internal pressure calibration standard at high temperatures and high pressures.

We present the parametrization of a new polarizable model for water based on Thole’s method [Chem. Phys. 59, 341 (1981)] for predicting molecular polarizabilities using smeared charges and dipoles. The potential is parametrized using first principles ab initio data for the water dimer. Initial benchmarks of the new model include the investigation of the properties of water clusters (n=2–21) and (hexagonal) ice Ih using molecular dynamics simulations. The potential produces energies and nearest-neighbor (H-bonded) oxygen–oxygen distances that agree well with the ab initio results for the small water clusters. The properties of larger clusters with 9–21 water molecules using predicted structures from Wales et al. [Chem. Phys. Lett. 286, 65 (1998)] were also studied in order to identify trends and convergence of structural and electric properties with cluster size. The simulation of ice Ih produces a lattice energy of −65.19 kJ/mol (expt. −58.9 kJ/mol) with an average dipole moment of 2.849 D. The calculated...

By use of inelastic incoherent neutron scattering techniques on ISIS at Rutherford-Appleton Laboratory, neutron spectra for a large range of recoverable high-pressure phases of ice have been measured. In order to understand the physical significance of the measured spectra and reproduce the experimental data, we have proposed a model, that requires two strengths of hydrogen bonding. In this paper, we describe some aspects of this model in detail and compare it with the existing potential models.

Since the observation of two intermolecular translational optic peaks at 28 and 37 meV using inelastic neutron scattering techniques, there has been a renewed effort to reproduce phonon density of states with these peaks by computer simulation. In this paper, we demonstrate our attempt to calculate the measured inelastic neutron spectrum using molecular dynamic techniques for a number of potentials, such as TIP4P, MCY, and KKY potentials. The results show that none of them can reproduce the double peaks seen in the measured spectrum.

The effect of using a realistic model for the electrostatic forces on the calculated structures of molecular crystals is explored by using atomic multipoles derived from an SCF 6-31G** wave function. This was tested on a wide ranging database of 40 rigid organic molecules containing C, H, N, and O atoms, including nucleic acid bases, nonlinear optic materials, azabenzenes, nitrobenzenes, and simpler molecules. The distributed multipole electrostatic model, plus an empirical repulsion-dispersion potential, was able to successfully reproduce the lattice vectors and available heats of sublimation of the experimental room temperature structure in almost all cases. Scaling the electrostatic energy to allow for the effect of electron correlation on the molecular charge density generally improved the lattice energies and the calculated structures to a lesser extent. However, omitting the anisotropic multipole moments usually gave very poor, sometimes qualitatively wrong structures, emphasizing the sensitivity of...

We describe a method for minimizing the lattice energy of molecular crystal structures, using a realistic anisotropic atom-atom model for the intermolecular forces. Molecules are assumed to be rigid, and the structure is described by the center of mass positions and orientational parameters for each molecule in the unit cell, as well as external strain parameters used to optimize the cell geometry. The resulting program uses a distributed multipole description of the electrostatic forces, which consists of sets of atomic multipoles (charge, dipole, quadrupole, etc.) to represent the lone pair, pi electron density, and other nonspherical features in the atomic charge distribution. Such ab initio based, electrostatic models are essential for describing the orientation dependence of the intermolecular forces, including hydrogen bonding, between polar molecules. Studies on a range of organic crystals containing hydrogen bonds are used to illustrate the use of this new crystal structure relaxation program, DMAREL, and show that it provides a promising new approach to studying the crystal packing of polar molecules. (C) 1995 by John Wiley and Sons, Inc.

Organic molecules with high non-linear optic coefficients have polar groups often linked by delocalised π-electron systems. Thus the charge distribution around many atoms in such molecules will be highly non-spherical. Current methods of predicting the crystal structures of these molecules, and thus the macroscopic second harmonic generation capability, take no account of these features as they use fixed charges at each atomic site to model the electrostatic interaction. A more realistic representation of the electrostatic interaction is to use a set of multipole moments on each atom, which may be derived from a distributed multipole analysis of an ab-initio wavefunction for the isolated molecule

Crystal structure prediction for organic molecules requires both the fast assessment of thousands to millions of crystal structures and the greatest possible accuracy in their relative energies. We describe a crystal lattice simulation program, DMACRYS, emphasizing the features that make it suitable for use in crystal structure prediction for pharmaceutical molecules using accurate anisotropic atom-atom model intermolecular potentials based on the theory of intermolecular forces. DMACRYS can optimize the lattice energy of a crystal, calculate the second derivative properties, and reduce the symmetry of the spacegroup to move away from a transition state. The calculated terahertz frequency k = 0 rigid-body lattice modes and elastic tensor can be used to estimate free energies. The program uses a distributed multipole electrostatic model (Q, t = 00,...,44s) for the electrostatic fields, and can use anisotropic atom-atom repulsion models, damped isotropic dispersion up to R(-10), as well as a range of empirically fitted isotropic exp-6 atom-atom models with different definitions of atomic types. A new feature is that an accurate model for the induction energy contribution to the lattice energy has been implemented that uses atomic anisotropic dipole polarizability models (alpha, t = (10,10)...(11c,11s)) to evaluate the changes in the molecular charge density induced by the electrostatic field within the crystal. It is demonstrated, using the four polymorphs of the pharmaceutical carbamazepine C(15)H(12)N(2)O, that whilst reproducing crystal structures is relatively easy, calculating the polymorphic energy differences to the accuracy of a few kJ mol(-1) required for applications is very demanding of assumptions made in the modelling. Thus DMACRYS enables the comparison of both known and hypothetical crystal structures as an aid to the development of pharmaceuticals and other speciality organic materials, and provides a tool to develop the modelling of the intermolecular forces involved in molecular recognition processes.

The siting of AI and of bridging OH groups in zeolite HZSM-5 is studied by defect energy minimization adopting the classical shell model. The small energy differences between the various lattice sites suggest that there would be little deviation from a random distribution of bridging hydroxyl groups and AI over the possible framework sites in contrast to previous suggestions.

The structure and properties of the four possible bridging hydroxyl groups in silicon-rich H-faujasite (zeolite Y) are studied by lattice-energy minimization carried out within the classical shell model. The most likely proton-accepting sites are the O1 and O3 oxygens. Their hydroxyl stretching frequencies are shown to be responsible for the characteristic high-frequency (HF) and low-frequency (LF) bands, respectively, in the infrared spectra of H-faujasites. The variation of the OH stretching frequencies for the four isolated sites is correlated with changes of local geometry such as the OH bond length and the SiO(H)Al and the SiOH angles. However, there appears to be no correlation with differences in the acid strength as evidenced by the values of the calculated deprotonation energy of the O(1)H and O(3)H hydroxyl groups for which only a minor difference is predicted.

We are extending the use of the method of static simulation of ionic lattices to modelling phosphates and condensed phosphates, the mineral components of many biominerals whose structure, crystallinity and reactivity can be influenced by substitutions and other defects. Parameters for the interatomic potentials of some phosphates and pyrophosphates have been determined and we present our results for the modelling of α-magnesium pyrophosphate, a representative compound of moderate complexity. The approach has been to treat the compound as an ionic solid with Mg2+ as the cation and P2O7 4 as a predominately covalent anion which is, however, ionically bound to the metal. Good agreement was found between the calculated and experimental structures for this ionic model using formal or residual charges, a Born-Mayer term for non-bonded repulsive interactions, whose parameters were calculated using the electron gas technique, a van der Waals dispersion term and three body terms to account for the directi...

Using appropriate inter and intramolecular atom-atom potential functions we have simulated the crystal structures of the C-form of n-dodecanoic and B- and C-forms of n-octadecanoic acids by lattice energy minimization. The molecular packing, in particular the intermolecular hydrogen bonds, and the lattice energies are in good agreement with available experimental evidence while the relative energies of the two polymorphic phases of n-octadecanoic acid are correctly predicted. Full unit cell relaxation reveals a systematic discrepancy with experimental values which is interpreted in terms of recognized anisotropy in thermal expansion of such structures. This work confirms the transferability of atom-atom potentials from small molecular crystals to far larger related systems.

This paper reports a novel methodology for the free-energy minimization of crystal structures exhibiting strong, anisotropic interactions due to hydrogen bonding. The geometry of the thermally expanded cell was calculated by exploiting the dependence of the free-energy derivatives with respect to cell lengths and angles on the average pressure tensor computed in short molecular dynamics simulations. All dynamic simulations were performed with an elaborate anisotropic potential based on a distributed multipole analysis of the isolated molecule charge density. Changes in structure were monitored via simulated X-ray diffraction patterns. The methodology was used to minimize the free energy at ambient conditions of a set of experimental and hypothetical 5-fluorouracil crystal structures, generated in a search for lattice-energy minima with the same model potential. Our results demonstrate that the majority ( approximately 75%) of lattice-energy minima are thermally stable at ambient conditions, and hence, the free-energy (like the lattice-energy) surface is complex and highly undulating. Metadynamics trajectories (Laio, A.; Parrinello, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562) started from the free-energy minima only produced transitions that preserved the hydrogen-bonding motif, and thus, further developments are needed for this method to efficiently explore such free-energy surfaces. The existence of so many free-energy minima, with large barriers for the alteration of the hydrogen-bonding motif, is consistent with the range of motifs observed in crystal structures of 5-fluorouracil and other 5-substituted uracils.

We describe the derivation of a potential model for ferroelectric lithium niobate, by means of fitting parameters in the potential to the experimentally determined structure and properties of the material. In doing so we highlight the strengths and limitations of the technique, compared to theoretical methods for calculating the parameters. We also see that it is important to include ionic polarisation directly in the fit, owing to the role of polarisation in stabilising the ferroelectric structure.

The lattice energies of a series of organic dihydrogenphosphate salts capable of second harmonic generation (SHG) have been calculated. These calculations, coupled with empirical data, indicate that a minimum of 20–25% of the lattice energy arises from hydrogen-bond interactions. Hydrogen bonding is shown to be a strong enough force to have a profound effect on the overall packing and crystal geometry of such ionic materials, and is thus an important factor to consider for crystal engineering.

The technique of calculating lattice dissociation energies using static, minimum lattice energy, ionic models has been extended to allow for multiple occupancy of the ionic sites. A particular lattice site can have a fraction x of an ionic species A and a fraction y of an ionic species B, where the position of each can be relaxed separately along with the unit cell dimensions until an equilibrium is reached. Various degrees of long and short range order can be modelled. This technique has been applied to the mineral sillimanite, Al2SiO5, to calculate the effect on the lattice energy of (Al, Si) ordering over the tetrahedral sites. It is found using this method that (Al, Si) ordering with space group Pbmn stabilizes the material by 29.25 kcal/mol (Aliv-O-Aliv), with respect to the completely disordered material.

Manganese, zinc and calcium diphosphates have been modelled using atomistic simulation techniques. The ionic model was used for the cation–anion interactions. Parameters for the short-range interatomic potentials previously calculated using the electron gas methods for the diphosphate anion of α-magnesium diphosphate, were used together with specific metal oxygen parameters. The structural data for each compound were reproduced well, indicating that the anion parameters were transferable. These studies present new opportunities for modelling important aspects of biominerals which are not readily accessible to experimental studies.

Molecular dynamics simulation techniques have been used to investigate behaviour of sorbed CH4 and C2H4 in zeolite ZSM-5. Our calculations allowed for framework in addition to molecular motions. Diffusion trajectories were obtained for the sorbed molecules. Simulated diffusion coefficients were in acceptable agreement with experiment.

To investigate the effects of angle bending terms on the structure of amorphous silica, three different potential models have been developed for the molecular dynamics simulation of vitreous silica.

Inter-atomic potential models have been derived for La2CuO4. The models correctly reproduce the structure of the orthorhombic phase, and yield calculated phonons dispersion curves showing a soft mode. Defect simulations show a large energy barrier to the Cu2+ disproportionation reaction, but find that doping of the material and oxidation of the doped material are energetically favoured. The calculations show that the latter takes place with formation of Cu3+ ions. Bipolaron formation is also investigated.

The first results are reported on the precise location of hydrogen atoms in a complicated crystal structure by using diffraction of polarized neutrons from dynamically polarized protons in the sample. Two methodologies exploiting the spin-dependent scattering power of protons are briefly illustrated.

The authors consider various possible hole-coupling mechanisms in the high-Tc superconductor, doped La2CuO4. Negative-U processes are energetically unfavourable. It is possible that bipolaronic species may be stabilised when antiferromagnetic energies and coupling terms arising from the distortions of the CuO6 octahedra are taken into account.

We review the current status of potential models for oxide materials, with emphasis on the applications of potentials in simulation studies. We pay special attention to transition-metal oxides, silicates and ternary metal oxides. Extensions of potentials beyond the two-body formulation are discussed.

We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.

We use an approach based upon the atomistic or Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the infrared and Raman vibrational frequencies of forsterite. We investigate a variety of interatomic potentials, and find that although all the potentials used reproduce the structural and elastic behaviour of forsterite, only one potential (THB1) accurately predicts its lattice dynamics. This potential includes ‘bond-bending’ terms, that model the directionality of the Si-O bond, which we suggest plays a major role in determining the structural and physical properties of silicates. The potential was derived empirically from the structural and physical data of simple oxides, and its ability to model the lattice dynamics of forsterite is a significant advance over previous, force-constant models, which have been simply derived by fitting to the spectroscopic data that they aim to model. The success that we have had in predicting the lattice dynamics of forsterite indicates that the potential provides the previously elusive yet fundamental, quantitative link between the microscopic or atomistic behaviour of a mineral and its macroscopic or bulk thermodynamic properties.

Proton nuclear magnetic resonance experiments on ammonia adsorbed on Graphon are described. Proton chemical shifts, spin–lattice relaxation times and linewidths have been measured as a function of temperature and coverage.The variation of the signal from adsorbed fluid ammonia has been used to measure adsorption isotherms, the isosteric heat of adsorption and melting curves of solid ammonia in contact with Graphon. All these results agree well with gravimetric and neutron scattering experiments described in three earlier papers.The chemical shift of adsorbed ammonia is large (39 p.p.m. to high field) and can be attributed to the effects of anisotropy in the susceptibility of graphite on ammonia molecules adsorbed close to the graphite basal planes. A semi-quantitative model of the effect shows that, above ≈ 195 K (the melting point of bulk solid ammonia), there can only be limited multilayer formation. As the temperature increases, the chemical shift decreases because ammonia molecules are promoted to layers further from the surface. At low temperatures the chemical shift also decreases because ammonia starts to aggregate at heterogeneities on the surface where there is no susceptibility effect. This is correlated with neutron scattering results. The two experiments taken together show the gradual transition with decreasing temperature from anisotropic fluid, through aggregates of ammonia similar to the super-cooled liquid, to bulk solid, the whole process resembling heterogeneous nucleation.The linewidths are abnormally large at all temperatures and coverages. There are two sources of the broadening, diffusion through local magnetic field gradients in the sample, and the possible restriction of the motion of the adsorbate to two dimensions. Other mechanisms which have been considered by other authors are eliminated by considering the neutron scattering results.The spin–lattice relaxation time of adsorbed ammonia is abnormally small (≈ 20 ms) and shows some evidence of a minimum corresponding to a correlation time of 2 × 10–9 s. Partial deuteration shows that the mechanism is intramolecular dipolar relaxation. Consideration of the frequency spectrum of the diffusive motions of adsorbed ammonia, known from quasielastic neutron scattering, indicates that the slow correlation time may arise from a slow rate of jumping between the molecular layers of adsorbate.

Neutron diffraction experiments determining the structure of the adsorbate in the system ammonia on Graphon are described. The experiments were done over a range of temperatures from 80 K to just above the melting point of bulk ammonia at ≈ 200 K, and a range of adsorbed volumes of vapour equivalent to surface coverages between 0.25 and 6 statistical layers. These ranges overlap those used for the isotherm measurements described in the previous paper.The results show that, on cooling to temperatures well below its bulk melting point, most of the ammonia desorbs from the surface to form bulk crystallites. Formation of the crystallites takes place over a wide range of temperature and is very sensitive to the amount of ammonia present. Melting point curves determined by neutron diffraction are shown to be closely connected with the capacity of the surface to adsorb ammonia. The results are interpreted in terms of an equilibrium between bulk solid and adsorbed fluid. Analysis of the shapes of the melting point curves and comparison of these with the isotherm measurements establishes a clear link between adsorption and heterogeneous nucleation.Addition of methane to the surface makes it less able to adsorb ammonia and the ammonia crystallites are stable at correspondingly higher temperatures.

Interatomic potentials, which include bond-bending terms, allow us to model accurately the properties of α-quartz and the structures of other polymorphs of SiO2.

The methodology for obtaining potential-energy function parameters by empirical procedures is discussed, and the strengths and limitations of the technique are highlighted. Studies of three important and structurally complex oxides using lattice-simulation techniques that incorporate the Mott–Littleton method, based upon empirically derived potentials are described. These are the high-Tc superconductor La2CuO4, the electro-optic device material LiNbO3 and the sensor and pigment material TiO2.

Incoherent neutron quasielastic and inelastic scattering experiments on the system ammonia on Graphon are described. The range of temperatures and coverages used for the experiments were similar to those for the isotherm measurements and diffraction experiments described in two previous papers.The freezing of ammonia on the surface was followed by very high resolution quasielastic experiments. In agreement with the neutron diffraction results, ammonia does not solidify until well below the freezing point of bulk ammonia. The freezing point depends on the coverage and freezing takes place over a range of temperature. There are qualitative differences between the freezing curves obtained by the two techniques because the diffraction experiment only observes the formation of crystalline ammonia while the quasielastic experiment does not distinguish amorphous and crystalline solid.Quantitative analysis of the quasielastic spectra shows that the solid ammonia is in equilibrium with an adsorbed fluid. The character of the adsorbed fluid is slightly different above and below the melting point of bulk ammonia (195 K). Above the melting point, it is an extremely mobile anisotropic fluid, the quasielastic spectra being best explained by a model of diffusion in two dimensions obeying Fick's law. The diffusion coefficient parallel to the surface is three times larger than for bulk liquid ammonia. The difference results from changes in both the activation energy and frequency factor for diffusion. Some diffusion may be occurring perpendicular to the surface but the resolution of the instrument does not allow it to be well characterized. Rotational diffusion is rapid in the adsorbed fluid indicating that the surface does not orient the ammonia molecules.Below 195 K the fluid starts to form clusters in which the ammonia molecules diffuse much less freely, having a diffusion coefficient comparable to that calculated by extrapolation of the properties of the bulk liquid. The clusters are probably the immediate precursors of the solid that is formed as the temperature is lowered. The formation of such clusters may therefore be an important step in the process of heterogeneous nucleation.The difference in the dynamical properties of the adsorbed ammonia above and below 195 K accounts qualitatively for differences in the heats and entropies of adsorption in the two temperature ranges.Inelastic spectra of the ammonia confirm much of the picture given above. At very low temperatures the spectrum is different from that of bulk ammonia. This is consistent with the presence of some amorphous solid and possibly also with molecules vibrating independently of each other in the surface field.

Isotherms of ammonia on a graphitized carbon black (Vulcan III) have been measured at temperatures of 163.4, 172.6, 182.0, 191.5, 201.5 and 211.0 K, using a microbalance. They are all of Type III. Values of isosteric heats and differential entropies of adsorption have been determined. The isosteric heat is comparable with the latent heat of evaporation of the liquid but much less than the latent heat of sublimation of the solid. This has the effect that, below the melting point of bulk ammonia, the amount adsorbed at a given relative pressure decreases with decreasing temperature. This type of behaviour is uncommon and seems to be characteristic of systems in which heterogeneous nucleation takes place.