Showing papers on "Lattice energy published in 2001"
TL;DR: In this paper, the authors investigated the properties of α-, β- and γ-polymorphs of glycine in terms of their properties and properties during the phase transition from two-dimensional to three-dimensional growth.
Abstract: X-ray, DSC and solution calorimetric investigations were carried out for α-, β- and γ-modifications of glycine. Particular attention was paid to kinetic and thermochemical aspects of γ- → α-phase transition. The temperature of this phase transition turned out to be sensitive to a) conditions under which the crystals of the γ-modification were grown, b) tempering of crystals c) form (geometry) of crystals. Kinetics of this phase transition of single crystals of γ-phase in rhomboedric form can be described by the equation for two-dimension nuclei growth, whereas for crystals of triangle geometry the equation for three dimension growth is valid. On the basis of energy parameters describing growth of α-form in γ- →α-phase transition, the kind of structure defects, which are responsible for this phase transition, was estimated. Taking into account the ΔsolHm, the absolute values of the lattice energies of the investigated polymorphs indescending order are follows: γ->α->β-modification. The obtained results are discussed with respect to the peculiarity of the crystal lattice structures, particularly the network of hydrogen bonds. The β-modification of glycine is monotropically related to the other forms, whereas γ-and α-polymorphs are enantiotropically-related phases.
183 citations
TL;DR: In this paper, the crystal structures and bulk moduli of the known or proposed TiO2 polymorphs were simulated using static lattice energy minimisation and two independent forcefields, namely a variable charge Morse potential and a fixed charge rigid ion potential.
116 citations
TL;DR: In this paper, the sensitivity of anisotropic atom−atom potentials 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.
Abstract: 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...
104 citations
TL;DR: In this article, a systematic search of direct space was employed to assess every possible packing arrangement of the asymmetric unit within the experimentally determined unit cell within the pharmaceutical material anhydrous theophylline (3,7-dihydro-1,3-dimethyl- 1H-purine-2,6-dione).
Abstract: When determining crystal structures of organic molecular materials from high-resolution powder diffraction data, the key step is the generation of reliable trial structures fur final refinement. The subject of the study reported here is the pharmaceutical material anhydrous theophylline (3,7-dihydro-1,3-dimethyl- 1H-purine-2,6-dione), which contains both oxygen and nitrogen as possible hydrogen bond acceptor atoms. A systematic search of direct space was employed to assess every possible packing arrangement of the asymmetric unit within the experimentally determined unit cell. Trial structures were ranked in terms of calculated lattice energy and weighted residuals from a comparison of calculated and experimental X-ray diffraction profiles. The systematic search found two packing arrangements with different intermolecular hydrogen-bonding motifs within the same unit cell. In one, denoted NH. . . N, the amino hydrogen is hydrogen bonded to the aldimine nitrogen, and in the other, denoted (NHO)-O-. . ., to the carbonyl oxygen neighboring the imidazole ring. These trial structures were "virtually indistinguishable" in terms of calculated lattice energy or X-ray profile fit. Solid-state NMR spectra of a commercial sample not only confirmed immediately that there was only one molecule in the crystallographic asymmetric unit but also produced distinctive C-13 and N-15 chemical shifts. The experimentally determined N-15 chemical shifts showed considerably better agreement with values from ab initio calculations for the trial crystal structure with N--H N hydrogen bonding. In these calculations, representative chains of three hydrogen-bonded molecules were employed as models for the (NHN)-N-. . . and (NHO)-O-. . . trial crystal structures. In addition, a more sophisticated analysis of the lattice energy hypersurfaccs. using a distributed multipole based intermolecular potential, indicated that the N-(HN)-N-. . . trial structure is the more stable. It was noted that the NH N packing motif identified by our studies is observed in a single-crystal determination for theophylline reported independently while our investigations were ongoing. Our study shows how the potential for polymorphism in a "given unit cell'' may be assessed successfully by combining several complementary experimental and theoretical approaches.
86 citations
TL;DR: In this article, the lattice energies of a number of chiral drugs with known crystal structures were calculated using the Dreiding II force field and correlated with experimental data to probe the underlying intermolecular forces responsible for the formation of racemic species, racemic conglomerates, or racemic compounds.
57 citations
TL;DR: A survey of the molecules which have been used in crystal structure prediction studies is presented in this article, which highlights the need for more theoretical and experimental collaboration to understand what determines whether a molecule's crystal structure will be so favorable that other polymorphs are unlikely.
Abstract: A survey of the molecules which have been used in crystal structure prediction studies is presented. The results of these studies have been analysed in terms of whether the experimentally observed crystal structures are found at or near the global minimum in the lattice energy. The results suggest that whilst some crystal structures can be predicted just on the basis of lattice energy searches, there is yet insufficient experience to judge for which molecules this energetic criterion is sufficient, within the limitations of current force-field accuracy. The molecules chosen to test crystal structure prediction methods appear to be biased away from the types that would be expected to be readily predictable and suitable for crystal engineering. The survey highlights the need for more theoretical and experimental collaboration to understand what determines whether a molecule's crystal structure will be so favourable that other polymorphs are unlikely.
57 citations
TL;DR: This method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy and it was found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii.
Abstract: The formation and physicochemical properties of polymer electrolytes strongly depend on the lattice energy of metal salts. An indirect but efficient way to estimate the lattice energy through the relationship between the heterolytic bond dissociation and lattice energies is proposed in this work. The heterolytic bond dissociation energies for alkali metal compounds were calculated theoretically using the Density Functional Theory (DFT) of B3LYP level with 6-311+G(d,p) and 6-311+G(2df,p) basis sets. For transition metal compounds, the same method was employed except for using the effective core potential (ECP) of LANL2DZ and SDD on transition metals for 6-311+G(d,p) and 6-311+G(2df,p) calculations, respectively. The dissociation energies calculated by 6-311+G(2df,p) basis set combined with SDD basis set were better correlated with the experimental values with average error of ca. ±1.0% than those by 6-311+G* combined with the LANL2DZ basis set. The relationship between dissociation and lattice energies was found to be fairly linear (r>0.98). Thus, this method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy. We also found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 827–834, 2001
49 citations
TL;DR: In this article, the fluorine anions are randomly distributed in two distinct crystallographic sites among three possible sites in the oxyfluoride lattice, that is, the equatorial site of NbO6 octahedra (Oequatorial) and the central site of two corner-sharing nbO 6 octahedral structures along the c axis (Ocenter), where the substituted fluorine does not prefer to occupy the Oapex site, which interacts ionically with an alkali metal cation (A+) in the interlayer.
Abstract: New Dion−Jacobson-type layered perovskite oxyfluorides, ASrNb2O6F (A = Li, Na, and Rb), have been prepared by conventional solid-state reaction, and their fluorine distribution in an anion sublattice was studied by performing 19F MAS NMR spectroscopic analyses and lattice energy calculations to probe the evolution of interlayer cationic conductivitiy upon fluorine substitution. From these comparative studies, it is found that the fluorine anions are randomly distributed in two distinct crystallographic sites among three possible sites in the oxyfluoride lattice, that is, the equatorial site of NbO6 octahedra (Oequatorial) and the central site of two corner-sharing NbO6 octahedra along the c axis (Ocenter). On the other hand, the substituted fluorine does not prefer to occupy the Oapex site, which interacts ionically with an alkali metal cation (A+) in the interlayer, because the (Nb−Oapex) bond is too strongly covalent to be replaced by the ionic Nb−F bond. We have also compared systematically the Li and ...
49 citations
TL;DR: In this paper, lattice-dynamical calculations were performed for an idealized natrolite structure Na4(Al4Si6O20)4H2O containing 46 atoms in the primitive unit cell (Z = 2), and the calculated frequencies in the Raman and IR spectra were fitted to the observed frequencies.
Abstract: Polarized single-crystal Raman scattering and powder infrared absorption spectra of Fdd2 orthorhombic natural natrolite (Na1.88 K0.02 Ca0.04 )(Al1.96 Si3.03 O10)⋅2.03 H2O from Khibiny, Kola peninsula, were measured. Using short-range models, lattice-dynamical calculations were performed for an idealized natrolite structure Na4(Al4Si6O20)4H2O containing 46 atoms in the primitive unit cell (Z = 2). By varying the valence force constants, the calculated frequencies in the Raman and IR spectra were fitted to the observed frequencies. On considering their calculated intensities as well, nearly all the observed bands (especially those corresponding to the A1 modes) could be unambiguously assigned and interpreted. The external vibrations of H2O could be correctly assigned using deuter- ated samples. The strongest Raman band at 534 cm-1 corresponds to a breathing mode of the four-membered alumi- nosilicate ring. The calculated bulk modulus (52.7 GPa at zero pressure) is close to the experimental value of 47 ± 6 GPa. The natrolite structure has some advantages upon other zeolites to understand the amorphization mechanism, because samples of this mineral surrounded by a non-penetrating medium show no crystal phase transitions with increasing pressure. Lattice energy minimization calculated with variable unit-cell dimensions shows the crystal structure to become unstable at about 5.5 GPa, thereby apparently explaining the amorphization process at 4-7 GPa. This instability is connected with shear acoustic modes coupled with soft internal framework vibrations.
45 citations
TL;DR: In this article, the influence of the intermolecular potential on predicted growth morphologies is investigated and compared with the attachment energy model for e-caprolactam, β-succinic acid, hexamethylene tetramine (I43m), pentaerythritol (I4), and urea (P421m).
Abstract: The influence of the intermolecular potential on predicted growth morphologies is investigated. We compare the morphologies calculated with the attachment energy model for e-caprolactam (C2/c), β-succinic acid (P21/c), hexamethylene tetramine (I43m), pentaerythritol (I4), and urea (P421m). Five different model potentials that have been developed for modeling organic crystal structures are used. These show a progressive improvement in the accuracy of the electrostatic model and a variety of empirical parametrizations of the repulsion−dispersion terms. Although the lattice energies and absolute attachment energies vary quite significantly, there are only minor variations in the predicted morphologies. The assumptions in the attachment energy model generally contribute more to any discrepancy between the predicted and observed morphologies than the variations between different model potentials that could be considered appropriate for modeling. However, the attachment energy model readily provides a reason...
45 citations
TL;DR: In this paper, the authors studied both experimentally and theoretically the losses induced by parametric excitation in far-off-resonance optical lattices and developed a model based on the actual atomic Hamiltonian in the lattice and introduced semi-empirically a broadening of the width of lattice energy bands which can physically arise from inhomogeneities and fluctuations of the lattices, and also from atomic collisions.
Abstract: We study both experimentally and theoretically the losses induced by parametric excitation in far-off-resonance optical lattices. The atoms confined in a one-dimensional sinusoidal lattice present an excitation spectrum and dynamics substantially different from those expected for a harmonic potential. We develop a model based on the actual atomic Hamiltonian in the lattice and we introduce semiempirically a broadening of the width of lattice energy bands which can physically arise from inhomogeneities and fluctuations of the lattice, and also from atomic collisions. The position and strength of the parametric resonances and the evolution of the number of trapped atoms are satisfactorily described by our model.
TL;DR: In this paper, the effect of lattice vibration as a driving force for sorbate diffusion in zeolites has been investigated and a generalized Langevin equation has been derived with the magnitude of the lattice vibrations captured by two parameters.
Abstract: A method of estimation of the effect of lattice vibration as a driving force for sorbate diffusion in zeolites is proposed. A realistic lattice model is employed to cut off unrealistic long vibrational modes and eliminate feedback due to lattice periodicity. A generalized Langevin equation for sorbate motion is then derived with the magnitude of the lattice vibration captured by two parameters, μ and ν, which can be readily computed for any system. The effect of lattice vibration is then estimated for a variety of sorbate–zeolite pairs. Lattice vibration is found to be a negligible driving force for some systems (e.g., methane and xenon in silicalite) and an important driving force for other systems. In the latter case, the lattice vibration can provide either linear stochastic Langevin-type force (e.g., for benzene in silicalite) or nonlinear deterministic force (e.g., for argon in sodalite).
TL;DR: In this paper, the authors presented a paper entitled "Analı́tica and Quı ǫ mica Fı à mica Inorgánica: Analımica Analímica Fí mica Quı mica (AQF à à f à MICA) in the context of Chemistry at the University of the Patagonia of Argentina.
Abstract: School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom Departamento de Quı ́mica, Universidad Nacional de la Patagonia SJB, Ciudad Universitaria, (9000) Comodoro Rivadavia, Argentina Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Quı ́mica Inorgánica, Analı́tica y Quı́mica Fı́sica, Pabello ́n 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina CLRC, Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom ~Received 31 October 2000; published 5 February 2001 !
TL;DR: In this paper, the pressure-induced sequence of phase transitions of BaF2 fluorite was studied within the shell-model approach and the model reliability was confirmed by a comparison with experimental results reported at ambient conditions, which were in good agreement with the model predictions.
Abstract: The pressure-induced sequence of phase transitions of the BaF2 fluorite was studied within the shell-model approach. This fluorite crystal presents two pressure-induced phase transitions at approximately 3 and 15 GPa. The interatomic potentials were calculated by using relevant physical properties measured at ambient conditions. These potentials were used to minimize the lattice enthalpy at high hydrostatic pressures. By comparing the enthalpies and lattice parameters of the three possible structures, it was possible to describe the complete phase transition sequence of the material. The model reliability is confirmed by a comparison with experimental results reported at ambient conditions, which are in good agreement with the model predictions.
TL;DR: In this article, it was shown that the equilibrium lattice parameter of a carbon polymorph with space group symmetry would be 3.08 A, and hence very different from that of diamond.
01 Jan 2001
TL;DR: In this article, the formation and stability of the substituted pyromorphites was investigated using ab initio quantum mechanics, which involved evaluating the lattice energy of the minerals using AB initio Quantum Mechanics.
Abstract: Soils contaminated with lead pose significant risk to human as well as terrestrial and aquatic ecosystems Theoretical phase relationships and field observations suggest that the interaction of lead and phosphorus to form pyromorphites Pb 5 (PO 4 ) 3 X (X= OH - , Br - , Cl - , or F - ) is an important buffer mechanism controlling the migration and fixation of lead in the environment We report a molecular modeling approach to investigate the formation and stability of the substituted pyromorphites, which involved evaluating the lattice energy of the minerals using ab initio quantum mechanics The lattice energy values are used in a Born-Haber thermodynamic cycle to calculate the heat of formation of the minerals The Gibbs free energy of the substituted pyromorphites is then calculated from the change in entropy and heat of formation The systems investigated in this study include partial and total substitution of Pb 2+ by Cd 2+ and Zn 2+ cations in chloropyromorphite (Pb 5 (PO 4 ) 3 Cl ) Results indicate the unstable nature of the substituted Pb pyromorphite The stability of the substituted minerals is found in the order Pb-pyromorphite > Cd-pyromorphite > Zn-pyromorphite
TL;DR: DFT calculations at the B3LYP/LanL2DZ level reveal that the three molecular structures observed in the solid state correspond closely to three minima on the gas-phase potential energy surface and are more stable than the alpha and gamma conformations, respectively.
Abstract: The reaction of yttrium trichloride with lithium 1-methylboratabenzene (1/2) in toluene (110 degrees C, 3 days) afforded the donor-free dinuclear sandwich complex [(C(5)H(5)BMe)(2)Y(mu-Cl)](2) (1) in 85% yield as pale-yellow crystals. By means of single crystal and powder diffraction methods, three conformational polymorphs, alpha-1 [P2(1)/n (No. 14), monoclinic, a = 6.6124(8) A, b = 14.352(9) A, c = 14.120(1) A, beta = 95.57(1) degrees, V = 1333.7(9) A(3), Z = 2], beta-1 [P2(1)/a (No. 14), monoclinic, a = 8.542(2) A, b = 13.712(6) A, c = 11.76(1) A, beta = 102.60(4) degrees, V = 1344.5(13) A(3), Z = 2], and gamma-1 [Pbca (No. 61), orthorhombic, a = 20.091(5) A, b = 13.527(3) A, c = 9.976(2) A, V = 2711.2(11) A(3), Z = 4], were characterized in the solid state of 1. The molecules in the three phases vary remarkably in the rotational position of boratabenzene ligands with differences of 91.1, 133.1, and 24.9 degrees between each pair. DFT calculations at the B3LYP/LanL2DZ level reveal that the three molecular structures observed in the solid state correspond closely to three minima on the gas-phase potential energy surface. The beta conformation is 2.8 and 7.2 kJ/mol more stable than the alpha and gamma conformations, respectively. Lattice energy minimizations predict that the alpha-1 phase is about 5.5 and 18.7 kJ mol(-)(1) more stable than the beta-1 and gamma-1 modifications, in agreement with the packing coefficients and the molecular volumes of the three crystal structures. While the alpha-1 and beta-1 modifications have comparable total energies, the gamma-1 form is less stable. The total energy differences among the polymorphs are greater than generally expected.
TL;DR: The M-point polarizable potential (MPP) is a modification of the potential constructed by Dang and Chang [J. Chem. Phys. 106 (1997) 8149].
TL;DR: In this paper, the lattice distortion caused by the doping ions has been characterized by the change of lattice parameters obtained from X-ray diffraction patterns, and the width of exponential absorption band tail.
TL;DR: The main features of a subcell catalogue for the packing of molecules with long alkyl chains were developed in the period of 1950-1975 based on single crystal studies.
Abstract: The main features of a subcell catalogue for the packing of molecules with long alkyl chains were developed in the period of 1950–1975 based on single crystal studies. In the last number of years many deviations from these typical packing modes in a subcell lattice were found and analyzed, particularly for surfactant molecules. From early on, energy approximations were already presented, but these became more dominant with the progress in hardware and software developments. Recently, new inputs are coming from the calculation of lattice energies for chain packing modes in connection with new experimental results. In this contribution, the most common chain packing modes already suggested in published papers are presented. The different packing modes are analyzed using lattice energy calculations. The results are discussed using a presentation method that allows us to find out interrelations between various packing modes.
TL;DR: In this article, damage in III/V semiconductors caused by processing in high and low-density plasmas is investigated employing material-specific analytical methods, and it has been shown that the gentlest ambient to etch Ga-containing 3/V compounds is Ar/Cl2.
Abstract: Damage in III/V semiconductors caused by processing in high- and low-density plasmas is investigated employing material-specific analytical methods. To distinguish the various regimes of damage, four different ambients out of the palette of plasma constituents were chosen: as borderline cases, argon which is associated with hard etching causing crystal lattice disordering until complete amorphization, and hydrogen which is the prototype of soft etching causing bond breaking between the lattice constituents without changing the crystal site, and the two model ambients Ar/Cl2 and Ar/CH4/Cl2/H2 which are widely used to etch Ga-containing compound semiconductors. Several probes and methods are employed which are selectively sensitive to the different kinds of damage: damage of the crystal lattice with photoluminescence, scanning electron microscopy and scanning transmission electron microscopy, and the concentration gradient of a chemically reactive species (hydrogen) with secondary ion mass spectrometry. Its reaction with lattice atoms (acceptor passivation), is recorded by capacitance/voltage and Hall measurements. For irreversible damage (knock out of atomic lattice sites), there can be detected only one damage zone reaching not deeper than 30 nm. The depth is inversely proportional to the lattice energy and proportional to the kinetic energy of the projectiles. In contrast to this rather shallow damage, the penetration depth of hydrogen amounts up to several microns. Since the subsequent passivation of acceptor atoms is a chemical reaction, it can be reverted at elevated temperatures. It has turned out, with all methods applied, that the gentlest ambient to etch Ga-containing III/V semiconductors is Ar/Cl2. This is not restricted to the smoothness of the surfaces but refers to all quantitative measurements which are applied in this study.
TL;DR: In this article, the crystal structures of the several complex oxides including MgAl 2 O 4, α-al 2 O 3, α-Al 3 O 3, θ-Al 2 o 3, r-al 3 O 5 O 12, and YAlO 3 are fully optimized by means of first-principles total energy calculations with relaxation of all internal parameters.
Abstract: The crystal structures of the several complex oxides including MgAl 2 O 4 , α-Al 2 O 3 , θ-Al 2 O 3 , r-Al 2 O 3 , Y 2 O 3 , Y 3 Al 5 O 12 , and YAlO 3 are fully optimized by means of first-principles total energy calculations with relaxation of all internal parameters. This is achieved by the implementation of a simple scheme based on the finite difference method for the energy gradient. In most cases, the predicted lattice constants are within 0.5% of the experimental values. The bulk moduli and the pressure coefficients for these crystals are calculated from fitting the energy vs volume data to the equation of state. Because all of the internal parameters of the crystal are optimized at different volumes, the calculated ground-state properties are found to be in excellent agreement with measured data. For the polymorphs of the Al 2 O 3 system, comparison to other existing calculations are made and discussed.
TL;DR: In this article, the phase boundaries between β-Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where ΔG(P,T )=0] as the measured phase boundary.
Abstract: Enthalpies of drop solution (ΔHdrop-sol) of CaGeO3, Ca(Si0.1Ge0.9)O3, Ca(Si0.2Ge0.8)O3, Ca(Si0.3Ge0.7)O3 perovskite solid solutions and CaSiO3 wollastonite were measured by high-temperature calorimetry using molten 2PbO · B2O3 solvent at 974 K. The obtained values were extrapolated linearly to the CaSiO3 end member to give ΔHdrop-sol of CaSiO3 perovskite of 0.2 ± 4.4 kJ mol−1. The difference in ΔHdrop-sol between CaSiO3, wollastonite, and perovskite gives a transformation enthalpy (wo → pv) of 104.4 ± 4.4 kJ mol−1. The formation enthalpy of CaSiO3 perovskite was determined as 14.8 ± 4.4 kJ mol−1 from lime + quartz or −22.2 ± 4.5 kJ mol−1 from lime + stishovite. A comparison of lattice energies among A2+B4+O3 perovskites suggests that amorphization during decompression may be due to the destabilizing effect on CaSiO3 perovskite from a large nonelectrostatic energy (repulsion energy) at atmospheric pressure. By using the formation enthalpy for CaSiO3 perovskite, phase boundaries between β-Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where ΔG(P,T )=0] as the measured phase boundary. The calculations suggest that the phase equilibrium boundary occurs between 11.5 and 12.5 GPa around 1500 K. Its slope is still not well constrained.
TL;DR: The structure of Ba4Er3F17 can be described as an ordered anion-rich fluorite type structure as mentioned in this paper, and the excess fluorine content is directly correlated with the occurence of Er6F36 clusters.
TL;DR: In this article, the bonding, charge transfer and structure of all X 3 P·BY 3 (X, Y=Cl, Br, I) and their fragments have been studied using density functional (B3LYP) and ab initio methods (MP2, CCSD(T)).
Abstract: Donor–acceptor complexes X 3 P·BY 3 (X, Y=Cl, Br, I) and their fragments have been studied using density functional (B3LYP) and ab initio methods (MP2, CCSD(T)) with effective core potentials. The bonding, charge transfer and structure of all X 3 P·BY 3 is discussed on the basis of natural bond orbital (NBO) analysis. The estimated free molecular enthalpies of dissociation indicate that all X 3 P·BY 3 represent thermodynamically labile species which are stabilized by the lattice energies in the solid state. The total distortion energy increases in accordance with the decreasing complex dissociation enthalpy. With regard to halogen exchange between the PX 3 and BY 3 moiety in X 3 P·BY 3 (g) adducts, only the complexes where Y=X and where X represents a heavier halogen than Y are thermodynamically stable.
TL;DR: A theoretical analysis of lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl is presented in this paper.
Abstract: A theoretical analysis of the lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl, and of the off-center equilibrium position adopted by those impurities in their ground ns2 electronic configuration is presented. The calculations are based in the cluster approach, and involve large active clusters embedded in an accurate quantal representation of the crystalline environment. The charge compensation problem is dealt with by considering several allocations of a cationic vacancy in the host lattice. The obtained distortions involve in all cases the concerted movement of a large number of host crystal ions. Those distortions are presented and discussed in terms of simple packing and Madelung considerations.
TL;DR: Parameters for the thioamide group have been derived for the molecular mechanics CFF91 force field by fitting to ab initio charges and dipoles and satisfactorily reproduced the bond lengths, bond angles, torsional angles, and nonbonded distances of all the crystal structures.
Abstract: When a sulfur atom is used to substitute for the oxygen in peptide bonds, its bulkiness should restrict the conformational space available to an amino acid. This conformational restriction as well as the ability to confer resistance to enzymatic degradation in the body means that thio-substituted amino acids are potentially useful building blocks for drug design. To simulate the effects of thio substitution, force field parameters for sp sulfur are required. In this article, parameters for the thioamide group have been derived for the molecular mechanics CFF91 force field (available at http://www.ludwig.edu.au/archive/tran). The bond increment charges were obtained by fitting to ab initio charges and dipoles. The van der Waals parameters were obtained by fitting to high-resolution crystallographic data, and the nonbonded parameters were verified by comparing with experimentally derived lattice energy. The bonded parameters were derived by least-square fits to the ab initio calculated energy surfaces, i.e., conformational energy as well as their first and second derivatives of seven model thioamide molecules. When the sp sulfur parameters were tested on a set of seven X-ray crystallographic structures from the Cambridge Structural Database, they satisfactorily reproduced the bond lengths, bond angles, torsional angles, and nonbonded distances of all the crystal structures.
TL;DR: In this paper, the relation between cathode performance and thermodynamic stability of lithium manganese spinels, LiMn2O4, Li2Mn4O9, Li4Mn5O12, as cathode active materials for lithium secondary battery, has been investigated.
Abstract: The relation between cathode performance and thermodynamic stability of lithium manganese spinels, LiMn2O4, Li2Mn4O9, Li4Mn5O12, as cathode active materials for lithium secondary battery, has been investigated. The heat of dissolution of these samples was measured by the solution calorimetry method. The standard enthalpy of formation, ΔfH0, the enthalpy change of reaction, ΔH, and the enthalpy change per mole of atoms for the formation reaction, ΔHR, were calculated from the heat of dissolution. |ΔHR| decreased in order of Li4Mn5O12>Li2Mn4O9>LiMn2O4, a larger |ΔHR| meaning a thermodynamically more stable sample. In addition, lattice energies of these samples were calculated for a unit-cell. The trend order for lattice energies was the same as that for |ΔHR|. A more thermodynamically stable sample makes a stable crystal structure and leads to a good cycle performance.
TL;DR: In this article, the lattice energies of the as-synthesized fluoride-containing chabazite-like aluminophosphate (AlPO4-34F) and of the corresponding metal-substituted materials [MeAPO- 34F, Me = Mn(II), Co(II, Ni(II)] have been calculated in order to investigate the Al-site preference the transition metal substitution in the AlPO434F.
Abstract: The lattice energies of the as-synthesized fluoride-containing chabazite-like aluminophosphate (AlPO4-34F) and of the corresponding metal-substituted materials [MeAPO-34F, Me = Mn(II), Co(II), Ni(II)] have been calculated in order to investigate the Al-site preference the transition metal substitution in the AlPO4-34F. The calculations show that the transition metal ions in MeAPO-34F should preferentially occupy octahedral Al3+ sites, and kinetic reasons are suggested as an explanation for the actual preference of tetrahedral sites. The lattice energies have also been calculated for the calcined AlPO4-34F material (AlPO4-34) and the rehydrated-calcined product (AlPO4-34h). The AlPO4-34 is found to be less stable than either AlPO4-34F or AlPO4-34h, which is consistent with the fact that AlPO4-34 can only be prepared starting from AlPO4-34F.
TL;DR: In this article, the energy loss for Ne atoms scattered with keV energies from a LiF(001) surface under channeling conditions, i.e., glancing angles of incidence, was studied via computer simulations using classical trajectories.