Solvent selection is an important factor in chemical process efficiency, profitability, and environmental impact. Prediction of solvent phase behavior will allow for the identification of novel solvent systems that could offer some economic or environmental advantage. A modified cohesive energy density model is used to predict the solid-liquid-equilibria for multifunctional solids in pure and mixed solvents for rapid identification of process solvents for design of crystallization processes. Some solubility data at several temperatures are also measured to further test the general applicability of the model. Gas-expanded liquids have potential environmentally advantageous applications as pressure tunable solvents for homogeneous and heterogeneous catalytic reactions and as novel solvent media for anti-solvent crystallizations. The phase behavior of some carbon dioxide/organic binary systems is measured to provide basic process design information. Solvent selection is also an important factor in the anti-solvent precipitation of solid compounds. The influence of organic solvent on the solid-liquid equilibria for two solid pharmaceutical compounds in several carbon dioxide expanded solvents is explored. A novel solvent system is also developed that allows for homogeneous catalytic reaction and subsequent catalyst sequestration by using carbon dioxide as a miscibility switch. The fundamental biphasic solution behavior of some polar organics with water and carbon dioxide are investigated.

This thesis is about the kinetics of anisotropic acidic/hydrothermal dissolution of apatite bulk single crystal vs. nanorods, and the kinetic phase change of dense nanocondensates of SnO2 vs. Ni-dissolved SnO2 prepared by laser ablation condensation technique. In the first regard, directional dissolution of a natural (OH,F,Cl)-bearing apatite has been studied at various solution pH values (0~3) and 30 oC. This apatite showed abnormally high O-H stretching frequencies due to the substitution of Cl for OH. The advance of dissolution front indicated that steady-state directional dissolution for pH = 0-2 followed an apparent rate law of rate(mole / m2h)¡×kaH+n, where the rate constants (k) are 2.15 and 1.61; and the rate orders (n) are 1.44 and 1.30 for [0001] and <11 0> directions, respectively. Previous study, however, indicated a smaller n value (n = 0.55~0.70) for fluorapatite powders at higher pHs. A nonlinear pH dependence of logarithmic dissolution rate at a wide pH range implied that the surface active sites and/or rate-determining steps have changed when the acidity of solution and/or the composition of the apatite were changed. The opening of etch pits on basal planes further indicated that the dissolution rates along the three principal directions have the following relationship: [0001] > <11-20> > <10-10> for pH=0-1, but the order was reversed for pH > 3. As a comparison, static immersion of needle-like hydroxyapatite nanoparticles in neutral hydrothermal solution at 100oC caused preferential dissolution along the crystallographic c-axis to form nanorods with a lower aspect ratio. The anisotropic dissolution behavior is due to diffusion-controlled rapid dissolution at the sharp tip, and interface-controlled dissolution at side surfaces in terms of active sites. Extensive dissolution was accompanied with amorphization via explosive generation of dislocations, forming corrugated surface with both negative and positive curvature regions. The amorphous residue was significantly Ca and OH depleted when treated in the hydrothermal solution at pH=3. The BET specific surface area of the apatite nanoparticles remained 45¡Ó1 m2/g after immersion in neutral solution at 100oC for 36 h, but drastically decreased to 24.5 m2/g in acidic (pH =3) solution at 100oC for 8 h due to coalescence of the partially amorphized apatite powders. The specific surface area and average pore size also remained nearly unchanged for the dry pressed powders subject to firing at 100oC, but decreased and increased, respectively when sintered shortly at 600oC in air. BJH measurements at 77 K indicated the N2 adsorption/desorption hysteresis loops shift toward high relative pressure for sintered/hydrothermally etched powders indicating a higher activation energy of forming overlain liquid-like nitrogen layers. This can be attributed to a lower surface energy of the powders due to their shape change and/or partial amorphization. Alternatively, desorption through cavitation via the small voids could occur, in particular for such treated samples with characteristic bimodal pore size distribution. In the second subject, dense SnO2 with fluorite-type related structures were synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Sn target. Combined effects of rapid heating to very high temperatures, nanophase effect, and dense surfaces account for the condensation of fluorite-type structure which transformed martensitically to baddeleyite-type accompanied with twinning, commensurate shearing and shape change. Alternatively Pa-3-modified fluorite-type hardly survived transformation to a-PbO2 type and rutile type in the dynamic process analogous to the case of static decompression. In addition, the rutile-type SnO2 nanocondensates have {110}, {100} and {101} facets, which are beneficial for {~hkl} vicinal attachment to form edge dislocations, faults and twinned bicrystals. The {011}-interface relaxation, by shearing along <011> directions, accounts for a rather high density of edge dislocations near the twin boundary thus formed. The rutile-type SnO2 could be alternatively transformed from orthorhombic CaCl2-type structure (denoted as o) following parallel crystallographic relationship, (0 1)r//(0 1)o; [111]r//[111]o, and full of commensurate superstructures and twins parallel to (011) of both phases. Furthermore, SnO2-NiO solid solution (ss) condensates were fabricated by laser ablation on Ni-Sn target at 1.1 J/pulse and oxygen flow of 50 L/min. AEM observations indicated that the particles were more or less coalesced/agglomerated as nano chain aggregate or in close packed manner. The Ni-rich condensates have rock salt structure with defect clusters not in paracrystalline distribution as would otherwise develop into the spinel phase. The Sn-rich condensates are predominantly rutile-type with minor baddeleyite-type, which are vulnerable to martensitic transformation/relaxation to form {101} incommensuare faults as well as epitaxial twin variants of rutile upon rapid cooling and/or electron irradiation. Islands of metallic Ni-Sn-NiSn were partially oxidized/solidified when deposited on silica glass.

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The behavior of sp-valent solids and liquids under compression is a field of intense re- search. At high pressure, they often undergo phase transitions to new structures with novel properties such as superconductivity, high-energy density, and superhardness. Furthermore, knowledge of these materials is essential for understanding the structure and evolution of planets. Molecular systems such as nitrogen and carbon dioxide are particularly interesting as energetic materials: their strong molecular bonds break under compression spawning transformations to exotic polymeric phases. We have used first-principles theory and molecular dynamics to make predictions for the properties of dense nitrogen, carbon dioxide, magnesium silicate, and magnesium oxide. For nitrogen, we provide evidence for a rare first-order liquid-liquid phase transition; only the second such transition seen in an elemental fluid. New finite-temperature structure search techniques have been developed and applied to predict a thermodynamically stable polymeric metal phase of solid nitrogen. Regarding carbon dioxide, we have computed its high-pressure liquid phase diagram over a broad pressure-temperature range, revealing rich structural diversity. We have also designed new free energy methods to explore the stability of free CO2 under deep mantle conditions. Lastly, first-principles molecular dynamics and finite-temperature free energy methods were used to predict a high-pressure phase separation transition in liquid MgSiO3 and also characterize the high-pressure phase diagram of MgO, including its melting curve.

In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT). The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures. The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach. The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

Crystallographically solids with well defined crystal structures are normally assumed to be highly ordered. However, it is not uncommon to find considerable degree of disorder amongst many of these crystalline substances. Disorder among crystalline substances often arise from the rotational motion which leads to the well known class of plastic crystalline substances. Typically, globular molecules such as methane, carbon tetrachloride or adamantane exhibit plastic crystalline phase with significant amount of orientational disorder. In many other substances, disorder arises from torsional motion as in the case of biphenyl, p- or o-terphenyls, stilbene or azobenzenes. In case of molecules with flexible segment, such as alkanes or surfactants, motion of the terminal methyl group or terminal ethyl group is responsible for the observed disorder. Chapter 1 discusses various aspects of disorder in crystals. A new pressure induced solid phase of biphenyl is reported at room temperature. Isothermal-isobaric ensemble variable shape simulation cell Monte Carlo calculations are reported on biphenyl at 300K as a function of pressure between 0-4 GPa. The potential proposed by Williams for inter-molecular and Benkert-Heine-Simmons(BHS) for intramolecular interactions have been employed. Different properties indicating changes in the crystal structure, molecular structure, distributions of inter- and intra-molecular energy are reported as a function of pressure. With increase in pressure beyond 0.8 GPa, the dihedral angle distribution undergoes a change from a bimodal to an unimodal distribution. The changes in IR and Raman spectra across the transition computed from ab initio calculations are in agreement with the experimental measurements. It is shown that at pressures larger than 0.8 GPa, competition between inter-molecular interactions with intra-molecular terms v/z., conjugation energy and the ortho-ortho repulsion favors a planar biphenyl due to better packing and consequently a predominant inter-molecular term. The exact value of the transition pressure will depend on the accuracy of the inter- and intra-molecular potentials employed here. p-terphenyl has been modeled at 300K and atmospheric pressure with different potential models. Modified Fihppini parameters for mtermolecular interactions and BHS potential for inter-ring torsion predict the structure of p-terphenyl reasonably well. Pressure variation calculations are carried out with this combination of inter- and intra-molecular potential. The structure as a function of pressure upto 5 GPa has been compared with experimental structure provided by Puschnig et al. The transformation of functional form of the potential energy curve (associated with the inter-ring flipping) from W-shaped to [/-shaped form as a function of pressure has been observed. This is in excellent agreement with previous studies of polyphenyls including biphenyl and p-hexaphenyl. The complete planarization of molecules occurs when the pressure range is 1.0 GPa-1.5 GPa. Molecular simulation of solid stilbene in the isothermal-isobaric ensemble with variable shape simulation are reported. Structure has been characterized by means of lattice parameters and radial distribution functions. Simulations show existence of pedal-like motion at higher temperatures in agreement with the recent X-ray diffraction measurements by Ogawa and co-workers and several others previously. Difference in energy between the major and minor conformers, barrier to conformational change at both the crystallographic sites have been calculated. Temperature dependence of the equilibrium constant between the two conformers as well as the rate of conversion between the con-formers at the two sites have been calculated. These are in agreement with the recent analysis by Harada and Ogawa of non-equilibrium states obtained by rapid cooling of stilbene. Volume and total intermolecular energy suggest existence of two transitions in agreement with previous Raman phonon spectroscopic and calorimetric studies. They seem to be associated with change from order to disorder at the two sites. Ab initio calculations coupled with simulations suggest that the disorder accounts for only a small part of the observed shortening in ethylene bond ength. A Monte Carlo simulation with variable shape simulation cell has been carried out on stilbene. The study attempts to investigate the disorder at various pressures upto 4 GPa. It is seen that the population of minor conformers at sites 1 and 2 decrease with increase in pressure. Population of minor conformers at site 2 decreases to zero by 1.5 GPa. In contrast, the population of minor conformers at site 1 remains finite for the runs reported here. It is seen that the population of minor conformers at site 1 is higher than at site 2 at room temperature which is to be expected on the basis of the activation energies. Associated changes in the unit cell as well as molecular conformation are discussed. Isothermal-isobaric ensemble Monte Carlo simulation of adamantane has been earned out with variable shape simulation cell. Low temperature crystalline phase and the room temperature plastic crystalline phases have been studied employing the Williams potential. We show that at room temperature, the plastic crystalline phase transforms to the crystalline phase on increase in pressure. Further, we show that this is the same phase as the low temperature ordered tetragonal phase of adamantane. The high pressure ordered phase appears to be characterized by a slightly larger shift of the first peak towards lower value of r in C-C, C-H and H-H rdfs as compared to the low temperature tetragonal phase. Co-existence curve between the crystalline and plastic crystalline phase has been obtained approximately upto a pressure of 4 GPa. We investigate the equation of state, variation of lattice parameters and the distortion of molecular geometry of low temperature phase of adamantane upto 26 GPa pressure. A rigid and a flexible model of adamantane have been studied using variable shape simulation within the isothermal-isobaric ensemble. Including six low frequency modes obtained from density functional theory carried out on a single-molecule has incorporated the flexibility. These calculations used Becke 3-parameter method and Lee-Yang-Parr electron correlation functional with 6-31G(d) basis set. The simulated equation of state and variation of c/a ratio as a function of pressure are compared with the experimental results. The results are in good agreement with high pressure experiments. Nature of distortion in molecular geometry obtained from the calculation are also in good agreement with the experiment.

Although the general instability of the iron nitride γ′-Fe4N with respect to other phases at high pressure is well established, the actual type of phase transitions and equilibrium conditions of their occurrence are, as of yet, poorly investigated. In the present study, samples of γ′-Fe4N and mixtures of α Fe and γ′-Fe4N powders have been heat-treated at temperatures between 250 and 1000 °C and pressures between 2 and 8 GPa in a multi-anvil press, in order to investigate phase equilibria involving the γ′ phase. Samples heat-treated at high-pressure conditions, were quenched, subsequently decompressed, and then analysed ex situ. Microstructure analysis is used to derive implications on the phase transformations during the heat treatments. Further, it is confirmed that the Fe–N phases in the target composition range are quenchable. Thus, phase proportions and chemical composition of the phases, determined from ex situ X-ray diffraction data, allowed conclusions about the phase equilibria at high-pressure conditions. Further, evidence for the low-temperature eutectoid decomposition γ′→α+ε′ is presented for the first time. From the observed equilibria, a P–T projection of the univariant equilibria in the Fe-rich portion of the Fe–N system is derived, which features a quadruple point at 5 GPa and 375 °C, above which γ′-Fe4N is thermodynamically unstable. The experimental work is supplemented by ab initio calculations in order to discuss the relative phase stability and energy landscape in the Fe–N system, from the ground state to conditions accessible in the multi-anvil experiments. It is concluded that γ′-Fe4N, which is unstable with respect to other phases at 0 K (at any pressure), has to be entropically stabilised in order to occur as stable phase in the system. In view of the frequently reported metastable retention of the γ′ phase during room temperature compression experiments, energetic and kinetic aspects of the polymorphic transition γ′⇌ε′ are discussed.