Performance of six functionals (LDA, PBE, PBESOL, B3LYP, PBE0 and WC1LYP) in the simulation of vibrational and dielectric properties of crystalline compounds. The case of forsterite Mg2SiO4

The performance of six different density functionals (LDA, PBE, PBESOL, B3LYP, PBE0, and WC1LYP) in describing the infrared spectrum of forsterite, a crystalline periodic system with orthorhombic unit cell (28 atoms in the primitive cell, Pbmn space group), is investigated by using the periodic ab initio CRYSTAL09 code and an all-electron Gaussian-type basis set. The transverse optical (TO) branches of the 35 IR active modes are evaluated at the equilibrium geometry together with the oscillator strengths and the high-frequency dielectric tensor 8. These quantities are essential to compute the dielectric function ϵ(ν), and then the reflectance spectrum R(v), which is compared with experiment. It turns out that hybrid functionals perform better than LDA and GGA, in general; that B3LYP overperforms WC1LYP and, in turn, PBE0; that PBESOL is better than PBE; that LDA is the worst performing functional among the six under study

Raman modes in Pbca enstatite (Mg2Si2O6): an assignment by quantum mechanical calculation to interpret experimental results

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid Hamiltonian WC1LYP.[1] The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ~5 cm1. The quantum mechanical simulation allowed the assignment of Raman features to specific vibrational modes. This enabled to assess quantitatively the contributions of internal (tetrahedral stretching) and external (tetrahedral chains and M1 and M2 cations) vibrations. Moreover, the mass substitution of 56Fe for 24Mg in the M1 and M2 sites and of 30Si and 18O for the 28Si and 16O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate the major experimental peaks to specific structural vibrations, and to link the changes in crystal structure to those modes with pressure, temperature and composition. The results provide new clues to identify most suitable peaks for the investigation of the intracrystalline ordering of Fe and Mg in the M1 and M2 sites, and of Al in the tetrahedral and octahedral sites. Moreover we have been able to identify those peaks which are related to structural features, like tetrahedral bond distances

Probing the Multiple Structures of Vaterite through Combined Computational and Experimental Raman Spectroscopy

First-principles Raman spectra have been computed for several new vaterite structural models that have been recently proposed, and compared with spectra recorded on a set of biogenic, geological and synthetic samples. This set includes new measurements collected on Herdamania momus spicules (Great Barrier Reef, Queensland, Australia), which are known to have purity and crystallinity that are higher than for other biogenic samples. Overall, due to the close structural connection between the various models, the computed Raman spectra are found to be broadly similar. However, the spectra obtained for the two most stable models (monoclinic C2 and trigonal P3221, corresponding to two different polytypes of vaterite) exhibit features that are in excellent agreement with the experimental spectra, whereas the other theoretical structures show minor peaks that are not observed experimentally. When comparing the spectra for the two lowest energy structural models (C2 and P3221), the differences are too small to discriminate between these candidates. The Raman spectrum of Herdamania momus is of higher quality with respect to spectra obtained in previous studies on other biogenic samples. However, there is no significant and systematic difference with respect to samples of geological and synthetic origin

Atomistic origins of mechanical amorphization of SiO2 using solid-state 29Si NMR and local electronic structures for SiO2 high pressure phases using ab initio calculation

학위논문 (석사)-- 서울대학교 대학원 : 지구환경과학부, 2017. 2. 이성근.Silica (SiO2), the most abundant material on the earth, is a major component of the crust and mantle. Therefore the structure of silica affects the dynamic process of crust or the structural properties of the earth interior. The structure of silica changes not only by pressure-temperature change but also mechanical energy like friction without melting. To understand the properties of interior of rocky planets and the geological process, the detailed understanding of the structure of silica with various conditions is needed. In this study, we performed two major subject related to structure of silica. The first study is about atomistic origin of the mechanical amorphization of silica with an experimental methodology. There have been reported the reduction of fault strength during fault slip, and one of the causes is reported as the formation of the amorphous silica and the silica gel layer on the fault plane. The formation of amorphous silica is due to the frictional energy between the fault surfaces, and the formation of silica gel layer is due to reaction with the water present in the layer or air. In previous, studies on frictional experiments using quartzite rocks, reported the formation of silica and silica gel and also the decrease of friction coefficient, and the observation of the silica gel layer in natural faults has been reported. However, due to the complex structure of amorphous silica and the analytical method limitations, the atomic structure of the amorphized silica and the detailed origin of the mechanical amorphization process have not been clearly identified. In this study, silica was amorphized with mechanical energy by ball mill method, and the resulting amorphous structure was analyzed by high resolution nuclear magnetic resonance (NMR) spectroscopy. The morphology of the amorphized silica and the phases formed by other elements were analyzed using XRD, HR-TEM, and EDS-mapping method. Solid-state nuclear magnetic resonance spectroscopy (NMR) is suitable for the analysis of complex amorphous structures because it yields atomic environments in short range order around specific atoms and provides quantitative information on atomic unit bonding. The 29Si MAS NMR spectra of amorphous silica milled at different rates present that the spectra of samples milled above 600 rpm show a broad amorphous peak over -80 to -120 ppm. These amorphous peaks indicate the presence of Q2 and Q3 structures in the mechanically amorphized silica. The results indicate that the mechanical amorphization of silica occurs only above a certain energy level, and that the amorphization process results in a change in the short range atomic structure and imply the presence of reaction with other elements. This results help to understand the atomic structure of the mechanically amorphized silica and the mechanical amorphization process that occurs without melting in the fault plane. The second study is an electronic structure and detailed origin of spectral feature of O K-edge XRS for crystalline silica using computational computation. Crystalline silica undergoes various phase transition according to pressure-temperature change. Therefore, many studies on the structure of high-pressure phase silica have been reported to understand the internal structure of rocky planets in high temperature and high pressure environment. The most powerful method of in situ high-pressure study for electronic structure is O K-edge x-ray Raman spectroscopy (XRS) using diamond anvil cell and as experimental limitation above ~70 GPa, the computational methods is used to understand the XRS spectrum. However, the detailed relation between spectral feature of O K-edge XRS and structure is still in debate. In this study, we calculated electronic structures and O K-edge XRS spectra for various crystalline silica using ab initio calculation, and proved the origin of O K-edge XRS spectrum. Previous studies have suggested that the origin of the O K-edge XRS spectrum is attributed to the number of Si atoms or the number of O atoms. However, in this study, we have found that the O K-edge XRS spectra of hp-cristobalite with 4-coordination Si, of penta-SiO2 with 5-coordination Si and of stishovite with 6-coordination Si are similar, and O site resolved K-edge XRS spectra the penta-SiO2 revealed that the coordination of atoms does not directly affect the O K-edge XRS spectral feature. These results not only provide an understanding of the electronic structure of high-pressure phase silica, but also provide clear criteria for O K-edge XRS spectral analysis.1. INTRODUCTION 1 2. THEORETICAL BACGROUNDS 3 2.1 NMR TECHNIQUES 3 2.2 BALL MILL METHOD 4 2.3 AB INITIO CALCULATIONS 5 3. ATOMISTIC ORIGINS OF MECHANICAL AMORPHIZATION BY BALL MILL EXPERIMENTS: INSIGHTS FROM SI-29 NUCLEAR MAGNETIC RESONANCE 7 3.1. INTRODUCTION 7 3.2. EXPERIMENTS 9 3.2.1 Sample Preparation and Analysis 9 3.2.2 Planetary Ball Mill 9 3.2.3 NMR Spectroscopy 10 3.2.4 X-ray Diffraction 10 3.3. RESULTS AND DISCUSSION 11 3.3.1. TEM EDS 11 3.3.2. X-ray Diffraction Patterns 11 3.3.3. Si-29 MAS NMR 12 3.4. CONCLUSION 13 REFERENCE 14 4. AB INITIO CALCULATIONS OF LOCAL ELECTRONIC STRUCTURES AND X-RAY RAMAN SCATTERING SPECTRA FOR SIO2 HIGH PRESSURE PHASE 16 4.1 INTRODUCTION 16 4.2 AB INITIO CALCULATIONS 16 4.2.1 Crystal structures 16 4.2.2 Calculation conditions 19 4.2.3 PDOS and O K-edge XRS calculations 20 4.3 RESULTS AND DISCUSSION 21 4.3.1 O K-edge x-ray Raman scattering 21 4.3.2 Correlation between spectrum and crystal structure 22 4.4 CONCLUSION 23 REFERENCE 25 TABLES 29 FIGURE 30 APPENDIX SECTION 42 ABSTRACT IN KOREAN 44Maste

First-principles Calculations on Doped Perovskites

Perovskite is a family of oxides have been received increasing attractions due to their great values in diverse applications. Among them, La-doped NaTaO3 shows particular interests as a highly efficient catalyst in the photo-dissociation reaction of water into H2 and O2, providing a potential clean and reusable energy source. The mechanism of the increasing in the catalytic efficiency by the La doping has not been fully revealed by neither precedent experiments nor calculations. Computational chemistry has been developed for decades to be a reliable and accurate predicting method in investigating chemical structures like molecules, solid states, etc. This method is capable of predicting the optimized and most stable atomic structures of periodic solid-state crystals, as well as the electronic structures like the band structures for semiconductors. In order to get an insight into the influence of the La doping upon the atomic structure and electronic structure of NaTaO3, we are applying the first-principles calculations on this doped system. Both the doped bulk systems and doped surfaces are treated in order to get a complete investigation. Otherwise, another correction for PM7 method is also introduced in order to reduce its error in predicting the band gap for semiconductor transition-metal oxides.Ph.D., Chemistry -- Drexel University, 201

Progettazione, sintesi e caratterizzazione in solido di forme cristalline di farmaci, pesticidi e fitofarmaci, in accordo con i principi di green chemistry

This doctorate was funded by the Regione Emilia Romagna, within a Spinner PhD project coordinated by the University of Parma, and involving the universities of Bologna, Ferrara and Modena. The aim of the project was: - Production of polymorphs, solvates, hydrates and co-crystals of active pharmaceutical ingredients (APIs) and agrochemicals with green chemistry methods; - Optimization of molecular and crystalline forms of APIs and pesticides in relation to activity, bioavailability and patentability. In the last decades, a growing interest in the solid-state properties of drugs in addition to their solution chemistry has blossomed. The achievement of the desired and/or the more stable polymorph during the production process can be a challenge for the industry. The study of crystalline forms could be a valuable step to produce new polymorphs and/or co-crystals with better physical-chemical properties such as solubility, permeability, thermal stability, habit, bulk density, compressibility, friability, hygroscopicity and dissolution rate in order to have potential industrial applications. Selected APIs (active pharmaceutical ingredients) were studied and their relationship between crystal structure and properties investigated, both in the solid state and in solution. Polymorph screening and synthesis of solvates and molecular/ionic co-crystals were performed according to green chemistry principles. Part of this project was developed in collaboration with chemical/pharmaceutical companies such as BASF (Germany) and UCB (Belgium). We focused on on the optimization of conditions and parameters of crystallization processes (additives, concentration, temperature), and on the synthesis and characterization of ionic co-crystals. Moreover, during a four-months research period in the laboratories of Professor Nair Rodriguez-Hormedo (University of Michigan), the stability in aqueous solution at the equilibrium of ionic co-crystals (ICCs) of the API piracetam was investigated, to understand the relationship between their solid-state and solution properties, in view of future design of new crystalline drugs with predefined solid and solution properties