Solid State and Structural Chemistry Unit (SSCU)

A Study of electronic structure and semiconductor-metal transitions in perovskite oxides

A Study of electronic structure and semiconductor-metal transitions in perovskite oxides Chainani, A In this chapter we discuss the semiconductor-metal transition in La???Sr?CoO?. We study the electrical resistivity, X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), Bremsstrahlung isochromat spectroscopy (BIS) and X-ray absorption spectroscopy (XAS) of well-characterized samples of La???Sr?CoO?, x = 0.0, 0.1, 0.2, 0.3 and 0.4. The electrical resistivity was measured over a wide temperature range (2 K–300 K) to confirm the semiconductor-metal transition. At low temperatures (T < 30 K), we see a rise in the resistivity for the metallic compositions. This rise in resistivity has a power-law dependence characteristic of disorder effects. The XPS studies show systematic changes in the valence band and Co 2p core-level spectra consistent with the electrical and magnetic properties of the compounds. The UPS spectra show a finite intensity at the Fermi level for the metallic compositions. The BIS data show hole states developing within the band gap of LaCoO? for increasing Sr content. These states overlap the valence band for x > 0.2 leading to a semiconductor-metal transition. The results of XAS at the O K-edge also show the development of hole states induced by the Sr substitution. These states overlap the Fermi level for x > 0.2. Comparison of BIS and XAS results indicate a strongly mixed character for the ground state of LaCoO?. Analysis of the core-valence-valence (CVV) Auger spectra indicates Udd = 3.4 eV and Upp = 6.7 eV for the on-site Coulomb interaction energies in the Co 3d and O 2p manifolds, respectively. The Co 2p core-level spectrum of LaCoO?, simulated in terms of model many-body calculations, provides estimates for various parameter values, such as the charge-transfer energy ? = 4.0 eV and the hybridization between Co 3d and O 2p states, t? = 3.8 eV, and is consistent only with a low-spin configuration of the ground state. The large hybridization strength, t?, is comparable to the bare charge-transfer energy ? and the Coulomb interaction strength, and is in agreement with the mixed character of the parent insulator, as deduced from the analysis of the valence band spectra. Moreover, these parameter values indicate that the doped hole states will locally convert the formally low-spin Co³? state to high-spin Co?? state, in agreement with earlier magnetic measurements. In this chapter we study the electronic structure and semiconductor-metal transition in La???Sr?MnO? (x = 0.0–0.4) using various techniques. The electrical resistivity (?) measured over a wide temperature range (2 K–450 K) shows an unusual semiconductor-metal transition for x > 0.2. The metallic compositions exhibit a broad maximum in ? versus T, in the temperature range 150 K–250 K depending on the composition. Magnetic measurements show a Curie temperature for the doped compounds, which cannot be directly related to the resistivity anomaly. In the metallic phase, the resistivity over the entire temperature range remains several orders of magnitude higher compared to other metallic oxides. The high-temperature resistivity indicates an activated behaviour for all the compositions with very similar band gaps. At low temperatures (T < 30 K) we see an upturn in the resistivity which fits a T??·? dependence, indicating the importance of electron-electron interactions in the presence of disorder in the system. The conductivity above 100 K, for x > 0.2, fits an equation of the type: ? = ??e??°/²?? + A/(1 + BT) implying the total conductivity to be a sum of semiconducting and metallic components. Electron spectroscopy studies employing photoemission and inverse photoemission show negligible intensity at E? even for the metallic compositions, indicating an unusual semiconductor-metal transition, consistent with the resistivity data. BIS spectra show doped hole states developing as a function of Sr content, about 1.4 eV above E?. Soft X-ray absorption studies at the oxygen K-edge also show the formation of doped hole states above E? for increasing x, implying substantial oxygen 2p character of the doped states. The Mn 2p core-level spectrum of LaMnO?, analysed in terms of a model many-body configuration interaction calculation, gives parameter values for ? (= 5.0 eV), t? (= 3.8 eV) and Udd (= 4.0 eV). AES gives an estimate of Upp, the on-site Coulomb interaction strength in the oxygen 2p states, to be 6.8 eV. The values of ?, t? and Udd classify LaMnO? as a mixed character oxide with large electron-electron correlations. These parameter values also indicate that the doped hole states in this series should have considerably mixed Mn 3d–O 2p character, in conformity with BIS and XAS results. The absence of any perceivable density of states at E? even for the metallic compositions (x > 0.2) in conjunction with the anomalous resistivity behaviour suggests the interpretation that doping of Sr in place of La produces a local density of states with a gap, in conformity with bulk SrMnO? being an insulator; however, metallic percolating paths, presumably arising from the interface or the boundary of Sr-doped and undoped regions, are established at high enough Sr concentrations, resulting in an apparent semiconductor-metal transition as a function of temperature. In this chapter, we study the effect of Sr substitution in LaFeO? using various electron spectroscopy techniques. From resistivity measurements, it is found that the samples remain semiconducting up to 40% substitution of Sr for La. We determine the electronic structure of well-characterized samples of La???Sr?FeO? (x = 0.0–0.4) by X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), Bremsstrahlung isochromat spectroscopy (BIS) and X-ray absorption spectroscopy (XAS). We find systematic changes in the occupied and unoccupied density of states (DOS). The spectral features, particularly of the unoccupied states obtained from BIS spectra, indicate the probable reason for the absence of an insulator-metal transition in this series. AES provides us with estimates of Udd and Upp, the on-site Coulomb interaction strengths in Fe 3d and O 2p states, respectively. Fe 2p core-level XPS in conjunction with a model many-body calculation gives us the parameter values for the bare charge-transfer energy ? and the Fe 3d–O 2p hybridization strength, t? for LaFeO?. The interaction parameters thus obtained suggest a substantially different nature of the ground state in LaFeO?, as compared to an earlier analysis; we also discuss the character of the doped hole states in La???Sr?FeO?, based on these parameter values.

Addressing Subtle Physicochemical Features Exhibited by Molecular Crystals Via Experimental and Theoretical Charge Density Analysis

Tue, 19 Jun 2018 00:00:00 GMT

Addressing Subtle Physicochemical Features Exhibited by Molecular Crystals Via Experimental and Theoretical Charge Density Analysis Pal, Rumpa The thesis entitled “Addressing subtle physicochemical features exhibited by molecular crystals via Experimental and Theoretical Charge Density Analysis” consists of five chapters. An introductory note provides a brief description of experimental and theoretical charge density methodology, followed by its utilization in obtaining certain physical and chemical properties in molecular crystals. Chapter 1 addresses not so easily accessed molecular property arising due to electron conjugation, highlighting antiaromaticity in tetracyclones. A systematic study of six tetracyclone derivatives with electron withdrawing and electron donating substituents has been carried out using experimental and theoretical charge density analysis. A three pronged approach based on quantum theory of atoms in molecules (QTAIM), nucleus independent chemical shifts (NICS), and source function (SF) has been employed to establish the degree of antiaromaticity of the central five-membered ring in all the derivatives. Electrostatic potentials mapped on the is density surface reveal the finer effects of different electron withdrawing and electron donating substituents on the carbonyl group. Chapter 2 presents a temperature induced reversible first order single crystal to single crystal phase transition (Room temperature Orthorhombic, P22121 to low temperature Monoclinic, P21) in a  hybrid peptide, Boc-γ4(R)Val-Val-OH. The thermal behavior accompanying the phase transition of the dipeptide crystal was characterized by differential scanning calorimetry, visual changes in birefringence of the sample during heating and cooling cycles on a hot-stage microscope with polarized light. Variable-temperature unit cell check measurements from 300 to 100 K showed discontinuity in the volume and cell parameters near the transition temperature, supporting the first-order behavior. The reversible nature of the phase transition is traced to be due to an interplay between enthalpy and entropy. Chapter 3 brings out an unusual stabilizing interaction involving a cooperative -hole and ¬hole character in a short NCS···NCS bond. This chapter describes structural features of four isothiocyanate derivatives, FmocXCH2NCS; X=Leu, Ile, Val and Ala. Among these it is observed that only FmocLeuCH2NCS which crystallizes in a tetragonal space group, P41, (a=b=12.4405(5) Å; c= 13.4141(8) Å) transforms isomorphously to a low temperature form, P41, (a=b=17.4665(1) Å; c= 13.1291(1) Å). The characteristics of the phase transition have been monitored by Differential Scanning Calorimetry, variable temperature IR and temperature dependent unit cell measurements. The short NCS···NCS intermolecular interaction (3.296(1) Å) is analyzed based on detailed experimental charge density analysis which reveals the nature of this stabilizing interaction. Chapter 4 explains a comparative study of syn and anti conformations of carboxylic acids in peptides from both structural aspect and charge density features. Single crystal structures of four peptides having syn conformations [BocLeuγ4(R)Valγ4(R)ValOH, BocLeuγ4(R)ValLeuγ4(R)ValOH, Boc3(S)Leu3(S)LeuOH] and one with anti conformation, BocLeuγ4(R)ValValOH have been analyzed. Experimental charge density analysis has been carried out exclusively on BocLeuγ4(R)ValValOH having anti form, because of its rare occurrence in literature. However, low temperature datasets on the four peptides with syn conformations were collected and theoretical charge density analysis has been carried out on two of these compounds. Electrostatic potentials mapped on is density surface bring out a significant difference at the oxygen atoms of the carboxyl group in the two conformations. However, lone pair orientation of different types of Oxygen atoms in the two forms (urethane, amide, acid) doesn’t exclusively indicate the differences in the corresponding charge density features. Chapter 5 addresses the issue of how sensitive are the charge density features associated with amino acid residues when the backbone conformational angles are varied. Three model systems, 1, L-alanyl–L-alanyl–L-alanine dehydrate; 2, anhydrous L-alanyl–L-alanyl–L¬alanine and 3, cyclo-(D,L-Pro)2(L-Ala)4 monohydrate have been chosen for this evaluation. Compound 1 has ant parallel alignment of tripe tide strands, and compound 2 has parallel alignment. All the alanine residues in compound 1 and 2 are in the -sheet region of the Ramachandran plot, whereas, the four Alanine residues in the cyclic hex peptide 3 span different regions of the Ramachandran plot. Theoretical multipole modelling has been carried out in order to explore the plausibility of transferring multipole parameters across different regions of Ramachandran Plot. Appendix I contains a brief description of charge shift bonding in Ph-CH2-Se-Se-CH2-Ph, as determined based on both experimental and theoretical charge density analysis. Appendix II contains a reprint of a published article on “Conformation-Changing Aggregation in Hydroxyacetone: A Combined Low-Temperature FTIR, Jet, and Crystallographic Study”.

Analysis Of Intermolecular Interactions In Pharmaceutical Salts And Cocrystals

Wed, 17 Jul 2013 00:00:00 GMT

Analysis Of Intermolecular Interactions In Pharmaceutical Salts And Cocrystals Dasgupta, Archi The studies on cocrystals and salts presented in the the chapters clearly bring out the influence of intermolecular interactions as the main evaluators of the cocrystal-salt regime. The observations made in Chapter 2 indicate that in case if the cocrystal formation is through hydrogen bonds the location of the proton decides the nature of the complex in the energy landscape. The observation that the coformer controls the topology of intermolecular space as demonstrated in Chapter 3 provides insights into the importance of directionality rather than strength of intermolecular interactions. Indeed halogen bonding in cocrystals gain importance in this context.

Applications Of Multiple Quantum Methods In NMR For Determination Of Dipolar Couplings And Chiral Discrimination

Wed, 20 Feb 2013 00:00:00 GMT

Applications Of Multiple Quantum Methods In NMR For Determination Of Dipolar Couplings And Chiral Discrimination Hebbar, Sankeerth This thesis is about excitation, detection, properties and applications of multiple quantum coherences applied to different dipolar coupled spin systems. Major focus of the work is on spectral simplification, measurement of residual dipolar couplings and discrimination of enantiomers in chiral aligning media. The first chapter gives a brief account on the fundamentals of nuclear magnetic resonance spectroscopy and multiple quantum coherences. This includes a description of product operator and polarization operator formalisms of pulses and evolution of magnetization. Subsequently a detailed account of two dimensional multiple quantum – single quantum (MQ-SQ) correlation experiments is given. Demonstration of the homonuclear MQ-SQ pulse sequence on a weakly coupled spin system and analysis of the spectrum obtained are also discussed. Homo-nuclear multiple quantum studies carried out to obtain relative the signs of the couplings have been reported in the initial part of the second chapter. The technique has been applied on doubly labeled acetonitrile (13CH313C15N) aligned in a liquid crystalline medium. Special situations like ambiguity in the determination of relative signs of the couplings from the appearance of two dimensional MQ-SQ spectra and the explanation for the same are also discussed. Homo-nuclear MQ experiments on indistinguishable spins, like protons in a methyl group of 13CH313C15N oriented in liquid crystal, and distinguishable spins, like the two carbons in the same molecule, have been carried out. Different directions of approach in which these results need to be analyzed have been discussed. Subsequent part of the chapter is about the correlation of connected MQ-SQ coherences. These experiments are significant in reducing the cross-peaks further from the MQ-SQ spectra. This concept is extended for the discrimination of optical enantiomers dissolved in chiral aligning medium made of poly-Γ-benzyl-L-glutamate (PBLG) and CDCl3. In molecules of Chemical and biological interest one encounters several nuclei such as, 1H, 13C, 15N and 19F. It will be of general interest to determine magnitudes and relative signs of the couplings among these coupled nuclei by NMR experiments. Utilization of hetero-nuclear MQ Experiments in solving such problems is discussed in the third Chapter. Hetero-nuclear MQ experiments were carried out on dipolar coupled 13CH313C15N, with the aim of obtaining the values and signs of various hetero-nuclear couplings in the molecule. The splitting of transitions in the spectra of oriented molecules is always influenced by the sum of dipolar and scalar couplings. Hence precise determination of dipolar couplings requires the knowledge of scalar couplings. To determine the J couplings, experiments were carried out on the same molecule in isotropic medium. When many coupled nuclei are involved one has to carry out several experiments to derive all the spectral parameters. In circumventing this problem heteronuclear multiple quantum experiments involving more than two nuclei as active spins are advantageous. This reduces the number of experiments and thereby reducing the total experimental time. Second part of this chapter demonstrates how a triple resonance triple quantum experiment can provide majority of the couplings from a given coupled system. The feasibility of the experiment is demonstrated even for molecules containing natural abundant isotopes. Application of multiple quantum j-resolved technique for chiral discrimination and obtaining complete one dimensional spectrum of each enantiomer from their racemic mixture is discussed in the fourth chapter. The two dimensional experiment consists of a selective double quantum excitation period followed by selective refocusing during indirect time domain, isotropic mixing and nonselective detection of SQ transitions. Hence this pulse sequence is named as DQSERF-COSY (Double Quantum Selective Refocused Correlation Spectroscopy). The experiment exploits the existence of different intra-methyl couplings between the enantiomers dissolved in chiral liquid crystal medium to separate the one dimensional spectra of each enantiomer in different cross sections. This is possible due to the fact that all the nuclei in any one of the enantiomers are coupled among themselves and there is no inter molecular interaction between the two enantiomers. Also one can extract all the couplings between protons in each enantiomer, which can subsequently be utilized for determination of the residual dipolar couplings, structure and orientation parameters.