Showing papers on "Crystal published in 1981"

Demonstration of efficient nonlinear optical crystals with vanishing molecular dipole moment: Second‐harmonic generation in 3‐methyl‐4‐nitropyridine‐1‐oxide

Abstract: Previous research toward more efficient nonlinear optical organic crystals has aimed at increasing the first‐order hyperpolarizabilitiy of individual molecular units. We show that the general features derived from these studies (high conjugation, charge transfer, and molecular asymmetry) leave room for the cancellation of the molecular dipole moment. This can be usefully exploited in two directions: improving the crystal growth conditions and a possibility of acting on the crystal point group structure through otherwise hidden parameters. Pyridine‐1‐oxide, owing to the electronic ’’push–pull’’ property of the N–oxide bond, allows for substitution in the 4 position of donor as well as acceptor groups, among which the nitro group ensures dipole cancellation. The possibility of charge transfer from or toward the N–oxide bond according to the nature of the radical in the 4 position is investigated and confirmed by means of second‐harmonic generation experiments in powder or solution (DCFISH) on a number of compounds. 3‐methyl‐4‐nitropyridine‐1‐oxide is shown to be the best candidate, among related compounds, for crystal growth. A detailed study of its linear and first‐order nonlinear properties is presented. Crystalline samples grown in solution are seen to be of excellent optical quality. Its efficiency compares to that of the best available phase‐ matchable organic crystals [d = (23±3)×10−9 esu]. Besides, the optical broadband phase‐matching potential of the crystal could be useful in a number of applications like frequency doubling of ultrashort pulses in the near infrared.

Organic molecular crystals : their electronic states

Abstract: 1. Introduction: Characteristic Features of Organic Molecular Crystals.- 1.1 Interaction Forces in Molecular Crystals.- 1.2 The Atom-Atom Potential Method.- 1.3 Aromatic Hydrocarbons - Model Compounds of Organic Molecular Crystals.- 1.3.1 Anthracene.- Anthracene as Model Compound.- Molecular Structure.- Basic Molecular Parameters.- Crystal Structure.- Elastic and Optical Properties.- Metastable Phases in Anthracene.- 1.3.2 Naphthalene.- Molecular Structure.- Basic Molecular Parameters.- Crystal Structure.- Elastic and Optical Properties.- 1.3.3 Higher Linear Polyacenes.- Tetracene and Pentacene.- Hexacene.- 1.3.4 Other Model Aromatic Compounds.- 1.4 Specific Properties of Electronic States in a Molecular Crystal.- 1.5 Basic Characteristics of Electronic Conduction States in Molecular Crystals.- 1.5.1 Band Theory Approach.- 1.5.2 Hopping Versus Band Model.- 1.5.3 Band-to-Hopping Transition.- 1.5.4 Electronic Polarization and Charge Carrier Self-Energy.- 1.5.5 Other Types of Interaction.- 2. Electronic States of an Ideal Molecular Crystal.- 2.1 Neutral Excited States in a Molecular Crystal.- 2.2 Ionized States in a Molecular Crystal.- 2.2.1 The Lyons Model of Ionized States.- 2.2.2 A Modified Lyons Model.- 2.3 Electronic Polarization of a Molecular Crystal by a Charge Carrier.- 2.3.1 Some General Considerations.- 2.3.2 Dynamic and Microelectrostatic Approaches to Electronic Polarization in Molecular Crystals.- 2.4 Electrostatic Methods of Electronic Polarization Energy Calculation in Molecular Crystals.- 2.4.1 Microelectrostatic Methods of Zero-Order Approximation.- 2.4.2 Method of Self-Consistent Polarization Field.- 2.5 Determination of Molecular Polarizability Tensor.- 2.5.1 Experimental Methods.- 2.5.2 Theoretical Methods.- 2.6 Selection of Molecular Polarizability Components bi. for Electronic Polarization Energy Calculations.- 2.7 Extended Polarization Model of Ionized States in Molecular Crystal s.- 2.7.1 Intrinsic Electronic Polarization of a Molecule by a Localized Charge Carrier.- 2.7.2 Vibronic Relaxation and Ionic State Formation.- 2.7.3 Extended Polarization Model Including Ionic States of Electronic Conductivity.- 2.7.4 Dynamic Electronic Polaron States in a Molecular Crystal.- 2.8 Charge Transfer (CT) States in Molecular Crystals.- 2.8.1 General on CT States.- 2.8.2 Evaluation of CT-State Energies in Anthracene and Naphthalene Crystals.- 2.8.3 CT States in Photogeneration Processes.- 2.8.4 CT States in Recombination Processes.- 2.9 Experimental Determination of Energy Structure Parameters in Molecular Crystals.- 2.10 Energy Structure of an Anthracene Crystal.- 2.11 Energy Structure of Aromatic and Heterocyclic Molecular Crystals.- 3. Role of Structural Defects in the Formation of Local Electronic States in Molecular Crystals.- 3.1 Statistical Aspects of the Formation of Local States of Polarization Origin.- 3.2 General Considerations on the Role of Specific Structural Defects.- 3.3 Point Defects (Vacancies) in Molecular Crystals, Their Crystallographic and Electronic Properties.- 3.4 Dislocation Defects, Their Role in Local State Formation.- 3.5 Energetics of Dislocations in Molecular Crystals.- 3.5.1 Discrete Configuration of Dislocations.- 3.5.2 Basic Elastic Properties of Anthracene and Naphthalene Crystals.- 3.5.3 Energy Estimates for Basal Edge Dislocations in an Anthracene Crystal.- 3.6 Atomic and Molecular Models of the Dislocation Core.- 3.6.1 Models of Spherical Atoms and Molecules.- 3.6.2 Polyatomic Molecular Models.- 3.7 Dislocation Alignments and Aggregations, Their Configurational and Energetic Properties.- 3.7.1 Interaction Between Dislocations.- 3.7.2 Dislocation Alignments.- 3.7.3 Dislocation Ensembles.- 3.8 Grain Boundaries, Their Energetic Characteristics.- 3.8.1 Energy of Grain Boundaries in Molecular Crystals.- 3.8.2 Relative Lattice Compression on Grain Boundaries of an Anthracene Crystal.- 3.9 Stacking Faults in Molecular Crystals.- 3.9.1 General on Stacking Faults.- 3.9.2 Stacking Faults in Anthracene-Type Crystals, Their Energetic Characteristics.- 3.9.3 Calculations of Equilibrium Configuration of Molecules in Stacking Faults of an Anthracene Crystal.- 3.10 Formation of Predimer States in the Regions of Extended Structural Defects of Anthracene-Type Crystals.- 3.11 Some More Complex Two- and Three-Dimensional Lattice Defects in Molecular Crystals.- 3.12 Observation of Structural Defects in Molecular Crystals.- 3.12.1 Optical Low Resolution Technique.- 3.12.2 Electron Microscopy and Diffraction Techniques.- 3.12.3 X-Ray Methods.- 3.13 Main Characteristics of Dislocation Defects in Some Model Molecular Crystals.- 3.13.1 Dominant Types of Dislocatioas in Anthracene Space Group Crystal s.- 3.13.2 Density of Dislocations in Anthracene Crystals, Its Dependence on Crystal Growth and Treatment.- 4. Local Trapping Centers for Excitons in Molecular Crystals.- 4.1 Theory of Exciton States in a Deformed Molecular Crystal.- 4.2 Electron Level Shifts in Hydrostatically Compressed Molecular Crystal s.- 4.3 Formation of Local Exciton Trapping Centers in Structural Defects of a Crystal.- 5. Local Trapping States for Charge Carriers in Molecular Crystals.- 5.1 Electronic Polarization Energy of a Compressed Anthracene Crystal.- 5.2 Formation of Local Trapping Centers for Charge Carriers in Structural Defects of a Real Molecular Crystal.- 5.3 Energy Spectrum of Local States of Polarization Origin in Stacking Faults of an Anthracene Crystal.- 5.4 Local Surface States of Polarization Origin in Molecular Crystals.- 5.5 Local States of Polarization Origin in the Vicinity of a Lattice Vacancy.- 5.6 Local Charge Carrier Trapping in Covalent, Ionic and Molecular Crystal s.- 5.7 Randomizing Factors Determining Gaussian Distribution of Local States of Structural Origin.- 5.8 Investigation of Local Trapping States by Method of Space Charge Limited Currents (SCLC).- 5.8.1 General Considerations.- 5.8.2 Injecting and Blocking Contacts.- 5.8.3 Conventional SCLC Theories of Discrete and Exponential Approximation of Trap Distribution.- SCLC Theory for an Insulator With Discrete Trap Distribution.- SCLC Theory for an Insulator With Exponential Trap Distribution.- Applicability Limits of Diffusion-Free SCLC Theory Approximation.- 5.8.4 Criteria for Validity of SCLC Conditions.- 5.8.5 Difficulties in Interpreting Experimental CV Characteristics in Terms of Discrete and Exponential Trap Distribution Models.- 5.9 Phenomenological SCLC Theory for Molecular Crystals with Gaussian Distribution of Local Trapping States.- 5.9.1 Conceptual Basis.- 5.9.2 Basic SCLC Theory Equations.- 5.9.3 Validity Range for Different Analytical SCLC Approximations.- 5.9.4 SCLC Dependence on Dispersion Parameter a.- 5.9.5 SCLC Temperature Dependences for Ge (E) and Gg (E) Distributions.- 5.9.6 SCLC Dependence on Et Value.- 5.9.7 Validity Criteria for Exponential and Gaussian Approximations.- 5.9.8 CV Characteristics for Two Sets of Gaussian Trap Distribution.- 5.10 Gaussian SCLC Approximations of Experimental CV Characteristics.- 5.10.1 Analytical Approximations.- 5.10.2 Differential Method of Analysis of CV Characteristics.- 5.11 SCLC Theory for Spatially Nonuniform Trap Distribution.- 5.12 Investigation of Local Trapping States by Thermally Activated Spectroscopy Techniques.- 5.13 Other Experimental Methods for Local Trapping State Study.- 5.14 Correlations Between Distribution Parameters of Local Trapping States and Crystalline Structure.- 5.15 Local Lattice Polarization by Trapped Charge Carrier in Molecular Crystals.- 5.16 Guest Molecules as Trapping Centers in a Host Lattice.- 6. Summing Up and Looking Ahead.- References.- Additional References with Titles.

The crystal and molecular structure of oxovanadium(V) orthophosphate dihydrate, VOPO4,2H2O

Abstract: The structure of a tetragonal crystal of the title compound was determined by X-ray diffraction. The space group was found to be P4/nmm with a 6.202(2) and c 7.410(1) A and Z 2. The structure consists of infinite sheets of distorted VO6 octahedra and PO4 tetrahedra linked by shared oxygen atoms. Shared water molecules link these sheets together. A short V-O(4) bond, 1.567(5) A, indicates the presence of the cation VO3+.

Free ion and crystal field parameters for REOCl : Eu3+

Abstract: The fluorescence spectra of Eu3+ activated rare earth oxychlorides REOCl : Eu3+ (RE = Y, La, Gd) were recorded and analyzed at 4.2, 77, and 300 K. For each compound more than 100 transitions between the Stark levels of the 4f6 configuration were observed and assigned. The energy level schemes established from the spectra show a relatively strong crystal field effect. The five nonzero crystal field parameters for the C4v point site symmetry occupied by the rare earth ion were determined, yielding a mean square deviation of a few cm−1. The free‐ion parameters were also determined with the aid of excitation spectra. The effect of truncation of the basis set of the 2S+1LJ levels on the energy level simulation of the excited levels was also studied.

X-ray diffraction and calorimetric study of anhydrous and hydrated N-palmitoylgalactosylsphingosine (cerebroside).

Abstract: Differential scanning calorimetry and X-ray diffraction of anhydrous and hydrated N-palmitoylgalactosylsphingosine (NPGS) show evidence of complex polymorphic behavior and interconversions between stable and metastable structural forms. Anhydrous NPGS exhibits three lamellar crystal forms (A, B, and B') at temperatures below 143 degrees C and a liquid-crystal form between 143 and 180 degrees C before melting to an isotropic liquid at 180 degrees C. The crystal B leads to liquid-crystal transition is accompanied by an enthalpy change, delta H, of 11.2 kcal/mol of NPGS, while a relatively small enthalpy change (delta H = 0.8 kcal/mol) marks the liquid-crystal leads to liquid transition. The A and B' crystal forms do not hydrate readily at room temperature. When heated, crystal form A in the presence of water undergoes an exothermic transition at 52 degrees C to produce a thermodynamically stable hydrated crystal E form. X-ray diffraction shows that this stable bilayer crystal form has a highly ordered hydrocarbon chain packing arrangement; melting to the bilayer liquid-crystal form occurs at 82 degrees C with a large enthalpy change, delta H = 17.5 kcal/mol of NPGS. A complex liquid-crystal leads to crystal transition is observed on cooling; the cooling rate independent exotherm involves the transition of the hydrated liquid crystal to an intermediate metastable crystal form identical with anhydrous crystal form A. The subsequent cooling rate dependent step involves the conversion of the metastable crystal form A to the stable crystal form E. We suggest that hydrated crystal form E is stabilized by both a highly ordered chain packing mode and a lateral intermolecular hydrogen bonding network involving the sphingosine backbone, the galactosyl group, and interbilayer water molecules. Although disruption of both the specific hydrogen chain packing and H-bonding networks occurs at the high enthalpy transition to the bilayer liquid-crystal L alpha form, these two types of interactions are not reestablished simultaneously on cooling. First, recrystallization of the hydrocarbon chain accompanies removal of water from the lipid interface, leading to "dehydrated" metastable crystal form A. This is followed by a time-dependent, temperature-dependent hydration process which allows a rearrangement of the hydrogen-bonding matrix. Alterations in the NPGS-NPGS and NPGS-water interactions accompany further changes in the hydrocarbon chain packing and lead to the formation of the stable E form.

Two-wave mixing and energy transfer in Bi(12) SiO(20) crystals: application to image amplification and vibration analysis.

Abstract: The energy transfer between two beams (signal and reference, respectively) writing a dynamic-volume hologram in photorefractive BSO crystals is applied to the image amplification of a diffuse object. The image intensity transmitted by the crystal is amplified 10 × in the presence of the pump reference beam. The crystal is used in the drift recording mode (applied electric field, E0 = 10 kV cm−1; fringe spacing, Λ = 3 μm), and beam coupling is induced by the nonlocal response of the crystal that is due to the fringe displacement at a constant speed. We have applied this two-wave mixing configuration to a real-time optical-processing operation; the related energy transfer and stationary image amplification permit the mode pattern visualization of a vibrating structure.

Electron-gas theory of ionic crystals, including many-body effects

Abstract: The electron-gas theory of ionic crystals is extended to include nonadditive (many-body) interactions arising from the simultaneous overlap of the densities of several ions. The theory is used to predict equilibrium geometries, lattice energies, and pressure-induced phase transitions for the fluorides and oxides of the alkali and alkaline earth metals. The results are in good agreement with electron-gas calculations assuming additive, two-body interactions between pairs of ions. This comparison shows that the simultaneous overlap of three or more ions has only a small effect on these crystals, and that for many purposes the simpler two-body approximation is adequate. The predictions of lattice distances and energies agree with experiment to a typical accuracy of 1 or 2%. In order to obtain this accuracy, it is necessary to calculate the electron densities for the anions in a stabilizing potential well whose size is determined self-consistently by the electrostatic potential of the crystal at the anion site. This shrinkage of the anion by the crystal is also a kind of many-body interaction, but it can be handled within the framework of an effective two-body interaction involving the stabilized anions. The corresponding cation expansion effect is also calculated, and shown to be negligible for the alkali and alkaline earth metal ions.

Zirconia ceramics and a method of producing the same

Abstract: of the Disclosure Zirconia ceramics consisting mainly of ZrO2 and Y2O3 in a molar ratio of Y2O3/ZrO2 of 2/98~7/93 and consisting of crystal grains having a mixed phase consist-ing essentially of tetragonal phase and cubic phase or having a phase consisting essentially of tetragonal phase, the average size of the crystal grains being not larger than 2 µm, has a high strength and is little in the deterioration of the strength due to the lapse of time, and is suitable as a solid electrolyte for oxygen concentration cell, machine parts for internal combustion engine, thermistor, cutting tool and other industrial materials.

Crystalline A-dna: the X-ray analysis of the fragment d(G-G-T-A-T-A-C-C).

Abstract: An A-DNA type double helical conformation was observed in the single crystal X-ray structure of the octamer d(G-G-T-A-T-A-C-C), 1, and its 5-bromouracil-containing analogue, 2. The structure of the isomorphous crystals (space group P6$_{1}$) was solved by a search technique based on packing criteria and R-factor calculations, with use of only low order data. At the present stage of refinement the R factors are 31% for 1 and 28% for 2 at a resolution of 2.25 angstrom (0.225 nm). The molecules interact through their minor grooves by hydrogen bonding and base to sugar van der Waals contacts. The stable A conformation observed in the crystal may have some structural relevance to promoter regions where the T-A-T-A sequence is frequently found.

Multicomponent Oxides in Oxygen Potential Gradients

Abstract: The steady-state demixing of initially homogeneous oxides in an oxygen potential gradient is studied theoretically and experimentally. In ternary oxides which are stable (i.e., not oxidized or reduced) in the oxygen potential range in question, the crystal between two different oxygen potentials is shifted with respect to the oxygen lattice system. If DA> DB≫ DO, the crystal is normally enriched in AO at the side of the higher oxygen potential. Solutions of the transport problem are presented and the predicted effects are verified experimentally. The following systems are studied: Complete oxide solid solutions (A, B)O; A-B-O systems with limited solid solution ranges and miscibility gaps; A-B-O systems with compound (e.g., spinel) formation.

Observation of selective desorption of one‐phonon inelastically scattered He atoms from a LiF crystal surface

Abstract: Measurements of angular distributions for scattering of low energy (≊20 meV) monoenergetic (Δv/v = 0.8% FWHM) He beams from a LiF (001) crystal along the 〈100〉 and 〈110〉 directions and from a NaF (001) crystal along the 〈100〉 direction reveal a large number of mostly small maxima and minima between the specular and diffraction peaks. Typical intensities of these features are about 10−3 of the specular peak. Time‐of‐flight spectra of the scattered atoms at the incident angles corresponding to the maxima reveal that the atoms were inelastically scattered by single phonons. From the known bound state energies for both systems He–LiF and He–NaF, we find that most of the maxima can be explained by a two step process in which the atoms are first elastically selectively adsorbed by diffraction into an in‐plane bound state and then are subsequently desorbed as a result of a single phonon inelastic process. From the observed half‐widths the lifetimes of atoms in the trapped states are found to depend on the vibrational quantum number and for the weakest bound state is as large as 60×10−12 sec. Evidence for minima involving selective adsorption into out‐of‐plane states is also presented.