Showing papers on "Crystal published in 1969"

Under what conditions can a glass be formed

Abstract: Summary Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, - T, the liquid volume, v], and the seed density, ps and upon a set of materials constants: the reduced crystal–liquid interfacial tension, α the fraction, f, of acceptor sites in the crystal surface, and the reduced glass temperature, Trg . The glass-forming tendency will be greater the larger are - T and Trg and the smaller are v]. ps, and f. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass temperature. T...

Significant precision in crystal structural details. Holly Springs hydroxyapatite

Abstract: The degree to which physically significant precision in crystal-structural details can be obtained with selected routine procedures has been assessed and some illustrative applications to analyses of bonding effects and of impurity substitutions have been made. Least-squares refinements with X-ray data for three single crystals of mineral hydroxyapatite and with neutron diffraction data for a fourth of the same origin yielded R = 2% in each case (_> 40 parameters adjusted and > 500 reflections). Seemingly minor extinction corrections improved the mutual agreement among separate measures of some parameters from 4a to the final la (typically <5%) found for all, even anisotropic thermal, parameters in the X-ray cases. Final results were insensitive to reasonable changes in the weighting scheme. Comparisons of X-ray and neutron results, which generally agreed within 2~r, showed systematic differences associated with the oxygen atoms bonded to phosphorus in the phosphate group. Analysis of the final R-value also suggested (1) real differences, among the crystals, smaller than the o-'s associated with individual parameters and (2) either a systematic inadequacy of the refinement model, or similar residual systematic errors (such as thermal diffuse scattering contributions), in both neutron and X-ray data. Direct refinement for the degree of fluorine substitution for OH in Cal0(PO4)6(OH)2 led to the same result, 8 at.% substitution, with both X-ray and neutron data. Analysis of the decrements found with both X-ray and neutron data in the apparent site-occupancy factors for the Ca atoms showed that a simple substitution of Mg2+ for Ca 2÷ at the same site is not in itself a sufficient substitutional model for this case.

Least‐squares refinement of the crystal and molecular structures of DNA and RNA from X‐ray data and standard bond lengths and angles

Abstract: tion, there are two other short contacts, namely N . . . O(1) (II0rI), 3.084 A and N . . . O(1), 2.752 A. The atom O(1) belongs to the same molecule while the atom O(1) (II0ia) lies approximately on the line C(2)-N. The arrangement of this latter type is very similar to that of carbonyl oxygen [O(1\")] around the N H + group in valine hydrochloride (Parthasarathy, 1966) or in 7-glycine (Iitaka, 1961).

Crystal-chemical characterization of clinopyroxenes based on eight new structure refinements'

Abstract: Comparison of new crystal-structure refinements for spodumene, LiFe3tSi,O" synthetic ureyite, acmite, diopside, augite, a CZ/c omphacite, and a PZ omphacite with published refinements for jadeite, johannsenite, NaIn'+Si,O" fassaite, and another PZ omphacite indicates that the structures are strikingly similar throughout the range of chemical compositions. The "end-member" clinopyroxenes are ordered within the limits of resolution. Bonding can be satisfactorily explained in terms of an essentially ionic model without invoking additional covalent effects. Except for PZ omphacites, the charge and size of the Mz cations determine the structure type: if Mz is Ca or Na, CZ/c is expected; if Mz is Li, CZ; and if Mz is Mg or Fe", PZ,jc or orthorhombic. Precise bond distances for ordered "end-member" clinopyroxenes can be used in determining T and M1 site contents of intermediate-composition clinopyroxenes, and ferrous iron is disordered in such clinopyroxenes. "Ideal" PZ omphacite, Cao.,Na,,.,Mgo.5Alo.,Si,O,, is expected to have Mg and AI fully ordered in the M1 octahedral chains, but in the Mz sites the maximum amount of order in alternating sites is jCa, j Na, and jNa, j Ca.

CZOCHRALSKI GROWTH AND PROPERTIES OF YAlO3 LASER CRYSTALS

Abstract: Czochralski growth characteristics of a new laser host crystal, YAlO3, and measurements of its optical, thermal, and mechanical properties are reported. Stimulated emission from Nd3+ in YAlO3 has been observed.

Ferroelectric Tungsten Bronze‐Type Crystal Structures. II. Barium Sodium Niobate Ba(4 + x)Na(2 − 2x)Nb10O30

Abstract: Ferroelectric Ba(4+x)Na(2 − 2x)Nb10O30, with a Curie temperature of 833°K, has a tungsten bronze‐type structure and crystallizes in the orthorhombic system, with subcell lattice constants a = 17.59182 ± 0.00001, b = 17.62560 ± 0.00005, and c = 3.994915 ± 0.000004 A at 298°K. The space group is Cmm2, and there are two formulas in the subcell. The c axis of the true cell is double that of the subcell. The integrated intensities of 6911 reflections within a reciprocal hemisphere of radius (sinθ)/λ = 1.02 A−1 were measured with PEXRAD, 1873 symmetry‐independent structure factors being significantly above background. The metal atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from metal‐phased Fourier series. The final agreement index between measured and calculated structure factors is 0.0579. The structure differs only in detail from previously determined tetragonal tungsten bronze structures. In the general formula (A1)2(A2)4C4(B1)2(B2)8O30, the B1 and B2 sites are fully occupied by Nb, the A2 sites by Ba and the A1 site by 87.0% Na and 6.5% Ba. Evidence from chemical analysis, x‐ray density calculations and the present determination suggests that the best approximation to the formula of the crystal studied is Ba4.13Na1.74Nb10O30. The Ba and O atoms at z ≃ 12 are disordered in a manner similar to the O atoms in the Ba layer in barium strontium niobate. The four crystallographically independent Nb atoms, each in octahedral coordination, are linked to O atoms by distances ranging from 1.765 ± 0.021 to 2.270 ± 0.021 A, with a mean value of 1.989 A. Ba is 10 coordinated, with Ba–O distances ranging upward from 2.671 ± 0.013 A. Na is 12 coordinated, with Na–O distances ranging from 2.660 ± 0.014 to 2.990 ± 0.015 A, with a mean of 2.788 A. The Nb‐atom displacements from the mean oxygen planes lie in the range 0.171–0.205 A; the parabolic relation with Curie temperature predicts a displacement of 0.204 A. The measured value of Ps at room temperature is 40 μC cm−2: the linear relation between displacement and polarization predicts a saturation value of 44–53 μC cm−2. All metal atoms are displaced from the oxygen planes in the sense given by the macroscopic positive polarity.

Anisotropy of the Optical Constants and the Band Structure of Graphite

Abstract: Detailed experimental results have been obtained on the reflectivity of both natural and pyrolytic graphite single crystals in the visible and ultraviolet regions. Results have been taken for both $E\ensuremath{\perp}c$ and $E\ensuremath{\parallel}c$ at near-normal incidence, and as a function of angle of incidence for radiation having its electric vector both perpendicular and parallel to the plane of incidence. These latter results, used in conjunction with an analysis of the Fresnel relations for anisotropic materials, permit extremely accurate values of the optical constants to be obtained both parallel and perpendicular to the crystal planes. The results show that ${k}_{z}$, the absorption index perpendicular to the cleavage planes, is identically equal to zero at all energies below about 5 eV, as expected from the two-dimensional approximation of the energy bands in graphite. A three-dimensional calculation is performed to explain the above picture. The optical constants here obtained explain the electron-energy-loss experiments on graphite and require the absence of the low-energy plasmon along the $c$ axis. The newly developed Fresnel analysis presented here can find general application to all highly anisotropic materials.

On the Rates of Growth of Grains and Crystals in South Polar Firn

Abstract: The size of firn crystals as a function of age has been investigated in thin sections to a depth of 49 m at the South Pole. Grain cross-sections increased in size from 0.24 mm2 at 0.1 m depth to 0.63 mm2 at 10 m. Crystals, as distinct from grains, increased in size from 0.18 to 0.43 mm2 over the same interval, implying that grains are generally composed of just one or two crystals rather than several as is frequently contended. The mean crystal cross-section increased linearly with the age of the firn at a rate of 0.0006 mm2 year−1; in 388 year old firn at 49 m the crystal size measured 0.63 mm2. Analysis of crystal-growth data from other locations in Antarctica and Greenland also revealed a strong linear relationship between the mean cross-sectional arcas (D 2) of crystals (in mm2) and their ages in years (t), i.e. . The fact that the temperature dependence of the crystal growth rate K can be expressed very satisfactorily in an equation of the form K = K 0 exp (E/RT) confirms predictions that crystal growth in firn is essentially analogous to grain growth in metallic and ceramic sinters. An extrapolation of available data indicates that crystal growth rates in dry firn could be expected to vary by two orders of magnitude (0.0003 to 0.03 mm2 year−1) over the temperature range −60° to −15°C. A method of utilizing crystal growth-mean annual temperature data to determine accumulation rates in snow is demonstrated.

The Shape and Riming Properties of Ice Crystals in Natural Clouds

Abstract: Individual ice crystals have been collected and replicated. The effect of the falling motion is found to enhance the growth along the “c” axis of columns and the dendritic growth of plane ice crystals. It is found that the onset of riming of ice crystals is controlled by crystal size; for columnar crystals the most sensitive parameter is the length along the “a” axis, while for plane crystals it is the diameter. Freely-falling ice crystals are oriented so as to present the maximum resistance to motion. Some consideration is also given to the collection efficiency of columnar ice crystals for cloud droplets.

EFFICIENT, TUNABLE OPTICAL EMISSION FROM LiNbO3 WITHOUT A RESONATOR

Abstract: We report the first observation of tunable stimulated optical emission from a nonlinear crystal without the use of an external resonator, and with greater than 50% conversion of the pump beam to the tunable frequency. Opposite faces of a LiNbO3 crystal were polished flat and parallel to provide for multiple reflections of the signal radiation (Stokes) inside the crystal. The A1 symmetry 248‐cm−1 polariton mode was excited with a Q‐switched ruby laser, and the signal frequency was tuned by varying the angle of incidence of the laser beam relative to the normal to the crystal surface. Operation was at room temperature; no crystal damage was observed over most of the tuning range; and laser depletion occurred within the first few nanoseconds of the Q‐switched pulse. The difference between the Stokes frequency and laser frequency was varied from 42 to 200 cm−1, and the corresponding idler wavelength was varied from 238 to 50 μ.

Lattice vibrations and the accurate determination of structure factors for the elastic scattering of X‐rays and neutrons

Abstract: Two basic assumptions are usually made by crystallographers in the evaluation of X-ray or neutron structure factors. The first is the validity of the harmonic approximation in deriving the Debye–Waller factors which account for the effect of lattice vibrations in reducing the intensities of the Bragg reflexions. The second assumption is that the contribution to the measured intensity of thermal diffuse scattering (TDS), which rises to a peak at the reciprocal lattice point, can be ignored. Anharmonicity and TDS can each give rise to appreciable intensity effects, and so must be allowed for in accurate work. Anharmonicity can be taken into account by treating the crystal as a system of independent anharmonic oscillators, with each atom vibrating in a potential field whose symmetry conforms with the site symmetry of the atom. The effect of TDS can be calculated approximately using first-order (one-phonon) harmonic scattering theory, together with a knowledge of the elastic constants of the crystal. Calculations of both types of correction are considered in detail for a cubic crystal, and are applied to the analysis of X-ray diffraction measurements on KCl and of neutron measurements on BaF2.

Nucleation and Growth of Crystals at High Supersaturation

Abstract: Homogeneous nucleation followed by diffusion controlled crystal growth was observed in highly supersaturated solutions of BaSO4, BaCrO4, BaMoO4, BaWO4, BaCO3, PbSO4, PbCrO4, PbCO3, SrSO4, and CaWO4. In the range of concentrations where the number of homogeneously nucleated particles is larger than the number of heteronuclei the induction period decreases from 1 to 10−4 sec, and the number of particles found per cm3 increases from 107 to 1012 with increasing concentration. Theoretical maps showing the rate determining mechanisms of crystal growth are drawn for different values of the physical parameters, primarily crystal size, supersaturation ratio, and interfacial tension. The techniques used for rapid mixing and for film recording are described.

Infra-red absorption due to localized modes of vibration of impurity complexes in ionic and semiconductor crystals

Abstract: A brief account is given of the theory of the vibrations of an imperfect crystal containing isolated impurities and pairs of impurities, and the strength of the induced infra-red absorption is discussed. An analysis is then made, by the methods of group theory, of the energy levels and expected optical transitions of an anharmonic localized oscillator vibrating in a static potential well with various point symmetries. These results are compared with the available experimental data for pairs of impurities and more complicated impurity aggregates in ionic crystals with the fluorite and rocksalt structures, and also for similar defects in the covalent crystals silicon, germanium and some III–V compound materials.

Facetting and optical perfection in Czochralski grown garnets and ruby

Abstract: Studies of ruby (Al2O3/Cr3+) and rare-earth aluminium garnet single crystals, in particular the mixed garnets formed between Y3Al5O12 (YAG) and Dy3Al5O12 (DyAG), have shown that the formation of facets on the solid/liquid interface, which give rise to a strained central core within the crystals, is dependent upon the shape of the solid/liquid interface. Development of the strained and facetted core can be prevented by modifying the growth conditions to produce a flat solid/liquid interface and as a result the optical perfection of the crystals is greatly improved. Certain crystals, e.g. DyAG, grow naturally with aflat interface, and in the present work this has been shown to be due to the optical absorption characteristics of this material. In other materials, e.g. YAG and ruby, the interface shape can be controlled by the rate at which the growing crystal is rotated. The changes in temperature gradient produced in a YAG melt by changes of crystal rotation rate have been measured, and their effect upon crystal perfection is described.

Single crystals of poly(L-glutamic acid)†

Abstract: Lamellar single crystals of alkaline earth salts of poly(L-glutamic acid) have been grown by precipitation from dilute aqueous solution and studied by optical and electron microscopy and by x-ray and electron diffraction. The calcium, strontium and barium salts were crystallized in the β form above room temperature and could be converted to crystals of β-poly(L-glutamic acid) by washing in dilute hydrochloric acid. The magnesium salt, on the other hand, was crystallized in the α form at or below room temperature but could not be converted into crystals of α-poly(L-glutamic acid) by washing in hydrochloric acid. The crystalline lamellae are very thin (thicknesses range from 25 to 60 A in β crystals and are about 100 A in α crystals) and the polypeptide chains are oriented normal to the planes of the lamellae. It is clear from the disparity between crystal thickness and molecular length that the molecules crystallize by folding at the upper and lower surfaces of the crystals. Conformations of the molecules at these folds are discussed briefly.

The Crystal and Molecular Structure of Carbazole

Abstract: The crystal structure of carbazole has been refined with three-dimensional X-ray data. Single crystals obtained from the melt are orthorhombic with four molecules in a unit cell of the dimensions a=7.772, b=19.182 and c=5.725 A; space group is Pnma. Positional and thermal parameters were obtained by the least-squares method. The final discrepancy index R is 10.9% excluding zero intensity data. The e.s.d.’s in bond lengths not involving hydrogen atoms are 0.006–0.010 A, and the corresponding errors in bond angles are about 0.5°. The molecule is planar within the deviation of 0.03 A. A crystallographic mirror plane normal to the b-axis passes through the nitrogen atom and is perpendicular to the molecular plane. Comparison of the observed and theoretical bond lengths is made with discussion thereof. Some physical properties related to the crystal structure are also discussed by comparing its structure with that of fluorene. With the help of thermodynamical data, it has been argued that the hydrogen bond of ...

Monte Carlo Simulation of Vacancies in Rare‐Gas Crystals

Abstract: The thermodynamics of vacancy formation in rare‐gas crystals is studied by using Monte Carlo simulation of a many‐body system. In the calculations, we assume a pairwise‐additive Lennard‐Jones potential and measure the change in Helmholtz free energy associated with reversibly adding a particle to a crystal containing a single vacancy. The addition is carried out by varying a coupling parameter joining one particle to the others in the crystal. Results for both 32‐ and 108‐particle systems show that in a macroscopic (rare‐gas) crystal near the triple point (1) the fraction of lattice sites vacant is about 1/3000, and (2) relaxation of particles neighboring a vacant site is less than 1% of an equilibrium interparticle spacing. These calculations are in excellent agreement with the earlier work of Glyde.