Showing papers on "Crystal published in 1971"
TL;DR: In this article, a review of solid-state photochemistry can be found under four parallel headings: first, analysis of the topochemical postulate according to which the course of the solid state reaction and the stereochemistry of the photodimer (if any) can be predicted from the configuration and nearest-neighbour geometry of closest monomer molecules in the crystal lattice; secondly, the study of the lOCUS of the reaction, that is the dependence of the course or (dimerization, cis trans isomerization) on crystal texture (dislocation
Abstract: Following a brief historical review of solid-state photochemistry current research is discussed under four parallel headings: first, analysis of the topochemical postulate according to which the course of the solid-state reaction and the stereochemistry of the photodimer (if any) can be predicted from the configuration and nearest-neighbour geometry of closest monomer molecules in the crystal lattice; secondly, the study of the lOCUS of the reaction, that is the dependence of the course of the reaction or (dimerization, cis — trans isomerization) on crystal texture (dislocation, grain and phase boundaries); thirdly, a better understanding of the packing principles of organic molecules in their crystal structure to enable us to construct photoreactive (or lightstable) crystal structures of any given monomer as well as mixed crystals of potentially codimerizable monomers for synthetic purposes or energy-transfer studies. Several approaches to this problem of 'crystal engineering' based on generalizations of packing modes of primary amides and of dichiorophenyl derivatives are outlined, and their application to a systematic photochemistry of the solid state discussed.
1,644 citations
TL;DR: In this paper, a rectangular prism with edges in principal crystal directions is prepared with electrodes on the corners of one face, and voltage-current ratios for opposite pairs of electrodes permit calculation of components of the resistivity tensor.
Abstract: A rectangular prism with edges in principal crystal directions is prepared with electrodes on the corners of one face. Voltage‐current ratios for opposite pairs of electrodes permit calculation of components of the resistivity tensor. The method can use small samples, and is best suited to materials describable by two or three tensor components. Examples are given of measurements of V2O3–Cr and oriented amorphous graphite.
574 citations
TL;DR: In this article, it was shown that overlap and covalency make the dominant contributions to the observed field, the electrostatic contributions only being inportant for the potential components with low angular dependence (i.e., the n=2 parameters).
Abstract: Recent progress in understanding the origin of the lanthanide crystal field is summarized. The basic assumption of the crystal field parametrization is shown to be that the crystalline environment can be represented as a one-electron potential, and the consequences of removing this assumption are traced. It is further shown that overlap and covalency make the dominant contributions to the observed field, the electrostatic contributions only being inportant for the potential components with low angular dependence (i.e. the n=2 parameters). In some circumstances it is found that the observed parameters can usefully be analysed into superposed contributions from the neighbouring ions in the crystal. The importance of crystal field concepts in related problems is emphasized as well as the stimulus crystal field theory gives to the development of formalisms for dealing with non-orthogonal basis states.
568 citations
TL;DR: The physical properties of many of the rare-earth intermetallic compounds have been discussed in terms of the role that the magnetic exchange and crystal field interactions play in determining these properties.
Abstract: The physical properties of many of the rare-earth intermetallic compounds have been collected together. They are discussed in terms of the role that the magnetic exchange and crystal field interactions play in determining these properties. It is pointed out that in this vast number of materials there is an ideal chance of establishing which of several second-order terms are effective in determining structural stability.
405 citations
TL;DR: In this article, the x-ray scattering by two different rutile crystals, within a hemisphere of reciprocal space having radius (sin θ) /λ=1.02 A−1, was measured at 298°K with PEXRAD.
Abstract: The x‐ray scattering by two different rutile crystals, within a hemisphere of reciprocal space having radius (sin θ) /λ=1.02 A−1, was measured at 298°K with PEXRAD. The integrated intensities of 780 reflections from each crystal were determined, resulting in 136 symmetry independent Fmeas from Crystal 1 and 122 Fmeas from Crystal 2. The crystal structure was refined by the method of least squares: The agreement factor R for all Fmeas is 0.0286, and the single position parameter x(0)=0.30479±0.00010. Normal probability plot analysis shows the least squares derived standard deviations are underestimated by 33%, and the two sets of Fmeas contain an appreciable parallel bias. The lattice constants, measured at or corrected to 298°K on two different crystals, are a=4.593659±0.000019 and c=2.958682±0.000008 A. The Ti–O distances are 1.9800±0.0009 and 1.9485±0.0005 A. Pairs of shorter bonds define O–Ti–O angles of 81.21° and 98.79°, forming a plane normal to the two longer bonds. The resulting octahedral Ti–O bo...
277 citations
TL;DR: In this article, an analysis of the Knoop indentation process is presented which establishes a clear relationship between the effective resolved shear stress (t 0 9), in the bulk of the crystal beneath the indenter, and the observed hardness.
Abstract: The results of this work, and those published by other researchers who have used Knoop indentation measurements, confirm that the nature of anisotropy in hardness is essentially determined by the crystal structure and the primary slip systems which accommodate dislocation motion during indentation. Materials belonging to the same class of crystal structure and having common slip systems possess similar anisotropic properties. The varying extent of work-hardening or fracture, associated with indentations, does not appear to influence the anisotropy— although twinning on the basal planes of hexagonal closepacked metals may have a significant effect. An analysis of the indentation process is presented which establishes a clear relationship between the ‘effective resolved shear stress’ (t 0 9), in the bulk of the crystal beneath the indenter, and the observed hardness. Directions which correspond to the minimum values of t9 e , on specific crystallographic surfaces, are those of maximum hardness and conversely. The analysis is shown to be equally applicable to a wide range of crystalline solids including nonmetallic materials, of various crystal structure, and typical f.c.c., b.c.c. and c.p.h. metals. Finally, anisotropy in hardness can be used to identify active slip systems in those crystals where it is possible for dislocations to move on more than one system.
232 citations
TL;DR: In this article, the elastic constants and the dispersion curves for phonons propagating along (0,0, xi ) and ( xi, xi, 0) are calculated using a rigid ion model with short range central axially symmetric forces and long range Coulomb forces which gives satisfactory agreement with the experimentally observed frequencies.
Abstract: For Pt. II see abstr. A71824 of 1970. The polarized infrared reflection absorption and laser Raman spectra have been recorded at both 300K and 100K for single crystals of SnO2. The dielectric parameters epsilon ' and epsilon " have been derived and the optically active transverse and longitudinal k=0 phonon frequencies found. The elastic constants and the dispersion curves for phonons propagating along (0,0, xi ) and ( xi , xi ,0) are calculated using a rigid ion model with short range central axially symmetric forces and long range Coulomb forces which gives satisfactory agreement with the experimentally observed frequencies. The principal force constants and the effective charges on the ions are compared with those for other crystals with the rutile structure. The multiphonon infrared absorption is compared with theoretical predictions based on selection rules and the calculated phonon dispersion curves. The hydroxyl ion impurities are shown to be oriented parallel to the crystal's optic axis.
226 citations
TL;DR: In this article, the x-ray data of three homologous series of poly(n-alkyl acrylates, poly(N-nalkyl-acrylamides) and poly(vinyl esters) was used to determine the temperatures of fusion, melting transitions, and derived entropies of homopolymers.
Abstract: Heats of fusion, melting transitions, and the derived entropies of fusion were obtained by differential scanning calorimetry for examples from three homologous series of homopolymers having long side chains. Homopolymers having side-chain lengths between 12 and 22 carbon atoms were chosen from the poly(n-alkyl acrylates), the poly(N-n-alkyl-acrylamides) and the poly(vinyl esters). The data demonstrated that only the outer paraffinic methylene groups were present in the crystal lattice. This was concluded because phase diagrams obtained for mixtures of structurally different monomers and homopolymers, as well as for selected copolymers, showed only isomorphism in the polymeric examples. In addition, scanning curves, reflecting the distribution of crystallite sizes, became narrower as the side chains became longer. The critical chain length required to maintain a stable nucleus in the bulk homopolymers was a constant value for each homologous series. It varied between 9 to 12 carbon atoms. When heats of fusion were determined in the presence of methanol, main-chain restraints were freed, thus permitting more methylene groups to enter the crystal lattice. Hence, the heats of fusion, the crystallinity, and melting points increased above that of the bulk state. The magnitude of the contribution to the heats of fusion by each methylene group indicated that the hexagonal paraffin crystal modification prevailed in these homopolymers, in agreement with x-ray data from the literature.
215 citations
TL;DR: In this paper, the 36 k=0 modes of vibration were classified using group theoretical methods using kramers-kronig theory and the dielectric parameters were derived from the reflection spectrum using kramer-krusig theory.
Abstract: This paper reports polarized infrared and raman studies on zircon, zrsio4. The 36 k=0 modes of vibration are classified using group theoretical methods. The dielectric parameters epsilon minutes and epsilonseconds have been derived from the reflection spectrum using kramers-kronig theory. Measurements have also been made on the elements of the raman polarizability tensor, and the angular dependence of the scattered intensity has been studied for several bands. The crystal field splittings of the vibrational levels of the silicate ions are large, but some simple force constant considerations lead to the conclusion that the silicate ions are essentially molecular units.
212 citations
TL;DR: In this article, the origin of laser-induced refractive changes (laser damage) in LiNbO3 has been identified as iron impurities, which were measured using holographic techniques as the iron impurity content of crystals was varied.
Abstract: The origin of laser‐induced refractive changes (laser damage) in LiNbO3 has been identified as iron impurities. Quantitative measurements of the damage were made using holographic techniques as the iron impurity content of crystals was varied. Holographic diffraction efficiencies from as high as 0.44 to lower than 10−6 have been achieved with crystals 0.2 cm thick, using 10 W/cm2 of 5145‐A radiation, both by varying the iron impurity concentration and by varying the valence state of these impurities. The previously observed effects of field annealing, crystal coloration, oxidation and reduction, and the role of OH‐ ions are all accounted for.
168 citations
TL;DR: In this paper, a model for the surface structure of rutile is proposed, based on infra-red studies of a crystalline sample prepared by the combustion of Ti(iso-PrO)4.
Abstract: A model for the surface structure of rutile is proposed, based on infra-red studies of a crystalline rutile sample prepared by the combustion of Ti(iso-PrO)4. It is suggested that the exterior surfaces of the rutile crystals correspond to three low index crystal planes—namely the (100), (101) and (110). Of these the first two are capable of adsorbing molecular water as ligand coordinated to Ti4+ surface ions, whereas the (110) crystal face adsorbs water dissociatively leading to the presence of equal quantities of two types of OH– ions. One of these types is associated with a surface Ti4+ ion which is five coordinate with respect to lattice oxide ions whereas the other type is bound to a surface Ti4+ ion which is only four fold oxide ion coordinate. It is possible to rationalize the observed thermal dehydroxylation and dehydration properties of the oxide and also account for its pyridine adsorption properties.
TL;DR: In this article, it was shown that even if the waves produced correspond to a uniform distribution of directions of wave vector, there may be a large enhancement of energy flow in some crystallographic directions compared to the average.
Abstract: The generation of thermal acoustic waves by a heated metallic film on the surface of a dielectric crystal at low temperatures is discussed. It is shown that even if the waves produced correspond to a uniform distribution of directions of wave vector, there may be a large enhancement of energy flow in some crystallographic directions compared to the average. This effect arises because of elastic anisotropy. Explicit expressions are given for the enhancement expected in the principal directions of cubic crystals.
TL;DR: In this article, an edge-defined, film-fed growth (EFG) technique is described, by which continuous single crystals having virtually any cross-sectional shape can be grown from the melt, by a thin melt layer sandwiched between the growing solid liquid interface and the plane top surface of the rigid capillary structure.
TL;DR: In this paper, the dielectric constants and refractive indices of a 5PbO · 3GeO2 crystal, which was grown from the melt by by the Czochralski method, were measured as a function of temperature.
Abstract: Dielectric constants and refractive indices of a 5PbO · 3GeO2 crystal, which was grown from the melt by by the Czochralski method, were measured as a function of temperature. The crystal was identified as a new type of ferroelectric with spontaneous polarization Ps = 0.046 C/m2, coercive field Ec ∼ 2.3 × 106 V/m at room temperature, and Tc = 177°C.
TL;DR: In this article, the optical and nonlinear optical properties of a new nonlinear crystal CdGeAs2 have been presented for second harmonic generation and parametric interactions over most of its transparency range from 24 to 18 μm.
Abstract: In this paper, we present the optical and nonlinear optical properties of a new nonlinear crystal CdGeAs2 The chalcopyrite structure (42 m) crystal is positive birefringent with Δn equal to 01 It is phasematchable for second harmonic generation (SHG) and parametric interactions over most of its transparency range from 24 to 18 μm In addition, it has the largest known nonlinear coefficient for a phasematchable crystal with the exception of tellurium The measured nonlinear coefficient is 34 times GaAs The calculated doubling efficiency for SHG of a CO2 laser is 10 times that of tellurium The crystal promises to be a very useful nonlinear material for parametric interactions between 3 and 18 μm
TL;DR: In this paper, it was shown that under favorable conditions these crystal planes can be identified, and their extent estimated, from photoelectric emission measurements on heterogeneous nickel surfaces, and that these surfaces are usually not sufficiently homogeneous to permit the work function of the preferred orientation to be determined accurately.
TL;DR: In this article, a tubular, annular, or ribbon-shaped capillary orifice is used to maintain the liquid level constant during growth and to determine the shape of the growing crystal.
TL;DR: In this paper, the deuteron-ordered structure of D2O ice II has been shown to be stable in the donor O···O−O angles, but not exactly symmetrical in the O−O −O angles.
Abstract: D2O ice II has a fully deuteron‐ordered structure in which two crystallographically independent types of water molecule form a tetrahedrally linked network of H bonds, with the deuterons lying near but not on the O···O centerlines. Orientation of the molecules is nearly, but not exactly, symmetrical in the donor O···O···O angles. The D–O–D angles (average, 105.4°) do not differ significantly from the angle in D2O vapor (104.5°) although the corresponding O···O···O angles are smaller (average, 93.8°). Agreement between D–O–D and O···O···O angles is improved about 6° (on average) by a distortion of the bond network, from symmetry R3c to R3. The approach to a match between D–O–D and O···O···O angles is only partly responsible for the existence of proton ordering in ice II. Twinning of R3 individuals is established by variations in certain diffraction intensities from crystal to crystal. Mean O–D bond length (0.98 A) is slightly longer than in the vapor (0.957 A). Variations among the individual O–D distan...
TL;DR: In this paper, the thermal conductivity of an infinite, one-dimensional harmonic crystal is investigated in a model system for which exact analytic results can be obtained, and it is shown that thermal conductivities approach infinity as least as fast as N 1/2.
Abstract: Energy transport is investigated in a model system for which exact analytic results can be obtained. The system is an infinite, one‐dimensional harmonic crystal which is perfect everywhere except in a finite segment which contains N isotopic defects. Initially, the momenta and displacements of all atoms to the left of the defect region are canonically distributed at a temperature T, and the right half of the crystal is at a lower temperature. This initial nonequilibrium state evolves according to the equations of motion, and ultimately a steady state is established in the vicinity of the region containing the defects. The thermal conductivity is calculated from exact expressions for the steady state energy flux and thermal gradient. For a crystal in which the N isotopic defects are distributed at random but in which the overall defect concentration is fixed, we demonstrate that the thermal conductivity approaches infinity as least as fast as N1/2. A Monte Carlo evaluation of the thermal conductivity for a given defect‐to‐host mass ratio and concentration is carried out for a series of random configurations of N defects for N in the range, 25 ≤ N ≤ 600. The thermal conductivity is proportional to N1/2 within the statistical uncertainty except for slight deviations at the smallest values of N.
TL;DR: In this article, the detailed structure within the two-dimensional projection of the unit cell of a Ti2Nb10O29 crystal, showing the arrangement of the metal atoms, was revealed by careful use of a high-resolution electron microscope.
Abstract: The detailed structure within the two‐dimensional projection of the unit cell of a Ti2Nb10O29 crystal, showing the arrangement of the metal atoms, has been revealed by careful use of a high‐resolution electron microscope. The image shows good agreement with the model of the structure deduced from x‐ray diffraction studies.
TL;DR: In this paper, a simple sample technique which makes it possible to record Raman spectra of highly absorbing solids is described, where the main feature is a rotating sample system containing pressed crystal powders, on which the laser beam is focused.
Abstract: A simple sample technique which makes it possible to record Raman spectra of highly absorbing solids is described. The main feature is a rotating sample system containing pressed crystal powders, on which the laser beam is focused. The relative motion between laser focus and sample surface avoids heating and decomposition of the crystal powder. With this technique it is, for instance, possible to obtain a Raman spectrum of solid potassium permanganate, displaying a progression of strongly resonance enhanced overtones of the totally symmetric permanganate mode as well as a well-defined weakly enhanced spectrum for the other internal modes.
TL;DR: In this paper, a method of computing the rate of growth of nonspherical ice crystals by deposition is described, and predictions of growth characteristics over the temperature range of −1 through −35C are shown and compared with meager experimental data available within this temperature range.
Abstract: A method of computing the rate of growth of ice crystals by deposition is described. Predictions of growth characteristics over the temperature range of −1 through −35C are shown and compared with meager experimental data available within this temperature range. The growth rate equations are based on Fick's law of diffusion. In applying this law to nonspherical ice crystals, the analogy between an electrostatic potential field surrounding a charged body and the vapor-density field surrounding a similarly shaped ice crystal is utilized. Allowances are made for effects of forced convection caused by the movement of the crystal through the air and the change in shape and bulk density of the ice crystal; these effects are predicted to be of significant importance. A simple, empirical equation using two temperature-dependent parameters was found to describe the crystal growth rate as a, function of mass. For many cases where only the rate of mass conversion from vapor to solid is required, the simple ...
TL;DR: In this article, an n-component substitutional solid solution in a perfect crystal can be characterized uniquely by n−1 independent composition functions, and the SRO parameters are linearly but not quadratically independent, which can be used to reduce the uncertainty relative to the interpretation of diffraction experiments realized with less than the optimum number of radiations.
Abstract: An n-component substitutional solid solution in a perfect crystal can be characterized uniquely by n−1 independent composition functions. There are n(n−1)/2 distinct short-range-order parameters αij(i,j = 1,2,... n−1) corresponding to these (n−1) independent compositions. The SRO parameters are linearly but not quadratically independent: for example, in ternary systems, the following equation holds Σ[(1 − \overline c_{1}) (1 − \overline c2)α11α22 − \overline c1 \overline c2α12α21] = 0 in which \overline c1 and \overline c2 are the average compositions pertaining to atomic species 1 and 2, the summation extending over all significant correlation ranges in the crystal. This relation can be used to reduce the uncertainty relative to the interpretation of diffraction experiments realized with less than the optimum number of radiations.
TL;DR: CdSiP2 as discussed by the authors is a typical member of the AIIBIVC2V family with chalcopyrite structure and the lattice constants of this tetragonal crystal are a
Abstract: CdSiP2 is a typical member of the AIIBIVC2V family with chalcopyrite structure. The x‐ray scattering for luminescent and nonluminescent material is essentially identical: The atomic order at the Cd and Si sites is at least 99.8% complete. The lattice constants of this tetragonal crystal are a = 5.680 ± 0.001 and c = 10.431 ± 0.003 A at 298°K. The space group is I42d, and there are four formulas per unit cell. A total of 2370 reflections from a nonluminescent crystal, and 2156 from a luminescent CdSiP2 crystal, were measured with PEXRAD. Least squares refinement, based on 320 symmetry‐independent reflections from both crystals, led to a final agreement factor of 0.047. Normal probability plot analysis shows the least squares derived standard deviations to be underestimated by a factor of 3.0, and the two sets of measured structure factors to possess a small parallel bias. The bond angles about the Si atom do not differ significantly from the regular tetrahedral angle of 109.47°. Regular tetrahedral bond a...
Patent•
08 Feb 1971TL;DR: Algebraic combination of the frequencies of two or more selected modes of piezoelectric crystal resonator vibrations yields a total frequency signal output which has a substantially zero temperature coefficient of frequency.
Abstract: Algebraic combination of the frequencies of two or more selected modes of piezoelectric crystal resonator vibrations yields a total frequency signal output which has a substantially zero temperature coefficient of frequency.
TL;DR: In this article, it was found that the intensity of a reflection from a polyoxymethylene single erystal decays exponentially after a short period, of reproducible duration, when the form of decay is variable.
Abstract: Irradiation in the electron microscope causes many changes in crystalline polymers, but the most easily measurable of these is the change in the diffraction pattern from single crystals. It has been found that the intensity of a {1010} reflection from a polyoxymethylene single erystal decays exponentially after a short period, of reproducible duration, when the form of decay is variable. The intensity of all electrons emerging from the column except those assosciated with the directly transmitted beam behaves in the same way. The period of variable decay corresponds to the period when diffraction contrast remains visible on the dark-field image. The decay of diffraction from polyethylene is more complex because radiation damage produces long-range disorder in the lattice, and this is accompanied by motion of the crystal, even when it is firmly supported on a carbon film. Measurement of these decay curves have provided the basis for comparison of radiation damage rates at different beam voltages a...