Crystals In Question Research Articles

Re-investigation of ‘minasgeraisite-(Y)’ from the Jaguaraçu pegmatite, Brazil and high-temperature crystal chemistry of gadolinite-supergroup minerals

AbstractThe chemical composition (including B, Be and Li), the Raman spectrum and the crystal-structure evolution (at the temperature range 27–1000°C) of a Mn-bearing, Bi-rich gadolinite-subgroup mineral from the Jaguaraçu Pegmatite, Brazil (type-locality of minasgeraisite-(Y)) was studied. Elemental mapping revealed that the crystal investigated has complex chemical zonation with various Bi (~8–24 wt.% Bi2O3), Ca (~8–10 wt.% CaO) and Y (~11–17 wt.% Y2O3) content. The sample investigated has all the specific features of the chemical composition of minasgeraisite-(Y), except Ca excess and, thus, should be considered as hingganite-(Y). The Raman spectrum of the sample under study has bands at 140, 179, 243, 350, 446, 519, 559, 625, 902, 973, 3224, 3353, 3532 and 3763 cm–1, and is similar to that of hingganite-(Y) / -(Nd). Crystal-structure refinement confirmed that the crystal in question should be considered as hingganite-(Y) and is in line with the previously obtained data on gadolinite-subgroup minerals from the Jaguaraçu Pegmatite. High-temperature single-crystal X-ray diffraction studies revealed that the mineral starts to decompose above 800°C. We can conclude that beryllosilicates are most stable at high-temperature conditions within the gadolinite supergroup and that species with a higher M-site occupancy have higher stability upon heating.

Ultra-precise surface processing of LYSO scintillator crystals for Positron Emission Tomography

The Lutetium-Yttrium Oxyorthosilicate (LYSO) is one of the most widely used scintillation crystal in the high-performance Positron Emission Tomography (PET) systems. The quality of the surface finish of the LYSO has an important impact on the light output, the decoding performance, the energy resolution and timing resolution of the PET detectors and systems. In this paper, we present an ultra-precise method for processing the surface of LYSO crystals. The hardness and elastic modulus of the crystals were initially measured using Nano indentation technology. The scintillators were fixed onto the plate in sparse, serried and continuous arrangements and polished using an alumina (Al2O3) and cerium oxide (CeO2) polishing solution with particles of varying size. We used a magnetorheological-polishing technique to polish the LYSO crystals. The polishing solution here included hydroxyl iron powder and hard abrasives. The hardness and elastic modulus of the crystals in question was, respectively, 11.18 ± 0.50 and 155.78 ± 4 gigapascals (GPa). A 3D optical surface profiler (3D-OPS) and an atomic force microscope (AFM) were used to evaluate the quality of the polished surfaces. The average roughness of Ra 0.55 nm measured by 3D-OPS was achieved using a precise plate grinding and polishing technique. The magnetorheological-polishing method also obtained an excellent roughness of Ra 0.75 nm (3D-OPS). Our report of the use of these processing technologies can serve as a foundation for further in-depth research regarding the optimal techniques for scintillator surface processing.

Quantum-chemical analysis of the thermodynamic isotope effect in quasi-one-dimensional H-bonded Pb(H/D)PO4 ferroelectrics

The thermodynamics of the structural phase transition of H-bonded ferroelectric materials, Pb(H/D)PO4, were considered in terms of the pseudo-spin Ising model with inclusion of tunneling and longrange effects. The pseudo-spin Hamiltonian parameters needed for analysis of the transition were determined by a procedure based on an independent quantum chemical method. A simplified scheme for the selection of model clusters was proposed, which allows the application of various quantum chemical methods, including high-level methods (CCSD/6-311+G** and so on), in the calculations of double-well potential profiles and Slater parameters. The calculation results were discussed in terms of two statistic models: molecular field approximation (MFA) and Bethe cluster method (BCM). The theoretical estimates of critical transition temperature for both systems are discussed and it is shown that the (semi)quantitative reproduction of experimental data is possible only in terms of BCM taking into account the tunneling effects. The explanation is given for the observed isotope effect caused by very pronounced increase in the critical transition temperature upon deuteration (ΔTс ≈ 140 K). The crucial role belongs to the difference between tunneling effects in the ferroelectric crystals in question. It is emphasized that the observed differences between the crystal lattice and H/D bond geometries, including the mutual orientation of the bonds, must be accurately included in the calculations.

Optical properties of phenanthrene: A DFT study for comparison between linear and nonlinear regimes

Abstract The present study tries to determine the optical characteristics as well as the electronic structure of phenanthrene as an important nonlinear organic crystal. We have performed our calculations within the frame work of DFT. Also, we have used bootstrap exchange-correlation kernel (within the framework of TDDFT) to estimate the excitonic effects. According to the results of our study, the investigated crystal has a band structure with low dispersions which is a sign of low intermolecular interactions. In addition to the high values of linear and nonlinear susceptibilities, the crystal in question has a wide range of transparency as well as sufficient anisotropy which make it promising crystal for nonlinear optical applications. Our TDDFT calculations show that the influence of excitonic effects on optical properties can be very dramatic, particularly near the band edge. In addition, the crystal in question shows extremely small wavelengths of plasmon peaks. Furthermore, this study also covers the 2ω/ω intra- and inter-band contributions to the dominant nonlinear susceptibilities. Findings indicate that these contributions have opposite signs at higher energies and nullify each other. Our calculations show that χ x x z , χ x z x and χ z x x have largest values of nonlinear response but χ x x z is the dominant component at IR-VIS region. Moreover, the current study shows significant similarities between linear and nonlinear spectra, when we draw linear one as a function of both ω and 2ω. Finally, our simulation reproduces the experimental results very well.

Understanding molecular crystals with dispersion-inclusive density functional theory: pairwise corrections and beyond.

CONSPECTUS: Molecular crystals are ubiquitous in many areas of science and engineering, including biology and medicine. Until recently, our ability to understand and predict their structure and properties using density functional theory was severely limited by the lack of approximate exchange-correlation functionals able to achieve sufficient accuracy. Here we show that there are many cases where the simple, minimally empirical pairwise correction scheme of Tkatchenko and Scheffler provides a useful prediction of the structure and properties of molecular crystals. After a brief introduction of the approach, we demonstrate its strength through some examples taken from our recent work. First, we show the accuracy of the approach using benchmark data sets of molecular complexes. Then we show its efficacy for structural determination using the hemozoin crystal, a challenging system possessing a wide range of strong and weak binding scenarios. Next, we show that it is equally useful for response properties by considering the elastic constants exhibited by the supramolecular diphenylalanine peptide solid and the infrared signature of water libration movements in brushite. Throughout, we emphasize lessons learned not only for the methodology but also for the chemistry and physics of the crystals in question. We further show that in many other scenarios where the simple pairwise correction scheme is not sufficiently accurate, one can go beyond it by employing a computationally inexpensive many-body dispersive approach that results in useful, quantitative accuracy, even in the presence of significant screening and/or multibody contributions to the dispersive energy. We explain the principles of the many-body approach and demonstrate its accuracy for benchmark data sets of small and large molecular complexes and molecular solids.

An interactive process in the mechanism of the thermally stimulated luminescence of anion-defective α-Al2O3 crystals

The role played by deep traps in the appearance of specific features of the thermally stimulated luminescence (TL) of anion-defective α-Al2O3 single crystals is reported. The existence of deep traps has been proved by direct observations of the TL peaks associated with these traps. From the experimental evidence it is adduced that the degree of occupancy of the deep traps affects the basic features of the main TL peak at 450K. A model of an interactive trap system is proposed. The main difference of this model from those described in the literature is that it allows for the temperature dependence of the probability of the charge carriers to be captured at the deep traps. The proposed model was used to calculate the basic TL parameters of the main peak (the TL output and the shape of the glow curve) of the crystals in question as a function of the heating rate and the occupancy of the deep traps. The calculated values are similar to those observed experimentally.

Optical and electric properties of crystals with alternate structures

Calculation of the electronic structure as well as of optical and electric properties of alternate crystals is discussed in terms of the tight-binding method. The crystals in question are of the A, B type where the electron state bands arise as a result of interaction between nearest neighbors. General formulas have been derived for the calculation of dielectric functions. A simple and effective scheme of parameterization of optical transition integrals has been developed.

Phenomenology of a Devil's Staircase

Abstract A phenomenological approach is used to discuss the successive phases of a crystal corresponding to various values of the wave-vector for normal coordinates belonging to one optical branch of the crystal's phonon spectrum. Phase diagrams are constructed in terms of the variables which determine the minimum of the branch, i.e. the soft mode. The effect of external forces of different symmetry on the diagram is clarified. Comparisons are made with experimental data for some crystals in which a sequence of phase transitions, with the participation of an incommensurate phase, is observed. The crystals in question are thiourea, betaine calcium chloride dihydrate, tetramethylammonium metal tetrahalogenides and potassium selenate.

The tuning algorithms used by the Donner 600 crystal tomograph

The authors describe the computer algorithms used to adjust the energy thresholds and timing delays in the Donner 600 Crystal tomograph. The adjustments are made using 1890 computer controlled digital-to-analog converters (DACs) and an orbiting positron source. The energy threshold for each crystal is adjusted by measuring the counting rate of each crystal-crystal coincidence as a function of the DAC settings that control a pulse height window for the crystal in question. The DAC settings that correspond to the 511-keV photopeak are noted, thus determining the conversion from DAC setting to energy and allowing the DACs to be set to any desired energy window. The DAC settings controlling the timing delay for each channel are systematically adjusted to maximize the overall event rate. The authors discuss the convergence of the tuning algorithms and report on the photomultiplier tube gain and timing variations over a period of 18 months. >

Gruneisen parameters of some cubic crystals using third-order elastic constants

The third-order elastic constants of 26 cubic crystals and two mixed crystals belonging to octahedral class have been evaluated from the available literature data on the hydrostatic pressure variation of the corresponding second-order constants using the method elaborated [O. H. Prasad and M. Suryanarayana Acustica 38, 192–193 (1977)] wherein the third-order constants can be evaluated without any reference to uniaxial stress measurements. The evaluated third-order constants for each of the crystals are used in the well-known equations [W. P. Mason and T. B. Bateman, J. Acoust. Soc. Am. 36, 644–652 (1964)] to obtain the 39 mode gammas all of which add up to give the Gruneisen parameter γ of the crystal in question. The correctness of the evaluated parameters is verified by evaluating the thermal expansivity α of each crystal using the familiar equation connecting α with γ The crystals studied are NaCl, NaBr, NaF, LiF, KCl, KBr, KI, KF, RbCl, RbBr, RbI, AgCl, AgBr, MgO, CsBr, Nb, Fe-α, Ag, Cu, Au, Al, Ge, Si, ZnSe, ZnTe, RbAg4I5, and mixed crystals AgCl-AgBr and KCl-KBr near room temperature and in a few cases at different temperatures and pressures.

From finite to infinite crystals: Analytic solution of simple tight binding model of finite sc, fcc and bcc crystals of arbitrary size

The electronic structure of finite sc, fcc and bcc crystals having a rectangular parallelepiped geometry is investigated here. In all three cases, analytic formulae for the energies and wave functions are obtained. It is shown that the results for the fcc and bcc lattice in the tight binding first nearest neighbour approximation can directly be obtained from those for the sc lattice in the third nearest neighbour approximation. Further it is shown for the crystals in question that the concept of the Brillouin zone has a good meaning not only for the infinite crystals but also for the finite ones. The mutual relation of real and Born-Karman boundary conditions and the transition finite-infinite crystal in both directions are investigated and discussed in detail.

Kinetics of the motion and multiplication of dislocations in potassium chloride

The kinetic characteristics of the motion and multiplication of dislocations in KCl are analyzed. The assumption to the effect that the two processes act in parallel is discussed and given a sound basis. Accepting this assumption, certain physico-mechanical characteristics of the crystals in question may be calculated. It may in particular be shown that the mean distance between the centers of dislocation multiplication in the shear stress range 100–160 g/mm2 is independent of the applied stress and approximately equals 75μ.

The electric field in a circular semiconductor plate placed in a magnetic field

The current distribution in an isothermic isotropically conducting plate of circular form is investigated theoretically and experimentally, in the absence and in the presence of an external magnetic field that is perpendicular to the plate. The general solution of the Riemann-Hilbert boundary value problem has been obtained under these conditions. The analysis of this solution points to experimental possibilities of determining parameters of a crystal under consideration such as the specific electric conductivity (in the absence and in the presence of an external magnetic field), the mobility of current carriers in it, and others. All the basic results of the calculations undertaken were experimentally verified and quantitatively confirmed in a series of tests carried out on homogeneous monocrystalline n-germanium (with the specific resistivity of 1.1 ohm cm) at room temperature. It is known that investigations into the galvanomagnetic phenomena (longitudinal and transverse magneto-resistance, the usual, planar and longitudinal Hall effects and others) at the present time constitute not only a means of determining the characteristics of the parameters of the crystals in question (concentration of current carriers, their mobility, etc.) [1], but serve also as a proven and simple means of obtaining important information about the zone structure of crystals [2–5]. Such broadening of the circle of problems affecting the sphere of galvanomagnetic investigations already begins not to correspond to the established traditions of carrying out these investigations on test pieces of rectangular shape (as a rule, in the form of parallelepipeds). This lack of correspondence is greater due to a number of completely logical causes, certain requirements as to the geometrical dimensions of such test pieces (the ratio of length to width) [6] can far from always be satisfied. We note in this connection that in the study of galvanomagnetic phenomena in impulsive magnetic fields, for example, the use of test pieces of circular form would simplify the use of working volumes of small diameter. This, in the final analysis, is equivalent to broadening the scale of magnetic fields that can be used. The replacement of a rectangular plate by a circular disc enables us also to simplify a measurement of the parameters of semiconductor crystals which usually are obtained in circular form. Below we present theoretical and experimental investigations into the problem of measuring the galvanomagnetic effects in conducting crystals having a circular form.

Optically Active Lattice Vibrations in Wurtzite-Type Crystals of Zinc Oxide and Cadmium Sulfide

By the use of a set of equations previously derived by Tsuboi and a set of Raman data recently published by Damen, Porto, and Tell, a set of reasonable values of force constants in the wurtzite-type ZnO and CdS crystals has been obtained. It was shown that, in the infrared-active A1- and E1-type vibrations, an appreciable amount of electrostatic force is involved besides the short-range interatomic forces through the chemical bonds. Such an electrostatic force is considered to be caused by a macroscopic polarization due to an in-phase displacement of the ions in the crystal in question, and this should not be involved in the infrared-inactive E2-type vibrations. The amount of this particular electrostatic force has also been estimated.

The effect of microsegregation on the generation of dislocations in zinc single crystals

The applicability of Tiller's considerations on the production of dislocations is proved. The density of dislocations appearing during impurity microsegregation increases with increasing rate of growth as a consequence of the corresponding change in the effective distribution coefficient. The real value of ΔC at the microsegregation boundaries is at least twice as great as the average value of the concentration of impurities in the crystal in question.

The study of crystal growth with the electron microscope III. Growth-step patterns and the relationship of growth-step height to molecular structure in n -nonatriacontane and in stearic acid

The exact relationship between molecular association in solution and growth-step height was obtained from measurements of shadowcast electron micrographs. Crystals of the paraffin n -nonatriacontane, C 39 H 80 , give a value of growth-step height of 45 ± 10 Ȧ, the unimolecular value. Similarly, crystals of stearic acid, C 17 H 35 COOH, give a value of 47 ± 10 Ȧ, the bimolecular value. There is thus strict correspondence between the unimolecular nature of n -nonatriacontane molecules in solution and unimolecular n -nonatriacontane growth-steps and between the bimolecular nature of stearic acid molecules in solution and bimolecular stearic acid growth-steps. In crystals of both compounds, grown from a dislocation of length any multiple of the length of the units in solution, multimolecular growth-steps were not encountered. Instead the growth pattern dissociated into a succession of unimolecular steps in n -nonatriacontane crystals and bimolecular steps in stearic acid crystals. There is thus strong evidence that the size of the unit in solution and not the length of the parent dislocation is the responsible factor in deciding the height of growth-steps. The crystals examined show a variety of growth-step patterns—spirals, layers, halfsteps and flat-tops. In layered crystals which show no sign of spiral growth there is strong internal evidence that the crystals have in fact grown by a spiral mechanism. A series of micrographs showing crystals in a state of transition makes it clear that the screw dislocation responsible for growth has been cancelled out by the formation of an equal and opposite dislocation. Numerous examples of this cancellation process have been encountered either at the end of all growth or as a relapse when one of the dislocations in a pair of the same sense has been cancelled during growth. This evidence for the growth and movement of dislocations in real crystals shows that crystals with molecularly flat surfaces may well be produced by a mechanism based on screw dislocations. The fact that no spiral growth-steps can be observed is no longer adequate proof that a spiral growth mechanism did not produce the crystal in question.

On the Presence of Free Sodium in Sodium Chloride Crystals Containing Color Centers and Color Center Precursors

The presence of ``free sodium'' in NaCl crystals containing color centers and color center precursors, as indicated by the results of Hacskaylo and Groetzinger, has been confirmed by the use of a more precise method of chemical analysis. The present measurements were carried out by dissolving the crystals in question in a solution containing an indicator, p-nitrophenol, and determining the resulting change in the optical density of the solution at a wavelength of 415 mμ. The ratio of the number of ``free sodium'' atoms to vacancy pairs was about one for crystals which had been subjected to electrolysis and about one-tenth for crystals irradiated with x-rays, independent of whether these crystals had been first subjected to electrolysis.