Crystals Of Different Compositions Research Articles

Mg in plagioclase: Experimental calibration of a new geothermometer and diffusion coefficients

The temperature-sensitive exchange of Mg between plagioclase (Pl) and clinopyroxene (Cpx) has been studied experimentally, accounting for different anorthite-contents in plagioclase (XAn) and various silica activities (aSiO2) in the system. The partitioning of Mg between plagioclase and clinopyroxene was determined over a temperature range of 1100–1200°C, using plagioclase single crystals of different compositions (XAn=0.5–0.8), surrounded by different clinopyroxene-bearing matrix powders to account for different silica activities from 0.55 to 1.0. The experimental design also allows the diffusivity (DMgPl) of Mg in plagioclase under these conditions to be determined. Both KMgPl/Cpx (defined as KMgPl/Cpx=CMgPl/CMgCpx) and DMgPl decrease with temperature and increase with aSiO2. Isothermal data for different XAn in plagioclase show a linear increase of lnKMgPl/Cpx with increasing XAn, but DMgPl appears to be insensitive to XAn. The partitioning data allow a new geothermometer to be calibrated, which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist:T[K]=(-9219+2034XAn)/(lnKMgPl/Cpx-1.6-lnaSiO2).Application of this geothermometer to experimental data from this study reproduces the experimental temperatures within ±20°C. Diffusion of Mg in plagioclase is described byDMgPl[m2s-1]=1.25×10-4[m2s-1]·exp(-320,924[J mol-1]/(RT))·(aSiO2)2.6.

Pulse Shape Discrimination in Impure and Mixed Single-Crystal Organic Scintillators

Neutron/gamma pulse shape discrimination (PSD) utilized for detection of high-energy neutrons with organic scintillators was investigated using a model system of mixed diphenylacetylene-stilbene single crystals of different compositions. The results of the studies, which include experimental tools of crystal growth and characterization combined with computer simulation, showed that the presence of impurities with lower bandgap energies can be a major factor influencing PSD properties of organic materials. Depending on the concentration, an impurity may suppress or increase the rate of excited triplet state interaction leading, respectively, to a complete disappearance or enhancement of PSD, consistent with a percolation threshold. The results are applied to produce novel materials with controlled decay characteristics. Single crystals with a large fraction of delayed light and enhanced PSD have been grown for high energy neutron detection, while crystals with suppressed delayed light were produced for use as low-afterglow scintillators for energetic neutron detection in time-of-flight experiments.

Determination of the effective coercive field of ferroelectrics by piezoresponse force microscopy

The effective coercive field Ec for ferroelectric domain reversal is usually determined in a capacitor-like geometry by increasing an applied electric field until poling occurs. Here we present a different method based on local poling with the tip of a scanning force microscope and analyzing the dependence of the domain size on the poling parameters. This method for determining Ec is of importance because for many samples the standard technique fails, either because they are too small in size, or because they are slightly conductive. Results obtained on lithium niobate crystals of different composition conform to literature values.

Influence of BaTiO3 Content on the Structure and Properties of Na0.5Bi0.5TiO3 Crystals

Lead‐free Na0.5Bi0.5TiO3–x%BaTiO3 single crystals of different compositions (x=0, 2, 4, 4.5 and 5.5) were grown using a top‐seeded solution growth method. Temperature‐dependent X‐ray diffraction revealed the presence of a high‐temperature tetragonal phase that existed over a narrow composition range of 0<x<2, at temperatures far in excess of the ferroelectric Curie temperature Tc. Polarized light microscopy images confirmed the presence of tetragonal domains in NBT that persisted until Tc+250°C; whereas for x>4, rhombohedral domains were found that persisted on heating to only about Tc‐150°C. Investigations of the properties revealed a dramatic increase in the dielectric constant (upto 500%), a decrease in the coercive field (300%), and an increase in the longitudinal piezoelectric constant (1000%) with modest increases in x, as the high‐temperature tetragonal ferroelastic phase was suppressed and as the morphotropic‐phase boundary approached.

Dynamic Holography Method for Nondestructive Testing of Optical Homogeneity of Transparent Media

The potential of the dynamic holography method for testing the phase optical homogeneity of media transparent in visible light has been considered. This method is based on imaging samples in the scheme of two-wave mixing of the second-harmonic radiation of a Nd:YAG laser and image amplification in a photorefractive barium-strontium niobate crystal. Examples of identifying phase inhomogeneities at a level of 10-5 cm-1 and less in crystals of different compositions are presented.

Micro- and nanostructures in lithium niobate single crystals doped with lanthanides

Lithium niobate single crystals doped with lanthanides (Gd, Er) and nominally pure single crystals of congruent and stoichiometric compositions have been grown under time-dependent thermal conditions. Regular growth domain microstructures and periodic nanostructures have been investigated by optical microscopy and atomic force microscopy with a step from 10 to 100 nm. Comparative investigations of the Raman spectra of lithium niobate single crystals of different compositions have been performed.

Raman spectra of photorefractive lithium niobate single crystals

We have conducted comparative studies of the Raman spectra of lithium niobate (LiNbO3) crystals of different compositions for excitation in the visible and near IR regions. We have observed that the photorefractive effect is one of the factors leading to line broadening. For this reason, the linewidths may be greater upon Raman excitation in the visible region than for excitation in the near IR region. This may be explained by formation in the crystal, when illuminated by laser radiation in the visible region, of a three-dimensional sublattice of nanostructures and microstructures (with refractive index and other physical parameters different from the parameters of the host crystal) from which photorefractive light scattering occurs. Formation of nanostructures and microstructures makes an additional contribution (besides the contribution due to random and dynamic disorder in the arrangement of the structural units) to the broadening of the Raman lines in the visible region of the spectrum. Illumination of the crystal by radiation in the near IR region does not induce a sublattice of nanostructures and microstructures, due to a significantly smaller photorefractive effect.

Characterization of gel grown iron‐manganese‐cobalt ternary levo‐tartrate crystals

AbstractIn the present investigation, iron–manganese–cobalt ternary levo‐tartrate crystals of different compositions have been grown by single‐diffusion gel growth technique in silica hydrogel medium. The metallic composition in the crystals was estimated by EDAX. The coloration of the crystals changed with composition of metallic content. The powder XRD study suggested the crystalline nature and indicated the presence of some extra phases. The grown crystals were characterized by FT‐IR spectroscopy, TGA, dielectric and Vibrating Sample Magnetometer (VSM) studies. The FT‐IR study suggested the presence of O‐H, C=O, C‐O and metal‐oxygen bonds. The effect of composition of metallic content was observed in certain absorption regions in FT‐IR spectra. The thermal stability of the crystals was studied by thermogravimetry and the kinetic and thermodynamic parameters of dehydration were calculated. The effect of composition of ternary levo‐tartrate was observed in dielectric study. The dielectric study was carried out in the frequency range of applied field from 500 Hz to 1 MHz. The variations in dielectric constant, dielectric loss, a.c. resistivity and a.c. conductivity with frequency of applied field were studied. VSM study suggested that all crystals were of paramagnetic nature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Manifestation of the specific structural features of lithium niobate single crystals of different composition in their Raman spectra

The Raman spectra of nominally pure lithium niobate single crystals of congruent, close to stoichiometric, and stoichiometric compositions and congruent lithium niobate single crystals doped with Gd3+, Y3+, and Mg2+ ions have been investigated. Weak lines whose widths anomalously decrease with an increase in cation sublattice disordering at a change in the single crystal composition were found for the first time. These lines may be indicative of fine ordering processes involving structural units of the cation sublattice, as a result of which this sublattice is disordered as a whole.

Effect of gamma irradiation on the optical spectra of undoped, Mg-doped, and Gd-doped LiNbO3 single crystals

We have measured the optical spectra of gamma-irradiated lithium niobate crystals of different compositions: nominally undoped and doped with rare-earth (Gd) and alkaline-earth (Mg) metals in the range 0.003–0.27 wt %. The difference in behavior between gamma-irradiated and vacuum-annealed nominally undoped lithium niobate single crystals has made it possible to elucidate the mechanisms underlying the formation of electronic and point defects in the crystals and to develop a model for the processes involved. The effects of gamma dose, doping level, and the nature of the dopant on the optical absorption of gamma-irradiated doped lithium niobate single crystals are analyzed.

Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition

Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO· nAl 2O 3 by 20 keV Ar + ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6)×10 17 ions/cm 2 was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg + ions in the 4s 2S excited state and Al atoms in the 5p 2P 0 excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardment-induced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO· nAl 2O 3 ( n=1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets.

NaBi(WO4)2: In Cherenkov crystals for electromagnetic calorimetry

NaBi(WO4)2:In (called NBW) colorless Cherenkov crystals were synthesized and studied. A decrease in the concentration of color centers provides a 50-nm blueshift of the optical transmission spectrum of NBW crystals. Upon exposing these crystals to gamma rays at a dose of 3 · 107 rad, the optical transmission spectra in these crystals remain almost the same. The energy resolution (calculated by the Monte Carlo method) of calorimeters based on NBW crystals of different compositions is discussed.

Understanding the scintillation efficiency of cerium-doped LSO, LYSO, YSO and LPS crystals from microwave study of photoconductivity and trapping

Photoconductivity and trapping effects were studied in various Ce-doped scintillating crystals (LSO, LYSO, YSO and LPS) by means of the microwave resonant cavity technique. In all the investigated crystals, at 300K and under 355nm excitation, transitions from the 4f ground state of Ce3+ ions to low energy levels of the 5d excited configuration give rise to a photoconductivity signal. Temperature- and excitation power-dependent measurements indicate that the photoionization mechanism results from the competition between thermal activation and two-photon (two-step) absorption. However, depending on the crystals, the relative weight of these two possible routes leading to electronic delocalization varies greatly. Moreover, four mixed Lu2−xYxSiO5:Ce (LYSO) crystals of different compositions and with various scintillation yields show different behaviors, in-between Y2SiO5:Ce (YSO) and Lu2SiO5:Ce (LSO). Our measurements concerning trapping and photoconductivity dynamics show that trap-related perturbations affecting the electronic transport during the scintillation process is a major source of losses for the light yield of these scintillating crystals.

Quantitative evaluation of the electro-optic effect and second-order optical nonlinearity of lithium tantalate crystals of different compositions using Raman and infrared spectroscopy

Nominally pure lithium tantalate (LiTaO3) crystals with different chemical compositions were characterized by Raman scattering, infrared reflection, and infrared absorption spectroscopy. In these crystals specific phonon bands are correlated to electro-optic and nonlinear optical properties. The electro-optic and nonlinear optical coefficients were determined and the data are compared with literature values obtained by direct measurements. Our measurements are in good accordance with the literature data, which opens up a new possibility for fast and easy estimation of electro-optic and nonlinear optical performance of newly developed LiTaO3-based materials modified by stoichiometry.

Tip-induced nanostructuring of alloy surfaces with an electrochemical scanning tunneling microscope

Tip-induced nanostructuring with an electrochemical scanning tunneling microscope (EC-STM) was applied to alloy surfaces in order to resolve the reasons for the unusual stability of tip-induced metal clusters to anodic oxidation. Therefore Au thin films on glass and AuCu-alloy single crystals of different composition and surface orientation were used as substrates. The experiments give evidence that the three main factors for cluster formation and stability are the attractive interaction between tip and substrate, alloy formation during the cluster formation process as well as additional stabilization by adsorption of an underpotential deposition layer. Experimental results on the three aspects are presented and discussed.

Characteristic features of the change of domain structure in mixed [(NH4)1−x Rbx]3H(SO4)2 crystals in the vicinity of superprotonic phase transition

X-ray diffraction measurements of mixed [(NH4)1−xRbx]3H(SO4)2 crystals of different compositions are performed. The characteristics of the change of the domain structure in the vicinity of the supeprotonic-ferroelastic phase transition in crystals of different compositions are studied and compared with the variations of the crystal structure in the course of gradual substitution of ammonium by rubidium. This phase transition is theoretically described based on the phenomenological theory of a high-temperature phase transition.

Manifestation of a photorefractive effect in Raman spectra of lithium niobate crystals of different compositions

The ordering of structural units in a cation sublattice and the photorefractive properties of lithium niobate single crystals of different compositions: nominally pure with different [Li]/[Nb] ratios and doped with non-photorefractive cations Mg2+, Gd3+, and Y3+, are studied by their Raman spectra. It is shown that at low concentrations of Mg2+, Gd3+, and Y3+, the magnitude of the photorefractive effect is determined by the ordering of the structural units of the cation sublattice. It is found for the first time that the intensity of a Raman line corresponding to the bridge valence vibrations of oxygen atoms in the NbO6 octahedra is sensitive to the dipole ordering of the cation sublattice.

Infrared imaging of calcified tissue in bone biopsies from adults with osteomalacia

Osteomalacia is a pathological bone condition in which there is deficient primary mineralization of the matrix, leading to an accumulation of osteoid tissue and reduced bone mechanical strength. The hypothesis that there are no qualitative or quantitative differences in osteomalacic bone mineral or matrix compared to disease-free bones was tested by examining unstained sections of polymethyl methacrylate (PMMA) embedded iliac crest biopsies using Fourier transform infrared imaging (FTIRI) at approximately 6-μm spatial resolution. Controls were seven female subjects, aged 36–57, without apparent bone disease. The experimental group consisted of 11 patients aged 22–72, diagnosed with osteomalacia. The spectroscopic parameters analyzed in each data set were previously established as sensitive to bone quality: phosphate/amide I band area ratio (mineral content), 1660/1690 cm−1 peak ratio (collagen cross-links), and the 1030/1020 cm−1 peak ratio (mineral crystallinity). The correspondence between spectroscopic mineral content (phosphate/amide I ratio) and ash weight was validated for apatite crystals of different composition and crystallite size. The FTIRI results from the biopsies expressed as color-coded images and pixel population means were compared with the nonparametric Mann–Whitney U test. There were no significant differences in the cortical parameters. Significant difference was found in the mineral content of the trabecular regions with a lower mean value in osteomalacia (P = 0.01) than in controls. Mineral crystallinity tended to be decreased in the trabecular bone (P = 0.09). This study supports the hypothesis that, in osteomalacia, the quality of the organic matrix and of mineral in the center of bone does not change, while less-than-optimal mineralization occurs at the bone surface. This study provides the first spectroscopic evaluation of whole bone mineral and matrix properties in osteomalacia, demonstrating that there are few differences in collagen cross-links between biopsies from patients with osteomalacia and from individuals without histological evidence of bone disease.

Optical transitions in magnesium aluminate spinel crystals of different compositions exposed to irradiation

Optical absorption of magnesium aluminate spinel single crystals of different compositions MgO · nAl 2O 3 ( n=1.0 and 2.5) were investigated after irradiation with neutrons, X-rays and the UV-light to elucidate existence of specific absorption bands related to lattice defects. The radiation induced absorption envelope is shifted to higher energy and is larger in intensity in non-stoichiometric spinel in comparison with stoichiometric one at the same irradiation conditions. Deconvolution of difference optical spectra of irradiated crystals shows the shift of the F-type centers from 4.75 and 5.3 eV in stoichiometric crystals to 5.08 and 5.63 eV in MgO · 2.5Al 2O 3 for F +- and F-centers, respectively. The absorption bands at 3.1 and 3.78 eV are present in both stoichiometric and non-stoichiometric crystals and are identified as V-type centers. By changing the irradiation time and using isochronal annealing it was revealed that additional bands appear in optical absorption spectra at 4.15 eV for MgO · 1.0Al 2O 3 crystals and at 4.46 eV for MgO · 2.5Al 2O 3 crystals. From the shift of the band energy position and the high intensity of this band in non-stoichiometric spinel crystals, it could be identified with electronic centers related to lattice defects.

Composition dependence of the OH-stretch-mode spectrum in lithium tantalate

The infrared absorption spectrum of the OH-stretch-mode in lithium tantalate is measured for crystals of different compositions. The spectrum exhibits two prominent lines which are representative for ideal and for Li-deficient sites in the crystal, respectively. Position, shape, and intensity of the two lines thus depend on crystal composition in a characteristic way. A comprehensive quantitative description is given which shows that the technique can be used as a sensitive non-destructive tool to rate the composition of lithium niobate. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)