Crystals Of Different Compositions Research Articles

Nanoscale investigation of polar structure of (1-x)PbMg1/3Nb2/3O3-xPbTiO3 single crystals

ABSTRACT (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) single crystals of different compositions were studied via piezoresponse force microscopy (PFM). Piezoelectric contrast has been observed in 0.9PMN-0.1PT above the temperature of structural transition indicating spatial distribution of polarization and was attributed to the existence of polar clusters. The domain patterns were found to change drastically with increasing Ti content. In particular, the piezoresponse images of 0.8PMN-0.2PT combine both relaxor and ferroelectric features and in 0.65PMN-0.35PT crystals only micron-sized ferroelectric domains were observed. The evolution of the polar structures with increasing temperature and under external electric field was also investigated. The nature of the observed phenomena is discussed based on the current understanding of the relaxor state in ferroelectrics and possible influence of PFM instrumentation

Defects and radiation induced electronic processes in magnesium aluminate spinel of different compositions

The time dependence of the formation and decay of irradiation-induced optical absorption centers in magnesium aluminate spinel single crystals of different compositions (MgO - 1.0Al 2 O 3 and MgO - 2.5Al 2 O 3 ) was investigated. The kinetics of accumulation of X-ray irradiation-induced absorption bands is consistent with the mechanism of trap filling with free charge carriers through the conduction band. The model includes Coulomb blocking effects on spatially correlated defects. The observed two-stage decay of absorption bands after termination of X-ray irradiation is explained by electron hole recombination between centers of two different distances and/or different potential barriers. UV-irradiation confirms the existence of charge exchange between complex spatially correlated defects.

Optical properties of nonlinear potassium lithium niobate crystals

Crack-free potassium lithium niobate (KLN) single crystals of different compositions have been grown by the resistance-heating Czochralski technique. The crystal structure was studied using synchrotron radiation X-ray powder diffraction, and the crystal compositions were determined by X-ray fluorescent spectroscopy. Homogeneous and crack-free samples of about 6×6×7 mm 3 were used to study the lattice vibration spectra by Raman and infrared reflection spectroscopy. The results show a strong influence of Li ions in the C sites of the KLN crystal on the characteristic Raman and infrared reflectivity spectra of the Nb–O octahedral ions. In particular, the symmetric bend vibrational mode, ν 5, is split into three Raman peaks; the antisymmetric bend vibrational mode, ν 4, in the infrared spectrum is split into three peaks as well. The antisymmetric stretch vibrational mode, ν 3, is broadened greatly and slightly split. The crystals obtained have good quality and improved second harmonic generation properties.

Kinetics of precipitation of surfactants. II. Anionic surfactant mixtures

AbstractPrecipitation kinetics were measured for calcium‐induced precipitation of mixtures of two anionic surfactants. The overall time required for precipitation to occur increased dramatically in specific ranges of compositions for the surfactant mixtures when compared to single components. Adsorption of the nonprecipitating surfactant onto the precipitate surface was shown to be responsible for this remarkable synergism. The higher the supersaturation of surfactant monomers, the more rapidly precipitation occurred. Under conditions where both surfactants were supersaturated, precipitation sometimes occurred in stepwise fashion, where crystals of different composition were formed with different induction times. Image analysis of the crystalline precipitate showed that crystal habit was affected when the two surfactants were mixed, indicating that processes such as adsorption and coprecipitation (most likely by inclusion) were occurring. When the crystals were allowed to age in solution for a period of 1 wk, the crystalline phase from the mixed surfactant solutions was found to separate into two types of crystals, which resembled week‐old crystals formed from single surfactant systems.

Optimization of lithium niobate for advanced applications by variation of extrinsic and intrinsic defect subsystems

A detailed study of magnetic resonance (EPR, ENDOR) spectra of lithium niobate (LN) crystals with different compositions, diverse modifiers and various probe impurities was carried out. Several new chromium and iron centers have been found. Possible impurity locations and charge compensation mechanisms for crystals of different compositions are proposed. The obtained results clearly show a quantitative and qualitative difference of impurity related material properties for congruent LN, crystals with extremely low concentration of intrinsic defects and magnesium doped LN. It is suggested that the strong interrelation found between subsystems of intrinsic and extrinsic defects can be used to optimize LN crystal properties for advanced applications.

Complexes in the Ni2+-imidazole-RN(CH2COO−)2 systems. The crystal structures of tris(imidazole)iminodiacetatonickel(II) monohydrate, hexa(imidazole)nickel(II) bis(N-methyliminodiacetato)nickelate(II) hexahydrate, and tetra(aqua)bis(imidazole)nickel(II) bis(N-benzyliminodiacetato)nickelate(II)

Crystals of different compositions, namely, [Ni(Ida)(Im)3] ⋅ H2O (I), [Ni(Im)6][Ni(Mida)2] ⋅ 6H2O (II), and [Ni(Im)2(H2O)4][Ni(Bida)2] (III), have been precipitated from aqueous solutions of the Ni2+-Lig 2−-Im systems, where Lig 2− is Ida, Mida, and Bida, respectively. The crystal structures of I–III are determined by X-ray diffraction analysis (R = 0.0307, 0.0348, and 0.0302 for 3061, 4706, and 2882 reflections, respectively). Crystals I are built of monomeric mixed-ligand complexes and molecules of crystallization water, which are interlinked by hydrogen bonds into a three-dimensional framework. In II and III, the ligands Lig 2− and Im form charged complexes separately. In II, the cationic and anionic layers of the complexes alternate along the c-axis. Numerous hydrogen bonds involving molecules of crystallization water link the layers into a three-dimensional framework. In III, the cationic and anionic complexes, which serve as proton donors and acceptors, respectively, are bound into layers parallel to the xy plane.

Physico‐chemical Characterization of a Novel Tricyclic β‐Lactam Antibiotic

GV118819X, a novel tricyclic β‐lactam antibiotic of GlaxoWellcome, is a racemic mixture of two diastereoisomers, A and B. Of the two diastereoisomers, only A is available as a pure compound. By analyzing mixtures of GV118819X and A, a partial phase diagram is constructed, which indicates the presence of a eutectic when the A fraction is ∼39%. Moreover, the melting enthalpies of the eutectic mixture and of diastereoisomer B can be estimated. With the exception of the pure A form, all mixtures undergo important modifications in morphology and microstructure as a consequence of thermal treatments, which induce melting/amorphization of the eutectic, and crystallization of the A form. Analyses of the sieved fractions of GV118819X demonstrate that it consists of acicular crystals of different composition, with the larger crystals having a larger A fraction than the smaller ones. Grinding causes melting/amorphization of the eutectic and, following hours‐long treatments, the formation of a substantial fraction of submicron particles with unusually low melting temperatures. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89: 232–240, 2000

Composition dependence of surface and bulk defects generated in CuZnAl single crystals after pseudoelastic cycling

Abstract The effect of composition on the microstructural evolution after fatigue is presented for CuZnAl single crystals. Fatigue associated to stress-induced martensitic transformations has been shown to induce several types of defects in poly and single crystals of CuZnAl alloys, the grain boundaries playing the main role in the polycrystals. In this work attention is paid to the formation of defects on the surface and in the bulk for single crystals of different compositions, and similar defects to those found for transformation temperatures M s around room temperature and e / a =1.48 are seen for e / a =1.43 but after a much smaller number of cycles. However, changes in the mechanical behavior are not observed at least up to 1000 cycles, even though the changes on the surface and in the bulk for the latter composition are striking.

Martensitic transformation in Ni-Mn-Ga alloys

Abstract Single crystals of different compositions in shape memory alloys Ni-Mn-Ga have been studied by electron and low temperature X-ray diffraction as well as by differential scanning calorimetry. It is shown that the cooling-induced martensitic phases are long-periodic ones modulated along the (110) directions by a transverse wave of atomic shifts with 5 and 7 atomic layers periodicity for the alloys studied exhibiting a martensitic transformation at 180 K and 446 K, respectively. The transformation heats appeared to be about 10 times different for both alloys.

Validity of the Lattice‐Parameter Vegard‐Rule in Cd1‐xZnxTe Solid Solutions

AbstractCd1‐xZnxTc crystals of different composition (0 ⩽ x ⩽ 1) were grown by the vertical Bridgman method and by synthesis in Te excess. After careful surface preparation of crystal slices, their Zn:Cd ratios were ascertained by wavelength‐dispersive X‐ray spectroscopy (WDXS), and the lattice parameters of the same slice regions were measured by X‐ray diffractometry. The Vegard rule concerning the linear lattice‐parameter dependence on composition is – in contrast with earlier literature data – exactly valid within the limits of error (Δx ⩽ ±0.01 and Δa/a ⩽ 3.7. 10−4) and follows the equation a(x) = (0.64822 ‐ 0.03792x) nm. After annealing some slices at different temperatures and controlled partial pressure conditions in ordcr to find possible phase separations or cation ordering effects, neither the lattice parameters were changed nor additonal interferences were found. A published occurrence of a rhomhohedral phase could not be confirmed by means of powder diffraction analysis. The results are discussed in relation to own electron diffraction investigations and to EXAFS literature data.

Lattice dynamics and related diffusion properties of intermetallics: I. Fe3Si

The phonon dispersion of iron-rich Fe-Si alloys of D03 structure (space group Fm-3m) has been studied by inelastic neutron scattering. The measurements were carried out on two crystals of different composition: Fe75Si25 at 20 and 930 degrees C, and Fe80Si20 at 20, 930 and 1100 degrees C. The respective degree of order was determined by powder diffraction measurements. A general decrease of phonon frequencies with increasing temperature is found, the temperature dependence being strongest for transverse phonons with ( xi xi 0) propagation. With decreasing order, phonon gaps close and the highest optical band degenerates into a broad distribution of inelastic intensity due to disorder scattering. The dependence of the dispersion on the alloy composition is not very pronounced. The migration enthalpies as well as several other thermodynamic quantities have been calculated from the densities of states. Although the low frequencies of the transverse phonons indicate particularly low migration barriers, they cannot explain the pronounced change of the diffusivity with composition. This composition dependence is tentatively explained by high vacancy concentrations.

Pyroelectric coefficients of LiNbO3 crystals of different compositions

Pyroelectric coefficients of LiNbO3 crystals of different compositions

Alloying effects on x-ray absorption edges in nickel-platinum single crystals

Electronic structure modifications of platinum and nickel produced by alloying these two metals were studied on a series of single crystals of different composition by X-ray absorption spectroscopy, using synchrotron radiation. Both Pt and Ni absorption edges show simultaneous structure changes. These modifications are located mainly several electronvolts above the Fermi level. The variation in hole number in the Pt 5d band was estimated by several analysis methods which all conclude to an electron transfer from nickel to platinum. Conversely, the modifications observed on the Ni K edge are situated at the neighborhood of E f and are interpreted as a population rearrangement between 3d, 4s, and 4p levels

CVT growth and characterization of Znl-xMnxCr204 single crystals

Single crystals of Znl-xMnxCr204 have been grown by the chemical vapour transport (CVT) technique using chlorine as the transport agent. The zinc chromite grew with little difficulty. The presence of manganese chromite liquid caused the ampoules to crack when the crystals rich in manganese were grown. In addition, when the spinels were grown with nominal starting compositions x > 0.5, the condensation of MnCI2 caused the crystals grown to be depleted in manganese. A thermodynamic study of the closed tube system has been done in order to determine the most favourable conditions for growth and calculate the ranges of temperature and chlorine pressure most suitable to avoid the segregation of manganese. Single crystals of different compositions were analyzed by X-ray diffractometric methods. Magnetic susceptibility measurements suggest a spin-glass-like behaviour which is explained by the existence of a cluster-glass state.

MARTENSITIC TRANSITION ENTROPY CHANGE AND ELASTIC CONSTANTS OF Cu-Al-Be ALLOYS

Measurements of the enthalpy (AH) and entropy (AS) changes associated with the martensitic transformation of Cu-Al-Be single crystals of different compositions, were performed by differential scanning calorimetry. In addition, the elastic constants of the body-centered-cubic parent phase single crystals were measured as a function of temperature, using the ultrasonic pulse-echo frequency varying - phase detection method, and the value of the elastic anisotropy (A=CdC7) was computed at the martensitic transformation temperature. The results are in good agreement with a theoretical expression obtained according to models previously proposed, which relates AS to the elastic anisotropy of the parent phase at the martensitic transformation temperature.

Optical and harmonic generation properties of barium sodium niobate crystals of different compositions

A comparison was made of three compositions of barium sodium niobate single crystals grown by the Czochralski method using Donets-1 apparatus with an improved stabilization of the output power of an hf oscillator. The absorption spectra were recorded. Measurements were made of the relative yield of the second harmonic of YAG:Nd laser radiation and of the absorption coefficient at the wavelengths of 0.63 and 1.15 μ. The best values of all three criteria were obtained for Ba2.09Na0.72Nb5.02O15 crystals.

Structure and some properties of niobium carbide single crystals

High-purity niobium carbide single crystals of different compositions within the range of homogeneity of the carbide were grown by crystallization from a melt. X-ray diffraction studies and metallographic examinations revealed that the single crystals obtained contained both perfect regions and regions distorted by thermal stresses and composition heterogeneities. As a result of x-ray diffraction studies, the presence of substructures of the first and second orders in the single crystals was established. Maximum disorientation was found to attain 0.2–0.5° for blocks of the first order and to be about 0.05° for blocks of the second order. The density of dislocations in the niobium single carbides was on the order of 105 cm−2. Within the limits of niobium carbide compositions investigated no differences in mechanical properties (microhardness and resistance to cracking) were detected. It was established that in single crystals of compositions NbC0.78-NbC0.82 room-temperature deformation took place over the {111} 〈110〉 system. The room-temperature fracture of the single crystals was mainly brittle in character.

The influence of the stoichiometry on the 153Eu nuclear spin-spin relaxation in EuO single crystals

The 153Eu spin-spin relaxation for two spherical EuO single crystals of different composition has been measured for two saturating field values of 2 and 6 T. The relaxation can be described by two time constants, a short one increasing with the magnetic field, arising from the Suhl-Nakamura coupling and a long one, due to the dipolar coupling, which is field independent. It is shown that the number of nuclei which are relaxed due to the dipolar coupling increases at increasing magnetic fields, in agreement with the Suhl-Nakamura theory. For the sample which is nearly stoichiometric the relative number of nuclei which is relaxed due to the SN coupling is much larger than for the sample which contains an excess of Eu atoms.

Diffusion of Br- ions in KCl-KBr mixed crystals

The diffusion of Br- ions in KCl-KBr mixed crystals of different compositions is studied at temperatures from 519 to 599°C by the capacitance method. In addition to the composition dependence of the activation energy associated with the diffusion process, the dependence of the chemical diffusion coefficient on composition is determined at different temperatures. The experimental results are in good agreement with those derived from the diffusion theory of non-uniform thermodynamics.

Lattice Constants for KMg(1−x)MnxF3 Crystals

The preparation of crystals of the form KMg(1−x)MnxF3 spanning the entire range of composition is reported. Crystal growth parameters are given. Lattice constants for nine crystals of different compositions were measured at room temperature using the Debye-Scherrer x-ray powder patterns. Measured values of the lattice constants conform to Vegard's law within experimental error.