New schemes for recording electron diffraction patterns of hexagonal and monoclinic crystals

Some new schemes for recording electron diffraction patterns of hexagonal crystals rotating around the axes lying in the (hk0) plane of the reciprocal lattice and monoclinic crystals rotating around the a and b axes of the direct lattice and the a* axis of the reciprocal lattice have been developed. Formulas for interpreting electron diffraction patterns are reported. The electron diffraction patterns obtained based on these schemes were used to solve the 2H and 3R polytypes of CdInGaS₄ crystals and the 3R polytype of Zn_1.5In₃Se₆ crystal with the parameters a = 4.046 and c = 59.292 Å, sp. gr. R3m.     

A contribution to the indexing of transmission electron diffraction patterns

A method for the indexing of TED spot patterns is presented. The reduced cell of the TED pattern defined by the two shortest vectors, which enclose an acute angle, serves as reference cell for the indexing. The d-values of both the cell defining vectors and of their sum and difference vectors are determined being the four largest d-values, which can be determined from the pattern. These four d-values are compared with suitable d-value sets of the possible crystalline phases and probable phase identifications are made. Generally, there exist several indexing variants for the individual d-values of the separate phase identification possibilities. It is possible to find the correct indexing by the determination of the indices of the sum and difference vectors, occasionally, for one pair of the indexing variant from the vectors defining the reference cell and by comparison of these indices with the indexing variants for the d-values of the sum and difference vectors of the corresponding phase identification variant. The measuring and calculation of the intervectorial angles formerly necessarily hasn't to be made in this method. However, the determination of the sum and difference vectors involves it. The reliability of the indexing can be increased in a strong manner if two or three different reciprocal lattice planes of the specimen under investigation are recorded in the corresponding TED patterns and if, in the following, these patterns can be indexed by the mentioned method without any doubt.     

Investigation of crystal lattice defects of PbZrO3 single crystals by transmission electron microscopy

The investigation of PbZrO₃ single crystal structure was carried out by the method of transmission electron microscopy (TEM). It is shown that extended planar defects, slip lines, twins, and morié patterns are observed in PbZrO₃.     

Neutron diffraction and X-ray diffraction investigations of langasite crystals

Crystals of langasite La₃Ga₄(GaSi)O₁₄ grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La₃Ga_5.14Si_0.86O₁₄ seed in an (Ar + O₂) atmosphere (the 〈 0001 〉 growth direction) can be written as(La_2.85(2)□_0.15)(Ga_0.95(2)□_0.05) Ga₃(Ga_1.15 Si_0.85(5))(O_13.72□_0.28(7)) and(La_2.89(1)□_0.11)·(Ga_0.98(1)□_0.02) Ga₃(Ga_1.06Si_0.94(4))(O_13.81□_0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La _2.97(4)□_0.03) Ga(1)(Ga_2.94(9)□_0.06)(Ga_0.7(1) Si_1.3)(O_13.9□_0.1(1)), which is grown from the La₃ Ga₅SiO₁₄ seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ?growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.     

Investigations on the defect depth of electroerosive-cut Te single crystals by reflection electron diffraction (RHEED)

Results of the experimentally determined defect depth due to electroerosive working of Te single crystals as a function of crystal orientation and electric cutting energy by RHEED are given. The abrasion of the disturbed surface layer was made by chemical etch polishing as well as by bombardment with Ar⁺ ions. The defect depths determined from the RHEED patterns depend on the crystal orientation and the applied cutting energy. In case of the lowest cutting energy, it has a value of about 3 μm for the (0001)-orientation and 6 μm for the (101 0)-orientation, the corresponding values for the highest cutting energy are about 8 μm and 20 μm, respectively. It is discussed in how far defect depths determined from the interpretation of RHEED patterns correspond to the “real defect depths” which can be determined by other measuring techniques.     

Relative angular precision in electron backscatter diffraction: A comparison between cross correlation and Hough transform based analysis

In the present study the relative angular resolution of an electron backscatter diffraction system based on Hough transform analysis has been determined with a silicon single crystal wafer. The resolution is found to be better than 0.1° and can be easily improved by repetition of measurements. A test measurement on a BaFe₂As₂ thin film, where disorientations of 0.1° and less are present, was performed using the cross correlation electron backscatter diffraction technique. The same measurement is evaluated with the Hough transform technique. Comparing both techniques give evidence of a relative resolution of better than 0.1°. However, in specimen areas with strain inhomogeneities a deviation along one rotation axis can be observed.     

Flux growth of ZnO crystals doped by transition metals

ZnO crystals doped with Cr, Mn, Fe and Co were grown by the flux method. The prepared crystals revealed no phase separation detectable by X-ray diffraction. Structure properties were characterized by Raman and photoluminescence spectroscopy. For ZnO:Co, Mn and Cr, no spontaneous ferromagnetic moment was observed up to T=2 K whereas for the ZnO:Fe crystals the m(H) curves suggest the existence of 5 nm superparamagnetic iron clusters. At low temperatures the m(H) curves can be interpreted as a superposition of major paramagnetic and minor antiferromagnetic contribution. The paramagnetic part corresponds to the presence of Co²⁺, Fe³⁺, Mn²⁺ ions and small Cr atom clusters.     

Evaporation of doped NaCl crystals

In the present paper an investigation of the influence of impurity ions on the evaporation of NaCl crystals is reported. Monovalent impurities little influence the evaporation rate. The evaporating structures have changed only when there were extremal differences in the values of ionic radii. However, the bivalent impurities substantially influence the evaporation rate. This unfluence depends upon the fact whether the impurities are in non-associated or associated state with cation vacancies, respectively. This fact also affects the shape of evaporating structure.     

Reliability of high‐resolution electron backscatter diffraction determination of strain and rotation variations using phase‐only and cross correlation

This contribution analyzes the reliability of strain and rotation variation determination using cross‐ as well as phase‐only correlation of experimental wide‐angle electron backscatter diffraction (EBSD) patterns. For both rotation and three‐point bending the resulting displacement gradient tensor components are examined in terms of magnitude and statistical scatter as a function of various correlation procedure parameters and signal‐to‐noise ratio. It is shown that on the one hand the Fourier filter width has a major impact on the strain results. At higher noise level a smaller filter width has to be applied for obtaining maximum precision. On the other hand, the influence of the degree of overlap of the regions of interest positioned in the patterns is less important. For both rotation and bending experiments the cross‐correlation variant yields a smaller standard deviation with respect to phase‐only correlation, in particular for elevated noise level. The difference is attributed to the stronger propagation of noise effects in the course of phase‐only correlation function calculation and fitting. In the rotation experiment a standard deviation of ∼1.0 × 10⁻⁴, averaged over the displacement gradient tensor components, and a rotational precision of ∼1.5 × 10⁻⁴ rad have been achieved by using optimized evaluation settings.     

From electron diffraction by clays to modular crystallography

Being an independent method of structure analysis, electron diffraction provides original data which are essential contributions to all the fields where this method is used. An important place of electron diffraction among other structural methods is illustrated by the results of the studies once started on clay minerals and continued on the objects extremely important for structural mineralogy, which stimulated the development of a new field—modular crystallography.     

X-Ray diffraction study of KTiOPO<Subscript>4</Subscript> single crystals doped with hafnium

Single crystals of KTi_{1 − x} Hf _x OPO₄ (x = 0.015(2), 0.035(1), and 0.128(1) are reinvestigated by precision X-ray diffraction at room temperature. It is found that the implantation of hafnium atoms in the crystal structure of KTiOPO₄ does not lead to significant changes in the framework and affects only the positions of the potassium atoms in the channel. Our studies reveal the displacements of the potassium atoms from their main and additional positions in the structure of pure KTP in all three structures studied. The largest displacements from the K1′ and K1″ additional positions are observed in the structure with x = 0.035. At this hafnium concentration, the occupancy of the main positions of potassium atoms decreases and the occupancy of the additional positions increases in relation to those in KTP. This redistribution of potassium atoms enhances the nonuniformity of distribution of the electron density in the vicinity of their positions, which is probably responsible for the increase in the nonlinear susceptibility of KTP crystals that contain 3.5% hafnium in relation to crystals of pure KTP.     

Light diffraction by acoustically induced domains in nematic liquid crystals

The phenomenon of light diffraction by a system of linear domains formed in planar layers of nematic liquid crystals in an oscillating Couette flow, acoustically induced at sound frequencies, is investigated.     

Accurate X-ray diffraction studies of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium

Single crystals of potassium titanyl phosphate doped with 4% of niobium (КТР:4%Nb) and 6% of niobium (KTP:6%Nb) are studied by accurate X-ray diffraction at room temperature. The niobium atoms are localized near the Ti1 and Ti2 atomic positions, and their positions are for the first time refined independent of the titanium atomic positions. Maps of difference electron density in the vicinity of K1 and K2 atomic positions are analyzed. It is found that in the structure of crystal КТР:4%Nb, additional positions of K atoms are located farther from the main positions and from each other than in КТР and KTP:6%Nb crystals. The nonuniform distribution of electron density found in the channels of the КТР:4%Nb structure is responsible for ~20% increase in the signal of second harmonic generation.     

Real structures of doped lead telluride crystals

A relationship between the real crystal structure and the doping level was proposed based on experimental data on the real structures of PbTe crystals with different Cr, Co, Ni, and In contents. Two composition regions were revealed for these crystals. One region is characterized by the presence of impurity atmospheres at dislocations as the main type of impurity inhomogeneity, whereas impurity-rich microprecipitates prevail in the second region. Possible reasons for the formation of solid solutions supersaturated with impurity ions were analyzed. The influence of thermal treatment on the real structure and galvanomagnetic properties of the crystals was investigated.     

Absorption spectra of molybdenum doped YAG crystals

Absorption spectra of molybdenum doped YAG crystals annealed in hydrogen, vacuum, oxygen and argon and exposed to an UV irradiation are given. Yellow or brown colour of the crystals is attributed to the O⁻ centre formation connected with the reduction of molybdenum ions to a lower valency state. Most intensive yellow colour was shown by crystals, the argon annealing of which was followed by a treatment in oxygen at rather low temperature (∼1000 °C) and by UV irradiation. Two types of O⁻ centres are suggested Blue colour of the samples annealed in argon is attributed to e⁻ centres.     

X-ray diffraction by polydomain crystals modulated by transverse waves of atomic displacements. 1. Single-wave modulation in crystals

The intensity distributions of diffuse X-ray scattering caused by domains characterized by the transverse wave of atomic displacements polarized in the (010) plane and propagating along the [001] direction are calculated by numerical methods analyzed in the kinematical approximation. The regular changes in the intensity and half-width of the satellite peaks are revealed as functions of the wave amplitude and the average domain size. The characteristic features of diffraction are considered in the case where the defect-density wave in the crystal affects the parameters of the displacement wave.     

Vapor growth of doped mercuric iodide crystals by the Temperature Oscillation Method

Crystals of HgI₂ are grown from the vapor phase using the Temperature Oscillation Method. Techniques to dope the crystals with Hg, I₂, NH₄I, SbI₃, PbI₂, and TeI₄ during growth are described. Evaluation of the external morphology indicates that HgI₂ can incorporate a relatively large excess of Hg. Increasing the iodine concentration in the crystals introduces morphological break- down of the vapor/solid interface.