G. V. Fetisov

SOFTWARE AND METHODS FOR PRECISE X-RAY-ANALYSIS

The software package PPXA (programs for precise X-ray analysis) is described, which provides crystallographers with increased precision and reliability of the results of X-ray analysis. The package includes programs for: testing the quality of an automatic four-circle X-ray diffractometer before X-ray experiments; optimization of X-ray diffraction experiments; preliminary processing of experimental data; precise structure analysis up to electron density distribution. The programs for diffractometer testing allow the experimenter to check the stability and precision of the instrument, the homogeneity of the X-ray beam and the detector sensitivity. The programs for preliminary data processing include: corrections for absorption, taking into account the intensity distribution in the X-ray beam cross section; correction or elimination of the simultaneous reflection effect; background subtraction, correcting the effects of the scan-interval cut-off; correction for thermal diffuse scattering (TDS) for crystals in cases of both known and unknown elasticity constants. The programs for precise crystal structure refinement include options for the determination of primary- and secondary-extinction parameters, atomic coordinates, thermal-vibration parameters in harmonic and anharmonic approximations, occupation of atomic positions, atom-core and valence-shell occupation parameters. The programs were written for a MicroVAX-II computer with a VAX/VMS operating system and designed to work with CAD-4 diffractometers and the SDP-Plus software package. A suitable dialogue with a menu system and detailed users instructions are available.

An X-ray diffractometer for studying the effect of external fields on the structure and electron distribution of single crystals

A new four-circle X-ray diffractometer (RMD) for single crystals is described that has only one (horizontal) axis of specimen rotation and a movable X-ray tube and detector. The diffractometer is characterized by the equatorial geometry. The equatorial plane rotates about the χ axis lying in this plane whereas the ϕ axis is fixed. This permits the use of various external (electromagnetic, magnetic etc.) fields applied to specimens along the ϕ axis. The angles between this direction and all the other crystallographic directions in the specimen remain constant in the course of intensity measurements. The designed RMD diffractometer allows the study of structural changes and electron distribution variations in a crystal subjected to external actions. The geometry and the design of the goniometer are considered in detail. Experiments carried out on the RMD diffractometer have demonstrated its efficiency for precision X-ray structure analysis.

Modeling of Bragg Intensity Profiles. 1. Allowance for Crystal Mosaicity

The model of the instrumental function of a fourcircle X-ray diffractometer suggested earlier [Chulichkov et al. (1987), Kristallografiya, 32, 11071114] is complemented by the introduction of the crystal mosaicity. This improved model is used to develop a method for the reconstruction of the mosaicity function profile f(w) from the experimental intensity profile l(w) measured on a diffractometer. The method consists of the reduction of the experimental l(w) distribution to the form it would have if it were measured on a diffractometer with an instrumental function close to the 6 function. The suggested method for determining f(w) is tested on Si crystals with dislocation densities Nd=3×10 ~° and 2x 1011 m -2.

THE INFLUENCE OF LASER RADIATION ON X-RAY DIFFRACTION IN FERROELECTRIC CRYSTALS WITH NONLINEAR OPTICAL PROPERTIES

The influence of laser radiation (λ = 532, 630 nm) on X-ray diffraction in nonlinear ferroelectric LiNbO3:Fe(0.01–0.02%) and triglycine sulfate (3C2H5NO2.H2O4S) crystals has been studied. The laser radiation was applied to the sample on a four-circle X-ray diffractometer through a flexible optical fiber. The end of the light guide was fixed in a special attachment installed on the goniometer head and allowed the laser beam to be oriented along some chosen direction in a crystal and to maintain this orientation during the whole process of data collection over the reflection sphere. The output power of laser radiation at the end of the light guide was 7 to 10 mW (or about 0.3 W cm−2). The sample irradiation by laser light led to a visible increase of intensity for some reflections (up to 34%). A noticeable difference in the extinction parameters of the samples was found between the sets of reflections measured under normal conditions and those measured under laser irradiation.

Correction for absorption and beam inhomogeneity in X-ray single-crystal diffractometry : method of analytical integration

A computational method of absorption correction in X-ray diffractometry for single crystals in the shape of convex polyhedra bathed in an inhomogeneous and in a homogeneous X-ray beam is suggested. An absorption correction is calculated for each reflection of the data set using the measured coordinates of the specimen vertices and the experimentally measured intensity distribution in the primary beam. The program ABSCOR is written in Fortran and may be readily adopted to any four-circle diffractometer and data format. The results of the method described for crystals of different dimensions and different absorption coefficients are given. It is shown that beam inhomogeneity strongly influences the reflection intensity of large non-isometric crystals.

On the interpretation of X‐ray diffraction effects in ferroelectric crystals subjected to laser irradiation

In an earlier X-ray investigation of ferroelectric single crystals of LiNbO3:Fe and triglycine sulfate (3C2H5NO2. H2SO4) subjected to laser irradiation [Zhukov, Fetisov & Aslanov (1991). J. Appl. Cryst. 24, 74–76], a large increase (greater than 20%) in the integrated intensities of Bragg reflections was found. An attempt to explain the observed effects is given here with the aid of additional experimental results and consideration of the abnormal photovoltaic effect (APE) followed by a deformation of the domain structure of the crystal.

Profile Analysis Application in the Modelling of Multiwave Diffraction

The results of intensity profile analysis of Bragg reflections are used for the calculation of the reflectivity Q(dO0., tro)= W(AO0., (ro.)(l~2Lp)o. in the energy transfer equation for multiwave X-ray diffraction in crystals. The diffraction profiles in the profile analysis are fitted by different analytical functions and the fitting results are used for modelling the multiwave diffraction. The results of modelling multiwave diffraction in Si and V3Si crystals with different grades of perfection demonstrate that the method suggested here is sensitive to the content of defects in crystals and can be used not only for simultaneous reflection correction in X-ray structure analysis but also for estimation of single-crystal perfection.