New approaches to scaling data measured on a CCD diffractometer

Abstract

New methods have been developed for scaling the intensities of diffraction reflections measured on a diffractometer with a CCD detector. The algorithm involves cyclic alternation of the refinement of a structure model from experimental data corrected for anisotropic effects with the refinement of anisotropic parameters from initial data for a fixed structure model. The first method consists of refining scale factors for the intensities after they have first been corrected for anisotropic effects of known physical nature (thermal diffuse scattering, absorption, extinction etc.). It has been shown that the majority of the observed biases of intensities are caused by the characteristics of the CCD detector and the technique used for data collection on the diffractometer, rather than by the properties of the sample. The second (alternative) method of fixed scaling has been implemented here. This method does not require refinement of empirical scale factors but uses intensity correction maps obtained by averaging the results derived from several experiments with different crystals. The first map describes the non‐uniformity of the response over the surface area of the detector. The second map accounts for distortions dependent on the setting angles of the goniometer. The methods were checked for data measured on an Oxford Diffraction Xcalibur Sapphire 3 CCD diffractometer. The proposed methods significantly improve the results of the refinement and make it possible to obtain a structure model that is reproduced in repeated investigations of samples of the same compound.

Ab Initio calculation of elastic and electromechanical constants of langasite family crystals

“…An X ray diffraction analysis of the crystals under consideration [17] yielded good (better than 1%) agreement between the experimental and calculated 2 2 ,…”

“…The main calculation parameters were the absolute zero temper ature, the local density approximation (LDA) [7], Troullier-Martins type pseudopotentials [14], Teter exchange correlation functional [15], and a plane wave energy cutoff (50 Ha); a 6 × 6 × 6 MonkhorstPack grid [16] of Brillouin zone k point sampling was used in the calculations of 80 bands. The initial models of the atomic structure of langasite family crystals were specified using the results of an X ray diffraction study [17]. First, the total energy of the ground state was minimized by varying the unit cell parameters and atomic coordinates (with a potential divergence of 10 -20 ).…”

“…The calculated elastic constants were used in the correction to ther mal diffusion scattering for the Ba 3 TaGa 3 Si 2 O 14 crystal [17]. The results of the ab initio calculation appeared very useful for establishing the effect of the structure on the piezoelectric properties of langasite family crystals [18].…”

Software for HUBER-5042 diffractometer with Displex DE-202 helium cryostat

“…First, the duration of the experiment in the high-angle range of reflections from crystals with small or intermediate unit cells is not much longer. Second, the use of diffractometers with point detectors leads to much smaller systematic measurement errors [7]. Thus, the entire set of characteristics of Huber diffractometer makes it quite appropriate for solving many problems.…”

Crystal-structure refinement of Sr3Ga2Ge4O14

“…The absorption cor rection for ellipsoidal samples was applied according to [13]. The CCD data were scaled according to the method described in [14]. The extinction coefficient [15,16] and the contribution of half wavelength radi ation [17] were refined.…”