The quality of diffraction data obtained using silver and molybdenum microsources has been compared for six model compounds with a wide range of absorption factors. The experiments were performed on two 30 W air-cooled Incoatec IµS microfocus sources with multilayer optics mounted on a Bruker D8 goniometer with a SMART APEX II CCD detector. All data were analysed, processed and refined using standard Bruker software. The results show that Ag Kα radiation can be beneficial when heavy elements are involved. A numerical absorption correction based on the positions and indices of the crystal faces is shown to be of limited use for the highly focused microsource beams, presumably because the assumption that the crystal is completely bathed in a (top-hat profile) beam of uniform intensity is no longer valid. Fortunately the empirical corrections implemented in SADABS, although originally intended as a correction for absorption, also correct rather well for the variations in the effective volume of the crystal irradiated. In three of the cases studied (two Ag and one Mo) the final SHELXL R1 against all data after application of empirical corrections implemented in SADABS was below 1%. Since such corrections are designed to optimize the agreement of the intensities of equivalent reflections with different paths through the crystal but the same Bragg 2θ angles, a further correction is required for the 2θ dependence of the absorption. For this, SADABS uses the transmission factor of a spherical crystal with a user-defined value of μr (where μ is the linear absorption coefficient and r is the effective radius of the crystal); the best results are obtained when r is biased towards the smallest crystal dimension. The results presented here suggest that the IUCr publication requirement that a numerical absorption correction must be applied for strongly absorbing crystals is in need of revision.

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Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination

Comparative study of defect structures in lithium niobate with different compositions

ABSORB: A computer program for correcting observed structure factors from absorption effects in crystal structure analysis

High-pressure single-crystal structure refinement requires intensity data that have been corrected for the various absorption effects in the pressure cell. In this contribution, the methods for calculating the absorption corrections for a transmission-geometry diamond-anvil pressure cell are reviewed. The ideas presented in this paper have been implemented in a software package for Microsoft Windows, Absorb6.0, available at http://www.crystal.vt.edu/.

Absorption corrections for diamond-anvil pressure cells implemented in the software package Absorb6.0

The reaction of the Er3+ ion with polycarboxylate ligands in gel media leads to coordination polymers exhibiting various structural types and dimensionalities. Five Er3+/1,4-benzenedicarboxylate-based coordination polymers have been obtained in such conditions. Four out of the five are new. Their crystal structures are reported and compared herein. Compound 1, namely, Er2Ter3(H2O)6, where H2Ter symbolizes the terephthalic acid, crystallizes in the space group P1 (No. 2) with a = 7.8373(10) A, b = 9.5854(2) A, c = 10.6931(2) A, α = 68.7770(8)°, β = 70.8710(8)°, and γ = 75.3330(12)°. It has already been reported elsewhere. The last four compounds are new. Compound 2, namely, Er2Ter3(H2O)6·2H2O, crystallizes in the space group P121/a1 (No. 14) with a = 6.7429(2) A, b = 22.4913(7) A, c = 9.6575(3) A, and β = 91.6400(18)°. Compound 3, namely Er2Ter3(H2O)8·2H2O crystallizes in the space group P1 (No. 2) with a = 7.5391(2) A, b = 10.0533(3) A, c = 10.4578(3) A, α = 87.7870(10)°, β = 82.5510(11)°, and γ = 86.28...

H. A. Levy

Papers

High-speed computation of the absorption correction for single crystal diffraction measurements