Biomedical subjects

Thomas E Weirich

Publications and source records attributed to Thomas E Weirich.

4 recordsLinked to original sources

PubMed2005

Ab initio determination of the framework structure of the heavy-metal oxide Cs(x)Nb2.54W2.46O14 from 100 kV precession electron diffraction data.

The present work deals with the ab initio determination of the heavy metal framework in Cs(x)(Nb, W)(5)O(14) from precession electron diffraction intensities. The target structure was first discovered by Lundberg and Sundberg [Ultramicroscopy 52 (1993) 429-435], who succeeded in deriving a tentative structural model from high-resolution electron microsopy (HREM) images. The metal framework of the compound was solved in this investigation via direct methods from hk0 precession electron diffraction intensities recorded with a Philips EM400 at 100 kV. A subsequent (kinematical) least-squares refinement with electron intensities yielded slightly improved co-ordinates for the 11 heavy atoms in the structure. Chemical analysis of several crystallites by EDX is in agreement with the formula Cs(0.44)Nb(2.54)W(2.46)O(14). Moreover, the structure was independently determined by Rietveld refinement from X-ray powder data obtained from a multi-phasic sample. The compound crystallises in the orthorhombic space group Pbam with refined lattice parameters a=27.145(2), b=21.603(2), and c=3.9463(3)A. Comparison of the framework structure from electron diffraction with the result from Rietveld refinement shows an average agreement for the heavy atoms within 0.09 A.

Journal Article↗

PubMed2003

First-principles calculations as a tool for structure validation in electron crystallography.

The crystal structures of Ti(11)Se(4) [Weirich, Ramlau, Simon, Hovmöller & Zou (1996). Nature (London), 382, 144-146] and Ti(45)Se(16) [Weirich (2001). Acta Cryst. A57, 183-191] determined previously from selected-area electron diffraction (SAED) data have been checked for their correctness by means of total energy calculations within the non-local density functional theory. The reliability of the used method was verified by test calculations carried out for the structurally related compound Ti(8)Se(3), which is well known from single-crystal X-ray diffraction [Weirich, Pöttgen & Simon (1996). Z. Kristallogr. 212, 929-930]. For Ti(8)Se(3), structural models from both experiment and calculation show a perfect match (average agreement 0.01 A). This proves that the geometrical optimized models from first-principles calculation can be used as a reliable alternative when good-quality X-ray results cannot be obtained. Calculations carried out for the two structures determined from electron crystallography yielded average improvement of the atomic coordinates of 0.04 and 0.09 A for Ti(11)Se(4) and Ti(45)Se(16), respectively. The optimized cell parameters of the monoclinic structures (both space group C2/m, No. 12) are a = 25.51, b = 3.43, c = 19.19 A, beta = 117.9 degrees for Ti(11)Se(4) and a = 36.31, b = 3.45, c = 16.59 A, beta = 92.1 degrees for Ti(45)Se(16). These results prove that crystals that are too small for single-crystal X-ray diffraction and are difficult to solve by powder diffraction may nevertheless be amenable to accurate structure determination by electron diffraction structure analysis using data from standard SAED and the assumption of quasi-kinematical scattering. Moreover, this study shows that geometry optimization by first-principles calculations is the perfect tool for validation and improvement of complex structural models, which are suspected to have errors owing to the poor quality of experimental data.

Journal Article↗

PubMed2003

Characterization of Co25Ag75 and (Co90Al10)28Ag72 granular films by electron diffraction, high-resolution transmission electron microscopy and electron spectroscopic imaging.

Series of sputter-deposited Co25Ag75 and (Co90Al10)28Ag72 giant-magnetoresistance granular films were characterized by electron diffraction, high-resolution transmission electron microscopy and electron spectroscopic imaging. Crystalline particles of fcc silver and hcp cobalt were detected in both thin-film systems before annealing. Annealing of (Co90Al10)28Ag72 films at 773 and 823 K yielded mixtures of fcc and hcp cobalt clusters and notably enlarged silver particles. In addition, crystallites of bcc Ag3Al were detected in the sample annealed at 823 K. The mesoscopic structure of the as-deposited films was investigated by dark-field imaging showing columnar growth-domains for silver. The columns were preserved during thermal treatment up to 773 K, whereas annealing at 823 K destroyed these domains.

Alloys↗

PubMed2002

Structure and stability of alpha- and beta-Ti2Se. Electron diffraction versus density-functional theory calculations.

The alpha structure as well as the new beta modification of Ti(2)Se were recently characterized by electron diffraction structural analysis of nanosize crystallites. In this study, both structures are investigated by means of total energy calculations within the non-local density-functional theory in order to validate the experimental results. The calculated parameters for both modifications are in excellent agreement with data determined from electron microscopy. From the calculated equation of states, beta-Ti(2)Se is predicted to be a high-pressure modification. The present investigation proves by a well established non-crystallographic method that unknown structures, which are not accessible by other standard crystallographic techniques, can be solved and refined with high accuracy using electron diffraction data.

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