REACTION KINETICS ON FORMATION OF In- AND Zr,Ca- SUBSTITUTED YIGs BY "IN SITU" POWDER XRD. A.Preisinger, K.Mereiter and L.Petrás. Institute of Mineralogy, Crystallography and Structural Chemistry Tech. University of Vienna, A-1060 Vienna, Getreidemarkt 9, AUSTRIA.

Solid state reactions taking place during formation of differently substituted Yttrium-Iron Garnets (YIG) from the corresponding oxides were studied with high temperature X-ray powder diffraction method up to 1400deg.C. The measurements were carried out in a newly developed high temperature chamber PMP1600 which provided good temperature uniformity in the sample, and reduced the recrystallization caused grain size effects by spinning the samples of compacted pellets. Reaction kinetic studies were based on Rietveld refinements of ,,in situ" high temperature diffraction patterns. Different diffusion processes and thermochemical reactions could be observed. The phase compositions at different temperatures were determined with standardless quantitative analysis, the substitution rates were concluded from both refinement of the site occupancy factors and unit cell volume changes, subtracting thermal expansions. In this way the distribution of all cations in the numerous phases (starting oxides, intermediate phases and end-products) could reasonably be traced on. The reaction paths were observed to be distinctly different when samples presintered at lower temperatures were ground. The influence of the substituents on the reaction kinetics was clarified in systems with compositions corresponding to garnets Y₃Fe_2-xIn_xFe₃O₁₂, and Y_3-xCa_xFe_2-xZr_xFe₃O₁ in the range x=0 to 1.0 for In, and x=0 to 2.0 for the Zr,Ca substitution resp. Zr was found to be able to substitute Fe of octahedral coordination completely, but no In substitution could be reached for x1. The angular dependencies of peak broadenings have special composition dependent behaviours: Garnets with Zr,Ca substitution close to x=0, 1 and 2 gave sharp reflections also at high 2[[theta]] angles, while those with substitution rates in between showed reflections highly broadening with 2[[theta]] angles. An attempt was made to explain these peculiarities with the phenomenon of paracrystallinity [1].

1. H. Ludwiczek, A. Preisinger, A. Fischer, R. Hosemann, A. Schönfeld, W. Vogel: J. Catalysis, 51 (1978) 326-337