Herstellung und Charakterisierung von Einkristallen der kubischen Laves-Phasen Se-Al2_{2}

Im Kristall-Labor des Instituts für Festkörperforschung wurden folgende Legierungseinkristalle für die verschiedensten Experimente gewünscht.La Al$_{2}$ (La Ce) Al$_{2}$ (La Er) Al$_{2}$ Y Al$_{2}$ (Y Ce) Al$_{2}$ Ce Al$_{2}$ Auftraggeber waren: 1. Das Institut für Festkörperforschung Jülich 2. Der Sonderforschungsbereich Köln - Aachen - Jülich / Im folgenden Bericht sollen die Herstellungs- und Charakterisierungsverfahren erläutert werden. 1. Probenpräparation 2. Art und Wahl der Kristallzüchtung 3. Probencharakterisierung 4. Messungen und Ergebnisse der Experimente Aus der Literatur war bis dahin nur ein Hersteller solcher Einkristalle bekannt. Aus den Veröffentlichungen konnten nur wenige Daten und Einzelheiten entnommen werden /1/2/3/4. Es war deshalb notwendig, vorher einige grundlegende Dinge zu erlernen, z. B. Probenpräparation, Wahl des Tiegelmaterials, Impflingherstellung. Die Problemstellung war, Einkristalle in verschiedenen Größen und Orientierungen herzustellen, von 2 $\varnothing$ mm 50 mm lang, bis 10 $\varnothing$ mm 80 mm lang, deren Stabachsen oder sein sollten

Züchtung und Charakterisierung von reinen und dotierten SE-Al2_{2}-Einkristallen

Single crystals of the compounds CeAl$_{2}$, LaAl$_{2}$, La(Ce)Al$_{2}$, La(Er)Al$_{2}$, La(Gd)Al$_{2}$, La(Tm)Al$_{2}$, TmAl$_{2}$ and YAl$_{2}$ ... were grown by the Czochralski-technique for studies of the low-temperature electronic and magnetic properties of RE-Al$_{2}$ cubic Lavesphases. These highly exothermic compounds were prepared by melting smaller portions of the elements together in a cold crucible. For the crystal growth they were remelted in inductively heatedtungsten crucibles. In the initial phases of the project the crucibles from which crystals were pulled were in a long quartz tube around which an induction coil was wound /2/. Later both the crucible and coil were placed inside a large water-cooled stainless steel vessel. In all cases atmospheres of well-purified argon were used.Crystals with lengths up to 80 mm and diameters between 2 and 20 mm and oriented along (100), (110), (121) and (111) were successfully grown. The structures were characterized with x-, $\gamma$- and n-rays. The homogeneity was checked metallographically and the metallic and gaseous impurities were analyzed by mass-spectrometry and vacuum extraction. We found that grooves and secondary nucleation on the surface were induced by a fluctuating melt surface. This effect wasvery pronounced in the first apparatus. The rocking curves measured with $\gamma$-rays consisted usually of several sharp peaks with a total width up to 60'. x-ray topograms revealed that the subgrains were mainly located at the surface. The fluctuations were almost completely suppressed in the new unit by reducing the pressure fluctuations in the large vessel, by improving the smoothness of the pulling rod, by installing the apparatus on an anti-vibration foundation and by cleaning the frequency spectrum of the RF-generator. These precautions lead to an increased structural quality and chemical homogeneity of the crystals. Now the rocking-curves usually show only one peak with a width below 10'. In addition the tungsten contamination was lowered by using polished crucibles. With increased argon purity, the concentration of the dissolved gaseous elements in the crystals - mainly oxygen and hydrogen - could be reduced to 500 ppm