RESUMEN
The structure of the Co2MnAl-type Heusler alloy in the form of a melt-spun ribbon was studied by electron microscopy, electron back-scattered diffraction (EBSD), and X-ray diffraction. The melt-spun ribbon consists of a homogeneous single-phase disordered Heusler alloy at the wheel side of the ribbon and an inhomogeneous single-phase alloy, formed by cellular or dendritic growth, at the free surface of the ribbon. Cellular growth causes the formation of an inhomogeneous distribution of the elemental constituents, with a higher Co and Al concentration in the centre of the cells or dendritic arms and a higher concentration of Mn at the cell boundaries. The EBSD analysis shows that the columnar crystals grow in the <111> crystal direction and are declined by about 10° against the direction of the spinning.
RESUMEN
Precise research on the RbF-Al2O3 system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb3AlF6, RbAlO2, Rb2Al22O34, and new phase, Rb2Al2O3F2, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO3F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10-6 S cm-1. The similar structural arrangement of O3Al-O-AlO3 and FO2Al-O-AlO2F tetrahedra of the conduction planes in Rb2Al22O34 and Rb2Al2O3F2 were confirmed by 27Al NMR measurements. A thermal analysis of the RbF-Al2O3 system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF3-Al2O3-Rb2O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb2Al2O3F2 is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.