TEMPERATURE-DEPENDENT SINGLE-CRYSTAL INVE STIGATIONS OF ALPHA-NA3HG

In contrast to beta-Na3Hg (rhomboedrally distorted Li3Bi-type) alpha-N a3Hg crystallizes in a hitherto poorly understood variant of the Na3As -type. Based on temperature dependent measurements of poly- and single crystalline samples (- 100-degrees-C < T < + 35-degrees-C) we show, t hat in particular the sodium atoms (Na1) located in the region of the octahedral Hg6-holes show a pronounced temperature dependent dynamical behaviour. To a lesser extend this is also true for the tetrahedrally coordinated Na-atoms (Na2). With increasing temperature the former on es more and more approach the centers of the opposite triangular faces of mercury atoms, limiting the Hg6-octahedra along [001]. Occupation of the latter positions by sodium atoms would lead to unusual short in teratomic distances d(Na-Hg). However before reaching this unreasonabl e situation alpha-Na3Hg decomposes under formation of beta-Na3Hg.