RE-EXAMINATION OF THE CA-GA SYSTEM AND CRYSTAL-STRUCTURE OF CAGA4, A MONOCLINIC DISTORTION OF THE BAAL4 TYPE

The gallium-rich region of the Ca-Ga system (64.9-84.6 at.% Ga) was re-examined by X-ray methods. In addition to the CaGa2 phase with the CaIn2 structure type, another phase, CaGa2+x, was found in a limited homogeneity range, being 0.1 &le x &le 0.4, with crystal structure derived from the AlB2 type. The mechanism of solid solubility formation was clarified by single crystal refinement. The new phase Ca3Ga8 was identified by powder diffraction data as isotypic with Eu3Ga8. The structure of CaGa4, determined by single crystal methods, can be described as a new monoclinic distortion of the BaAl4 type, related to the low temperature form of CaAl4. Crystal data of the nine intermediate phases present in the Ca-Ga system are given, and a revised form of the phase diagram is proposed.