The pseudobinary systems SrAg 1− xZn x, CaCu 1− xGa x and CaCu 1− xGe x and their use for testing structural maps

Abstract

The systems SrAg 1− x Zn x , CaCu 1− x Ga x and CaCu 1− x Ge x were studied by metallographic and X-ray methods in their isothermal sections at 723 K. A series of pseudobinary phases occurs in the whole SrAg 1− x Zn x system, while a large central polyphasic region results in the other two systems. All pseudobinary phases identified crystallize in the FeB- or CrB-type structures, or in stacking variants of these two parent types. The phases SrAg 0.9Zn 0.1 and CaCu 0.975Ga 0.025, corresponding to new stacking variants (Jagodzinski notations h 3chc and (hc) 2(hc 2) 2 respectively), were refined by single-crystal diffractometric data. The binary and pseudobinary equiatomic compounds containing calcium, strontium and barium are used as tests for several types of structural map, and an empirical rule on the formation of pseudobinary phases is proposed. The influence of the electron concentration is pointed out on the basis of the unusual trends shown by the crystallographic parameters of some pseudobinary phases. If phases with the same structure occur in two or more systems, their compositions correspond to the same electron concentration values. A trend in the percentage of hexagonal stacking with the electron concentration is also found.