The RG lines of MgCu2, AsNa3 and SiCr3 crystal-structure types. The binary systems in which compounds of these crystal structures occur are plotted on a (, ) map. = − and = (NA1/3 − NB1/3), where A is the minority element. Binary systems with compounds belonging to each structure type fall on a straight line on the map. From the overlap and lack of overlap between the lines, one can predict concomitant and mutually exclusive structure types in binary systems (Rajasekharan & Seshubai, 2010a). Most of the compounds of the MgCu2-type crystal structure have negative , and most of the compounds with SiCr3 and AsNa3-type structures have positive .