EQUIATOMIC TERNARY PHASES FORMED BY ALKALINE-EARTHS AND RARE-EARTHS

A survey is given on the occurrence of several structure types in nearly 300 RMX phases formed by the divalent metals Ca, Sr, Ba, Eu and Yb and nearly 1000 formed by the trivalent rare earths, Sc and Y, where M is a d-block element, Li or Mg and X is a p-block element. Structural maps are proposed for phases with valence electron concentrations of 7/3 and 8/3 electrons per atom, using as coordinates the combination of the elemental radii, r(R)/(r(M)+r(X)), the combination of the three electronegativities on the Miedema scale, phi(M)*+phi(X)*-phi(R)*, and the period number of the X element. Though many structure types are closely related and sometimes a certain overlap of the domains cannot be avoided, some predictions for ternary phases not yet studied can be made. Volume effects in the formation of these ternary phases were also analysed. The volume of an RMX compound can with good confidence be expressed as a mean of the volumes of the two binary compounds RM, and RX, calculated with equations already applied to binary systems of alkaline earths and rare earths.