Ordered and disordered phases in CaCu5 -type derived structures: Dumbbell cluster modeling with first-principles calculations

Abstract

We investigate the structural stabilities of CaCu5-type derived structures (Smm-nT5m+2n, T=Fe,Co) and predict metastable structures with the dumbbell cluster expansion model in which the dumbbell atoms are regarded as virtual single atoms. The metastable structures are concentrated from n/m=1/3 to 1/2, suggesting the existence of compounds with high magnetization. The disordered TbCu7-type structures are stabilized by the configurational entropy of the dumbbell clusters. The estimated solubility limits of the T-T dumbbell are close to the experimental results. The difference in the phase diagram for the Sm-Fe and Sm-Co systems is explained from their effective cluster interactions.