Crystal structures and phase relationships in magnetostrictive Tb_1-xDy_xCo₂ system

The crystal structures and phase relationships of Tb_1-xDy_xCo₂ alloys with 0 ≤ x ≤ 1 were investigated by synchrotron-based high-resolution x-ray powder diffraction. Three different crystal structures are observed in the system: all the compositions show cubic structure with space group  Fd¯3m at temperatures above the Curie temperature T_C; the Tb-rich side sample shows a rhombohedral structure with space group R¯3m the Dy-rich side sample has a tetragonal I4₁/amd space group. In situ measurements on the intermediate compound Tb_0.3Dy_0.7Co₂ show a rhombohedral to tetragonal structural transition, and the two phases coexist from 99 K to 111 K, where the so-called magnetic morphotropic phase boundary (MPB) is found. The coexisting phases are believed to induce the anomalous magnetostrictive effect in the MPB regime.