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 IMG ALIGN="MIDDLE" ALT="" SRC="cmab5cc9ieqn001.gif" at temperatures above the Curie temperature T _C; the Tb-rich side sample shows a rhombohedral structure with space group IMG ALIGN="MIDDLE" ALT="" SRC="cmab5cc9ieqn002.gifand the Dy-rich side sample has a tetragonalIMG ALIGN="MIDDLE" ALT="" SRC="cmab5cc9ieqn003.gifspace 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.