Pressure-induced stacking disorder and suppression of long-rangeSm-typeorder in medium-entropy rare-earth alloys.

Rare-earth medium-entropy alloys provide a platform for investigating how chemical disorder modifies the well-established pressure-induced structural evolution of close-packed 4flanthanides. Here, we study TbHoEr and TbHoDy using synchrotron x-ray diffraction in diamond anvil cells. Both alloys transform from the ambient hexagonal close-packed (hcp) structure to a double hexagonal close-packed (dhcp) phase, while no well-resolved bulkSm-typeintermediate phase is observed. For TbHoEr, compression to 70 GPa further reveals a high-pressure rhombohedralhR24phase. Unlike the constituent heavy lanthanides, however, both alloys bypass the intermediateSm-typephase. Two-dimensional diffraction images further reveal streak-like diffuse scattering in the transition region, indicating stacking disorder and limited stacking coherence along the close-packed direction. These observations indicate that the transformation proceeds through a stacking-disordered close-packed state rather than through a well-ordered bulkSm-typephase. We propose that configurational disorder, local lattice distortion, stacking-fault energetics, and transformation kinetics collectively suppress the development of long-rangeSm-typeorder. The results demonstrate that medium-entropy alloying can fundamentally modify pressure-induced stacking pathways in rare-earth materials under extreme conditions.