Stability, magnetism, and vibrations in MnTe_xSb_yBi_1-x-y alloys

We investigate the energetics, magnetic orderings, lattice, vibrational, and electronic properties of MnTe_xSb_yBi_1–x–y alloys, whose parent materials have distinct electronic and magnetic behaviors: MnTe being an antiferromagnetic (AFM) semiconductor and MnSb and MnBi being ferromagnetic (FM) metals. Using large, disordered supercells within density functional theory, we show that composition significantly impacts the structural and functional properties of these alloys. Broadening of vibrational spectra due to strong positional and bond variations is found at the AFM to FM transition in the MnTe_xSb_1–x alloys near x = 0.75, accompanying a semiconductor-to-metal transition similar to that found in measurements. Te-rich alloys also exhibit pronounced volume expansion when comparing relaxed AFM structures to their FM counterparts, highlighting strong spin-lattice coupling that could enable tunable magnetostructural responses near the AFM to FM transition. Furthermore, the virtual crystal approximation for phonons is validated against full disordered supercell calculations, reliably capturing vibrational entropies and free energy trends, and providing an efficient means of evaluating thermal stability across the compositional phase space. These findings emphasize the interplay between magnetism, electronic structure, and lattice structure and dynamics in MnTe_xSb_yBi_1–x–y alloys, offering a pathway for designing multifunctional magnetic materials with potential applications in information storage, spintronic devices, and tunable magnetic systems.