Defying immiscibility via epitaxial design

Thermodynamically driven immiscibility in materials science fundamentally limits the synthesis of many functional materials with desired mechanical, optical, electronic, and magnetic properties. Here, we demonstrate that epitaxial design can overcome these limits by stabilizing otherwise immiscible constituents within a fully miscible solid solution. Using hybrid molecular beam epitaxy (MBE), we control epitaxial strain as well as interface and surface energies, to drive the formation of a complete solid solution far beyond bulk solubility limits in the SnO₂-TiO₂ system. Through a combination of atomically precise synthesis, X-ray diffraction, X-ray spectroscopy, scanning transmission electron microscopy, and density functional theory calculations, we reveal the underlying mechanisms. Our results show how epitaxial thin films can defy the conventional rules of materials science governing bulk immiscibility, offering a powerful route to design materials with otherwise inaccessible compositions and functionalities.