Synthesis and Electronic Structure of the Fractionally Occupied Double Perovskite EuTa ₂ O ₆ with Ordered Europium Vacancies

Abstract 2D electronic states underline a wide range of exotic phenomena and provide potential for electronic devices. The ability to create and control these states often requires physical thinning or highly perfect interfaces. In this work, the “fractional double perovskite” A Ta ₂ O ₆ ( A : Eu ²⁺ ) is synthesized and characterized, where alternating A ‐site cations and A ‐site vacancies significantly impact the electronic structure, giving rise to a quasi‐2D electronic state within a 3D crystal framework. The intrinsic crystal anisotropy of A Ta ₂ O ₆ plays a pivotal role, underscoring how targeted structural modifications can facilitate the emergence of novel quantum states. Utilizing single‐crystal synthesis via molecular‐beam epitaxy, the crystal and electronic structures of A Ta ₂ O ₆ are investigated. X‐ray diffraction and electron microscopy reveal the layered A ‐site ordering. Synchrotron‐based diffraction shows the presence of three epitaxial twin variants of A Ta ₂ O ₆ domains, with preferential orientation along out‐of‐plane direction. Angle‐resolved photoemission spectroscopy, along with density functional theory calculations, provide direct insight into the electronic structure, unveiling the potential for engineered confined states within bulk materials. These findings highlight A Ta ₂ O ₆ as a platform for studying 2D‐like electronic phenomena in a 3D context, paving the way for novel device architectures.