Atomically Sharp Heterophase Interfaces in Cation-Eutaxy A-III-V Compounds

Atomically sharp interfaces between dissimilar crystal phases are typically achieved through vacuum-based epitaxial growth, whereas solution-based processes generally produce broadened reaction fronts and compositional gradients. Here, we show that wet etching of cation-eutaxy A-III-V compounds unexpectedly yields atomically sharp heterophase interfaces. Using CaGa2P2 as a model system, we demonstrate a composition-driven structural evolution from a stable cation-eutaxy phase to a metastable Ca-deficient cation-eutaxy phase, followed by the emergence of an atomically sharp interface between cation-eutaxy CaGa2P2 and zinc-blende GaP, and culminating in complete conversion to zinc-blende GaP. This behavior arises from the absence of thermodynamically stable intermediate phases, a large mixing enthalpy imposed by coordination mismatch, and strong curvature-driven interface flattening associated with directional III-V covalent bonding. The same transition pathway and atomic scale interface sharpness are reproduced in CaIn2P2 and CaGa2As2, establishing the generality of this mechanism across cation-eutaxy A-III-V compounds. These findings demonstrate that atomic-scale interface control is achievable in diffusion-limited solution processes and motivate future electrical and optical studies of devices exploiting such sharply defined heterophases.