Internal strain-driven bond manipulation and band engineering in Bi Sb YOCl photocatalysts with triple fluorite layers.

In extended solid-state materials, the manipulation of chemical bonds through redox reactions often leads to the emergence of interesting properties, such as unconventional superconductivity, which can be achieved by adjusting the Fermi level through, , intercalation and pressure. Here, we demonstrate that the internal 'biaxial strain' in tri-layered fluorite oxychloride photocatalysts can regulate bond formation and cleavage without redox processes. We achieve this by synthesizing the isovalent solid solution Bi Sb YOCl, which undergoes a structural phase transition from the ideal BiYOCl structure to the SbYOCl structure with (Bi,Sb)O rings. Initially, substitution of smaller Sb induces expected lattice contraction, but further substitution beyond > 0.6 triggers an unusual lattice expansion before the phase transition at = 1.5. Detailed analysis reveals structural instability at high values, characterized by Sb-O underbonding, which is attributed to tensile strain exerted from the inner Y sublayer to the outer (Bi,Sb)O sublayer within the triple fluorite block - a concept well-recognized in thin film studies. This concept also explains the formation of zigzag Bi-O chains in BiMOCl (M = Bi, La). The Sb substitution in Bi Sb YOCl elevates the valence band maximum, resulting in a minimized bandgap of 2.1 eV around = 0.6, which is significantly smaller than those typically observed in oxychlorides, allowing the absorption of a wider range of light wavelengths. Given the predominance of materials with a double fluorite layer in previous studies, our findings highlight the potential of compounds endowed with triple or thicker fluorite layers as a novel platform for band engineering that utilizes biaxial strain from the inner layer(s) to finely control their electronic structures.