Manipulating chemical bonding in a solid is crucial for controlling and realizing desirable properties. We have recently demonstrated that replacing the M³⁺ cation of Bi₂MO₄Cl, which contains triple-fluorite slabs, from Y³⁺ to a larger cation (La³⁺, Bi³⁺) induces tensile strain in the Bi–O square net, resulting in Bi–O bond cleavage to form double- and single-chain structures, respectively. In this study, we synthesized a solid solution of Bi₂(La_1–xBi_x)O₄Cl with almost uniform tensile strain, revealing an unexpected tetragonal (T) phase (0.15 ≤ x ≤ 0.35), along with an additional monoclinic single-chain phase (0.425 ≤ x ≤ 0.475). The emergence of the T phase with the elongated in-plane axis likely arises from the competition between the single and double chain structures. The T phase exhibits a narrower bandgap of 2.2 eV (vs Bi₂YO₄Cl), ascribed to the presence of Bi in the inner sublayer. Furthermore, the T phase undergoes a reentrant transition upon heating via the single-chain phase, forming a high-temperature tetragonal (T′) phase with partial Bi–O bond cleavage due to spatial and thermal fluctuations of the outer Bi cations. This study emphasizes the role of uniform tensile strain in promoting phase competition in triple fluorite layered systems, resulting in complex phase transitions under external stimuli. Controlling such delicate balance offers a pathway to engineering of physical properties of functional materials, such as visible-light photocatalysts.

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