K(UO₂)O₂(H₂O)Cl·2H₂O: Identification of Uranyl Peroxide Chloride as a Studtite Derivative

Uranyl peroxide chemistry has traditionally been limited to natural phases─studtite and metastudtite─or to synthetic uranyl peroxide complexes and clusters in which the uranyl equatorial plane is generally oxygen-coordinated. Herein, we report the first uranyl peroxide chloride compound of K(UO₂)O₂(H₂O)Cl·2H₂O, featuring chloride directly coordinated within the uranyl equatorial plane, a structural motif unprecedented in natural or synthetic uranyl peroxide materials. Structural and spectroscopic analyses combined with theoretical modeling reveal a distinctive studtite-like chain structure in which Cl^- coordinates directly in the equatorial plane of uranyl. K⁺ occupies interstitial positions and engages in both K⁺-Cl^- contacts and coordination to framework oxygen atoms. Compared with studtite, K(UO₂)O₂(H₂O)Cl·2H₂O displays elongated U═O_yl bonds and twisted uranyl-peroxide chains, arising from K-O interactions. Notably, substitution with Li⁺, Na⁺, or Cs⁺ chlorides failed to yield analogous structures, demonstrating the size-specific role of K⁺. K(UO₂)O₂(H₂O)Cl·2H₂O exhibited enhanced thermal stability, which was attributed to the interlayer water bound to K⁺. This work establishes a new topology in uranyl peroxide chemistry and demonstrates how anion incorporation and size-selective cation templating can stabilize unprecedented actinide phases relevant to chloride-rich nuclear waste environments.