Dimensional Reduction of Transition Metal Dichalcogenides by Oxide Insertion: Quasi‐1D Oxychalcogenide Semiconductors

Heteroanionic solids offer unique levers for tuning structures and properties, but accessing multianion phase space remains challenging due to mismatches in anion reactivity. Here, we describe a new family of A 2 Hf 3 Q 6 O (A = K, Rb, Cs; Q = S, Se) oxychalcogenides, the first reported in the A─Hf─Q─O quaternary space, which are prepared by reacting HfQ 2 with the corresponding alkali carbonate (A 2 CO 3 ). The resulting structures are dimensionally reduced variants of the 1T‐HfQ 2 parent phase, where rows of O atoms insert into the transition metal dichalcogenide (TMD) layers. This yields nanometer‐wide 1D‐[Hf 3 Q 6 ] ribbons that link into step‐like sheets through distinctive Hf─O─Hf bridges, which connect heteroleptic HfQ 5 O polyhedra at the ribbon edges. UV/Vis and photoluminescence spectra reveal tunable absorption and emission features spanning 1.8–2.6 eV. Calculated band structures show direct‐gap transitions and highly dispersive bands along the [Hf 3 Q 6 ] nanoribbons, yielding effective masses more than two orders of magnitude lower than in other crystallographic directions. Charge‐density plots further illustrate quasi‐1D electronic anisotropy, showing frontier orbitals confined to the nanoribbons due to covalent Hf─Q bonding. We anticipate heteroanion‐driven dimensional reduction can be used to engineer anisotropy in semiconductors and offers new routes to synthetically challenging multianion systems.