Significant Birefringence Enhancement via Cs ⁺ Alloying in Layered Perovskites: Synergistic Alignment of Organic Cations and Inorganic Framework

ABSTRACT High birefringence (Δ n ) is essential for miniaturizing polarization‐control optics, yet commercial birefringent crystals provide only limited optical anisotropy. Moreover, the underlying mechanisms for efficient birefringence modulation and the precise arrangement of functional units remain elusive. Here we report a rational chemical alloying strategy that triggers a record 23‐fold enhancement of birefringence (from 0.01 to 0.23 at 550 nm) in hybrid perovskites, TZ 2 PbBr 4 (TZ + = C 2 H 4 N 3 + ) and its Cs‐alloyed derivative TZ 2 Cs 5 Pb 4 Br 15 , achieving the highest amplification reported in any optical material. Cs + alloying reorients the [PbBr 6 ] 4− framework from (100) to (110) while enforcing parallel alignment of π ‐conjugated TZ + cations. First‐principles calculations identify this synergistic inorganic‐organic reorganization as the origin of the giant enhancement. These findings establish chemical alloying as a versatile route to deterministic control of optical anisotropy, opening avenues for the development of high‐performance integrated photonic devices.