Deep-Ultraviolet Nonlinear-Optical Crystals K 1– x A x Be ₂ BO ₃ F ₂ (A = NH ₄ , OH ₃ ) Realized by A-Site Cationic Solid Solution

Deep-ultraviolet (DUV) nonlinear-optical (NLO) crystals are essential for sub-200 nm coherent light. However, KBe2BO3F2 (KBBF) suffers from layered cleavage and a BO3 rotation-driven phase transition from R32 to R3̅c, which quenches second-harmonic generation (SHG). In contrast, isostructural NH4Be2BO3F2 (ABBF) retains the R32 phase during hydrothermal growth, offering both motivation and mechanistic insight for A-site engineering. This study elucidates the critical role of interlayer hydrogen bonding in suppressing the BO3 rotation that drives the detrimental R32 → R3̅c phase transition in KBBF-type crystals. Based on this mechanism, we propose a hydrogen-bond engineering strategy via A-site cationic alloying, leading to the design of two novel materials: K1–x(NH4)xBe2BO3F2 (KABBF) and H3OBe2BO3F2 (HBBF). KABBF is thermodynamically stable near x = 0.5 and maintains a phase-matched wavelength of 183 nm, while HBBF features an exceptionally high BO3 rotational barrier and enables DUV SHG output as short as 164 nm. Both materials retain wide bandgaps (>8.2 eV) and strong SHG effects (>0.4 pm/V) comparable to KBBF. These findings establish a chemically intuitive strategy to overcome the longstanding structural instability in KBBF and open new avenues toward high-performance DUV NLO materials.