Intersectant Coplanar [CO3] Groups with Stabilized Interlayer-Connection, Inducing Comparable Second Harmonic Generation Response, the Largest Band Gap and Suitable Birefringence in YCO4H crystal

A rare earth hydroxycarbonate DUV NLO crystal YCO 4H has been successfully grown via the hydrothermal method. YCO 4H crystallizes in the orthorhombic non-centrosymmetric space group P2 12 12 1, featuring a three-dimensional framework constructed by distorted [YO 9H 2] polyhedra connected by intersectant coplanar [CO 3] groups with van der Waals hydroxy action, which effectively suppresses the layered growth habit and favors the growth of large single crystals. Compared with typical DUV NLO carbonate crystals (e.g., ABCO 3F series, NaZnCO 3(OH), LiZnCO 3(OH)), YCO 4H exhibits superior comprehensive performance.The powder second-harmonic generation (SHG) intensity reaches approximately 1.6 times that of KDP under 1064 nm laser irradiation by Kurtz−Perry method, which is consistent with the SHG tensor calculation d14 ranges from 0.68 to 0.99 pm/V at different scissors operators. Compared with typical ABCO 3F series carbonates, the SHG response of YCO 4H is higher than that of RbCaCO 3F and CsCaCO 3F (1.11×KDP), while it is lower than that of KSrCO 3F, RbSrCO 3F, and KCaCO 3F (3.33–3.61×KDP). Overall, its SHG efficiency falls within the moderate range of reported carbonate NLO crystals, providing a good balance between SHG response and other key properties. Thermal analysis confirms that YCO 4H exhibits excellent thermal stability up to 700 K. First-principles calculations reveal that YCO 4H possesses a record-wide band gap of 7.86 eV (158 nm) among all known carbonate NLO crystals, which is much higher than that of the ABCO 3F series (6.27-6.62 eV), NaZnCO 3(OH) (~6.2 eV) and LiZnCO 3(OH) (>6.53 eV). YCO 4H also has a suitable birefringence of 0.064 @1064 nm, which is moderate and falls within the favorable range for DUV NLO crystals (0.06–0.12). Compared with the ABCO 3F series (0.034–0.116 @1064 nm), it is lower than most high-birefringence members (e.g., RbCaCO 3F: 0.116, KCaCO 3F: 0.112, KSrCO 3F: 0.105), but significantly higher than low-birefringence ones (e.g., Cs 3Ba 4(CO 3) 3F 5: 0.034). Such a moderate value is beneficial for achieving DUV phase-matching output. Two critical phase-matching wavelengths are quantitatively determined: Conventional birefringent phase-matching (BPM): shortest λPM =275 nm and Artificial periodic phase-matching (PPM): shortest λPM ≈158 nm (near the bandgap edge). These results demonstrate that YCO 4H achieves an excellent balance of ultra-wide bandgap, strong SHG response, and favorable phase-matching capability for DUV output. As a beryllium-free, thermally stable, and easily grown crystal, YCO 4H is superior to traditional ABCO 3F series carbonates and opens a new direction for the design and development of next-generation DUV NLO crystals.