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Deep‐ultraviolet (DUV) nonlinear optical (NLO) crystals are vital to the development of science and state‐of‐the‐art technologies, but the exploration of these important optoelectronic materials is hindered by the contradiction between NLO effect and energy band gap as well as the sensitivity of DUV phase‐matching to optical birefringence. Here, in order to achieve a well balanced DUV NLO performance, we chose the diamond‐like structure as the prototype template for the material design and synthesized the first DUV NLO hydroxycarbonate crystal, LiZn(OH)CO3. This compound simultaneously has a wide band gap (>6.53 eV), a strong second harmonic generation response (ca. 3.2×KDP), and a large birefringence (0.147 at 1064 nm) with the shortest DUV phase‐matching capability (λPM<190 nm) among all reported NLO carbonates. Moreover, the structure–property relationships in LiZn(OH)CO3 are analyzed by first‐principles calculations. This work presents an effective design strategy to push the NLO performance of hydroxycarbonates into the DUV region. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first‐principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well‐known NLO phosphate KH2PO4, the oxythiophosphate KH2PO3S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V−1), and large birefringence (ca. 0.1 at 1 μm) with short phase‐matching SHG output wavelength (≈214 nm). [ABSTRACT FROM AUTHOR]