To combine noncentrosymmetry, large second order nonlinear optical (NLO) susceptibility, deep UV transparency, and phase matchability simultaneously into a single structure is a great challenge. Especially, the relationship between the macrophase matchability and microstructure has been less investigated. Herein, we report a unique Ba2NaClP2O7 representing the first phase-matchable example of the fresnoite family, which exhibits excellent NLO properties, including short absorption edge (<176 nm, ca. 7.0 eV), strong second-harmonic generations at the incident lasers of 850-1110 nm (obv. 0.9-1.3 × KDP; cal. dave, 1.11 vs 0.96 pm/V of KDP), and large laser-induced damage threshold (1.4 × KDP). More interestingly, Ba2NaClP2O7 illustrates an unprecedented mechanism of phase matchability that is promoted by crystallographic disorder. The uniaxial symmetry breaking caused by the disorder of [P2O7]4- group results in a structural anisotropy that changes the band dispersions along k-paths of G-Y and G-X, resulting in an increase of refractive index n x and a n y decrease, eventually enlarging the birefringence.

中文翻译：

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Ba2NaClP2O7：由对称破坏及其独特的 Fresnoite 型结构引起的前所未有的相位匹配性

将非中心对称性、大二阶非线性光学 (NLO) 磁化率、深紫外透明度和相位匹配性同时结合到单个结构中是一个巨大的挑战。特别是，宏观相匹配性与微观结构之间的关系研究较少。在此，我们报告了一种独特的 Ba2NaClP2O7，代表了fresnoite 家族的第一个相位匹配示例，它表现出出色的 NLO 特性，包括短吸收边（<176 nm，约 7.0 eV）、入射激光处的强二次谐波产生。 850-1110 nm（观察值 0.9-1.3 × KDP；cal. dave，1.11 vs 0.96 pm/V of KDP）和大激光诱导损伤阈值 (1.4 × KDP)。更有趣的是，Ba2NaClP2O7 说明了一种前所未有的由晶体无序促进的相匹配机制。