The π-conjugated [C₃H_6+xN₆]^x+ (x = 0–3) cations are good functional groups, which are widely employed in the preparations of nonlinear optical (NLO) and birefringent materials due to their high hyperpolarizability and optical anisotropy. In this paper, the first melamine hydroxyborate [C₃H₇N₆]₃[B₃O₅(OH)₂] (MelBO-I) was synthesized by the boric acid melting method under the molar ratio of H₃BO₃:C₃H₆N₆ = 1:1. MelBO-I (P2₁/c) exhibits a two-dimensional (2D) {[C₃H₇N₆]₃[B₃O₅(OH)₂]}_∞ layer composed of [C₃H₇N₆]⁺ cations and [B₃O₅(OH)₂]^3– anions interconnected via hydrogen bonds. MelBO-I exhibits significant birefringence (Δn = 0.286@546 nm). Under the molar ratio of H₃BO₃/C₃H₆N₆ = 3:1, [C₃H₈N₆]₄[B₁₂O₁₉(OH)₆] (MelBO-II) was isolated. In MelBO-II (P2₁), highly polymerized [B₁₂O₁₉(OH)₆]^8– groups form a 3D network through hydrogen bonding, featuring 1D tunnels of 8-membered and 16-membered rings filled by [C₃H₈N₆]²⁺ cations. MelBO-II is the first noncentrosymmetric (NCS) bifunctional melamine borate with a moderate SHG response (0.4 × KDP) and large birefringence (Δn = 0.285@546 nm). The results indicate that incorporating [C₃H_6+xN₆]^x+ (x = 0–3) cations into borate can effectively induce birefringence. A high concentration of boric acid promotes the formation of large boric acid cluster anions and facilitates the transformation from the CS to NCS structure.

中文翻译：

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[C3H7N6]3[B3O5（OH）2] 和 [C3H8N6]4[B12O19（OH）6]：两种具有大双折射的三聚氰胺硼酸盐


π共轭 [C₃H_6+xN₆] ^x+ （x = 0–3） 阳离子是良好的官能团，由于其高超极化率和光学各向异性，被广泛用于非线性光学 （NLO） 和双折射材料的制备。本文以H₃BO₃：C₃H₆N₆ = 1：1的摩尔比，采用硼酸熔融法合成了第一种羟基硼酸三聚氰胺[C₃H₇N₆]_[B₃O₅（OH）₂]（MelBO-I）。MelBO-I （P2₁/c） 表现出二维 （2D） {[C₃H₇N₆]₃[B₃O₅（OH）₂]}_∞ 层由 [C₃H₇N₆] ⁺ 阳离子和 [B₃O₅（OH）₂]3^– 阴离子组成通过氢键互连。MelBO-I 表现出显着的双折射 （Δn = 0.286@546 nm）。在 H₃BO₃/C₃H₆N₆ = 3：1 的摩尔比下，分离出 [C₃H₈N₆] ₄[B₁₂O₁₉（OH）₆] （MelBO-II）。在 MelBO-II （P2₁） 中，高度聚合的 [B₁₂O₁₉（OH）₆]^8– 基团通过氢键形成 3D 网络，具有 8 元和 16 元环的 1D 隧道，填充物为 [C₃H₈N₆] ²⁺ 阳离子。 MelBO-II 是第一个非中心对称 （NCS） 双功能硼酸三聚氰胺，具有中等 SHG 响应 （0.4 × KDP） 和大双折射 （Δn = 0.285@546 nm）。结果表明，将 [C₃H_6+xN₆]^x+ （x = 0–3） 阳离子掺入硼酸盐中可以有效地诱导双折射。高浓度的硼酸促进大硼酸簇阴离子的形成，并促进从 CS 结构向 NCS 结构的转变。