Two structural polymorphs of CdP₂, including orthorhombic α-CdP₂ (Pna2₁) and tetragonal β-CdP₂ (P4₁2₁2/P4₃2₁2), were synthesized through one chemical transport reaction and one solid-state reaction, respectively. Their three-dimensional (3D) framework consisted of _∞¹[P^–] polyanion chains and CdP₄ tetrahedra. Remarkably, α-CdP₂ exhibited excellent comprehensive optical properties, including a strong SHG effect (3.2× AgGaS₂), large birefringence (Δn = 0.138), high LIDT (10.1× AgGaS₂), and wide IR transparent range, which suggested that α-CdP₂ was a promising IR NLO material. Theoretical studies also revealed that the large SHG effect and birefringence originated from the synergism of its parallel polar _∞¹[P^–] polyanion chains and distorted CdP₄ tetrahedra, in which the charge transfers from πp_z(P1–P2) to σ*(Cd–P1) and π*p_y(P1–P2) were responsible for the large SHG effect and birefringence. This indicated that P–P homoanion bonds could be adapted to design the IR NLO crystal with large SHG effects and moderate band gaps.

中文翻译：

---

α-CDP ₂：大SHG影响源自于并行的协同作用_∞ ¹ [P ^- ]聚阴离子链和扭曲CDP ₄四面体

CDP的两个结构多晶型物₂，包括斜方晶α-CDP ₂（PNA 2 ₁）和四方β-CDP ₂（P 4 ₁ 2 ₁ 2 / P 4 ₃ 2 ₁ 2），是通过一个化学传输反应和一种固体合成态反应。它们的三维（3D）框架包括了_∞ ¹ [P ^- ]聚阴离子链和CDP ₄四面体。值得注意的是，α-CDP ₂显示出优异的综合光学性能，包括强SHG效果（3.2×AgGaS ₂），大的双折射（Δ Ñ = 0.138），高LIDT（10.1×AgGaS ₂），和宽IR透明范围，这表明α-CDP ₂是一个有希望的IR NLO材料。理论研究还表明，大SHG效果和双折射源自其平行极性的协同作用_∞ ¹ [P ^- ]聚阴离子链和扭曲CDP ₄四面体，其中来自率Πp电荷转移_Ž（P1-P2），以σ*（ Cd–P1）和π* p _y（P1–P2）是造成较大的SHG效应和双折射的原因。这表明，P-P均阴离子可用于设计具有大SHG效应和中等带隙的IR NLO晶体。