Struvite-type AMgPO₄·6H₂O (A = NH₄, K): Two Natural Deep-Ultraviolet Transparent Nonlinear Optical Crystals

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play vital roles in diverse fields. Unfortunately, only the KBe₂BO₃F₂ crystal has found commercial applications so far. Therefore, the discovery of new DUV NLO crystals is still urgent. As we all know, digging into the properties of existing crystals is also an effective way to obtain new NLO crystals. Herein, two natural asymmetric orthophosphates AMgPO₄·6H₂O (A = NH₄, K) are proposed. Although their structures and some properties such as infrared spectra, thermal properties, and dielectric properties have been previously characterized, their NLO properties have not been reported. Thus, in this work, these two natural DUV transparent orthophosphates NH₄MgPO₄·6H₂O (NMP) and KMgPO₄·6H₂O (KMP) were successfully acquired by a simple slow evaporation method. The single-crystal X-ray diffraction data indicate that NMP and KMP are isomorphic and that both belong to the Pmn2₁ space group of the orthorhombic system. Remarkably, NMP and KMP possess short cutoff edges below 190 nm, and their second-harmonic generation (SHG) efficiencies are 0.62 and 0.80 times that of KH₂PO₄(KDP), respectively; furthermore, they can achieve type-I phase matching at 1064 nm.