Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Two Noncentrosymmetric Bismuth Phosphates: Rational Design Synthesis and Optical Properties

Developing strategies to rational design noncentrosymmetric structure still attract much interest for their applications in nonlinear optical and piezoelectric materials. Two noncentrosymmetric (NCS) alkaline earth metal bismuth phosphates have been successfully achieved via partial replacement of Bi3+ with Ca2+ or Sr2+ ions. BiCa(H0.5PO4)2 (designated as CaBiPO) and BiSr(H0.5PO4)2 (designated as SrBiPO), together with their solid solution Bi(Sr1-xCax)(H0.5PO4)2 (0 < x ≤ 0.5), crystallize in the NCS space group C2. Both CaBiPO and SrBiPO exhibit ultraviolet nonlinear optical (NLO) properties, and their second-harmonic generation effects belong to type-II phase matching. Meanwhile, they could also act as photoluminescence hosts in which the Eu3+-doping samples SrBiPO:xEu3+ (x = 0.02-0.2) emit orange light. The effect of different radius ions on the derivative structures and the structure-NLO property relationship has also been discussed in detail.

Rational Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Fluorinated Orthophosphate: BaZn(PO4)F

Rational design and tailored synthesis of noncentrosymmetric compounds with nonlinear optical (NLO) properties, especially in the deep-ultraviolet (deep-UV) region, remains a great challenge. Herein, we report on the development of a modified fluoro-solvo-hydrothermal method with two additive reagents (trimethylamine and NaF solution) as the solvents, using BaFe(PO₄)(OH) ( P2₁2₁2₁) as the prototype, for the rational design and tailored synthesis of the first deep-UV fluorinated orthophosphate, BaZn(PO₄)F. It crystallizes in the polar space group Pna2₁ and exhibits transparency down to deep-UV region (<190 nm) with SHG effect at 0.26 × KH₂(PO₄). Its structure is built from strictly alternating ZnO₄F trigonal bipyramids and PO₄ tetrahedra, resulting in a four-connected ABW-type zeolite framework. First-principles calculations confirm the deep-UV absorption edge and reveal that ZnO₄F plays an essential role in the NLO properties. The synergetic effect of Zn and F atoms leads to its more polar crystal structure, much deeper absorption edge, and better SHG effect than the prototype.

Structural diversities induced by cation sizes in a series of fluorogermanophosphates: A2[GeF2(HPO4)2] (A = Na, K, Rb, NH4, and Cs)

Germanophosphates, in comparison with other metal phosphates, have been less studied but potentially exhibit more diverse structural chemistry with wide applications. Herein we applied a hydro-/solvo-fluorothermal route to make use of both the "tailor effect" of fluoride for the formation of low dimensional anionic clusters and the presence of alkali cations of different sizes to align the anionic clusters to control the overall crystal symmetries of germanophosphates. The synergetic effects of fluoride and alkali cations led to structural changes from chain-like structures to layered structures in a series of five novel fluorogermanophosphates: A₂[GeF₂(HPO₄)₂] (A = Na, K, Rb, NH₄, and Cs, denoted as Na, K, Rb, NH4, and Cs). Although these fluorogermanophosphates have stoichiometrically equivalent formulas, they feature different anionic clusters, diverse structural dimensionalities, and contrasting crystal symmetries. Chain-like structures were observed for the compounds with the smaller sized alkali ions (Na⁺, K⁺, and Rb⁺), whereas layered structures were found for those containing the larger sized cations ((NH₄)⁺ and Cs⁺). Specifically, monoclinic space groups were observed for the Na, K, Rb, and NH4 compounds, whereas a tetragonal space group P4/mbm was found for the Cs compound. These compounds provide new insights into the effects of cation sizes on the anionic clusters built from GeO₄F₂ octahedra and HPO₄ tetrahedra as well as their influences on the overall structural symmetries in germanophosphates. Further characterization including IR spectroscopy and thermal analyses for all five compounds is also presented.