Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Anhydrous compounds are crucially important for many technological applications, such as achieving high performance in lithium/sodium cells, but are often challenging to synthesize under hydrothermal conditions. Herein we report that a modified solvo-/hydro-fluorothermal method with fluoride-rich and water-deficient condition is highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Two anhydrous phosphate germanium fluorides, namely, Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], with chainlike structures involving multiple fluorine substitutions, were synthesized using the modified solvo-/hydro-fluorothermal method. The crystal structure of Na3[GeF4(PO4)] is constructed by the common single chains ∞1{[GeF4(PO4)]3–} built from alternating GeO2F4 octahedra and PO4 tetrahedra. For K4[Ge2F9(PO4)], it takes the same single chain in Na3[GeF4(PO4)] as the backbone but has additional flanking GeOF5 octahedra via an O-corner of the PO4 groups, resulting in a dendrite zigzag single chain ∞1{[Ge2F9(PO4)]4–}. The multiple fluorine substitutions in these compounds not only force them to adopt the low-dimensional structures because of the “tailor effect” but also improve their thermal stabilities. The thermal behavior of Na3[GeF4(PO4)] was investigated by an in situ powder X-ray diffraction experiment from room temperature to 700 °C. The modified solvo-/hydro-fluorothermal method is also shown to be effective in producing the most germanium-rich compounds in the germanophosphate system.无水化合物（anhydrous compounds）在诸多技术应用中至关重要，例如实现锂/钠电池的高性能表现，但在水热条件（hydrothermal conditions）下通常难以合成。本文报道了一种基于富氟缺水环境的改性溶剂热-水热氟热法（solvo-/hydro-fluorothermal method），通过用氟（fluorine）取代羟基（hydroxyl groups）与水分子，可高效合成无水化合物。采用该方法，我们合成了两种具备多氟取代特征的链状结构（chainlike structures）磷酸氟锗化合物（phosphate germanium fluorides）：Na3[GeF4(PO4)]与K4[Ge2F9(PO4)]。其中Na3[GeF4(PO4)]的晶体结构（crystal structure）由常见的无限单链∞1{[GeF4(PO4)]3–}构筑而成，该链由交替排布的GeO2F4八面体（octahedra）与PO4四面体（PO4 tetrahedra）连接得到。对于K4[Ge2F9(PO4)]，其以Na3[GeF4(PO4)]中的相同单链作为骨架（backbone），通过PO4基团的氧顶点（O-corner）连接了额外的侧翼GeOF5八面体（flanking GeOF5 octahedra），最终形成树枝状锯齿形单链（dendrite zigzag single chain）∞1{[Ge2F9(PO4)]4–}。这些化合物中的多氟取代不仅凭借剪裁效应（tailor effect）促使其形成低维结构，还提升了其热稳定性（thermal stabilities）。我们通过原位粉末X射线衍射（in situ powder X-ray diffraction）实验，在室温至700 ℃的温度区间内对Na3[GeF4(PO4)]的热行为进行了研究。此外，该改性溶剂热-水热氟热法还可有效合成锗磷酸盐体系（germanophosphate system）中富锗程度最高的化合物。