New Iron(III) Phosphate Phases: Crystal Structure and Electrochemical and Magnetic Properties

Two new iron(III) phosphates, FePO4, have been synthesized from the dehydration of hydrothermally prepared monoclinic and orthorhombic hydrated phosphates FePO4·2H2O. The structures of both hydrates were redetermined from single crystal data. On dehydration, a topotactic reaction takes place with only those bonds associated with the water molecules being broken, so that both FePO4 phases have essentially the same Fe−P backbone frameworks as the corresponding hydrates. They are, respectively, monoclinic FePO4, space group P21/n, a= 5.480(1) Å, b = 7.480(1) Å, c= 8.054(1) Å, β = 95.71(1)°, and Z = 4; and orthorhombic FePO4, space group Pbca, a = 9.171(1) Å, 9.456(1) Å, c = 8.675(1) Å, and Z = 8. Both of these phases are thermally unstable relative to the trigonal quartz-like FePO4. The electrochemical studies find that the orthorhombic iron phosphate is more active than the monoclinic phase, while both are more active than trigonal FePO4. Both phases approach Curie−Weiss behavior at room temperature, with the monoclinic phase exhibiting stronger antiferromagnetic interactions due to Fe−O−Fe interactions. The electrochemical and magnetic data are consistent with the structures of these two compounds. The properties of these new iron phosphate structures are compared with other iron phosphate phases.本研究通过对水热合成得到的单斜晶系、正交晶系水合磷酸铁FePO4·2H2O进行脱水处理，成功制备了两种新型铁(III)磷酸盐FePO4。两种水合物的结构均通过单晶衍射数据完成了重新测定。脱水过程中发生拓扑化学反应，仅断裂与水分子相关的化学键，因此两种FePO4晶相的Fe-P骨架与对应水合物的骨架基本一致。两种晶相分别为单斜相FePO4（空间群P2₁/n，晶胞参数a=5.480(1) Å、b=7.480(1) Å、c=8.054(1) Å，β=95.71(1)°，Z=4）以及正交晶相FePO4（空间群Pbca，晶胞参数a=9.171(1) Å、b=9.456(1) Å、c=8.675(1) Å，Z=8）。相较于类三角石英结构的FePO4，这两种晶相均表现出热不稳定性。电化学研究结果表明，正交晶系磷酸铁的电化学活性优于单斜晶系相，且二者均优于三角晶相FePO4。室温下两种晶相均表现出近居里-外斯行为，其中单斜晶相因Fe-O-Fe桥联相互作用呈现出更强的反铁磁相互作用。电化学与磁学测试数据与这两种化合物的结构特征相符。本研究还将这两种新型磷酸铁结构的性能与其他已知磷酸铁晶相进行了对比。