Structural and Electrical Properties of Novel Cr/Fe Mixed Transition-Metal Phosphates.

The phosphate KCoCr(PO) and iron-substituted variants KCoCrFe(PO) ( = 0.25, 0.5, and 0.75) were synthesized by a solid-state reaction route, while a high substitution level of Fe was achieved. Their structures were refined using powder X-ray diffraction and indexed in a monoclinic system with a 2/ space group. A 3D framework with six-sided tunnels parallel to the [101] direction was formed in which the K atoms are located. Mössbauer spectroscopy confirms the exclusive presence of octahedral paramagnetic Fe ions, with isomer shifts increasing slightly with substitution. Electron paramagnetic resonance spectroscopy confirmed the presence of paramagnetic Cr ions. The activation energy, determined by dielectric measurements, shows that the iron-containing samples present higher ionic activity. Relative to the electrochemical activity of K, these materials could be good candidates for positive and/or negative electrode materials for energy storage applications.