Effect of transition metal ions on the dielectric properties of chromium potassium phosphates

Potassium ions are important for developing electrode materials because they have similar properties to lithium and sodium ions. Mixed chromium phosphates (${\mathrm{KM}}^{\mathrm{II}}\mathrm{Cr}{({\mathrm{PO}}_4)}_2$${\mathrm{KM}}^{\mathrm{II}}\mathrm{Cr}{({\mathrm{PO}}_4)}_2$) with substituted M^II sites using divalent elements (M = Ni, Co, Cu) were synthesized using a solid-state reaction method. The samples were analyzed using various techniques such as powder X-ray diffraction, Fourier transform infrared, Raman, and electron paramagnetic resonance spectroscopy. The proposed phosphates had a monoclinic phase structure with a P2₁/n space group, and they contained large tunnels occupied by K⁺ cations. The dielectric properties showed that the Ni-based phosphates had slower dielectric relaxation, while the Co and Cu-based phosphates had quicker polarization and depolarization processes. Additionally, the resistance of the grains decreased from Ni to Co to Cu-based phosphates, indicating easier charge movement in each material, consistent with the increase in conductive losses and a.c. conductivity when changing the M^II ions.