Novel Sol–Gel Synthesis of MgZr₄P₆O₂₄ Composite Solid Electrolyte and Newer Insight into the Mg²⁺-Ion Conducting Properties Using Impedance Spectroscopy

Magnesium zirconium phosphate, MgZr₄P₆O₂₄ (MZP), is a magnesium ion conducting ceramic material with potential for application as solid electrolyte in high temperature electrochemical sensor in non-ferrous scrap metal refining and virgin metal alloying operations. In this work, MZP was synthesized using a simple but novel and economical sol–gel route at a significantly reduced temperature. An insight into the calcination process and possible phase transformation at higher temperature was obtained using simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC). Phase identification of the synthesized material was studied after calcining the powder at 900 °C for 3 h using X-ray diffraction (XRD); a single monoclinic phase was observed at that temperature. However, a trace amount of possible minor second phase, zirconium oxide phosphate [Zr2(PO4)2O], was formed after heating at temperatures T ≥ 1000 °C. Impedance spectroscopy measurements on platinized sintered-MZP pellets were carried out in the frequency range 100 mHz to 32 MHz and in a temperature range of 30–800 °C to determine the electrical properties of MZP. Ionic conductivity of MZP was found to be equal to 7.23 × 10–3 Ω–1 cm–1 at relatively lower temperature, 725 °C. Furthermore, the Nyquist and modulus plots measured at 764 and 390 °C show single semicircles suggesting contribution from only grain interiors (GIs). The ion-hopping rate was calculated by fitting the conductance spectra to the power law variation, σac (ω) = σdc + Aωn. The ac and dc conductivity of MZP shows Arrhenius-type of behavior with activation energies in the range 0.84 ≤ Ea(eV) ≤ 0.87. SEM of the fractured MZP pellet sintered at 1300 °C for 24 h revealed a highly dense microstructure with clearly visible grain boundaries and low porosity which is in good agreement with the relative density of ∼99% determined using the Archimedes’ principle and the theoretical density calculated from the crystal structure. EDS confirms the presence of Mg, Zr, P, O in appropriate atomic ratio to yield MgZr4P6O24. Finally, TEM on MZP particles with crystallite size of ∼50 nm also confirmed MZP as stable at 900 °C with no observable second phase, Zr2(PO4)2O.