Abstract
Single crystals and powders of the orthophosphates A2Mg2Fe(PO4)3 with A = Na+ or Ag+ have been synthesized by solid-state reaction. The X-ray diffraction diagrams of Na2Mg2Fe(PO4)3 and Ag2Mg2Fe(PO4)3 were refined by pattern matching method. The two compounds crystallize in C2/c space group adopting the alluaudite type structure. Refinement of the crystal structure of the silver-containing compound leads to the non-stoichiometric chemical formula Ag1·67Mg2·33Fe0·89(PO4)3 while that of the sodium-containing compound leads to the stoichiometric chemical formula Na2Mg2Fe(PO4)3. The complex impedance plots are obtained in frequency range from 0.1 Hz to 1 MHz, between 573 K and 823 K Na2Mg2Fe(PO4)3 presents a conductivity of 0.25 × 10−5 S cm−1 at 773 K, with Ea = 0.81 eV. For Ag2Mg2Fe(PO4)3, a better ionic conductivity of 1.8 × 10−3 S cm−1 has been observed at the same temperature with Ea = 0.47 eV. The two alluaudite compounds present an intrinsic cationic conductivity type resulting from Na+ or Ag+ ions movement.
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