Ionic conductivity of crystalline and amorphous Na3Al2(PO4)2F3

The ionic conductivity of Na3Al2(PO4)(2)F-3 (space group I4/mmm) has been studied by impedance spectroscopy in the temperature range 25-650 degrees C for both the crystalline and the amorphous compounds. The de conductivity is mainly due to a long-range Na+ motion in the cavities formed by the arrangement of the [Al2O8F3] bioctahedra and the [PO4] tetrahedra of the material. According to the structure this motion would be bidimensional. The frequency dependence of sigma' shows that,for both materials, the Na+ motion is somehow correlated and that the increase of conductivity at high frequency may be ascribed to the relaxation of the neighbourhood of the mobile species after hops occurred. On the other hand, the frequency dependence of sigma ", for the amorphous compound, reveals at low frequency a dipolar relaxation which may be caused by the motion of small units of polyhedra present in this disordered material. This relaxation is thermally activated and does not appear in the crystalline compound which presents a long-range ordering. (C) Published by Elsevier Science B.V. All rights reserved.