NEW COPPER PHOSPHATES WITH THE NASICON OR ALLUAUDITE-TYPE STRUCTURES AS IONIC OR MIXED CONDUCTORS

The electrical properties of the copper(I) phosphates CuTi2(PO4)3, CuT iZr(PO4)3, CuZr2(PO4)3, Cu2ZrSc(PO4)3 and Cu2TiCr(PO4)3 which adopt th e NASICON structure are reviewed with comparison to their sodium analo gues. Two new compounds (Cu(I)Cu(II))1.35(Fe(II)Fe(III))3(PO4)3 and Cu (I)Cu(II)Mg3 (PO4)3 exhibiting the alluaudite-type structure have been prepared and characterised. Cu(I)Cu(II)Mg3 (PO4)3 shows mixed copper valency and mixed conductivity, with a roughly temperature independent transport number t(ion) (Cu2Mg3(PO4)3) = 0.3, and E(a) = 0.96 eV, sug gesting a coupling of the two transport mechanisms. A mechanistic comp arison is made with the mixed valence NASICON compound Na2.5ZrFe(II)0. 5Fe(III)0.5 (PO4)3.