SOLID STATE lOgES ELSEVIER Solid State Ionics 74 (1994) 119-123 New copper phosphates with the NASICON or alluaudite-type structures as ionic or mixed conductors T.E. Warner, W. Milius, J. Maier * Max-Planck-lnstitut fiir Festk~rperforschung, Heisenbergstrafle I, D-70569 Stuttgart, Germany Received 1 June 1993;acceptedfor publication 1 September 1994 Abstract The electrical properties of the copper(I) phosphates CuTi2(PO4)3, CuTiZr(PO4)3, CuZr2(PO4)3, Cu2ZrSc(PO4)3 and Cu2TiCr (PO4)3 which adopt the NASICON structure are reviewed with comparison to their sodium analogues. Two new com- pounds (CuXCu n) L3S (FenFe m) 3(PO4) 3and CuICullMg3 (PO 4) 3exhibiting the alluaudite-type structure have been prepared and characterised. CuXCullMg3 (PO4) 3shows mixed copper valency and mixed conductivity, with a roughly temperature independent transport number tion(Cu2Mg3(PO4)3) --0.3, and E.=0.96 eV, suggesting a coupling of the two transport mechanisms. A me- chanistic comparison is made with the mixed valence NASICON compound Na2.sZrFeno.sFemo. 5 (PO4)3. Keywords: Copper phosphates; NASICON;Alluaudite; Magneticsusceptibility;Electrical conductivity;Mixedvalency 1. Introduction The well known crystal structure of the superionic sodium conductor NASICON comprises of a rigid three-dimensional framework of PO4 or SiO4 tetra- hedra sharing corners with ZrO6 octahedra, provid- ing two sites for cations where both are involved in the conduction mechanism [1,2]. Partial substitu- tion of P by Si increases the ionic conductivity such that NaaZr2(SiO4)2PO4 has a very high ionic con- ductivity of0.19 S cm- ~at 573 K, with an activation energy of 0.25 eV [3]. Both the increase in the di- ameter of the "bottle-neck" between the two cation sites, as well as the increase in the effective charge carrier concentration are considered to be responsi- ble for the conductivity enhancement [4 ]. An equal Paper was presented at the SSI-9 Conference, September 12- 19, 1993, The Hague, The Netherlands. * Corresponding author. enhancement can be achieved by partially substitut- ing Zr 4+ by a trivalent cation e.g. Sc, Y, In, Fe or Cr [5]. With regards to the copper(I) compounds which adopt the NASICON structure, none of the pure sili- cate end members have been reported. The first pure phosphate end member CuZr2(PO4)3, was discov- ered by Yao and Fray in 1983 [6]. Two copper(I) phosphate solid solutions Cul+xTi2_xCrx(PO4)3 (0-<x_<l) and CuxNbl_ffil+x(PO4)3 (0_<x_<l) were prepared and structurally characterised by E1 Jazouli et al. in 1985 [ 7 ]. Oudet et al. [ 8 ] reported a single conductivity measurement taken at 573 K of 1.072× 10 -a S cm -~ for the common end member CuTi2 (PO4) 3. More recently, two new "copper(I) NASICON type" compounds CuTiZr (PO4) a and Cu2ZrSc (PO4)a were prepared and characterised by us together with CuTi2(PO4)3, CuTiCr(PO4)3 and CuZr2 (PO4) 3 [ 9 ]. Besides CuZr2 (PO4) 3 which is 0167-2738/94/$07.00 © 1994 ElsevierScienceB.V. All fights reserved SSDI 0167-2738 ( 94 )00196-0