INFLUENCE OF DEFECT MICROSTRUCTURE AND POLYTYPISM ON THE SILVER ION CONDUCTIVITY IN AgI : Al₂O₃ COMPOSITES

St. Adams ¹, J.-S. Lee ² and J. Maier ²

¹ Min.-Kristallograph. Institut, Universität Göttingen, Goldschmidtstr. 1 , D 37077 Göttingen, Germany; Email: sadams@gwdg.de
² Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D 70563 Stuttgart, Germany

Keywords: silver ion conduction, polytypism, space charge effect, valence maps, molecular dynamics

"Heterogeneous doping" of the moderate Ag⁺ ion conductor b-AgI with highly dispersed g-Al₂O₃ results in a conductivity enhancement by up to 4 orders of magnitude. While similar conductivity effects in AgCl and AgBr are quantitatively explained as space charge effects at conductor : insulator-interfaces, for AgI this model yields neither the magnitude of the effect nor the existence of a large hysteresis of the phase transition to the superionic high temperature phase a-AgI phase, which we confirmed by impedance spectroscopy, DSC and in situ powder diffraction.

Powder XRD revealed that besides hexagonal b-AgI and cubic g-AgI an additional AgI phase exists in the composites at room temperature. The volume fraction of this phase rapidly increases with the content of g-Al₂O₃: for >=30m/o Al₂O₃ virtually the whole AgI is converted to this phase [1]. This contrasts to earlier speculations, which associated the conductivity enhancement to the formation of highly conducting interface phases. The new phase was identified by Rietveld refinement as the 7H polytype of AgI (space group: P3m1, a=4.598A, c=26.271 A, stacking sequence hcchhch ). 7H-AgI had been suggested earlier - without structure refinement - for microcrystalline Ag_1-xI synthesized from acetone solutions [2].

The determination of possible Ag⁺ migration pathways by means of a valence sum pseudopotential approach (cf. [3]) shows, that in contrast to b-AgI, where infinite chains of octahedral interstitials act as migration pathways along the hexagonal axis, the conduction pathways of lowest activation energy for 7H-AgI are restricted to the a-b planes (cf. Fig. 1). Accordingly impedance spectroscopic studies showed, that the activation energy for the Ag⁺ transport in polycrystalline 7H-AgI (0.29eV) is the same as for the a-b-plane of -AgI single crystals. Molecular dynamics simulations again indicate a slightly increased Ag⁺ mobility within the marked a-b plane and support the relevance of stacking faults to the conductivity effect.

Fig. 1: Schematic representation of the stacking sequence in 7H-AgI (r.h.s.) and Ag valence sum map of 7H-AgI displaying the network of possible Ag⁺ migration pathways (l.h.s).

Crystallite sizes and and deformations are estimated from the Bragg peak broadening for composites with varying AgI contents and compared to SEM micrographs. The variation of the mean stacking fault distance is clearly correlated to the conductivity enhancement.

The conductivity enhancement may thus be attributed to a "mesoscopic multiphase effect" which had been predicted earlier as a special case of space charge effects[3]: if the domain size becomes smaller than the Debye length, space charge effects at opposite b:g interfaces interact and thereby extend through the whole "bulk". Whether the arrangement of the different AgI polymorph domains is highly ordered such as within the 7H-AgI polytype or more or less random (as in the composites with the highest Al₂O₃ contents) seems to play only a minor role.

[1] St. Adams, J.-S. Lee, J. Maier; Z. Kristallogr. Suppl. 15 (1998) 133 and J.-S. Lee, St. Adams, J. Maier, in prep.
[2] B.L. Davis, L.R. Johnson; Crystal Lattice Defects 5(1974) 235.
[3] St. Adams, J. Maier; Solid State Ionics 105 (1998) 67.
[4] J. Maier; Prog. Solid State Chemistry 23 (1995) 171.