Conductivity and Fluoride Ion Dynamics in r-PbSnF 4 ; 19 F Field-Cycling NMR and Diffraction Studies Eoin Murray, ² Dermot F. Brougham,* ,²,‡ Jovan Stankovic, § and Isaac Abrahams § School of Chemical Sciences, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland, National Institute for Cellular Biotechnology, School of Chemical Sciences, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland, and Centre for Materials Research, School of Biological and Chemical Sciences, Queen Mary, UniVersity of London, Mile End Road, London, E1 4NS, United Kingdom ReceiVed: NoVember 14, 2007; In Final Form: January 31, 2008 Fast-field cycling 19 F NMR relaxometry has been applied to investigate fluoride ion dynamics in the layered anionic conductor PbSnF 4 . Two dynamic processes, on different timescales, were shown to drive the 19 F relaxation. By considering the temperature dependencies of the NMR, conductivity, and diffraction data, a complete mechanism for fluoride transport can for the first time be proposed. The slower process is due to anion exchange between equivalent sites (F(2)) in the conducting fluoride plane, which lie between Sn and Pb layers. This is a diffusive process related directly to the mechanism of electrical conduction. The activation barriers for this motion agree closely with those determined from the temperature dependence of the DC conductivity. The faster process is due to non-diffusive exchange between the occupied sites (F(2)) and nominally vacant sites (F(1)), which lie between Sn planes. While this process does not directly limit conductivity, it generates vacancies on the F(2) sites and partial occupancy of the F(1) sites. Above 340 K, the fast process shows an increase in activation energy, as increased occupancy of F(1) sites necessitates the formation of a Frenkel defect on a significant proportion of the F(1) sites prior to an F(2)-F(1) jump. In this temperature range, the slow process shows a decrease in the activation energy, also observed in the conductivity data, due to increased numbers of vacancies in the F(2) sites which provide additional diffusive pathways through the conducting fluoride plane. The results demonstrate that R-PbSnF 4 is essentially a two-dimensional (anisotropic) conductor, in which nondiffusive fluoride exchange into sites normal to the conducting plane provides a high population of vacancies within the conducting plane, resulting in unusually high conductivity. Introduction PbSnF 4 , is a member of the MSnF 4 family (M ) Pb, Ba, and Sr) of layered anionic conducting solids. The conduction mechanisms across this family are of interest because of their high conductivities. PbSnF 4 has the highest ambient temperature anionic conductivity, 10 -3 S cm -1 , of any known material and has been studied using a range of techniques focusing on structural, thermal, electrical and dynamic properties. 1-3 Tech- nologically, PbSnF 4 is a viable electrolyte in solid-state oxygen sensors, as the alternative oxides have low room-temperature conductivity, which limits the sensor response time. 4 R-PbSnF 4 was first reported by Donaldson and Senior. 1 More recently, Ahmad and co-workers 5 observed a discontinuity in the temperature dependence of the conductivity at around 340 K and reported barriers to conductivity of 22 kJ mol -1 (0.23 eV) in the low-temperature range and 30 kJ mol -1 (0.31 eV) in the high-temperature range. Several authors have reported phase transitions, or other anomalous behavior, between 340 and 350 K in PbSnF 4 . Reau et al. 6 had earlier suggested a transition from a monoclinic (R) to a tetragonal () phase, while Denes et al. 7 suggested that there are no symmetry changes or phase transitions at this temperature. In a recent neutron diffraction study, Castiglione and co- workers 8 demonstrated that pure PbSnF 4 is tetragonal, in the space group P4/nmm, over the temperature range 298-591 K, and adopts an ordered fluorite-type structure. The structure may be described as being based on an ordered cubic close packed (ccp) array of cations (Sn 2+ and Pb 2+ ), resulting in a layered tetragonal structure, Figure 1. In the ideal fluorite structure, fluoride ions fully occupy all of the available tetrahedral interstices in the ccp lattice, with octahedral interstices remaining vacant. In R-PbSnF 4 , the room-temperature distribution of fluoride ions was found 8 to deviate from the ideal fluorite structure with three-quarters of the fluoride ions located in two crystallographic sites F(2) (4f) and F(3) (2b) corresponding to those in the ideal fluorite structure and the remaining fluoride ions located in an essentially octahedral site F(4) (2c) between the Sn and Pb layers. Ions located in the F(4) site were found to be preferentially coordinated to the Sn atoms. The remaining tetrahedral site, F(1) between adjacent Sn layers, shows no measurable occupancy at 298 K. The structure remains tetrago- nal above 340 K, but there is increased fluoride disorder, with greater anion density linking the F(2) and F(4) sites. While critical in determining the structure of the fluoride sub- lattice, diffraction studies can only provide spatially and temporally averaged atomic positions. Furthermore, refinement of the diffraction data is difficult because of large anisotropic thermal parameters and partial site occupancies. 19 F NMR spectroscopy is sensitive to fluoride motion and has been used to study the mechanism of conductivity in some members of the MSnF 4 group. In a recent report, 9 spin-lattice relaxation time measurements and 19 F and 119 Sn MAS NMR spectra were * Corresponding author. E-mail: dermot.brougham@dcu.ie. ² School of Chemical Sciences, Dublin City University. ‡ National Institute for Cellular Biotechnology. § University of London. BATCH: jp4c251 USER: rll29 DIV: @xyv04/data1/CLS_pj/GRP_jy/JOB_i15/DIV_jp7108708 DATE: February 29, 2008 10.1021/jp7108708 CCC: $40.75 © xxxx American Chemical Society PAGE EST: 6.2 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77