Varying temperature FT-IR and a.c. conductivity analyses of high-capacitance fast ion conducting HCl salt of hexaamidocyclotriphosphazatriene

Abstract

The HCl salt of hexaamidocyclotriphosphazatriene (HACTP) shows much of the characteristics of the well-established family of solid superionic conductors. The key feature revealing this trend is a Cole–Cole Z″– Z′ dependence tending sharply to a low resistance, especially above 343 K far below the decomposition temperature, a three orders of magnitude rise in the d.c. (10 −4–10 −1 Ω −1 cm −1) and a.c. (10 −7–10 −4 Ω −1 cm −1) conductivity, a low ionic relaxation energy of 0.16 eV and a high capacitance of 0.1 F. The electric permittivity loss shows evidence of significant contribution from d.c. conduction. At the molecular scale, the origin of these peculiar features is assigned to a thermally induced structural shift in the HACTP-HCl ring from the puckered C 3 v to the higher D 3 h symmetry. The thermally induced planar ring supports a better orientation and hence interaction of the conduction ions with the phosphazene ring in the solid state. This structural shift is explicit in a DTA-detected solid-state phase transition activated at 343 K and optimized at 443 K prior to the melting at 513 K and is probed, using structure probe solid state FT-IR (in the same temperature range as the a.c.-conductivity) and 1H NMR in solution. Spectral correlation of solid state FT-IR and solution 1H NMR is necessarily made for distinguishing solid state from intrinsic phenomena.