Gradual solid electrolyte transition in ( C 5 H 5 NH) Ag 5 I 6

In specific-heat measurements of the solid electrolyte (${\mathrm{C}}_{5}$${\mathrm{H}}_{5}$NH)${\mathrm{Ag}}_{5}$${\mathrm{I}}_{6}$, the disordering of the silver ions shows up in the form of a Schottky-type anomaly. A model calculation of the site occupancies of the ${\mathrm{Ag}}^{+}$ ions and the excess specific heat reproduces the main features of the gradual transition to the solid-electrolyte state in this compound and proves that correlations between ${\mathrm{Ag}}^{+}$ ions play an important role. Single-crystal conductivity measurements were carried out both along and perpendicular to the hexagonal axis. The large anisotropy of the conductivity at low temperatures ascertains that the motion of the ${\mathrm{Ag}}^{+}$ ions through the channels provided by the $c$ sites is much easier than the motion via pathways involving $m$ sites. In addition, the specific-heat measurements show the existence of a second-order phase transition which is probably associated with the orientational disordering of the pyridinium ions. Finally, single-crystal conductivity results of ${({\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N}\mathrm{H})}_{5}$${\mathrm{Ag}}_{18}$${\mathrm{I}}_{23}$ are presented which confirm the prediction of Geller et al. that this material is a two-dimensional solid electrolyte.