The crystal structure of the Cu+ ion conductor, (C5H5NH)2Cu5Br7

(C₅H₅NH)₂Cu₅Br₇, in which (C₅H₅NH)⁺ is the pyridinium ion, belongs to space group P2₁2₁2₁ with Z = 4 and a = 13.09 ± 0.03 Å, b = 14.04 ± 0.03 Å, c = 11.78 ± 0.02Å. The 20 Cu⁺ ions are distributed non-uniformly over 52 tetrahedral sites. The bromide tetrahedra share faces in such a manner that undulating channels are formed in the 100] and 010] directions, and right- and left-handed helical channels are formed in the c-direction. The channels are interconnected, thereby forming a three-dimensional solid electrolyte. A detailed examination of the conduction pathways and of Cu⁺ ion site occupancies leads to the predictions that σ₂ (|b) should be somewhat greater than σ₁ (|a) but σ₃ (|c) should be substantially less than σ₁. (σ_i is the specific conductivity in the ith direction.) The ratio of available sites to current carriers and the percentage of unit cell volume attributable to the conduction pathways are both rather low and the average conductivity is rather high relative to AgI-based solid electrolytes.