A 3D open-framework iron hydrogenophosphate showing high proton conductance under water and aqua-ammonia vapor.

Herein we report the first example of the proton conductivity of an open-framework metal phosphate (NH 3 (CH 2 ) 3 NH 3 ) 2 -[Fe 4 (OH) 3 (HPO 4 ) 2 (PO 4 ) 3 ]·4H 2 O under aqua-ammonia vapor. Its optimized proton conductivity is 5 × 10 -2 S cm -1 at 313 K and aqua-ammonium vapor from 1 M NH 3 ·H 2 O solution. That is approximately two orders of magnitude greater than the maximum value under water vapor (8.0 × 10 -4 S cm -1 at 317 K and 99% RH). The proton transfer mechanism has been proposed in terms of the structural analyses, activation energy calculations, and PXRD determinations.