Two different types of channels exhibiting distinct proton transport behavior in an open-framework aluminophosphate

A three-dimensional open-framework aluminophosphate, (NH₄)₂Al₄(PO₄)₄(HPO₄)·H₂O (1), shows two types of eight-membered polyhedral ring channels. The lattice water molecules occupy within the channel with (HO)PO₃ tetrahedra (channel-I), whilst the ammonium ions reside in another type of channel (channel-II). This aluminophosphate shows superior stability to water and NaOH aqueous solution, but unstable to H₂SO₄ aqueous solution. The proton conductance of 1 has been investigated under anhydrous condition and various relative humidity, indicating rather low conductivity under anhydrous condition even at elevated temperature (σ_dc = 9.05 × 10⁻¹³ S cm⁻¹ at 343 K and 4.25 × 10⁻¹⁰ S cm⁻¹ even at 473 K), and this situation demonstrates that both the ammonium ions and the protons in (HO)PO₃ tetrahedra have quite low mobility. The conductivity of 1 reaches to 4.0 × 10⁻⁵ S cm⁻¹ at 299 K, 2.57 × 10⁻⁴ S cm⁻¹ at 343 K under 99%RH, respectively. The greatly enhancement of proton conductivity at 99%RH is due to the formation of H-bond between H₂O molecules and (HO)PO₃ tetrahedra in the channel-I to assist proton hopping process, while the ammonium ions show negligible contribution to enhancement of proton conductivity.