Stability, structure, dynamics and thermal properties of C-type aluminium tris-dihydrogen phosphate

Among the fastest hydrogen ion conductors are various hydrogen-rich phosphates, such as phosphoric acid and CsH₂PO₄. This, in principle, makes Al(H₂PO₄)₃ a promising candidate for proton conduction within its hydrogen bonding network, especially in the medium-temperature range. Here, the phase-pure synthesis and structural details of the ferroelectric crystal C-type Al(H₂PO₄)₃ are reported, where density functional theory (DFT) calculations were used to study the thermodynamic stability of different Al(H₂PO₄)₃ polymorphs. Its ion conductivity, as determined by impedance spectroscopy, is an order of magnitude higher than previously reported. The thermal stability and decomposition route of Al(H₂PO₄)₃ were studied using XRD, thermal analysis, and solid-state NMR, yielding results different from those previously observed in various mixtures containing Al(H₂PO₄)₃. Heating produces an amorphous intermediate phase which converts into phase-pure monoclinic Al₂(P₆O₁₈), which at even higher temperatures converts into the cubic phase Al₄(P₄O₁₂)₃ and little AlPO₄. Rietveld refinements of C-type Al(H₂PO₄)₃ and Al₄(P₄O₁₂)₃ are provided. The results show that while the proton ion conductivity of C-type Al(H₂PO₄)₃ is hampered by the strong hydrogen-bonds, the structure features a Curie temperature significantly higher than room-temperature.