Thermodynamic Stability, Structure, Dynamics and Thermal Properties of C-type Aluminiumtrisdihydrogenphosphate

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 in 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 the decomposition route of Al(H2PO4)3 are different from those previously observed from different 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.