Although the fibrous polymorphic modification of titanium phosphate, π-Ti2O(PO4)2·2H2O (π-TiP) is known for decades, its
crystal structure has remained unsolved. Herewith we report the crystal structure of π-TiP at a room temperature,
determined from the synchrotron radiation powder X-ray diffraction, and corroborated by 31P solid state NMR and the
accurate density functional theory calculations. In contrast to the previously reported ρ-TiP polymorph, the as-synthesized
hydrated phase crystallizes in the monoclinic system (P21/c, a = 5.1121(2) Å, b = 14.4921(9) Å, c = 12.0450(11), β =
115.31(1)°, Z=4) and is composed of corner-sharing titanium octahedra and phosphate units arranged in a pattern distinct
of ρ-TiP polymorph. The unit cell was confirmed by SAED, while the formation of planar packing imperfections and stacking
faults along the [101] was revealed by HRTEM analysis. In situ dehydration study of π-TiP, monitored by high-temperature
powder X-ray diffraction, led to a new anhydrous monoclinic [P21/c, a = 5.1187(13) Å, b = 11.0600(21) Å, c = 14.4556(26),
β = 107.65(2)°, Z=4) phase that crystallizes at 500°C. The latter resembles the packing fashion of the parental π-TiP, albeit
titanium atoms are present both in distorted tetrahedral and octahedral coordination environments. Anhydrous π-TiP was
found to partially rehydrate at room temperature adopting reversibly the structure of the initial phase. The studies carried
out under different conditions of leaching and impregnation with H3PO4 showed that π-TiP exhibits an extrinsic proton
conductivity (1.3·10−3 S·cm−1 at 90 °C and 95% RH) due to the presence of protonated phosphate species bounded on the
particles surface, as revealed by 31P MAS-NMR spectroscopy data. The composite membranes of Chitosan (CS) matrices
filled with H3PO4-impregnated π-TiP solid show an increment of proton conductivity up to 4.5·10–3 S·cm–1, at 80 °C and 95%
RH, 1.8-fold higher than those of bare CS membranes
