Synthesis, structural characterization and high pressure phase transitions of monolithium hydronium sulfate

A three dimensional lithium hydronium sulfate LiSO ₄· H ₃O [1], [space group Pna2 ₁ a=8.7785(12) Å, b=9.1297(12) Å, c=5.2799(7) Å, V=423.16(10) Å ³] was synthesized via solvothermal methods using 1,5-naphthalenedisulfonic acid (1,5-NSA) as the source of sulfate ions. The structure of [1], determined by single crystal X-ray diffraction techniques, consists of corner sharing LiO ₄ and SO ₄ tetrahedra, forming an anionic 3-D open framework that is charge balanced by hydronium ions positioned within channels running along [001] and forming strong H-bonding with the framework oxygen atoms. Compound [1] undergoes two reversible phase transitions, involving reorientation of SO ₄ ^2- ions at pressures of approximately 2.5 and 5 GPa at room temperature, as evident from characteristic discontinuous frequency drops in the ν ₁ mode of the Raman spectra. Additionally, compound [1] forms dense β-lithium sulfate at 300 °C, as evident from temperature dependent powder XRD and combined reversible TGA-DSC experiments.