Nucleation & growth of α-Ti(HPO4)2·H2O single-crystal and its structure determination from X-ray single–crystal data

Abstract

α-titanium phosphate phase [α-Ti(HPO4)2·H2O; abbreviated as α-TiP] is a nanolayered metal phosphate with several industrial and recent biotechnological applications related to their nanolayered morphology and ion exchange capabilities. Structural wise, α-TiP is a tetravalent metal phosphate that tends to crystallize into microcrystalline powder. Here we report that the crystallization of single α-TiP crystals with suitable dimensions (>50 μm) for single-crystal X-ray diffraction could be effectuated using a prolonged hydrothermal treatment of a metallic titanium alloy (Ti–6Al–4V) in a highly concentrated phosphoric acid aqueous solution. Accordingly, the single-crystal X-ray diffraction analysis revealed the crystalline structure in a monoclinic space group, P21/c, with a = 8.6288(5) Å, b = 5.00546(17) Å, c = 19.1468(11) Å, and β = 127.555(9)°. Although the space group is the same crystallographic space group that was previously reported from the microcrystalline powder relying on powder X-ray diffraction (PXRD) and neutron powder diffraction, the obtained unit cell volume is slightly larger. Bulk of the obtained crystals were also subjected to detailed analyses using polarization optical microscopy (POM), scanning electron microscopy combined with energy dispersive X-ray microanalysis (SEM-EDX), Fourier transform Infrared Spectroscopy (FTIR) and thermal analysis (TG/SDTA). The single crystals are hexagonally shaped and exhibit the exact titanium to phosphorus ratio expected for α-TiP. The bulk sample PXRD pattern resembles perfectly that reported for the α-TiP polycrystalline powder with clearly sharper peaks. TGA-SDTA analysis revealed the expected thermal behaviour for α-TiP phase and demonstrated their higher thermal stability when compared to the polycrystalline powder.