Zirconium and Titanium Phosphates

. With a nanofibrous morphology, two polymorphs with the chemical formula: Ti2O(PO4)2·2H2O, called π-TiP and ρ-TiP, were synthesized around two decades ago . Although π-TiP and ρ-TiP are compounds with neutral lattices, they exhibit an outstanding ability to incorporate intracrystalline metal cations through ion exchange processes . Particularly, π-TiP reaction with alkali metal nitrates in the molten state results in the formation of fibrous phases charged with the reactant alkali metal . In terms of their crystallographic structures, the structure of ρ-TiP was reported shortly after being solved ab initio from synchrotron X-ray and neutron powder diffraction data (Figure 3a) . On the other hand, there were only some scientific hypotheses reported about the most probable structural ordering of π-TiP . The crystal structure of the ρ-TiP phase has two channels that run parallel to the crystallographic 

a

-axis in which the water molecules are housed. Recently, researchers succeeded to reveal the crystal structure of π-TiP . The π-TiP phase, obtained under mild hydrothermal conditions, crystallizes in the monoclinic system (space group 

P

21/c) in contrast to the previously described ρ-TiP (triclinic, 

P

-1). The π-TiP structure is made up of TiO6 and TiO4(H2O)2 octahedra, connected with orthophosphate groups, defining the anisotropic three-dimensional packing, consistent with the existence of preferential crystal growth directions. The properties of both polymorphs have scarcely been explored, probably because they were just synthesized less than 3 decades ago. Although both π- and ρ-TiP exhibit low accessible porosity when hydrated (11–16 m2·g−1) , Amghouz et al. showed that this can be changed significantly if they are treated thermally. In such a process, the crystallized water molecules are eliminated, and the resultant dehydrated phases exhibit an unusual adsorption capacity for nitrogen a little above ambient temperature . For the ρ-TiP, the thermal treatment at temperatures of 200–300 °C results in the generation of tetrahedral (ex-octahedral) titanium atoms (Figure 3b) that are responsible for the enhanced nitrogen adsorption capability . On the other hand, this treatment for the π-TiP was shown to lead to a change in the coordination environment of one of the titanium atoms, which goes from octahedral to tetrahedral, with the formation of the anhydrous Ti2O(PO4)2  . Beside their nitrogen adsorption properties, π-TiP has been recently investigated for its proton conductivity. π-TiP was doped in a chitosan matrix to study the proton conductivity of the resultant chitosan-based composite membranes (CS@π-TiP) . This study showed that doping the CS matrix with π-TiP (5 

w

/

w

%) results in a 1.8-fold rise in the proton conductivity compared to the bare membrane . Another explored property is their attraction to water. Yada et al. prepared super-hydrophilic thin films made up of micro- and nano-crystals of π-TiP with controlled morphologies that transform to superhydrophobic layers when π-TiP is modified with alkylamines . Similarly, Cai et al. showed that TiP thin films grown on titanium substrates can possess a hydrophilicity that can be switched to a super-hydrophobicity through increasing the preparation temperature . Recently, π-TiP has been reported to possess ultrastability as a Ca2+ storage material with a minimal dimensional change and almost no transformation in the crystallographic structure upon the insertion or extraction of Ca2+  . This exceptional stability that stands for over 1700 cycles of the Ca2+ insertion and extraction presents π-TiP as a viable electrode material for calcium-ion battery applications . Therefore, researchers can estimate that many attempts could be made in the near future to utilize these nanofibrous phases for some of the other applications that have been established for the nanolayered titanium phosphates.

Several titanium phosphate compounds have been reported with different morphologies and thus different plausible applications. With a nanofibrous morphology, two polymorphs with the chemical formula: TiO(PO·2HO, called π-TiP and ρ-TiP, were synthesized around two decades ago. Although π-TiP and ρ-TiP are compounds with neutral lattices, they exhibit an outstanding ability to incorporate intracrystalline metal cations through ion exchange processes. Particularly, π-TiP reaction with alkali metal nitrates in the molten state results in the formation of fibrous phases charged with the reactant alkali metal. In terms of their crystallographic structures, the structure of ρ-TiP was reported shortly after being solved ab initio from synchrotron X-ray and neutron powder diffraction data (a). On the other hand, there were only some scientific hypotheses reported about the most probable structural ordering of π-TiP. The crystal structure of the ρ-TiP phase has two channels that run parallel to the crystallographic-axis in which the water molecules are housed. Recently, researchers succeeded to reveal the crystal structure of π-TiP. The π-TiP phase, obtained under mild hydrothermal conditions, crystallizes in the monoclinic system (space group/c) in contrast to the previously described ρ-TiP (triclinic,-1). The π-TiP structure is made up of TiOand TiO(HO)octahedra, connected with orthophosphate groups, defining the anisotropic three-dimensional packing, consistent with the existence of preferential crystal growth directions. The properties of both polymorphs have scarcely been explored, probably because they were just synthesized less than 3 decades ago. Although both π- and ρ-TiP exhibit low accessible porosity when hydrated (11–16 m·g, Amghouz et al. showed that this can be changed significantly if they are treated thermally. In such a process, the crystallized water molecules are eliminated, and the resultant dehydrated phases exhibit an unusual adsorption capacity for nitrogen a little above ambient temperature. For the ρ-TiP, the thermal treatment at temperatures of 200–300 °C results in the generation of tetrahedral (ex-octahedral) titanium atoms (b) that are responsible for the enhanced nitrogen adsorption capability. On the other hand, this treatment for the π-TiP was shown to lead to a change in the coordination environment of one of the titanium atoms, which goes from octahedral to tetrahedral, with the formation of the anhydrous TiO(PO. Beside their nitrogen adsorption properties, π-TiP has been recently investigated for its proton conductivity. π-TiP was doped in a chitosan matrix to study the proton conductivity of the resultant chitosan-based composite membranes (CS@π-TiP). This study showed that doping the CS matrix with π-TiP (5%) results in a 1.8-fold rise in the proton conductivity compared to the bare membrane. Another explored property is their attraction to water. Yada et al. prepared super-hydrophilic thin films made up of micro- and nano-crystals of π-TiP with controlled morphologies that transform to superhydrophobic layers when π-TiP is modified with alkylamines. Similarly, Cai et al. showed that TiP thin films grown on titanium substrates can possess a hydrophilicity that can be switched to a super-hydrophobicity through increasing the preparation temperature. Recently, π-TiP has been reported to possess ultrastability as a Castorage material with a minimal dimensional change and almost no transformation in the crystallographic structure upon the insertion or extraction of Ca. This exceptional stability that stands for over 1700 cycles of the Cainsertion and extraction presents π-TiP as a viable electrode material for calcium-ion battery applications. Therefore, researchers can estimate that many attempts could be made in the near future to utilize these nanofibrous phases for some of the other applications that have been established for the nanolayered titanium phosphates.