Small is Beautiful: The Unusual Transformation of Nanocrystalline Layered α‑Zirconium Phosphate into a New 3D Structure

Nanosized α-zirconium phosphate, α-ZrP, undergoes a phase transition at 120 °C, which is not observed with microcrystalline α-ZrP in the same conditions, and which leads to a new 3D phase. The new compound, with formula Zr­(HPO4)2 (τ′-ZrP), consists of cubelike nanoparticles and has a tetragonal unit cell (space group P43212, a = 7.955 Å, c = 10.744 Å). The structure of τ′-ZrP is in close relationship with that of the already known τ-ZrP. Both structures are made of packed chains of eight-membered rings, composed of Zr atoms connected to bridging HPO4 groups. The main difference between the two structures concerns the different orientation of the uncoordinated P–OH groups, pointing into the channels. The in situ XRPD analysis on nanosized α-ZrP, performed at 120 °C as a function of time, provided information about the kinetics of the formation of τ′-ZrP, showing that the α-ZrP phase is directly transformed into τ′-ZrP. Moreover, τ′-ZrP is converted into α-ZrP at room temperature in the presence of water vapor. It was proved that the free phosphoric acid, which is originally present in small amounts in nanosized α-ZrP and τ′-ZrP, is necessary for the interconversion between the two phases. As a matter of fact, the removal of phosphoric acid, by washing α-ZrP and τ′-ZrP with anhydrous ethanol, inhibits the above conversion.