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.