Wardite (NaAl3(PO4)2(OH)4·2H2O) at High Pressure: Compressional Behavior and Structure Evolution

The high‐pressure behavior of wardite, NaAl3(PO4)2(OH)4·2H2O (a = 7.0673(2) Å, c =
19.193(9) Å, Sp. Gr. P41212), has been investigated by in‐situ single‐crystal synchrotron diffraction
experiments up to 9 GPa, using a diamond anvil cell under quasi‐hydrostatic conditions. This
phosphate does not experience any pressure‐induced phase transition, or anomalous compressional
behavior, within the pressure‐range investigated: its compressional behavior is fully elastic and all
the deformation mechanisms, at the atomic scale, are reversible upon decompression. A secondorder
Birch–Murnaghan Equation of State was fitted to the experimental data, weighted by their
uncertainty in pressure (P) and volume (V), with the following refined parameters: V0 = 957.8(2) Å3
and KV0 = −V0(dP/dV)P0,T0 = 85.8(4) GPa (βV0 = 1/KV0 = 0.01166(5) GPa−1). Axial bulk moduli were also
calculated, with: K0(a) = 98(3) GPa (β0(a) = 0.0034(1) GPa−1) and K0(c) = 64(1) GPa (β0(c) = 0.0052(1)
GPa−1). The anisotropic compressional scheme is: K0(a):K0(c) = 1.53:1. A series of structure
refinements were performed on the basis of the intensity data collected in compression and
decompression. The mechanisms at the atomic scale, responsible for the structure anisotropy of
wardite, are discussed.