| Summary: | The high-pressure behavior of wardite, NaAl<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>(OH)<sub>4</sub>·2H<sub>2</sub>O (<i>a </i>= 7.0673(2) Å, <i>c </i>= 19.193(9) Å, Sp. Gr. <i>P</i>4<sub>1</sub>2<sub>1</sub>2), 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 second-order Birch–Murnaghan Equation of State was fitted to the experimental data, weighted by their uncertainty in pressure (<i>P</i>) and volume (<i>V</i>), with the following refined parameters: <i>V</i><sub>0 </sub>= 957.8(2) Å<sup>3</sup> and <i>K</i><sub>V0</sub> = −<i>V</i><sub>0</sub>( ¶<i>P</i>/¶<i>V</i>)<sub>P0</sub>,<sub>T0</sub> = 85.8(4) GPa (β<sub>V0</sub> = 1/<i>K</i><sub>V0 </sub>= 0.01166(5) GPa<sup>−1</sup>). Axial bulk moduli were also calculated, with: <i>K</i><sub>0</sub>(<i>a</i>) = 98(3) GPa (β<sub>0(a)</sub> = 0.0034(1) GPa<sup>−1</sup>) and <i>K</i><sub>0</sub>(<i>c</i>) = 64(1) GPa (β<sub>0(c) </sub>= 0.0052(1) GPa<sup>−1</sup>). The anisotropic compressional scheme is: <i>K</i><sub>0</sub>(<i>a</i>):<i>K</i><sub>0</sub>(<i>c</i>) = 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.
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