Abstract
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 second-order 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(∂P/∂V)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.
Highlights
Wardite, ideally NaAl3 (PO4 )2 (OH)4 ·2H2 O, is a hydrous phosphate discovered at the end of the nineteenth century [1,2]
The aim of this study is to investigate the high-pressure behavior of wardite by means of in-situ single-crystal X-ray diffraction using a diamond anvil cell (DAC), under quasi-hydrostatic conditions
The evolution with P of bond distances and angles of the (H-free) structure model of wardite can be deduced on the basis of the data reported in Tables S2—S4 and in the Crystallographic Information Files (CIF)
Summary
Ideally NaAl3 (PO4 ) (OH)4 ·2H2 O, is a hydrous phosphate discovered at the end of the nineteenth century [1,2]. It is a hydrothermal mineral, which usually occurs in P-rich zones of granite pegmatites and in sedimentary iron-rich formations. Its crystal structure (as H-free model) was solved by Fanfani et al [3], on the basis of single-crystal X-ray intensity data (collected by the Weissenberg method) and recently reinvestigated by Gatta et al [4] by means of single-crystal neutron diffraction (at 20 K).
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