Data di Pubblicazione:
2019
Citazione:
A single-crystal neutron diffraction study of wardite, NaAl3(PO4)2(OH)4·2H2O / G.D. Gatta, A. Guastoni, O. Fabelo, M.T. Fernandez-Diaz. - In: PHYSICS AND CHEMISTRY OF MINERALS. - ISSN 0342-1791. - 46:4(2019 Apr), pp. 427-435. [10.1007/s00269-018-1013-7]
Abstract:
The crystal structure and crystal chemistry of wardite, ideally NaAl3(PO4)2(OH)4·2H2O, was investigated by single-crystal
neutron diffraction (data collected at 20 K) and electron microprobe analysis in wavelength-dispersive mode. The empirical
formula of the sample used in this study is: (Na0.91Ca0.01)Σ = 0.92(Al2.97Fe3+0.05Ti0.01)Σ = 3.03(P2.10O8)(OH)4·1.74H2O. The
neutron diffraction data confirm that the crystal structure of wardite can be described with a tetragonal symmetry (space
group P41212, a = b = 7.0577(5) and c = 19.0559(5) Å at 20 K) and consists of sheets made of edge-sharing Na-polyhedra
and Al-octahedra along with vertex-sharing Al-octahedra, parallel to (001), connected by P-tetrahedra and H bonds to form
a (001) layer-type structure, which well explains the pronounced {001} cleavage of the wardite crystals. The present data
show that four crystallographically independent H sites occur in the structure of wardite, two belonging to a H2O molecule
(i.e., H1–O6–H2) and two forming hydroxyl groups (i.e., O5–H3 and O7–H4). The location of the hydrogen atoms allows us
to define the extensive network of H bonds: the H atoms belonging to the H2O molecule form strong H bonds, whereas both
the H atoms belonging to the two independent hydroxyl groups form weak interactions with bifurcated bonding schemes.
As shown by the root-mean-square components of the displacement ellipsoids, oxygen and hydrogen atoms have slightly
larger anisotropic displacement parameters compared to the other sites (populated by P, Al and Na). The maximum ratio of
the max and min root-mean-square components of the displacement ellipsoids is observed for the protons of the hydroxyl
groups, which experience bifurcated H-bonding schemes. A comparative analysis of the crystal structure of wardite and
fluorowardite is also provided.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Wardite; Phosphates; Single-crystal neutron diffraction; Crystal chemistry;· Hydrogen bonding
Elenco autori:
G.D. Gatta, A. Guastoni, O. Fabelo, M.T. Fernandez-Diaz
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