Protons in the magnesium phosphates phosphoellenbergerite and holtedahlite: an IR and NMR study.
Two structurally related Mg phosphates, phosphoellenbergerite (P63mc) and holtedahlite (P63), have been investigated using a combination of high-frequency IR and 1H MAS, 31P MAS, and 31P CP MAS NMR spectroscopy on synthetic material. For phosphoellenbergerite, the two proton sites, H1 and H2, detected in single-crystal XRD studies, are confirmed. The protons on the first site, H1, delocalized around the threefold axis, form OH groups with the unshared apical O atom of PO4 tetrahedra. Those on the second site, H2, belong to Mg2O8OH dimers of face=sharing Mg octahedra, which form double chains. The spectroscopic data reveal addnl. protons (H3) in the phosphoellenbergerite. By combining both crystallog. and spectroscopic data, it can be inferred that the H3 protons are assocd. with Mg vacancies of the single chains of face-sharing Mg octahedra. Furthermore, a quant. inspection of signal intensities indicates that the proton sites in synthetic phosphoellenbergerite are not all satd. Therefore, the substitution P4+ = Si4+ + H+ might account for part of the Si-P replacement in the silicate-to-phosphate ellenbergerite series. The results obtained for the proton distribution in holtedahlite, Mg2PO4OH, were used as a ref. to facilitate the interpretation of the IR and NMR spectra of phosphoellenbergerite.
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