Protons in the magnesium phosphates phosphoellenbergerite and holtedahlite: an IR and NMR study

Two structurally related magnesium phosphates, phosphoellenbergerite ([P6.sub.3]mc) and holtedahlite ([P6.sub.3]), 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, Hl and H2, detected in single-crystal XRD studies, are confirmed. The protons on the first site, Hl, delocalized around the threefold axis, form OH groups with the unshared apical O atom of P[O.sub.4] tetrahedra. Those on the second site, H2, belong to [Mg.sub.2][O.sub.8]OH dimers of face-sharing Mg octahedra, which form double chains. The spectroscopic data reveal additional protons (H3) in phosphoellenbergerite. By combining both crystallographic and spectroscopic data, it can be inferred that the H3 protons are associated with Mg vacancies in the single chains of face-sharing Mg octahedra. Furthermore, a quantitative inspection of signal intensities indicates that the proton sites in synthetic phosphoellenbergerite are not all saturated. Therefore, the substitution [P.sup.5+] = [Si.sup.4+] + [H.sup.+] 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, [Mg.sub.2][PO.sub.4]OH, were used as a reference to facilitate the interpretation of the IR and NMR spectra of phosphoellenbergerite.