Hydroxyapatites as Versatile Inorganic Hosts of Unusual Pentavalent Manganese Cations

Contrary
to molecular species, only very few solids are reported
to host manganese (V) species. Herein, we report three new compounds
with a hydroxyapatite structural backbone built on the MnVO43– anion: Sr5[(Mn1–xSix)­O4]3(OH)1–3x (x = 0 and 0.053), Sr5(MnO4)3(OH)1–yFy (y = 0.90), and Sr5[(Mn1–xSix)­O4]3F1–3x (x = 0.058). These solids are fully characterized
using powder X-ray and neutron powder diffraction, scanning transmission
electron microscopy, electron energy loss spectroscopy (EELS), thermogravimetric
analysis, and magnetic measurements. Especially, we report for the
first time EELS Mn–L2,3 spectra of manganese with
the oxidation state (V). Contrary to other Mn­(V) oxides, these solids
and the nominal compound Sr5(MnO4)3OH do not comprise Ba2+ cations but rely only on Sr2+ cations, showing that barium is not a required element to
stabilize Mn­(V) species in inorganic solids. We show that by tuning
soft chemistry conditions on the one hand and post-treatment topological
transformation conditions on the other hand, Mn­(V) and hydroxyl groups
can be substituted by Si­(IV) and fluoride ions, respectively. Hence,
we deliver solids with a potentially wide composition range. These
compounds show significant oxygen anionic conduction, thus suggesting
the emergence of new functional materials built from high-oxidation
state manganese cations.