Sodium is in large excess versus manganese and strontium at all compositions probed (Na:Mn = 211 atomic ratio), so that Na + doping when moving from high to low q(H2O)Na can only be linked to the amount of water (pH2O value) introduced in the reaction medium, hence to the basicity of the hydroflux. The substitution of Sr 2+ by Na + may be related to the very low solubility of Sr 2+ at q(H2O)Na = 0.5 at the crystallization onset (see supplementary text 1). In such conditions, the large excess of Na + versus Sr 2+ in solution facilitates incorporation of the alkali cation. Another hypothesis this time related to the crystal growth step is that as p(H2O)Na increases, speciation in the hydroflux evolves: Sr may form some (hydr)oxoanions as reported for other metals, and the water-poor hydroflux may act as good alkali cation donor. Both hypotheses are in line with the formation of alkali-containing phases in hydrofluxes with low amounts of water, as exemplified by KSrMnO4 above and the speciation between BaNa[Rh(OH)6] and Ba3[Rh(OH)6]2 • H2O in the Ba-Rh-O system. High Resolution transmission electron images (HRTEM) of the NaxSr5-x(MnO4)3(OH)1-x (Figures ) show that the material is nanostructured and encompasses aggregated particles of 20-30 nm size (Figure , zone A) in agreement with XRD data. These particles are embedded into non-crystalline areas. From the corresponding fast Fourier transforms (FFT), all maxima can be indexed on the basis of the hexagonal lattice according to the cell parameters determined from XRD. Local particle by particle STEM-EDS analysis yields Sr:Mn and Na:Mn ratios of 1.7 and 0.04, consistent with WDXRF given measurement uncertainties, in both crystalline and amorphous areas. Such amorphous regions could not be detected by electron microscopy in the hydroxyapatite samples at higher q(H2O)A probably indicating a change in the crystallization mechanism. Indeed, in a strongly basic hydroflux, the very low solubility of Sr 2+ during the onset of crystallization (see supplementary text 1) would hinder nucleation from the liquid phase, causing a switch from a liquid-mediated mechanism to solid-state crystallization from an amorphous precursor phase precipitated at the beginning of the process, as is the case for calcium phosphate hydroxyapatite. Upon crystallization from the amorphous precursor, the morphology of the latter is retained and no anisotropic growth occurs. Therefore, we demonstrate control of the micro-/nanostructure of Mn(V) hydroxyapatites. Note however that the speciation of dissolved strontium in highly basic hydrofluxes may also change, as shown for other metals, and then impact the crystallization pathway by first triggering precipitation of an amorphous phase. Raman spectroscopy provides additional evidence of Mn(V) in the form of bands typical of [MnO4] 3-tetrahedra (Figure , Table ). Additional bands at 400-700 cm -1 region are related to a hexagonal SrMnO3 (h-SrMnO3), which highlights the presence of Mn(IV) species.