Strontium-substituted Ca₁₀(PO₄)₆(OH)₂ hydroxyapatite (HAp) powders, with Sr wt% concentrations of 2.5, 5.6 and 10%, were prepared by a solid-state synthesis method. The chemical composition of the samples was accurately evaluated by using inductively coupled plasma (ICP) spectroscopy. The morphology of the samples was analyzed via optical microscopy, while structural characterization was achieved through powder X-ray diffraction (PXRD) and infrared (FTIR) and Raman spectroscopy. The PXRD structural characterization showed the presence of the Sr dopant in the Ca1 structural site for HAp with a lower Sr concentration and in the Ca2 site for the sample with a higher Sr concentration. FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO₄) bands with increasing the Sr doping rate.

Europium-doped hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ (3% mol) powders were synthesized by an optimized chemical precipitation method at 25 °C, followed by drying at 120 °C and calcination at 450 °C and 900 °C. The obtained nanosized crystallite samples were investigated by means of a combination of inductively coupled plasma (ICP) spectroscopy, powder X-ray diffraction (PXRD), Fourier Transform Infrared (FTIR), Raman and photoluminescence (PL) spectroscopies. The Rietveld refinement in the hexagonal P6₃/m space group showed europium ordered at the Ca2 site at high temperature (900 °C), and at the Ca1 site for lower temperatures (120 °C and 450 °C). FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO₄) bands with increasing annealing temperature. PL spectra and decay curves revealed significant luminescence emission for the phase obtained at 900 °C and highlighted the migration of Eu from the Ca1 to Ca2 site as a result of increasing calcinating temperature.

Stony monuments must continuously be safeguarded from damage caused over time, in particular from the detrimental effects of weathering. One of the new environmentally-friendly (nano) materials for stone reinforcement, particularly suitable for marble and calcareous (limestone, sandstone) artifacts, is Ca₁₀(PO₄)₆(OH)₂ hydroxyapatite (HAp), which has a considerably lower dissolution rate and solubility compared to CaCO₃ calcite (the building block of marble materials): thus, HAp has been proposed for the protection of calcareous monuments against acidic rain corrosion. Promising results have been obtained, but further optimization is necessary as the treated layer is often incomplete, cracked and/or porous. Several parameters need to be optimized, in this way a homogeneous layer can be obtained, and consequently the formation of metastable can be avoided, soluble phases instead of HAp. These include: the pH of the starting solution; the effect of organic and inorganic additions in particular, that of ethanol, which is known to adsorb calcite, thus possibly favoring the growth of the HAp layer. The formation of HAp nanoparticles and their application on stony substrates has been investigated by means of a multi-methodological approach based on scanning electron microscopy, x-ray diffraction, small- and/or wide-angle x-ray scattering, Fourier-transform infrared spectroscopy, and finally, in situ measurements of laser-induced breakdown spectroscopy and acid attack preliminary tests on stony substrates.

Eu-doped (mol 3%) and undoped polycrystalline Ca₁₀(PO₄)OH₂ hydroxyapatite (HAp) [1-2] were synthesized by using the chemical-precipitation technique [3]. Some precautions were observed: the temperature was kept low at 25° C so that to inhibit the increasing of the average size of the particles; pH was kept constantly high (10 ± 0.05) in order to minimize the formation of secondary phases and to prevent the aggregation of the particles during their formation. The multi-methodological characterization achieved through powder X-Ray diffraction (PXRD) and photoluminescence (PL) techniques, showed that the Eu entered in Ca1 site in the dried (120 °C) doped samples and in those calcinated (450 °C) at low temperature: these samples show low crystallinity (3% and 7%, respectively), good luminescence and very low crystallite size (around 25 nm). On the contrary, Eu-doped sample calcinated at 900°C showed very high crystallinity (87%), with a crystallite size of 148 nm, while PL spectroscopy suggested that this sample presents the highest and narrowest emission bands. Specifically, the PL emissions peak at 573 nm, corresponding to the ⁵D₀–⁷F₀ of the Eu³⁺ transition in Ca2 site [4], was more than 10 times more intense than the emission peak at 592 of Eu³⁺ in Ca1 site, indicating the complete migration of Eu³⁺ ions in the Ca2 sites of HAp framework for the high temperature sample [3]. FTIR and Raman spectra showed slight band shifts with increasing annealing temperature of the samples [3]. Results show that low crystalline HAp obtained at 120° and 450°, could be employed as luminescent drug carriers, while high crystalline HAp, annealed at 900°, could be suitable materials for biological optical imaging. [1] Dorozhkin, S.V. Calcium Orthophosphates: Occurrence, Properties and Major Applications. Bioceram Dev Appl 2014, 4. [2] Hughes, J.M.; Rakovan, J. The Crystal Structure of Apatite, Ca₅(PO₄)₃(F,OH,Cl). Reviews in Mineralogy and Geochemistry 2002, 48, 1–12. [3] Baldassarre, F.; Altomare, A.; Corriero, N.; Mesto, E.; Lacalamita, M.; Bruno, G.; Sacchetti, A.; Dida, B.; Karaj, D.; Ventura, G.D.; Capitelli, F.; Siliqi, D. Crystal Chemistry and Luminescence Properties of Eu-Doped Polycrystalline Hydroxyapatite Synthesized by Chemical Precipitation at Room Temperature. Crystals 2020, 10, 250 . [4] Nikolaev, A.; Kolesnikov, I.; Frank-Kamenetskaya, O.; Kuz’mina, M. Europium concentration effect on characteristics and luminescent properties of hydroxyapatite nanocrystalline powders. Journal of Molecular Structure 2017, 1149, 323–331.