Structural Characterization of Calcium Phosphates by Means of X-ray Diffraction

Dear Colleagues,

Calcium phosphates have gained a lot of attention in the scientific community because of their presence in human beings (as bones, teeth, and various pathological calcifications) and their abundance in nature as mineral phases retrieved in geological settings. Owing to their structural characteristics, their synthetical counterparts are employed in many applications. In this sense, the Ca₅(PO₄)₃(OH,F,Cl) apatite group is the most investigated phase, showing multiple substitutions both at cationic and anionic sites. Hydroxylapatite (HA) is the mineral component of the bones and teeth (enamel, dentin), and it is currently employed as cement to fill gaps in bones, or as coating on prothesis to improve biocompatibility; at a nanometric scale, in recent years, HA has become popular as a nanoparticle drug delivery agent and synthetic nanoparticles of HA have been employed in the conservation of cultural heritage, with applications on marble and limestone substrates.

Fluororapatite (FA) and chloroapatite (ClA) phases, traditionally considered reliable petrogenetic indicators, are currently actively investigated for their properties of heavy metal sequestration, solid nuclear waste form, and water treatment. Moreover, apatite materials, when doped with rare earth elements (REE), manganese, or other activator elements, show fluorescent properties, hence being employed as laser materials. Last, natural apatite ores are a major source of phosphorus, in turn largely used to produce agricultural fertilizers, as well as several chemical products.

Similar considerations should be carried out for tricalcium phosphates Ca₃(PO₄)₂ (TCP), retrieved in human dental calculi, salivary stones, arthritic cartilage, and soft-tissue deposits. Synthetic TCP materials, displaying a wide range of cationic replacements, are used in bone cements/fillers and low load bearing implants, aiming to provide a degradable coating on metallic implants and to induce a favorable biological response, in order to increase osteointegration of the same implant. When TCP are doped with REE, they become phosphor materials owing to their luminescence properties, and areemployed like X-ray dosimeters because of their density, which is similar to that of human bones. TCP are also found in different geological environments, as terrestrial phase, whitlockite Ca₁₈Mg₂(PO₄)₁₂[PO₃(OH)]₂, and its dehydrogenated extraterrestrial analogue, merrillite Ca₁₈Na₂Mg₂(PO₄)₁₄, found in Moon rocks; relationships between the two phases, over investigations on apatite phases retrieved in meteorites, can better focus the role of phosphorous in the origin of life on Earth.

We invite colleagues to submit papers on calcium phosphate materials, both natural and synthetized, with possible substitutions both at Ca sites (Sr, Pb, REE, etc) and at anionic groups (V, As, etc), which relate to the methods and synthesis for novel phosphate nanomaterials, their structural characterization by means of X-ray diffraction, joined by other complementary techniques (SEM–EDS, FTIR, Raman, luminescence etc.), and possible applications/interests in biomedical sciences, materials, cultural heritage, optics, mineralogy, planetary sciences, etc., including:

- Ca₅(PO₄)₃(OH,F,Cl) apatite;

- Ca₃(PO₄)₂ tricalcium phosphate (TCP);

- Ca₉(Mg,Fe)(PO₄)₆(PO₃OH) whitlockite;

- Other Ca orthophosphate phases, such as CaHPO₄·2H₂O brushite, CaHPO₄ monetite, oxyapatite Ca₅(PO₄)₃O_1/2, etc.;

- Calcium diphosphates or polyphosphates of any technological interest.

Prof. Francesco Capitelli
Guest Editor

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