Special Issue "Structural Characterization of Calcium Phosphates by Means of X-ray Diffraction"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystalline Materials".

Deadline for manuscript submissions: 31 December 2019.

Special Issue Editor

Guest Editor
Dr. Francesco Capitelli

Institute of Crystallography-CNR, Monterotondo (Rome), Italy
Website | E-Mail
Interests: inorganic phosphates; materials; X-ray diffraction; apatite; tricalcium phosphates; minerals; rare earth elements

Special Issue Information

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)3O1/2, etc.;

- Calcium diphosphates or polyphosphates of any technological interest.

Prof. Francesco Capitelli
Guest Editor

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Keywords

  • Inorganic phosphates
  • Materials
  • X-ray diffraction
  • Structural characterization
  • Tricalcium phosphate (TCP)
  • Apatite
  • Rare earth elements

Published Papers (3 papers)

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Research

Open AccessArticle
Silver Doping Mechanism in Bioceramics—From Ag+:Doped HAp to Ag°/BCP Nanocomposite
Crystals 2019, 9(7), 326; https://doi.org/10.3390/cryst9070326
Received: 24 May 2019 / Revised: 20 June 2019 / Accepted: 23 June 2019 / Published: 26 June 2019
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Abstract
The results presented in this paper, based on the powder X-ray diffraction technique followed by Rietveld analyses, are devoted to the mechanism of silver incorporation in biphasic calcium phosphates. Results were confirmed by SEM observation. Samples were synthesized via the sol-gel route, followed [...] Read more.
The results presented in this paper, based on the powder X-ray diffraction technique followed by Rietveld analyses, are devoted to the mechanism of silver incorporation in biphasic calcium phosphates. Results were confirmed by SEM observation. Samples were synthesized via the sol-gel route, followed by heat treatments. Two incorporation sites were highlighted: Ca2+ replacement by Ag+ into the calcium phosphates (HAp: hydroxyapatite and β-TCP: tricalcium phosphate), and the other as metallic silver Ag° nanoparticles (formed by autogenous reduction). The samples obtained were thus nanocomposites, written Ag°/BCP, composed of closely-mixed Ag° particles of about 100 nm at 400 °C (which became micrometric upon heating) and calcium phosphates, themselves substituted by Ag+ cations. Between 400 °C and 700 °C the cationic silver part was mainly located in the HAp phase of the composition Ca10−xAgx(PO4)6(OH)2−x (written Ag+: HAp). From 600 °C silver cations migrated to β-TCP to form the definite compound Ca10Ag(PO4)7 (written Ag+: TCP). Due to the melting point of Ag°, the doping element completely left our sample at temperatures above 1000 °C. In order to correctly understand the biological behavior of such material, which is potentially interesting for biomaterial applications, its complex doping mechanism should be taken into consideration for subsequent cytotoxic and bacteriologic studies. Full article
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Open AccessArticle
X-ray Structure Refinement and Vibrational Spectroscopy of Metavauxite FeAl2(PO4)2(OH)2·8H2O
Crystals 2019, 9(6), 297; https://doi.org/10.3390/cryst9060297
Received: 11 May 2019 / Revised: 1 June 2019 / Accepted: 3 June 2019 / Published: 6 June 2019
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Abstract
In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl2(PO4)2(OH)2·8H2O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier [...] Read more.
In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl2(PO4)2(OH)2·8H2O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier transform infrared (FTIR) spectroscopy. Our new diffraction results allowed us to locate all hydrogen atoms from the structure refinements in the monoclinic P21/c space group. Metavauxite structure displays a complex framework consisting of a stacking of [Al(PO4)3(OH)(H2O)2]7− layers linked to isolated [Fe(H2O)6]2+ cationic octahedral complex solely by hydrogen bonding. The hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. Accordingly, strong hydrogen bonds, due to four coordinated H2O molecules, bridge the [Fe(H2O)6]2+ units to the Al/P octahedral/tetrahedral layer. The hydroxyl group, coordinated by two Al atoms, contributes to the intra-layer linkage. FTIR and Raman spectra in the high-frequency region (3700–3200 cm−1) are very similar, and show a complex broad band consisting of several overlapping components due to the H2O molecules connecting the isolated Fe(H2O)6 and the adjacent Al/P octahedral/tetrahedral layers. A sharp peak at 3540 cm−1 is assigned to the stretching mode of the OH group. The patterns collected in the low-frequency region are dominated by the stretching and bending modes of the PO43− group and the metal-oxygen polyhedra. Full article
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Graphical abstract

Open AccessArticle
New Ca2.90(Me2+)0.10(PO4)2 β-tricalcium Phosphates with Me2+ = Mn, Ni, Cu: Synthesis, Crystal-Chemistry, and Luminescence Properties
Crystals 2019, 9(6), 288; https://doi.org/10.3390/cryst9060288
Received: 19 April 2019 / Revised: 23 May 2019 / Accepted: 28 May 2019 / Published: 1 June 2019
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Abstract
Ca2.90Me0.102+(PO4)2 (with Me = Mn, Ni, Cu) β-tricalcium phosphate (TCP) powders were synthesized by solid-state reaction at T = 1200 °C and investigated by means of a combination of scanning [...] Read more.
C a 2.90 M e 0.10 2 + ( P O 4 ) 2 (with Me = Mn, Ni, Cu) β-tricalcium phosphate (TCP) powders were synthesized by solid-state reaction at T = 1200 °C and investigated by means of a combination of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, and luminescence spectroscopy. SEM morphological analysis showed the run products to consist of sub spherical microcrystalline aggregates, while EDS semi-quantitative analysis confirmed the nominal Ca/Me composition. The unit cell and the space group were determined by X-ray powder diffraction data showing that all the compounds crystallize in the rhombohedral R3c whitlockite-type structure, with the following unit cell constants: a = b = 10.41014(19) Å, c = 37.2984(13) Å, and cell volume V = 3500.53(15) Å3 (Mn); a = b = 10.39447(10) Å, c = 37.2901(8) Å; V = 3489.22(9) Å3 (Ni); a = b = 10.40764(8) Å, c = 37.3158(6) Å, V = 3500.48(7) Å3 (Cu). The investigation was completed with the structural refinement by the Rietveld method. The FTIR spectra are similar to those of the end-member Ca β-tricalcium phosphate (TCP), in agreement with the structure determination, and show minor band shifts of the (PO4) modes with the increasing size of the replacing Me2+ cation. Luminescence spectra and decay curves revealed significant luminescence properties for Mn and Cu phases. Full article
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