Apatite-type materials have attracted considerable attention due to their potential applications in optical, electronic, and energy devices. In this study, the synthesis and physicochemical characterization of the novel lacunary lead apatite Pb6Ca2K2(PO4)6 were investigated to evaluate its structural, optical, and dielectric properties. The compound was synthesized via solid-state reaction. Structural characterization, performed by powder and single-crystal X-ray diffraction, revealed that the compound crystallizes in the hexagonal P63/m space group. The single-crystal analysis further indicated that the apatite framework contains total anionic vacancies, which are stabilized by the stereochemically active lone electron pair of Pb2+ ions. Infrared spectroscopy confirmed the absence of (OH)/(CO) groups in the structure. Optical properties were examined through UV-visible absorption spectroscopy. The material exhibited a strong absorption in the UV region with characteristic absorption bands at 230 and 315 nm. The optical band gap was determined to be a direct band gap of 3.50 eV. Under excitation at 375 nm, fluorescence measurements revealed emission in the green region of the visible spectrum. Dielectric properties were studied using complex impedance spectroscopy over a range of temperatures and frequencies. Results indicated a phase transition around 740 K, likely associated with an order-disorder process. The electrical conduction mechanism is attributed to the thermally activated migration of Ca2+ ions through the structural tunnels along the c-axis. Overall, the structural stability, the semiconductor-like behavior, and the luminescent properties suggest that Pb6Ca2K2(PO4)6 is a promising candidate for technological applications in photonic devices, photovoltaic systems, batteries, and light-emitting diodes (LEDs).

A novel organic-inorganic metallo-phosphate complex, CoF2(H2O)Fe(C10H8N2)(HPO4)2, obtained through hydrothermal method, was characterized via single-crystal X-ray diffraction, infrared and Raman spectroscopies, and thermogravimetric analysis. UV and photoluminescence spectroscopy were used to determine the optical properties. Structural study revealed a metal-organic framework (MOF), with iron and cobalt octahedra and phosphate tetrahedra, and a (2,2 '-bipyridine) coordinated by Fe octahedron. These moieties form chains, connected via it-it and H bonds. In order to assess the structural stability and interactions between atoms, a crystal contact study was carried out through Hirshfeld surface analysis and 2D fingerprint diagrams, to investigate the tendency of intermolecular interactions to form crystal packing by using enrichment ratio. To better examine the structure, theoretical studies were carried out using density functional theory, followed by a detailed examination of atoms in molecules using the bond critical point approach. The material revealed clear fluorescence after excitation at 285 nm, with intense emission in the violet.

Clays have been exploited in the manufacture of diverse products from ceramics to paints, pharmaceuticals, plastics, cosmetics, and more. Thus, they can be used in many industrial applications, showing good adsorbent ability thanks to their lamellar structure, high cation exchange capacity, pore size distribution, and large surface area. For this reason, considerable attention has been paid to their in-depth characterization, for further integration in sectors such as biomedicine, construction, remediation, aerospace, and nanotechnology. For this aim, two samples of natural clays, ALO1 and PRE4, from the southeast part of Albania, were subject to a multi-methodological characterization, with the aim of addressing the use of such geomaterials in possible sensing applications. X-ray fluorescence analysis, morphological characterization of the samples, and energy-dispersive system spectroscopy pointed to an extreme mineralogical variety, with kaolinite in AL01 and montmorillonite in PRE4 as the most abundant phases. This fact was further confirmed by powder X-ray diffraction, showing a quartz content of 20%, a kaolinite content of 64%, and a muscovite content of 16% for ALO1; meanwhile, for PRE4, we found a content of quartz of 45%, a content of montmorillonite of 34.9%, and a content of clinochlore of 20%. Infrared spectroscopy and thermal analyses confirmed the presence of hydroxyl groups in both samples, suggesting a higher content in ALO1. Measurement of N2 adsorption isotherms on the clay samples yields specific surface areas of 87 m2/g for PRE4 and 32 m2/g for ALO1, pore volumes of 0.721 cm3/g for PRE4 and 0.637 cm3/g for ALO1, and similar pore sizes in the range of 6–12 nm. Electrochemical analysis highlighted a good conductivity of ALO1 and PRE4 when used for the modification of commercial carbon-based screen-printed electrodes. In detail, higher currents were registered by differential pulse voltammetry for the electrodes modified with the clays with respect to bare electrodes, as well as good repeatability of the measurements. In addition, a comparative study with nanomaterials, known for their good conductivity, was achieved, using carbon black and gold nanoparticles as a reference, showing that the conductivity of the clays was lower than but not so different from those of the reference materials.

Prismatic crystals of partially potassium substituted lead fluorapatite Pb5.09Ca3.78K1.13(PO4)6F0.87 were grown through a solid-state reaction. The structural study conducted by single-crystal X-ray diffraction revealed that the compound crystallizes in the hexagonal P63/m space group, with unit cell parameters a = b = 9.7190(5) Å, c = 7.1700(6) Å and V = 587.37(7) Å3(Z = 1), as well as final values amounting to R and wR of 0.0309 and 0.0546, respectively. The structural refinement demonstrated that Pb occupies both the (6h) and (4f) structural sites of hexagonal fluorapatite, K occupies the (6h) site, and Ca is placed on the (4f) site. Powder X-ray diffraction study indicated the absence of additional phases or impurities. Chemical analysis using atomic absorption spectrometry and energy-dispersive X-ray spectroscopy confirmed the expected chemical formula. The electrical conductivity measured over a wide temperature range was found to be governed by the ion mobility mechanism in the tunnel along the c axis (probably attributed to the fluorine ion located there). We, therefore, could infer from the analysis of the complex impedance spectra that the electrical conductivity of our apatite depends essentially on the temperature and frequency, which produces a relaxation phenomenon and semiconductor-like behavior. Moreover, the strong absorption in the UV-Visible region was substantiated through studies of the optical properties of the developed sample. Fluorescence spectra exhibited emissions in the orange regions when excited at 375 nm. The findings of the phenomena resulting from the emission and conduction of the apatite in question suggest its potential for application in various technological fields such as photovoltaic cells, optoelectronics, photonics, LED applications, catalysis and batteries.

Hydroxyapatite (HAp), ideal formula Ca10(PO4)6(OH)2, has unique physicochemical properties, including an excellent adsorption ability for functional biomolecules (e.g. nucleic acids, proteins) thanks to its specific large crystal surface. This property can be further improved with cationic and anionic replacements within the HAp framework. The adsorption of such biomolecules, indeed, can cause changes in the electric properties of the HAp surface in terms of resistivity and capacitance, generating the conditions for an improvement of the materials targeted for sensor applications. This work relates to the multiple routes for the synthesis of HAp materials, their electrochemical and structural investigations, and a short overview on the most well-known applications in sensor design. Moreover, with the aim of finding new promising HAp-based materials tailored for bioreceptor immobilization in biosensing, we underwent some doped-hydroxyapatite materials, specifically Sr-HAp, Gd-HAp, and Er-HAp, to a complete characterization. Electrochemical analyses, based on differential pulse voltammetry and cyclic voltammetry, evidenced improved analytical performances of HAp in terms of signal enhancement, repeatability, reproducibility, and reusability, in particular concerning the Er-HAp phase. A multi-methodological structural study, based on powder X-ray diffraction analysis, microscopy techniques (optical, electron, and fluorescence), energy dispersive X-ray spectroscopy (for chemical analyses), Fourier transform infrared spectroscopy, and absorption/fluorescence spectroscopies, showed the mechanism of doping replacement in HAp crystallographic sites, owing to the results of the Rietveld refinement from powder X-ray data, and a strong fluorescence for Sr-HAp.

Strontium-substituted Ca-10(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.

Eu-doped and undoped polycrystalline Ca10(PO4)OH2 hydroxyapatite [1] were synthesized by using the chemical-precipitation technique [1].Chemical composition of the samples was confirmed by using Inductively Coupled Plasma (ICP) spectroscopy.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, good luminescence and very low crystallite size.On the contrary, Eu-doped sample calcinated at 900°C showed very high crystallinity, 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 5 D0-7 F0 of the Eu 3+ transition in Ca2 site, was more than 10 times more intense than the emission peak at 592 of Eu 3+ in Ca1 site, indicating the complete migration of Eu 3+ ions in the Ca2 sites of HAp framework for the high temperature sample.FTIR and Raman spectra showed slight band shifts with increasing annealing temperature of the samples.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 [2]. Figure 1: Chemical-precipitation synthesis of hydroxyapatite

In the quest to find powerful modifiers of screen-printed electrodes for sensing applications, a set of rare earth-doped Ca10−xREx(PO4)6(OH)2 (RE = La, Nd, Sm, Eu, Dy, and Tm and x = 0.01, 0.02, 0.10, and 0.20) hydroxyapatite (HAp) samples were subjected to an in-depth electrochemical characterization using electrochemical impedance spectroscopy and cyclic and square wave voltammetry. Among all of these, the inorganic phosphates doped with lanthanum proved to be the most reliable, revealing robust analytical performances in terms of sensitivity, repeatability, reproducibility, and reusability, hence paving the way for their exploitation in sensing applications. Structural data on La-doped HAp samples were also provided by using different techniques, including optical microscopy, X-ray diffraction, Rietveld refinement from X-ray data, Fourier transform infrared, and Raman vibrational spectroscopies, to complement the electrochemical characterization.

Nowadays stony monuments, despite their excellent conservation considering their long-time story within time, must continuously be protected from damage caused mainly from the detrimental effects of weathering, meaning cold, warm, acid rains, etc..The design and the application of new environmentally friendly materials in the protection of such monuments is the challenge that many scientists are going to face all over the world.In this sense, nanomaterials (size 30-60 nm) have been applied in the last three decades in the maintenance of the world cultural heritage, with the goal to improve the consolidation and protection treatments of damaged stone which make up the monument.

The lack of information on structural basis where proteins are involved, as well as the biomineralization processes of different systems such as bones, diatom frustules, and eggshells, have intrigued scientists from different fields for decades. This scientific curiosity has led to the use of methodologies that help understand the mechanism involved in the formation of these complex structures. Therefore, this work focuses on the use of eggshell membranes from different species of ratites (emu and ostrich) and reptiles (two species of crocodiles) as a model to differentiate biocalcification and biosilicification by introducing calcium phosphate or silica inside the membrane fiber mantles. We performed this to obtain information about the process of eggshell formation as well as the changes that occur in the membrane during crystal formation. In order to identify and understand the early processes leading to the formation of the microstructures present in the eggshell, we decided to carry out the synthesis of silica-carbonate of calcium, barium, and strontium called biomorph in the presence of intramineral proteins. This was carried out to evaluate the influence of these proteins on the formation of specific structures. We found that the proteins on untreated membranes, present a structural growth similar to those observed in the inner part of the eggshell, while in treated membranes, the structures formed present a high similarity with those observed in the outer and intermediate part of the eggshell. Finally, a topographic and molecular analysis of the biomorphs and membranes was performed by scanning electron microscopy (SEM), Raman and Fourier-transform Infrared (FTIR) spectroscopies.

A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca9Mg(PO4)6[PO3(OH)], from Palermo Mine (USA), was achieved by means of a combination of electron microprobe analysis (EMPA) in WDS mode, single-crystal neutron diffraction probe (NDP) and single-crystal X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The crystal-chemical characterization resulted in the empirical formula (Ca8.682Na0.274Sr0.045)Σ9.000(Ca0.034□0.996)Σ1.000(Mg0.533Fe2+0.342Mn2+0.062Al0.046)Σ0.983(P1.006O4)6[PO3(OH0.968F0.032)Σ1.000]. Crystal-structure refinement, in the space group R3c, converged to R1 = 7.12% using 3273 unique reflections from NDP data and to R1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are a = 10.357(3) Å, c = 37.095(15) Å and V = 3446(2) Å3, and from XRD, the parameters are a = 10.3685(4) Å, c = 37.1444(13) Å and V = 3458.2(3) Å3. NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO3(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca3(PO4)2 and displays minor band shifts due to slightly different P-O bond lengths and to the presence of additional elements in the structure. A comparison between whitlockite, isotypic phases from the largest merrillite group, and its synthetic counterpart Ca3(PO4)2 is provided, based on the XRD/NDP and FTIR results.

The main objective of this study was to provide a preliminary analysis of the meteorite North West Africa (NWA) 12606 recently classified as ureilite, which was found in 2018 in Morocco, to unveil the possible presence of diamonds. The preliminary analysis of the surface of a meteorite fragment by scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) and optical microscopy has shown that it mainly consisted of olivine, minor pyroxene and carbon phases possibly including carbon in the form of diamonds. The results achieved are preliminary to a further deeper study of this meteorite as the diamond origin in ureilites is still an open issue debated among the scientific community due to its significant implications for the sizes of early Solar System bodies.

oral communication at 1st Conference on Crystallography, Structural Chemistry and Biosystems, (Catania)

Rare-earth-based Ca9RE(PO4)7 (RE = Nd, Gd, Dy) materials were synthesized by solid-state reaction at T = 1200 °C. The obtained tricalcium phosphate (TCP) materials are efficient light emitters due to the presence of RE3+ ions, although these ions are present at high concentrations. Moreover, in these host structures, these ions can be used as optical probes to study their local environments. Thus, photoluminescence (PL) emission spectra of the powder samples clearly indicated, for Dy3+ and Gd3+ ions, the presence of the RE3+ ion in low-symmetry sites with some local structural disorder, and the spectra show the presence of vibrational features (in the case of Gd3+). For the Nd3+ phase, emission bands are present around 900, 1050, and 1330 nm, originating from the 4F3/2 level. In general, these RE-TCP samples are interesting luminescent materials in the visible (Dy), UV (Gd), and NIR (Nd) regions, due to weak concentration quenching even for high concentrations of the emitting ion.

The scientific interest toward the structural, luminescence, and dielectric properties of tricalcium phosphate (TCP) materials, in particular, the doped TCP-based compounds, is mainly due to their biocompatibility and bioactivity behavior and the increasing number of their applications. In the literature, structural investigations of a doped beta-TCP compound are usually carried out by powder diffraction data under the assumption that it has a centrosymmetric whitlockite structure. Recent studies have shown that this assumption is not always fulfilled. For this reason, it is necessary to use methods of nonlinear and dielectric spectroscopy to prove the true symmetry of whitlockite-like phases. This work provides a detailed and comprehensive study on the structural, dielectric, and nonlinear properties of the polycrystalline Tb- and Ho-doped tricalcium phosphates, chemically Ca9Tb(PO4)(7) and Ca9Ho(PO4)(7). Their crystal structure was explored using powder X-ray diffraction, adopting an ab initio approach for the structure solution followed by the Rietveld refinement. For Ca9Tb(PO4)(7), the qualitative analysis of the experimental powder X-ray diffraction pattern revealed that the sample was not a single phase. Nevertheless, its crystal structure has been determined by Direct Methods. The presence of the dopant cation was omitted in the solution process by Direct Methods and considered in the successive Rietveld refinement step. The analysis of the occupancies has clarified the Tb3+ cation distribution in its preferential sites. The same pathway has been followed for the Ca9Ho(PO4)(7) sample, which corresponded to a single-phase structure. The dielectric properties of the two doped compounds (Tb3+ and Ho3+) were also examined, and reversible ferroelectric-paraelectric phase transitions were found at 862 and 872 K, respectively.

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.

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.

Luminescence properties of europium-doped Ca10-xEux(PO4)6(OH)2 (xEu = 0, 0.01, 0.02, 0.10 and 0.20) and gadolinium-doped hydroxyapatite Ca9.80Gd0.20(PO4)6(OH)2 (HA), synthesized via solid-state reaction at T = 1300 °C, were investigated using scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR), and luminescence spectroscopy. Crystal structure characterization (from unit cell parameters determination to refined atomic positions) was achieved in the P63/m space group. FTIR analyses show only slight band shifts of (PO4) modes as a function of the rare earth concentration. Structural refinement, achieved via the Rietveld method, and luminescence spectroscopy highlighted the presence of dopant at the Ca2 site. Strong luminescence was observed for all Eu- and Gd-doped samples. Our multi-methodological study confirms that rare-earth (RE)-doped synthetic hydroxyapatites are promising materials for bio-imaging applications.

Sodalite-group minerals are silicates belonging to the zeolite group hosting in their cages a variety of cations and anions. Four mineral species of the sodalite-group are known in nature, the main difference being connected to the identity of the chemical species in the cages, particularly the sulfur molecular arrangements. Notably, the sodalite-group minerals show different colours; lazurite, in particular, is the main constituent of the well-known lapis lazuli, a material used throughout the human history as a precious stone or as a blue pigment for artworks. In this paper, we exploit Raman and Fourier-transform infrared (FTIR) spectroscopy to highlight an existing connection between the chalcogen species entrapped within the samples originated from the volcanic region of Latium (Italy) and their final colour. We show that Raman spectroscopy is a very valuable tool in characterizing the extraframework content in these minerals. Further, our analysis demonstrates how the blue hue of the sodalite species is related to the S-3(-)/SO42- relative content. We finally show how most specimen are also rich in CO2, a feature addressed by combination with FTIR spectroscopy. These findings have a particular interest not only in geology, but also in Cultural Heritage studies.