Apatite Channel Research Articles

Thermally‐Induced Reentrant Order‐Disorder‐Order Transitions in ALa9(GeO4)6O2 Apatites

AbstractOxyapatites A10(GeO4)6O2 (A – alkaline, rare‐earth elements) are considered promising materials for applications in solid‐state lighting and non‐contact thermometry. The temperature dynamics of the oxygen sublattice have a crucial influence on the luminescent properties of these optical hosts. Here, the structural evolution with temperature of germanates with the general formula ALa9(GeO4)6O2 (A – Li, Na, K, Rb) is investigated, using X‐ray and neutron powder diffraction methods. These compounds crystallize in space‐group P63/m. NPD experiments helped to determine the oxygen and alkaline metal positions. Thermally‐induced reentrant structural transitions relating to the oxygen in the apatite channel are discovered for the first time. In the example of Eu‐doped RbLa9(GeO4)6O2 and KLa9(GeO4)6O2, it is demonstrated that the luminescence of the Eu3+ ion is influenced by small changes in the crystal structure.

Incorporation of Cu and Mn ions into hydroxyapatite channels and deposition of complex metal oxide

The Cu-and Mn-incorporated apatite-type phosphate, Ca10(PO4)6CuxMny(O,OH)2, was successfully synthesized by heating the hydroxyapatite under N2 at 900 °C after Cu- and Mn-nitrate aqueous solution loading with x = 0–0.2 and y = 0–0.2 onto it. In Ca10(PO4)6Cu0·2Mn0·2(O,OH)2, Cu occupies the apatite channel center site, and the Mn site shifts from the center. The deposition of Cu and Mn species was found to occur upon heating Cu0·2Mn0.2-Ca-HAp at 600–800 °C in air, accommodating the spinel-type complex metal oxide, (Cu, Mn)3O4 formation from the apatite channel. In addition, the CuO deposition from the Ca10(PO4)6Cu0·2(O,OH)2 apatite was observed for samples heated at 600–800 °C, whereas Mn3O4 was observed for the Ca10(PO4)6Mn0·2(O,OH)2 sample heated at 800 °C. These results suggest that the presence of Cu in the apatite channel facilitates the deposition of Mn species.

Location of Carbonate Ions in Metal-Doped Carbonated Hydroxylapatites

The environment model for the description of the location of carbonate ions in apatites predicts that approximately half of the carbonate occupies the apatite channel. This model relies on the influence of entities surrounding the carbonate on its IR spectrum and can be used to determine how various substituents affect the location and structure of that ion. Careful deconvolution (peak-fitting) of the asymmetric carbonate IR region was used to determine the percentage of A-type (channel) ions, A′-type (channel with either a Ca2+ vacancy or substitution of Na+ for Ca2+) ions, and B-type (substitution for phosphate) ions. In our previous applications of this model, we have looked at the effect of alkali metal ions, such as sodium, lithium, and potassium, the ammonium ion, and the rare earth europium ion. In the present work, we explore the incorporation of the first-row transition metal ions and find that they have little effect on the location of the carbonate ion. Like the un-substituted carbonated apatite, these apatites contain about half of the carbonate in the channel, at least in derivatives that contain up to half a mole of the metal ion per mole of apatite. Attempts to incorporate greater amounts of metal ions by aqueous ion-combination reactions generally lead to lower-resolution XRD patterns and IR spectra that produce greater uncertainties in the peak-fitting modeling.

Incorporation and deposition behaviors of Cu and Ni ions into hydroxyapatite channels by heat treatment

The incorporation of multiple metals into hydroxyapatite was investigated using Cu and Ni. Both Cu and Ni were successfully incorporated into hydroxyapatite, forming Ca10(PO4)6CuxNiy(O, OH)2 by heat treatment under N2 at 900 °C after metal aqueous solution loading with x = 0–0.3 and y = 0–0.2. The Cu occupied the channel center site, whereas the Ni shifted from the center of the Ca10(PO4)6Cu0.3Ni0.2(O, OH)2. The environment surrounding Ni in this apatite was different from that in Ca10(PO4)6Ni0.2(O, OH)2, suggesting the influence of Cu incorporation in the channel. CuO and NiO were deposited from Ca10(PO4)6Cu0.3Ni0.2(O, OH)2 at 600–900 and 700–900 °C by heating under 10% H2O air, respectively. The NiO deposition temperature was lower than that of Ca10(PO4)6Ni0.2(O, OH)2, suggesting that the coexistence of Cu in the apatite channel facilitated the Ni deposition.

Effect of H2O on Cu incorporation and deposition behaviors in hydroxyapatite by heat treatment

The effect of H2O in the atmosphere during heat treatment on Cu incorporation/deposition into hydroxyapatite was investigated after loading a Cu aqueous solution. Cu-incorporated apatite, Ca10(PO4)6Cux(O,OH)2 (Cux-Ca-HAp), was successfully synthesized from single-phase apatite under an atmosphere of low H2O concentration. The deposition of Cu from the apatite channel was facilitated under a H2O atmosphere at high temperatures. CuO deposition from Cu0.5-Ca-HAp under heat treatment in 10% H2O-air occurred at a lower temperature (600 °C) than that required when heating in dry air. The same deposition temperature is required when heat treatment in 10% H2O–N2 is used, indicating that the O2 concentration in the atmosphere is less conducive to the deposition of CuO than H2O vapor. In addition, we successfully synthesized single-phase apatite with a relatively high Cu content, up to 1.38 per apatite unit cell, by heat treatment under a dry N2 atmosphere.

Synthesis of Borate Doped La10Ge6O27: Confirming the Presence of a Secondary Conducting Pathway

The investigation of oxide ion conductivity in apatite germanates has attracted significant interest due to potential applications in SOFC electrolyte materials. These systems conduct via interstitial oxide ions, and a range of studies have indicated the importance of the GeO4 units in the conduction process. In this paper, we investigate the effect of boron incorporation on the structure and conductivity. Studies show that heat treatment of La10Ge6O27 with H3BO3 leads to an expansion in cell volume, attributed to incorporation of borate groups in the oxygen ion channels within the structure. For low levels of dopant i.e La10Ge6O27(BO1.5)0.5, a small enhancement in conductivity was observed attributed to a transition from a triclinic to a hexagonal apatite. For further increases in boron content, the conductivity was shown to decrease attributed to the blocking of the conduction pathway down the apatite channels. Interestingly, significant oxide ion conductivity was still observed, which provides the first experimental support for a secondary conduction mechanism perpendicular to the apatite channels proposed by prior modelling studies.

Crystal Chemistry and Antibacterial Properties of Cupriferous Hydroxyapatite.

Copper-doped hydroxyapatite (HA) of nominal composition Ca10(PO4)6[Cux(OH)2-2xOx] (0.0 ≤ x ≤ 0.8) was prepared by solid-state and wet chemical processing to explore the impact of the synthesis route and mode of crystal chemical incorporation of copper on the antibacterial efficacy against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) strains. Apatites prepared by solid-state reaction showed unit cell volume dilation from 527.17 Å3 for copper-free HA to 533.31 Å3 for material of the putative composition Ca10(PO4)6[Cu0.8(OH)0.4O0.8] consistent with Cu+ insertion into the [001] hydroxyapatite channel. This was less pronounced (528.30 Å3 to 529.3 Å3) in the corresponding wet chemical synthesised products, suggesting less complete Cu tunnel incorporation and partial tenancy of Cu in place of calcium. X-ray absorption spectroscopy suggests fast quenching is necessary to prevent oxidation of Cu+ to Cu2+. Raman spectroscopy revealed an absorption band at 630 cm−1 characteristic of symmetric O-Cu+-O units tenanted in the apatite channel while solid-state 31P magic-angle-spinning nuclear magnetic resonance (MAS NMR) supported a vacancy-Cu+ substitution model within the apatite channel. The copper doping strategy increases antibacterial efficiency by 25% to 55% compared to undoped HA, with the finer particle sizes and greater specific surface areas of the wet chemical material demonstrating superior efficacy.

Impact of fluoride for hydroxide substitution on the magnetic properties of a Co-based single-ion magnet imbedded in the barium apatite crystal lattice

The atomic group O–Co–O persists in the apatite channel and retains a high energy barrier for magnetization reversal.

Incorporation of fluorophosphate in apatite

The fluorophosphate ion has been widely used as a source of fluoride in dentifrices because of its anticaries properties. Because it is isoelectronic with sulfate, which is known to replace phosphate in apatite, fluorophosphate could be expected to react similarly. In an attempt to incorporate the fluorophosphate ion into hydroxylapatite, calcium and strontium hydroxylapatites were synthesized in both aqueous and nonaqueous solutions in the presence of fluorophosphate. The corresponding fluorophosphate salts (CaFPO3 and SrFPO3) were also synthesized and found to decompose over time to their corresponding fluorapatites. Samples were characterized using solid state 19F NMR, percent fluorine analysis, and unit cell parameter analysis. Hydrolysis of the fluorophosphate ion in various solutions was determined using solution 19F NMR. The presence of hydroxylapatite or calcium ions in an aqueous solution of fluorophosphate at 80 °C and pH 7 or 9 resulted in significant hydrolysis. Inclusion of Na2FPO3 as a reactant, along with calcium or strontium nitrate and sodium phosphate, produced only fluorapatite, a result of the hydrolysis of the FPO32− ion. With a larger ratio of fluorophosphate to phosphate and longer digestion times, the corresponding fluorophosphate salt (e.g., CaFPO3) formed in addition to the fluorapatite. Attempts to incorporate fluorophosphate into hydroxylapatite using nonaqueous solvents resulted in apatites with a larger weight percent of fluorine, but there was not sufficient evidence to conclude that the fluorophosphate ion was incorporated. The use of nonaqueous solvents, including DMSO, anisole, ethanol, several ionic liquids, 1:1 mixtures of anisole in glacial acetic acid, and 15-crown-5 ether in acetonitrile, produced apatite or fluorapatite with no evidence of incorporation of FPO3. It is convenient to explain the failure of both aqueous and nonaqueous syntheses by assuming that the fluorophosphate ion, perhaps in an unstable apatite phase, serves to provide fluoride to the apatite channel while the displaced hydroxide reacts with the fluorophosphate to form phosphate for the formation of fluorapatite.

Infrared spectra of carbonate apatites: Evidence for a connection between bone mineral and body fluids

The complex asymmetric stretch (ν3) region infrared (IR) spectrum of synthetic sodium- and carbonate-bearing hydroxylapatites (CHAP) has been interpreted using overlapped Gaussian distributions for individual carbonate ion species. There is now good agreement for the distribution of carbonate ions between phosphate (type B) and c -axis channel (type A) positions using three independent methods: X-ray structure site occupancies, out-of-plane bend (ν2) band areas, and asymmetric stretch (ν3) band areas; B/A ratios for a well-crystallized CHAP sample being 0.77, 0.78, and 0.75, respectively. The reported dominance of type B carbonate ions in bone mineral and dental enamel is attributed to the anomalous shift of type A band frequencies into the spectral region of type B, resulting from the substitution of Ca2+ by Na+ in the nearest-neighbor cation shell of the channel carbonate ions. The infrared spectra show that the hydrogencarbonate (bicarbonate) ion in apatite crystals is a channel species, as are its room-temperature decomposition products, type A carbonate and labile (type L) carbonate. The research suggests that bone mineral crystals may actively communicate with body fluids through the apatite channel, pointing to a possible role for the apatite channel in mediating acid-base reactions in the body.

Probing atomic scale transformation of fossil dental enamel using Fourier transform infrared and nuclear magnetic resonance spectroscopy: A case study from the Tugen Hills (Rift Gregory, Kenya)

A series of fossil tooth enamel samples was investigated by Fourier transform infrared (FTIR) spectroscopy, 13C and 19F magic-angle spinning nuclear magnetic resonance (MAS NMR) and scanning electron microscopy (SEM). Tooth remains were collected in Mio-Pliocene deposits of the Tugen Hills in Kenya. Significant transformations were observed in fossil enamel as a function of increasing fluorine content (up to 2.8wt.%). FTIR spectroscopy revealed a shift of the ν1 PO4 stretching band to higher frequency. The ν2 CO3 vibrational band showed a decrease in the intensity of the primary B-type carbonate signal, which was replaced by a specific band at 864cm−1. This last band was ascribed to a specific carbonate environment in which the carbonate group is closely associated to a fluoride ion. The occurrence of this carbonate defect was consistently attested by the observation of two different fluoride signals in the 19F NMR spectra. One main signal, at ∼−100ppm, is related to structural F ions in the apatite channel and the other, at −88ppm, corresponds to the composite defect. These spectroscopic observations can be understood as resulting from the mixture of two phases: biogenic hydroxylapatite (bioapatite) and secondary fluorapatite. SEM observations of the most altered sample confirmed the extensive replacement of the bioapatite by fluorapatite, resulting from the dissolution of the primary bioapatite followed by the precipitation of carbonate-fluorapatite. The ν2 CO3 IR bands can be efficiently used to monitor the extent of this type of bioapatite transformation during fossilization.

Molecular water in nominally unhydrated carbonated hydroxylapatite: The key to a better understanding of bone mineral

Despite numerous analytical studies, the exact nature of the mineral component of bone is not yet totally defined, even though it is recognized as a type of carbonated hydroxylapatite. The present study addresses the hydration state of bone mineral through Raman spectroscopic and thermogravimetric analysis of 56 samples of carbonated apatite containing from 1 to 17 wt% CO 3 , synthesized in H 2 O or D 2 O. Focus is on the relation between the concentration of molecular water (as distinguished from hydroxyl ions) and the concentration of carbonate in the apatite. Raman spectra confirm the presence of molecular water as part of the crystalline structure in all the aqueously precipitated carbonated apatites. TGA results quantitatively document that, regardless of the concentration of carbonate in the structure, all hydroxylapatites contain ~3 wt% of structurally incorporated water in addition to multiple wt% adsorbed water. We spectroscopically confirmed that natural bone mineral also contains structurally incorporated molecular H 2 O based on independent analyses of bone by means of spectral stripping (subtracting the spectrum of collagen from that of bone) and chemical stripping (chemically removing the collagen content of bone prior to analysis). Taken together, the above data support a model in which water molecules densely populate the apatite channels regardless of the abundance of hydroxyl vacancies. We hypothesize that water molecules keep the apatite channels stable even when 80% of the hydroxyl sites are vacant (typical in bone), hinder carbonate ions from substituting for hydroxyl ions in the channels, and help regulate chemical access to the channels (e.g., ion exchange, entry of small molecules). Our results show that bone apatite is not a “flawed hydroxylapatite,” but instead a definable mineralogical entity, a combined hydrated-hydroxylated calcium phosphate phase of the form Ca 10−x [(PO 4 ) 6−x (CO 3 ) x ](OH) 2−x ·nH 2 O, where n ~ 1.5. Water is therefore not an accidental, but rather an essential, component of bone mineral and other natural and synthetic low-temperature carbonated apatite phases.

Shedding light on bone material

The exact nature of the mineral component of bone is not yet totally defined, even though it is recognized as a type of carbonated hydroxylapatite. It is remarkable that such fundamental natural material, which forms all hard parts of the human body except for small portions of the inner ear, is not well understood. Authors Jill Pasteris, Claude H. Yoder, and Brigitte Wopenka have undertaken detailed and truly painstaking experiments to characterize bone material and shed light on its relationship to hydroxylapatite. The authors very effectively demonstrate, through Raman spectroscopic and thermogravimetric analysis of 56 synthetic samples of carbonated apatite containing from 1 to 17 wt% CO 3 , that bone material is not simply carbonated hydroxylapatite, but instead a definable mineralogical entity, a combined hydrated-hydroxylated calcium phosphate phase of the form Ca 10–x [(PO 4 ) 6–x (CO 3 ) x ] (OH) 2–x · n H 2 O, where n ~ 1.5. They hypothesize that water molecules keep the apatite channels stable even when 80% of the hydroxyl sites are vacant (typical in bone apatite, in contrast to hydroxylapatite), and hinder carbonate ions from substituting for hydroxyl ions in the channels, thus regulating chemical access to the channels. The results of this study are extremely important in many fields and will be of particular interest to those in medicine who study diseases of the bone.

The carbonate ion in hydroxyapatite Recent X-ray and infrared results

The location and orientation of the carbonate ion in the channel (A) and phosphate (B) positions of hydroxyapatite (CHAP) have been investigated by single-crystal X-ray structure and Fourier transform infrared (FTIR) spectroscopy, using crystals synthesized at high pressure. The type A carbonate ion is oriented in the apatite channel with two oxygen atoms close to the c-axis and the B carbonate ion is located near a sloping face of the substituted phosphate tetrahedron. Close comparison of FTIR and X-ray structure results shows that a Na-bearing CHAP containing approximately equal amounts of A and B carbonate ions is a realistic model for the overall crystal structure of biological apatite. However, the absence of distinct OH stretch and OH libration bands indicates that the hydroxyl content of biological apatite is disordered in respect to its orientation and precise location both in the channel and elsewhere in the structure.

Atom Probe Tomography of Apatites and Bone-Type Mineralized Tissues

Nanocrystalline biological apatites constitute the mineral phase of vertebrate bone and teeth. Beyond their central importance to the mechanical function of our skeleton, their extraordinarily large surface acts as the most important ion exchanger for essential and toxic ions in our body. However, the nanoscale structural and chemical complexity of apatite-based mineralized tissues is a formidable challenge to quantitative imaging. For example, even energy-filtered electron microscopy is not suitable for detection of small quantities of low atomic number elements typical for biological materials. Herein we show that laser-pulsed atom probe tomography, a technique that combines subnanometer spatial resolution with unbiased chemical sensitivity, is uniquely suited to the task. Common apatite end members share a number of features, but can clearly be distinguished by their spectrometric fingerprint. This fingerprint and the formation of molecular ions during field evaporation can be explained based on the chemistry of the apatite channel ion. Using end members for reference, we are able to interpret the spectra of bone and dentin samples, and generate the first three-dimensional reconstruction of 1.2 × 10(7) atoms in a dentin sample. The fibrous nature of the collagenous organic matrix in dentin is clearly recognizable in the reconstruction. Surprisingly, some fibers show selectivity in binding for sodium ions over magnesium ions, implying that an additional, chemical level of hierarchy is necessary to describe dentin structure. Furthermore, segregation of inorganic ions or small organic molecules to homophase interfaces (grain boundaries) is not apparent. This has implications for the platelet model for apatite biominerals.

Structural change in lead fluorapatite at high pressure

The structure of lead fluorapatite (PbFAP; Pb10(PO4)6F2), crystallized from the melt in a platinum capsule at 1,000C and 1 atm, has been investigated by single-crystal X-ray diffraction. Crystal data are a = 9.7638 (6), c = 7.2866 (4) A ˚ , space group P63/m, R = 0.043, Rw = 0.034. We have also studied the com- pressional behaviour of the c-axis channel of PbFAP up to 9 GPa at 25C, using a diamond-anvil cell, synchrotron X-radiation, and Rietveld powder structure refinement. Pressure-volume data for the channel polyhedron of PbFAP fitted to the third-order Birch-Murnaghan equation resulted in KT = 33.2 ± 1.2 GPa when KT 0 is fixed at 4. The c-axis channel of PbFAP is about twice as compress- ible as the unit-cell volume of PbFAP and the channel of calcium apatites. This is attributed to the anomalous nar- rowing of the channel of PbFAP with increase in confining pressure. Flexibility of the apatite channel is a key factor in the scavenging of toxic heavy metals by calcium apatites.

The crystal chemistry of the alkaline-earth apatites A10(PO4)6CuxOy(H)z (A = Ca, Sr and Ba)

The crystal chemistry of the cuprate apatites A(I)(4)A(II)(6)(PO(4))(6)Cu(x)O(y)(H)(z) (A = Ca, Sr and Ba) was investigated by powder X-ray (PXRD) and neutron diffraction (PND) and X-ray photoelectron spectroscopy (XPS). The refined crystal structures confirmed earlier X-ray diffraction studies that showed copper resides in the apatite channels and additionally, located hydrogen. For all materials copper is primarily divalent (Cu(2+)) but in the calcium and strontium analogues co-exists with minor Cu(3+). This is in contrast with a previous work where Cu(1+) and Cu(2+) were reported.

The Crystal Chemistry of Ferric Oxyhydroxyapatite

Ferric hydroxyapatites (Fe-HAp) and oxyapatites (Fe-OAp) of nominal composition [Ca(10-x)Fe(x)(3+)][(PO(4))(6)][(OH)(2-x)O(x)] (0 < or = x < or = 0.5) were synthesized from a coprecipitated precursor calcined under flowing nitrogen. The solid solubility of iron was temperature-dependent, varying from x = 0.5 after firing at 600 degrees C to x approximately 0.2 at 1000 degrees C, beyond which Fe-OAp was progressively replaced by tricalcium phosphate (Fe-TCP). Crystal size (13-116 nm) was controlled by iron content and calcination temperature. Ferric iron replaces calcium by two altervalent mechanisms in which carbonate and oxygen are incorporated as counterions. At low iron loadings, carbonate predominantly displaces hydroxyl in the apatite channels (Ca(2+) + OH(-) --> Fe(3+) + CO(3)(2-)), while at higher loadings, "interstitial" oxygen is tenanted in the framework (2Ca(2+) + (vac) --> 2Fe(3+) + O(2+)). Although Fe(3+) is smaller than Ca(2+), the unit cell dilates as iron enters apatite, providing evidence of oxygen injection that converts PO(4) tetrahedra to PO(5) trigonal bipyramids, leading to the crystal chemical formula [Ca(10-x)Fe(x)][(PO(4))(6-x/2)(PO(5))(x/2)][(OH)(2-y)O(2y)] (x < or = 0.5). A discontinuity in unit cell expansion at x approximately 0.2 combined with a substantial reduction of the carbonate FTIR fingerprint shows that oxygen infusion, rather than tunnel hydroxyl displacement, is dominant beyond this loading. This behavior is in contrast to ferrous-fluorapatite where Ca(2+) --> Fe(2+) aliovalent replacement does not require oxygen penetration and the cell volume contracts with iron loading. All of the materials were paramagnetic, but at low iron concentrations, a transition arising from crystallographic modification or a change in spin ordering is observed at 90 K. The excipient behavior of Fe-OAp was superior to that of HAp and may be linked to the crystalline component or mediated by a ubiquitous nondiffracting amorphous phase. Fe-HAp and Fe-OAp are not intrinsically suitable magnetic agents for drug delivery but may be useful in reactive cements that promote osteoblast proliferation.

Hydrogen-carbonate ion in synthetic high-pressure apatite

The hydrogen-carbonate ion [(HCO3)-] has been detected by Fourier transform infrared (FTIR) spectroscopy in the c-axis structural channel of Na-bearing type A-B carbonate apatite synthesized under conditions of high P (0.1-1 GPa), T (800-1350 °C), and p(CO2), and accounts for up to one-third of the total complement of channel carbonate. The hydrogen-carbonate ion is only loosely bound in the apatite channel, and breaks down on aging at room temperature. Volatile decomposition products are lost from the carbonate apatite structure, with CO2 more mobile than H2O. The mobility of small volatile molecules points to a possible role for the apatite channel in mediating acid-base reactions in restricted surficial environments and biological systems.

Phase transition and mixed oxide-proton conductivity in germanium oxy-apatites

La 9.75□ 0.25(Ge 6O 24)O 2.62 oxy-apatite shows a phase transition from triclinic to hexagonal symmetry at approximately 1020 K that has been characterised by high-temperature synchrotron X-ray and neutron powder diffraction, and ionic conductivity measurements. The crystal structure at 1073 K has been determined from joint Rietveld refinements of synchrotron X-ray and neutron powder diffraction data. The study shows that hexagonal-La 9.75□ 0.25(Ge 6O 24)O 2.62 contains interstitial oxygen at the position previously reported for other oxy-germanates. Changes in the oxide conductivity associated with this structural transition are discussed. The thermal analyses showed a weight loss on heating close to 600 K very likely due to water release. The synchrotron thermodiffractometric study shows an anomaly in the cell parameters evolution at that temperature, which indicates that this residual water is located into the apatite channels. The electrical characterisation under different atmospheres (dry and wet synthetic air) indicates that there is a significant proton contribution to the overall conductivity below 600 K, mainly under wet atmosphere.