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Hydroxyapatite (HA) is the main constituent of bone mineral in vertebrates and is widely used as a biomaterial in biomedical studies and medical clinics. The synthesis of this calcium phosphate phase is a crucial factor in its potential use in various applications as well as in understanding the nucleation and growth processes in biological mineralization (biomineralization). In this work, a sequential analysis of the different steps of HA synthesis in aqueous media was performed by using cryo-transmission electron microscopy (Cryo-TEM), which allows preservation of the specimens at different stages of the synthesis process, preventing their modification during drying. X-ray energy dispersive spectroscopy and electron diffraction, in addition to high-resolution imaging, were performed on vitrified samples. We showed that drying a specimen from the synthesis solution induces the crystallization of nanoparticles into an HA phase after 30 min of reaction, while when analyzed by Cryo-TEM, crystalline nanoparticles were detected only after 15 h. Calcium phosphate nanoparticles presented three different morphologies: irregular shapes characterized by low contrast in TEM (5 min), isotropic shapes with well-defined contrast (10 min) and needle-like shapes (after 30 min). The long axis of the HA needle-like nanoparticles can be associated with the c-axis direction of the hexagonal unit cell, which can thus be assigned to the preferential crystallization direction. However, a diffraction signal of the nanoparticles in vitreous ice that “mimics” the original solution was detected by Cryo-TEM only after 15 h of synthesis. These apparently controversial results are discussed in this manuscript considering various aspects of the use of Cryo-TEM in relation to the current concepts of calcium phosphate nucleation and growth.

In the present work, we investigated the nanostructure of calcium phosphate films formed during the initial stages of deposition (between 30 and 240 s; deposition rate of 3.7 nm/min) by radio-frequency magnetron sputtering. The sputtering deposition parameters and the plasma energy were previously adjusted to produce nearly stoichiometric nanocrystalline hydroxyapatite films after 180 min. Grazing-incidence X-ray diffraction and X-ray photoelectron spectroscopy analyses revealed that the films formed in the initial stages were composed of an amorphous calcium phosphate phase with a Ca/P ratio Ca/P = 1.60 ± 0.12, which is close to that of hydroxyapatite. Using high-resolution transmission electron microscopy, we demonstrated that the films were not a unique amorphous phase. An additional phase composed of dense calcium phosphate nanospheres was found in all periods and investigated. The nanospheres grew in size as the deposition time increased, and small nanospheres were always observed. This observation suggests the continuous formation of nanospheres at the surface of the film. Nanocrystalline regions were revealed by electron diffraction and high-resolution electron microscopy after a deposition of 60 s with a corresponding film thickness of 3.7 nm. Electron energy-loss spectroscopy analysis comparing individual nanospheres to the amorphous calcium phosphate layer showed differences in the oxygen K edge, suggesting a difference in the chemical bonding of the oxygen species. Nanospheres were observed in association with isolated crystals, suggesting that the nanospheres may participate in the crystallization of hydroxyapatite according to a heterogeneous nucleation process.

In this work, the synthetic hydroxyapatite (HAP) was studied using different preparation routes to decrease the crystal size and to study the temperature effect on the HAP nano-sized hydroxyapatite crystallization. X-ray diffraction (XRD) analysis indicated that all samples were composed by crystalline and amorphous phases . The sample with greater quantity of amorphous phase (40% of total mass) was studied. The nano-sized hydroxyapatite powder was heated and studied at 300, 500, 700, 900 and 1150 °C. All samples were characterized by XRD and their XRD patterns refined using the Rietveld method. The crystallites presented an anisotropic form, being larger in the (001) direction. It was observed that the crystallite size increased continuously with the heating temperature and the eccentricity of the ellipsoidal shape changed from 2.75 at 300 °C to 1.94, 1.43, 1.04 and 1.00 respectively at 500, 700, 900 and 1150 °C. In order to better characterize the morphology of the HAP the samples were also examined using atomic force microscopy (AFM), infrared spectrometry (IR) and thermogravimetric analysis (TGA).

Well characterized kaolin samples from different deposits of Brazil were studied by electron paramagnetic resonance. Three paramagnetic species attributed to iron in different chemical environments were identified in raw samples. A very intense and broad line with 0.2 mT linewidth was typical of large particles of iron oxides/hydroxides. A species at g = 2.0 with 0.04 mT line width was associated with superparamagnetic particles adsorbed to kaolinite surfaces or trapped between its lamellae. An isotropic and/or orthorhombic species in the g = 4.2 region belongs to structural Fe 3+ replacing Al 3+ in sites with different crystal-field symmetries. It was verified that large iron oxide particles were responsible for keeping the kaolin whiteness at levels below 50%. Superparamagnetic iron particles mainly affect whiteness in the range of 50% 86%) were determined by Fe 3+ iron content in the kaolinite structure. However, only a small part of this species can be removed from the kaolinite structure by traditional industrial processing techniques. These results showed that EPR is a powerful technique to assist in controlling kaolin quality in industrial processes.

Polycrystalline solid solution of phosphate and vanadate lead calcium apatites Pb 10 -x Ca x( VO 4)1( PO 4)5( OH )2 apatites [x = 0.0, 2.0, 4.0, 6.0, 8.0, and 10.0] was reduced with hydrogen gas at the temperature of 300 and 400°C for 1 h. All these compounds were investigated by XRD and ESR spectroscopy. Only one reduced oxidation state V +4 was detected. Two types of species like isolated vanadium (+4) and clusters of V 4+ had been observed. The stability of the apatites increases and the particle size of the crystal decreases with calcium doping concentrations. As the concentrations of Ca increase the intensity of ESR lines deviates due to the spin–lattice relaxation interactions. The deviation in the intensity of ESR spectra can be attributed as the coexistence of paramagnetic and antiferromagnetic phases in the apatites.

In this work we applied a new route to synthesize magnetic iron oxide nanoparticles into alginate polymer for future application as drug delivery system activated by magnetic external stimuli. Calcium-alginate was used to encapsulate iron oxide nanoparticles, and as scaffold for particle nucleation and its influence on particles size and magnetic properties were studied. The iron oxide mean sizes were between 4.3 and 9.5 nm. Iron is dispersed throughout the polymer matrix mainly as iron oxide particles, and a small fraction as iron (III) occupying calcium sites in the polymer network. The temperature dependence of the Mossbauer spectra is typical of superparamagnetic particles in agreement with the magnetic susceptibility data.

A solid solution of phosphate and vanadate lead apatites Pb 10( PO 4)6-x( VO 4)x( OH )2 (x = 0.5, 1.0, 2.0, 3.0, and 6.0) were reduced with hydrogen gas at temperature 300°C for one hour. All these compounds were investigated by XRD and ESR spectroscopy. Only one reduced oxidation state, V 4+, was detected. Two types of species-like isolated vanadium (+ 4) and clusters of V 4+ were observed. The EPR powder spectrum computer simulations were performed using the program Simfonia from Bruker. Spin Hamiltonian parameters were also calculated. As the concentrations of vanadium ions increase to 2.0, the intensity of the ESR lines increases, and x = 3.0 the intensity of the ESR line decreases sharply. After this concentration, i.e. x > 3.0, there is again an increase in the intensity. The variation of the line shape and intensity of the ESR spectra can be attributed to increases in the hopping rate of the charge carriers (polarons) in different apatite compositions.

Spin–lattice relaxation times, T 1 , for EPR signals created by irradiation of tooth enamel or carbonate-doped hydroxyapatite were studied by three-pulse inversion recovery and long-pulse saturation recovery. The recovery curves were modeled as the sum of two log-normal distributions of relaxation times. The dominant component in the EPR signal for tooth samples is assigned to CO 2 - replacing phosphate (B-sites) in the hydroxyapatite matrix. For this component the center of the T 1 distribution was 1.4 – 1.6 μ s at 294 K, and did not vary significantly with radiation dose or the source of the sample. The smaller, slower relaxing, component of the signals from the tooth samples had a T 1 distribution centered at 30 – 60 μ s and is assigned to an organic radical ( g ∼ 2.0045 ) . At 294 K the spin–spin relaxation time T 2 measured by two-pulse echo decay was dominated by motion of the CO 2 - and was independent of dose. The dose independence of T 1 and T 2 at ambient temperature provides the basis for using dose-independent microwave powers to record dosimetric tooth signals at microwave powers above the linear response regime.

Hydroxyapatite (HA) films have been widely recognized for their biocompatibility and utility in promoting biointegration of implants in both osseous and soft tissue. Conventional sputtering techniques have shown some advantages over the commercially available plasma spraying method; however, the as-sputtered films are usually amorphous which can cause some serious adhesion problems when post-deposition heat treatment is necessitated. In this paper we present an opposing radio frequency (RF) magnetron sputtering approach for the preparation of HA thin films on various substrates at low power levels. Using this alternative RF magnetron geometry, as-sputtered HA films are nearly stoichiometric, highly crystalline, and strongly bound to the substrate. Post-deposition heat treatment under 800 °C did not result in a marked improvement in the degree of crystallinity of the films. In addition, dissolution experiments show that as-sputtered films are more stable than annealed ones. As-sputtered films grown on amorphous silica substrates exhibit X-ray diffraction (XRD) patterns similar to those of randomly orientated HA powder. On the other hand, films deposited on oriented substrates such as Si(100) and Si(111) show a polycrystalline HA XRD pattern but with some strongly preferred orientations, indicating that HA crystallization is sensitive to the nature of the substrate. The results suggest that the opposing RF magnetron sputtering approach has some potential to produce high quality HA films on metallic implants.

Isolated iron ions and iron oxide nanoparticles entrapped in alginate microspheres were studied by 57 Fe Mfssbauer spectroscopy (MS), electron paramagnetic resonance (EPR) and magnetization measurements. Ferric alginate beads were prepared by dropping sodium alginate solution into ferric chloride solution (0.1 and 0.5 M) with stirring under Ar atmosphere. MS and EPR spectroscopy show that iron is absorbed in alginate as Fe(III). The EPR suggests that isolated Fe(III) substitutes sodium in the polymeric structure (narrow line paramagnetic species at g=4.23). This mechanism induces the linking of the alginate units and the formation of the alginate microspheres. During this process, superparamagnetic iron hydroxide particles (broad line species at g=2.22) are precipitated inside the polymer network. MS measurements performed in diluted and concentrated Fe(III) samples displays a paramagnetic doublet at 300 K, and two components at 4.2 K due to a paramagnetic site and blocked superparamagnetic particles. D 2004 Published by Elsevier B.V.

Immobilization of toxic metals by calcium phosphates is a promising technology for treating contaminated soil, water and wastes. A detailed study on the mechanisms of lead immobilization by hydroxyapatite has been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). For this, synthetic hydroxyapatite powder were submitted to a sorption process through exposure to an aqueous solution containing 917 mg L−1 of lead for times that varied from 3 min to 54 h. The results obtained reinforce the hypothesis that hydroxypyromorphite formation is the end of a kinetic process in which the hydroxyapatite crystals are continuously dissolved and recrystallized in order to form more stable structures with higher lead content. Consequently, the use of calcium phosphates to immobilize lead ions seems to be technically viable.

Archaeological artefacts made of obsidian are all the more precious for archaeologists as they are witnesses of cultural, social or 'economic' relationships between prehistoric populations. The main obsidian sources reachable in these times have been identified and characterized by chemical analysis and fission track dating. Because of iron presence (from 1% to 10% expressed in Fe 2 O 3 ) it is possible to use Mossbauer spectroscopy and electron spin resonance (ESR) on geological samples to characterize different sources. We studied by electron spin resonance about seventy obsidians coming from six Mediterranean volcanic islands. The complex spectra are mainly due to iron in different states and site location (and sometimes isolated Mn 2+ ions). X-band (9 GHz) spectra exhibit a signal at g = 4.3 with a shoulder at 9.8 ascribed to isolated Fe 3+ in the glassy matrix with a rhombic environment (C 2y ). Condensed clusters of Fe 3+ ions give rise to a resonance line at g = 2.0 whose position and width do not depend on temperature and in addition to this signal broad resonance lines are ascribed to micro-crystallites of mixed iron oxides (hematite, magnetite and related spinels, titanates, silicates) as inclusions into the amorphous matrix of obsidian. When temperature decreases from 300 to 5 K these signals shift towards weak fields. The contribution of every signal to ESR spectrum is different from one source to the other according to the different thermodynamic conditions that occurred during obsidian formation. It is thus possible to discriminate a priori one geological source using ESR spectra. At room temperature the first differences appear at X-band, low temperatures experiments may allow to remove some ambiguities.

Porous hydroxyapatite (HA) has already been widely used as a bone substitute due to its similarity with the mineral part of the bone. In this work, cylindrical tablets with micro and macro porosity were produced from stoichiometric and deficient hydroxyapatites by using naphthalene as porosifier agent. The influence of the processing parameters such as Ca/P ratio of start material, calcination temperature, and naphthalene content on the characteristics of porous calcium phosphate tablets was evaluated. Three mineral phases-HA, alpha-TCP (alpha tri-calcium phosphate), and beta-TCP (beta tricalcium phos-phate)-with variable contents were identified by x-ray diffraction (XRD) and Fourier-transformed infrared spectroscopy (FT-IR). Image analysis and density measurements were used to characterize sample porosity. As expected, the total porosity of the calcinated material is not dependent on the stoichiometry of the precursor hydroxyapatite. For calcium-deficient hydroxyapatite, the increase in naphthalene content contributes to stabilize alpha-TCP phase, altering the relative phases content.

Calcium phosphate granules were produced through the calcination of three hydroxyapatites (HAs) at 1150 degrees C: nearly-stoichiometric (NS-HA), calcium-deficient (CD-HA), and carbonated (Carb-HA). The characterization using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier-transformed infrared spectroscopy (FT-IR) showed that those granules exhibit strong differences in chemical composition, surface texture, and dissolution behavior. Sample dissolution in a simulated body fluid (SBF) and precipitation of a calcium phosphate layer on the granule surface were followed up for 7 days by chemical analysis of calcium content in SBF. After 80-min reaction, precipitation of new calcium phosphate phases predominates over the dissolution of original granules. Comparison between SEM images and XRD/FT-IR analysis revealed that the composition of the calcium phosphate layer and its precipitation rate depend on the HA composition and its heat treatment. Calcined carbonated apatite induces the precipitation of an octacalcium phosphate (OPC) layer, whereas a carbonated apatite layer is formed from calcined-deficient HA. The calcined nearly-stoichiometric HA exhibits low efficiency in producing carbonated apatite layer.

Geological sedimentary dolomite samples from the Superior Proterozoic are studied using electron paramagnetic resonance (EPR) spectroscopy. The complex spectra in the g=2.0 region is composed of Mn2+ lines and signals due to crystallization and radiation-induced defects. Measurements in microwave frequencies of 9.5 GHz (X-band) and 35 GHz (Q-band), and thermal and/or radiation treatments allowed identification of seven paramagnetic radicals in the g=2.0 region: (1) isotropic organic radical; (2) axial SO2 −; (3) axial PO2 0 or PO2 2−; (4) isotropic CO2 −; (5) axial CO2 −; (6) axial CO3 3−; (7) isotropic unknown line. The use of these paramagnetic centres as indicators of geological events is discussed.

The structural and magnetic properties of Fe/Zn films prepared by thermal evaporation have been studied by means of X-ray diffraction, vibrating-sample magnetometry and ferromagnetic resonance (FMR). For Fe layer thickness smaller than 20 A the saturation magnetization decreases with decreasing Fe thickness, which is an indication of the island growth of Zn and Fe–Zn interdiffusion at the layer interfaces. The effective field magnetization 4 πM eff of the Fe/Zn multilayers was determined from the FMR data in a rotating external magnetic field. The interface anisotropy constant of the Fe/Zn multilayers, K S , is found to be 1.0 erg/cm 2 at 300 K. This indicates the presence of a large perpendicular interface anisotropy and this may suggest that the largest part of K S originates from lattice misfit strain.

Synthetic A-type carbonated apatite prepared in controlled conditions was irradiated at room temperature with 60Co gamma rays. The ESR spectrum was associated to axial CO2- and orthorhombic CO3- species. Radicals used as dose markers in biological apatites are long-lived paramagnetic species. The stability of the post-irradiation signal of A-type apatite was investigated for almost 2 years. Measurements showed variations in the spectra attributed to unstable CO3- species, which can be eliminated by thermal treatments at 100 degrees C for 24 h. Results indicated the potential use of an A-type carbonated apatite as a dosemeter.

Synthetic A-type carbonated apatite samples were irradiated at room temperature with 60Co gamma rays. Their ESR spectra consist of the lines of CO2- and CO3- radicals of orthorhombic and axial symmetry. The measurements carried out immediately after sample irradiation showed that CO2- species are produced by decomposition of CO3- radicals. Intensity of the CO2- lines in the synthetic and enamel samples increases during the first 400 and 200 h after irradiation, respectively. The dependence of the EPR signal on the dose varies with carbonate content of the sample. The dose response curve for tooth enamel is steeper for the synthetic material.

A gamma-irradiated synthetic B-type carbonated apatite calcined at 300°C was studied by EPR and ENDOR. The room temperature EPR spectrum is composed of an axially symmetric species (g⊥=2.0025 and g‖=1.9971) attributed to rotating CO2− radicals. At low temperatures (T

Synthetic A-type carbonated apatite and enamel samples were irradiated using X rays and gamma rays of 137 Cs and 60 Co in the range of 47 keV to 1.25 MeV. The energy-dependent response of CO - 2 radicals induced by radiation in both systems is studied using electron paramagnetic resonance (EPR). No energy dependence was verified for synthetic apatites and enamel in the range of 58 to 1250 keV. The results are compared with the energy dependence of the same radicals induced in bones and enamel reported in the literature.

A γ-irradiated synthetic hydroxyapatite sample was studied by EPR and ENDOR. At room temperature the EPR spectra are composed of two species, both with axially symmetric -tensors (g⊥=2.0061 and g//=2.0018) and attributed to CO- radicals. One of these species presents a hyperfine structure with a nuclear spin I=1/2

Several irradiated biocarbonates were studied by magnetic resonance techniques. Seven paramagnetic species, attributed to CO 2 − , SO 2 − and SO 3 − were identified. Comparison between radiation induced defects in bioaragonites and aragonite single-crystals shows that isotropic and orhorhombic CO 2 − centers with broad line spectra are not produced in the latter samples. Vibrational and rotational properties of isotropic CO 2 − centers were studied from low temperature Q-band spectra. Vibrational frequency is determined from the 13 CO 2 − hyperfine spectrum and yielded v =1.54 × 10 13 s −1 . The correlation time for isotropic CO 2 − , τ c =1.2 × 10 −11 s ( T =300 K), is typical of radicals rotating in liquids. ENDOR and General Triple spectroscopy show that orthorhombic CO 2 − centers are surrounded by water molecules located in the second nearest CO 3 2− sites at 5.14, 5.35 and 6.02 A. Water molecules replacing carbonates or as liquid inclusion of growth solution in local crystal imperfections may be responsible for the variety of orthorhombic and isotropic CO 2 − species, respectively.

In this work we combine electron paramagnetic resonance (EPR), high-resolution electron nucleus double resonance (ENDOR) and general triple resonance (GTR) spectroscopies, to study the local environment of the CO2− groups created by ionizing radiation in fossil tooth enamel. We demonstrate that the CO2− groups occupy slightly modified phosphate sites in the hydroxyapatite lattice. In quaternary shark enamel we found these groups to be interacting with water molecules in the apetite channels. The absence of water molecules as first neighbors in mammalian samples indicate, however, that these molecules are not significantly responsible for the stabilization of CO2− dating centers in enamel.

Irradiated A-type carbonated apatites with carbonate content ranging from 1.45% to 4.84% are studied by using electron paramagnetic resonance (EPR). The EPR spectra are mainly constituted of lines associated to axial CO 2 − species ( g ⊥ =2.0028 and g ∥ =1.9973) and orthorhombic CO 3 − species ( g 1 =2.0170, g 2 =2.0090 and g 3 =2.0041). The production of CO 2 − species on gamma irradiation depends on the carbonate concentration and the hydroxyapatite stoichiometry. The lowest dose detection limit was achieved with stoichiometric samples and carbonate concentration of 3.7%.

Intensity of the 150 °C thermoluminescence peak of beta-irradiated carbonated synthetic A-type hydroxyapatite is approximately 12 times higher than that of the noncarbonated material. Deconvolution of the glow curve showed that this peak is a result of a trap distribution. An attempt was made to relate this thermoluminescence peak enhanced by carbonation with the ESR signal of the CO 2 − radical in natural or synthetic hydroxyapatite.

Synthetic porous ceramics can be used as three-dimensional scaffolds for bone repair. The aim of this work is to correlate process parameters with scaffolds structural characteristics. The factorial 2 level experimental design was chosen to study the effect of the Ca/P ratio (1.58 or 1.67), calcination temperature (1000 or 1150 °C) and porogen content (20 or 40%) on the calcium phosphate samples characteristics. The influence of such parameters was determined by X-ray diffraction, infrared spectroscopy and image analysis. It was observed that phase composition was basically a function of the Ca/P ratio of the raw material. The use of the porogen did not alter the hydroxyapatite (HA)/tricalcium phosphate (TCP) content, but induced changes in the relative content of TCP phase (alpha or beta). It is possible to design a porous sample with defined characteristics, and the model herein used can be considered as having a good predictive power.

In this preliminary work the EPR spectra of a small dental enamel block and a synthetic B-type hydroxyapatite in powder form, both irradiated with gamma rays, were analyzed in K-band. The spectra of the dental enamel block allow the analysis of independent components with different angular orientations, while the study of the dosimetric properties of the synthetic hydroxyapatites showed good performance of this spectrometer. K-band spectra show better resolution when compared to X-band, while using significantly less sample material.

Electron paramagnetic resonance, Mossbauer spectroscopy, and electronic structure calculations were combined in order to study the local geometry of Fe 2 + /Fe 3 + in Fe-doped hydroxyapatite. Atomistic simulations were carried out to obtain estimates of local geometry and lattice strain associated with fourfold, fivefold, and sixfold Fe sites. First-principles embedded cluster density functional calculations were performed to investigate the electronic structure associated with the substitution of calcium by Fe 2 + /Fe 3 + . Mossbauer isomer shift, quadrupole splitting, and the hyperfine magnetic field were calculated for each site and local coordination, for comparison to an experimental fit to a five-line model consisting of two bulk sites each for Fe 2 + and Fe 3 + and a surface hematitelike Fe 3 + species.

Polycrystalline CaSO 4 powder, doped with different elements but mainly rare earths, is one of the most interesting thermoluminescent (TL) materials. Although many electron spin resonance (ESR) analyses have been reported for these materials few studies have been published about the potential of CaSO 4 for ESR dosimetry; almost all studies used CaSO 4 :Dy with a very low Dy concentration as the material for TL measurements. Pure CaSO 4 from Merck was used to prepare CaSO 4 :Dy and CaSO 4 :P:Dy with different Dy concentrations. Samples were annealed at 600°C for 1 h before irradiation in a Gammacell 220 irradiator with a 60 Co gamma source at a dose rate of 100 Gy.min -1 . The ESR spectra of the pure CaSO 4 and CaSO 4 doped with P and Dy show the lines usually observed with these types of material, with the factor g around 2.036 and an intense line at g = 2.0011 found only in the pure material. This line, probably an axial SO 4 -, grows linearly with absorbed dose until 1.0 kGy and shows good stability with time. The line should be stabilized by matrix impurities because it can be removed by a simple treatment with hot sulphuric acid.

Electron spin resonance (ESR) spectroscopy is used to assess absorbed doses of six teeth belonging to victims of the highly irradiated group of Goiânia accident. The influence of the broad background signal at g=2.0040 as well as of the unstable fraction of CO − 2 radicals was taken into account in dose estimates. Three victims teeth showed absorbed doses comparable to those estimated by chromosomal analysis. For the other three teeth, the doses were higher by a factor of 1.3, 1.8 and 2.2.

It is known since more than thirty years that ionising radiations induce a signal in the Electron Spin Resonance spectrum of bones (Gordy et al. 1955). The stability of this signal at ambiant temperatures allowed applications to calcified tissue research (Ostrowski et al. 1980), nuclear bomb and accident dosimetry (Ikeya et al. 1984, Ikeya and Ishii 1989), geological dating (Ikeya 1988, Poupeau and Rossi 1989) as well as the detection and dosimetry of ionised foodstuffs (Dodd et al. 1985).

Solid solutions of phosphate and vanadate calcium apatites, Ca10(PO4)6−x(VO4)x(OH)2, were treated with hydrogen at high temperatures and studied by chemical analysis, XRD, FTIR, EPR and ENDOR spectroscopies. Only one reduced oxidation state, V4+, was detected. The extent of the reduction depended on the vanadium content as well as the treatment time. For solid solutions with x 1.5, the reaction results in apatite decomposition and the formation of a perovskite, CaVO3. EPR and ENDOR spectroscopy reveal that the V5+ → V4+ reaction induces strong changes in the vanadium site structure with the formation of a vanadyl bond and the loss of the nearest OH− group. Atomistic simulations were made to estimate local distortions around vanadium due to the reduction mechanism. Density functional calculations were performed to characterize the chemical environment induced by the V5+ → V4+ reaction at VO4 sites through Mulliken atomic-orbital population analysis as well as charge and spin density maps. A mechanism for the overall reaction is proposed.

Electron spin resonance (ESR), electron-nuclear double resonance (ENDOR) and general triple resonance were combined to study linewidth differences observed in the ESR spectra of CO2 - species in calcium carbonate and carbonated apatite. In the latter compound, the motion and a distribution of g-factors (g-strain) are dominant contributions to the observed linewidth. The linewidth temperature behavior follows previous models for CO2 - rotating in calcite. In apatite, these radicals rotate more freely and movement is not restricted to g x x -g z z plane. In carbonates theoretical fit of the linewidth at 9.5 and 34.0 GHz shows the existence of two orthorhombic spectra with slightly different g values and linewidths, which contribute to the observed linewidth difference between narrow and broad line species. Superhyperfine interactions with protons of water molecules, located probably near the second and the third nearest neighbor lattice sites, are responsible for minor contributions to line broadening of CO2 - spectrum.

Electron irradiation of the diamagnetic OsII(CN)64− ion in a KCl host lattice at 77 °K produces three paramagnetic species. Annealing experiments reveal that these species decay into at least four other species. EPR studies indicate that each species can be assigned to a pentacoordinated OsI(CN)5 complex, with 5d7 low spin configuration arising from the trapping of an electron in a dz2 orbital, associated with a charge compensating vacancy located at different lattice sites. The EPR spectra show a strong quadrupolar interaction. The g values, the magnetic hyperfine interaction, and a minimum value for the quadrupolar interaction with the 189Os nucleus are presented and discussed. Ligand hyperfine structure with four equivalent and one nonequivalent nitrogens is measured and the ligand fs and fp spin densities are calculated and discussed. The relative contribution of 5d and 6p electrons to the electric field gradient is discussed and compared with Mossbauer spectroscopy results.

A hidroxiapatita e o esmalte dental são, na atualidade, alguns dos materiais mais pesquisados pela técnica de espectroscopia por Ressonância Paramagnética Eletrônica ? RPE. O esmalte dental, que possui cerca de 97% de sua massa constituída por cristais de hidroxiapatita, se apresenta como o tecido humano preferencial para dosimetria de radiação com o uso desta técnica, principalmente no caso de exposições acidentais da população em geral. A análise de hidroxiapatitas sintéticas tem por finalidade auxiliar o entendimento do espectro do esmalte dental bem como desenvolver um dosímetro de radiação com boa eficiência e eficácia. O espectrômetro em banda-K utiliza uma freqüência intermediária aos espectrômetros em banda-X e em banda-Q, possuindo uma melhor resolução que banda-X sem as dificuldades, devido à alta sensibilidade, do banda-Q. O presente trabalho tem como objetivo analisar espectros em banda-K de um bloco de esmalte irradiado e de hidroxiapatitas sintéticas, comparando com espectros de banda-Q e banda-X presentes na literatura, e ainda verificar a possibilidade de se fazer dosimetria de radiação com o uso deste aparelho. Hydroxyapatite and tooth enamel are, nowadays, some of materials mostly investigated by the spectroscopic technique of Electron Paramagnetic Resonance ? EPR. Tooth enamel, that have about of 97% of its mass constituted by hydroxyapatite crystals, appears itself as the preferential human tissue for radiation dosimetry using EPR, mainly in case of accidental expositions by ordinary personnel. The analyses of synthetic hydroxyapatites have as the main goal to help the understanding of tooth enamel?s spectrum seeking the developing of a radiation dosimeter with good efficiency and accuracy. The K-band EPR spectrometer uses an intermediate frequency between the spectrometers in X-band and Q-band, possessing a better resolution than the X-band without presenting the Q-band?s difficulties. In this preliminary work the EPR spectra of a small block tooth enamel and synthetic hydroxyapatites irradiated were analyzed in K-band, comparing the results with X- and Q-band with published results, the possibility of performing radiation dosimetry at K-band was verified.