EPR investigations of SiC and SiOC nanometric powders

EPR investigations are performed in SiC and SiOC nanometric powders annealed between 1200 and 1800°C. By using different EPR frequency bands and a suitable spectra analysis, three quite different paramagnetic defects with well defined $\tilde g_i $ (i=1, 2, 3) and hyperfine $\tilde A_i $ (i=1, 2) tensors account for the EPR signal in these materials. The defects are characterized by $\tilde g_1 $ (g ₁ ^‖ =2.0046(3), g ₁ ^? =2.0023(3)), $\tilde g_2 $ (g ₂ ^‖ =2.0037(3), g ₂ ^? =2.0028(3)) and isotropic $\tilde g_3 $ (2.0030(3)) tensors. In SiC powders, the defects assignment is discussed with respect to the different SiC forms, namely α-SiC and β-SiC polytypes as well as amorphous SiC and carbon present in minor concentration in the network. In SiOC powders, the above defects are evidenced only at high annealing temperature (T _a≥1200°C) when the oxygen contained is highly reduced.     

EPR study of radiation-induced defects in carbonate-containing hydroxyapatite annealed at high temperature

Radiation-induced (γ or UV) paramagnetic defects in carbonate-containing hydroxyapatite (HAP) annealed at high (600–950 °C) temperature were studied by EPR. The complex spectra reveal the presence of different paramagnetic species. Their contributions were found to be strongly dependent on the annealing temperature as well as microwave power, thus, by the adjustment of experimental conditions some of the components can be eliminated that allowed to record EPR spectra caused by no more than two types of paramagnetic defects. All experimental spectra were analyzed using computer simulation. The parameters of the paramagnetic defects detected were determined, and the centers models were discussed. It was found that high-temperature annealing influences essentially the formation of radiation-induced defects in HAP. The СО₃³⁻, О⁻ centers and oxygen vacancy V_O⁻ were shown to be the main stable γ-induced defects in the HAP annealed at high temperatures. New paramagnetic defect with the parameters g_|| = 2.002, g_⊥ = 2.0135 was detected and tentatively identified as an O-related radical. The γ-induced EPR response from СО₃³⁻ radicals was found to be more intense than response from CO₂⁻ in non-annealed HAP. UV-irradiation was found to create smaller amounts of paramagnetic defects in comparison with γ-rays. Besides, oxygen vacancy V_O⁻ was not observed, while two other centers (СО₃⁻ and the center of unknown nature) appear in the UV-induced EPR spectra.     

NO 3 2− and CO 2 − centers in synthetic hydroxyapatite: Features of the formation under γ- and UV-irradiations

The EPR studies of synthetic hydroxyapatite containing carbonate and nitrate ions exposed to γ-ray and UV irradiations have been performed. It has been found that γ irradiation leads to the formation of both NO₃²⁻ and CO₂⁻ paramagnetic centers, while the UV irradiation induces only NO₃²⁻ centers. To explain this fact, the hypothesis has been proposed, according to which in the hydroxyapatites studied, there coexist complexes consisting of nitrate ions and shallow electron traps that serve as sources of secondary electrons during UV irradiation. The EPR spectroscopy parameters (g and A) of the detected centers have been determined and compared with similar centers in hydroxyapatite with a different impurity composition. The study of the thermal stability of the centers has demonstrated that, in the temperature range 20–300°C, the NO₃²⁻ centers formed by UV irradiation are more stable than the same centers created by γ-ray irradiation.     

Nitrogen-containing species in the structure of the synthesized nano-hydroxyapatite

Synthesized by the wet chemical precipitation technique, hydroxyapatite (HAp) powders with the sizes of the crystallites of 20–50 nm and 1 μm were analyzed by different analytical methods. By means of electron paramagnetic resonance (EPR) it is shown that during the synthesis process nitrate anions from the reagents (byproducts) could incorporate into the HAp structure. The relaxation times and EPR parameters of the stable axially symmetric NO ₃ ^2? paramagnetic centers detected after X-ray irradiation are measured with high accuracy. Analyses of high-frequency (95 GHz) electron-nuclear double resonance spectra from ¹H and ³¹P nuclei and ab initio density functional theory calculations allow suggesting that the paramagnetic centers and nitrate anions as the precursors of NO ₃ ^2? radicals preferably occupy PO ₄ ^3? site in the HAp structure.     

Effect of pre-annealing on NO32- centers in synthetic hydroxyapatite

Effect of pre-annealing of synthetic hydroxyapatite (HAP) on properties of γ- and UV- induced NO₃^2- centers was studied by electron paramagnetic resonance (EPR). Nitrate-containing hydroxyapatite powders ((N)HAP)) and the powders with an admixture of carbonate and nitrate ions ((C,N)HAP) were annealed in the temperature range T_ann = 20 °C ? 600 °C before irradiation. It was found that pre-annealing of (N)HAP samples changes the parameters of NO₃^2- centers while no changes took place in (C,N)HAP. Moreover, at the pre-annealing temperatures T_ann > 200 °C two new NO₃^2- centers were observed in (N)HAP samples; they are characterized by larger value of A_⊥ (3.67 and 4.41 mT) as compared to the known centers. It was also found that the dependence of NO₃^2- centers amount on T_ann is non-monotonous in both types of samples. Presumably this is caused by the escape of water molecules from HAP during the annealing and essential modification of the defect subsystem of HAP.     

A study of hydroxyapatite nanocrystals by the multifrequency EPR and ENDOR spectroscopy methods

Specimens of powders of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) with average crystallite sizes in the range of 20–50 nm synthesized by the wet precipitation method have been investigated by the multifrequency (9 and 94 GHz) electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) methods. In specimens subjected to X-ray irradiation at room temperature, EPR signals that are caused by nitrogen compounds have been observed. Numerical calculations performed in terms of the density functional theory show that the observed EPR signal is caused by the occurrence of paramagnetic centers, the structure of which is NO ₃ ^2? and which replace the positions of PO ₄ ^3? in the hydroxyapatite structure.     

Formation of conductive layers inside diamond by hydrogen ion implantation and subsequent thermal treatment at low or high pressures

(111) synthetic HPTP diamond plates are irradiated by H ₂ ⁺ 50 keV ions in the range of the fluences of 1?13 × 10¹⁶ sm^?2 and annealed in vacuum at 1 mPa (VPHT, 500?C1600°C) or at high HPHT parameters (4.0?C7.5 GPa, 1200?C1550°C). It is shown by measuring the layer conductivity and Raman light scattering that after VPHT annealing, a buried layer of glassy carbon 10?C100 nm thick with low resistance (??1 kOhm/??) is formed, followed by HPHT with high resistance (??1 MOhm/??) and hopping transport along defects.     

Defect sites in thin films of germanium dioxide irradiated with silicon ions

Optical transmission spectra of GeO₂ films irradiated with silicon ions and subjected to postimplantation annealing in the regime of silicon nanocrystal formation are analyzed. It is shown that point defects form in the films after irradiation with doses D ～ 10²⁰ m^?2: germanium electron centers, neutral oxygen vacancies, and Ge²⁺ centers, which have been annealed at a temperature of 1000°C for an hour. At D ≥ 1 × 10²¹ m^?2, more complex defects arise in the films, which are only partially annealed under the same conditions.     

Radiation phenomena in microinhomogeneous structures of flouroaluminate glass-like materials

The simulation of the fluoroaluminate matrix with small additives of phosphates has made it possible to create low-scattering glasses with a wide transparency window in the infrared region and to study the fundamental problem on the formation of radiation defects typical for the phosphate matrix (PO ₃ ^2? hole, PO ₃ ^2? electron, and PO ₄ ^2? hole defects). By comparing the spectra of induced optical absorption, electron paramagnetic resonance, and Raman scattering, it has become possible to investigate the generation of defects in the course of the successive formation of the phosphate matrix in fluoroaluminate glass. The feasibility of the formation of PO ₄ ^2? hole centers both in single tetrahedra and upon rupture of P-O-P bridge bonds with the simultaneous formation of PO ₃ ^2? electron centers has been shown. As a result of the revision of the nature and mechanisms of formation of radiation color centers in the studied glasses, it appears promising to introduce cerium (Ce³⁺) and europium (Eu³⁺) as protectors into these glasses.     

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO₂:80SiO₂ (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge²⁺ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm^-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent. PACS 82.50.Hp; 71.23.Cq; 81.20.Fw     

31P and1H powder ENDOR study of ozonide radicals in carbonated apatites,synthesized from aqueous solutions

An O ₃ ^? defect in Na⁺ CO ₃ ^2? containing apatite powder has been investigated with ENDOR after X-irradiation. The powder, synthesized by a hydrolysis of octo-calciumphosphate (OCP) and Na₂CO₃ was dried at 25°C until constant weight was reached. At low temperatures, both³¹P and¹H ENDOR spectra were recorded for different settings of the magnetic field (i.e., when the magnetic field is swept through the EPR O ₃ ^? spectrum). The ENDOR powder spectra were thoroughly analyzed using computer simulations based on the “orientation selection principle”. Interactions with two types of protons and two types of³¹P nuclei could be resolved. In this way, a detailed model could be established for the O ₃ ^? ion in the hydroxyapatite lattice. The defect is located between two successive vacant hydroxyl sites. The axis connecting the two outer oxygen atoms (g _y-axis) of the O ₃ ^? ion is found to be along the hexagonalc-axis of the lattice. The twofold axis of the defect ion (g _z-axis) is parallel to theb-axis of the lattice.     

Relaxation processes in colored crystals of LiF after the coaction of UV radiation and a shock wave

The relaxation of F ₂ ⁺ centers in radiation-colored crystals of lithium fluoride starting from the moment of the coaction of pulsed UV irradiation and a shock wave is investigated. It is shown that the annealing curve of these centers displays a nonmonotonic character at room temperature and is a consequence of three + processes. The first two are the processes of disintegration of unstable (short-lived) and stable F ₂ ⁺ centers, and the third process involves the formation of stable centers upon a random encounter of appropriate constituents in the course of their diffusive wandering over the lattice.     

Strong ultraviolet and violet photoluminescence from Si-based anodic porous alumina films

We have investigated photoluminescence (PL) from Si-based anodic porous alumina films formed by real-time controlled anodization of electron-beam evaporated Al films. As-anodized samples show three strong PL bands at 295, 340, and 395 nm. These bands blueshift and their intensities decrease after the samples are annealed. When the annealing temperature increases to 1000 °C, the blueshift becomes specially pronounced and meanwhile the structures of the films develop toward crystalline Al₂O₃. Based on discussions on the thermal annealing behaviors of the PL and PL excitation spectra, we suggest that optical transitions in oxygen-related defects, F⁺ (oxygen vacancy with one electron) centers, are responsible for the observed ultraviolet and violet PL. Received: 24 July 2000 / Accepted: 24 February 2001 / Published online: 3 May 2001     

The role of additional anions in microporous aluminosilicates with cancrinite-type framework

Microporous framework aluminosilicates with the cancrinite- and sodalite-type structures containing NO ₃ ^? , CO ₃ ^2? , SO ₄ ^2? and PO ₄ ^3? , as extra-framework anions were synthesized under hydrothermal conditions at temperatures from 160 to 200°C. SO ₄ ^2? exhibits the highest affinity to cancrinite structure at the competition of various X anions. Chemical composition, X-ray diffractometry characteristics, and IR spectra of the compounds obtained, as well as kinetics and mechanism of thermal transformations of nitrate counterpart of cancrinite, were studied.     

EPR study of carbonate derived and ozonide radicals in carbonated apatites synthesized from aqueous solutions

In this study, a series of Na⁺ and¹²CO ₃ ^2? containing apatites synthesized by a hydrolysis of octo-calciumphosphate (OCP) and dried at 25°C until constant weight, were examined with EPR after X-irradiation. A variety of different paramagnetic radicals was formed, giving rise to composite and hence complex EPR powder spectra. The spectra were successfully decomposed into their basic components using a multivariate statistical technique. The different components could be identified unambiguously. In this way, it was found that an O^?, an O ₃ ^? , a CO ₃ ^? , two types of CO ₂ ^? and two types of CO ₃ ^3? ions were formed upon X-irradiation. Also resonances from atomic hydrogen were observed. The most striking feature is the presence of the ozonide ion, which is only rarely observed in irradiated hydroxyapatites. A comparison is made between the EPR spectra of apatite samples prepared by hydrolysis of OCP on the one hand, and those of samples prepared by hydrolyzing monetite, and the tooth enamel spectrum on the other hand.     

Color center production by femtosecond-pulse laser irradiation in fluoride crystals

Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF₄ single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm² was determined in YLF and LiF, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F₂, F ₂ ⁺ , and F ₃ ⁺ . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived F ₂ ⁺ centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.     

Radiation defects in germanium. radiation defects annealed at ∼260°C in n-type germanium

Abstract

Point defects produced by neutron, electron or γ-ray irradiation were studied by electrical measurements. The defect levels were analyzed by DLTS technique. Annealing of radiation induced defects at about 260°C was obtained in 20 min isochronal annealings. Annealed fraction of the 260°C stage was obtained to be 85% in arsenic-doped crystals and independent of the species of irradiating particle. The value in antimony-doped and oxygen-doped specimens were 25 and 70%, respectively. The activation energy was found to be 1.6 eV and the annealing kinetics were first order. A tentative model for the defect responsible at the 260°C stage is a vacancy complex.     

Variation in the EPR characteristics of nanosized zirconia particles under exposure to X-ray radiation and annealing in hydrogen

The effects of various external factors on the properties of nanosized zirconia particles are studied using electron paramagnetic resonance. It is shown that x-rays initiate radiation-stimulated oxidation of chromium impurity ions according to the scheme Cr³⁺ → Cr⁵⁺. Annealing of samples in hydrogen at temperatures in the range 250–650°C brings about a substantial decrease in the fraction of chromium ions in the Cr⁵⁺ charge state, but subsequent annealing of these samples in air leads to an increase in the Cr⁵⁺ fraction. Samples annealed in hydrogen generate a singlet EPR signal with the g factor of 2.0033 ± 0.0005 originating from electrically conducting regions which are formed on the surface of zirconia nanoparticles during their annealing in hydrogen.     

Temperature dependences of the ion-electron emission of glassy carbons under 30-keV argon ion irradiation

The temperature dependences (?200°C < T ≤ 350°C) of the ion-induced electron emission yield γ and the structures of modified surface layers have been studied experimentally for SU-850, SU-1000, SU-1300, SU-2000, and SU-2500 glassy carbons under high-dose 30-keV Ar⁺ and N ₂ ⁺ ion irradiation. Glassy carbons manufactured using a relatively high heat-treatment temperature T _ht exhibit a stepwise increase in the electron yield at certain annealing temperatures T _a. The same behavior is observed for graphitized carbon materials. For low-temperature glassy carbons, the electron yield exhibits a monotonic increase with increasing irradiation temperature. The observed differences are related to the occurrence of different structural types of fullerene-like nanoparticles in the low-and high-temperature glassy carbons.     

Transformation of As-Grown Phosphorus-Related Centers in HPHT Treated Synthetic Diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P–C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073–2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C₁ separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073–2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen–phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen–phosphorus centers NP2 and NP3 with the supposed structures C₁–N–C–P and N–P–C₁, respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C₁. Annealing these samples in the temperature range of 2,073–2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen–phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g ≈ 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed.