Stability and electronic properties of carbon in α-Al2O3

The stability and electronic properties of carbon in α-Al₂O₃ are investigated using density functional theory. In the host lattice, the substitutional C prefers the Al site under the O-rich conditions, whereas the O site is preferred by carbon under the Al-rich conditions. The calculated results predict a direct relationship between the thermodynamic and optical transition levels with the degree of the local distortion induced by C in the alumina lattice. We also find C at the O site acts as a charge compensator to stabilize the F⁺ center, thereby enhancing the TL signal at 465 K. Also, C at Al site can serve as electron traps for TL emission process in α-Al₂O₃.     

Luminescence of F + and F centers in YAlO3

In YAlO₃ crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F ⁺ centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F ⁺ and F centers in oxides of the Al₂O₃-Y₂O₃ system is analyzed on the example of the compounds α-Al₂O₃, YAlO₃, and Y₃Al₅O₁₂. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.     

Green luminescence and EPR studies on Mn-activated yttrium aluminum garnet phosphor

Yttrium aluminum garnet (Y₃Al₅O₁₂) and Mn activated Y₃Al₅O₁₂ phosphors have been prepared by urea combustion route in less than 5 min. The phosphors are well characterized by powder X-ray diffraction, Scanning electron microscopy and Fourier-transform infrared spectroscopic techniques. Photoluminescence tests on the pure Y₃Al₅O₁₂ showed a strong green emission at 525 nm (2.36 eV) attributed to the strongly allowed transition of F⁺ center whereas in Mn²⁺ activated YAG the green emission at 519 nm is due to the ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. EPR studies have been carried out on Mn²⁺ activated Y₃Al₅O₁₂ phosphor at 300 and 110 K. From EPR spectra the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) indicates that the Mn²⁺ ions are in a moderately ionic environment. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

In-Vacuo thermal processing of α-Al2O3 single crystals in boron ambience and its implication on TL & OSL response

Single crystals of α-Al₂O₃ (10×10 mm², 0.4 mm thick) were annealed in vacuum at about 1500 °C in the ambience of boron. The OA studies on these samples showed bands at 203, 232 and 258 nm signifying that such a treatment leads to the formation of F and F⁺ centers in significant concentrations, these bands, however, were not found in the Al₂O₃ crystals processed in the similar manner in the absence of boron. The Al₂O₃:B samples were irradiated to different absorbed doses of ⁹⁰Sr/⁹⁰Y β-source and the continuous wave OSL (CW-OSL) was recorded on the samples using 470 nm blue light stimulation. These samples have shown a linear TL and CW-OSL response in the dose range of 20 mGy to 15 Gy. The minimum detectable dose, corresponding to 3σ limit of the variation of the output of the unirradiated dosimeters, was found to be 100 μGy. Irradiated samples stored in dark at room temperature for a period of two months show negligible fading. The TL and OSL sensitivities of the samples were found to be strongly dependent on process temperature and time. The TL response is marked by the absence of low temperature peak (<100 °C), unlike the case of α-Al₂O₃:C, implying that the boron doping does not lead to formation of shallow traps. The Al₂O₃:B samples show faster photoionisation cross-section as compared to α-Al₂O₃:C. This approach of processing of single crystal Al₂O₃ in the boron ambience thus represents a potential way of introducing dosimetrically pertinent defects in Al₂O₃ single crystals.     

Temperature behavior of the 6.05-eV band in optical absorption spectra of oxygen-deficient corundum

The effect of temperature on the 6.05-eV absorption band in α-Al₂O₃ has been studied in the 80–515 K region. The data obtained are analyzed in terms of a one-coordinate model with strong electron-phonon coupling. This band is shown to be formed by two peaks at 5.91 and 6.22 eV (T=293 K) originating from absorption at the F ⁺ and F centers, respectively. An analysis of the experimental temperature dependences has allowed us to calculate the energies of effective phonons responsible for the broadening and shift of the peaks. The energies calculated agree with the data obtained in other studies and lie in the region of corundum acoustic-vibration frequencies. The Huang-Rhys factors have been evaluated for both centers and found to be close to the estimates made by other authors. The results are discussed in detail and compared with independent data on optical absorption and luminescence of anion centers in colored and irradiated α-Al₂O₃ single crystals.     

Characterization of polycrystalline (Na,K)β/β′- alumina ceramics

K β′-alumina is unstable at >1300°C. Mixed alkali β′-alumina has a variable stability depending on the alkali ratio, [K⁺]/([Na⁺]+[K⁺]). For f(β)<[K⁺]/([Na⁺]+[K⁺]), the β′-Al₂O₃ phase decomposes to Kβ-Al₂O₃ 0997 0815 V 3 and a solid solution of Na β′-Al₂O₃ and K β′-Al₂O₃. For f(β)=[K⁺]/([Na⁺]+[K⁺], the ceramic consists of K β-Al₂O₃ and Naβ′-Al₂O₃ and for f(β)>[K⁺]/([Na⁺]+[K⁺]), the excess Na⁺ after Na β′-Al₂O₃ dissolves in the β phase, giving Na β-Al₂O₃/K β-Al₂O₃ solid solution and Na β′-Al₂O₃. These sequences were confirmed by measuring the dependence of the c-axis lattice parameters of β- and β′-Al₂O₃ phases on the f(β), and the change of these parameters during the ion-exchange of Na⁺ and K⁺ ions.     

Ionoluminescence and formation of color centers in α-Al2O3 single crystals under proton irradiation

This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: α-Al₂O₃ with an unperturbed lattice and α-Al₂O₃:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and F⁺-types are formed in the α-Al₂O₃ lattice under irradiation with 5,7 MeV protons.     

Synthesis, characterization, optical absorption, luminescence and defect centres in Er3+ and Yb3+ co-doped MgAl2O4 phosphors

The Er³⁺–Yb³⁺ co-doped MgAl₂O₄ phosphor powders have been prepared by the combustion method. The phosphor powders are well characterized by X-ray diffraction (XRD) and energy dispersive (EDX) techniques. The absorption spectrum of Er³⁺/Er³⁺–Yb³⁺ doped/co-doped phosphor powder has been recorded in the UV–Vis–NIR region of the electro-magnetic spectrum. The evidence for indirect pumping under 980 nm excitation of Er³⁺ from Yb³⁺ was observed in the MgAl₂O₄ matrix material. Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the thermally stimulated luminescence (TSL) process in MgAl₂O₄:Er³⁺ phosphor. Three defect centres were identified in irradiated phosphor by ESR measurements which were carried out at room temperature and these were assigned to an O^? ion and F⁺ centres. O^? ion (hole centre) appears to correlate with the low temperature TSL peak at 210 °C and one of the F⁺ centres (electron centre) is related to the high temperature peak at 460 °C.     

Radiation-induced electrical and optical processes in materials based on Al2O3

The irradiation of dielectrics induces electric charging of microscopic regions in the bulk, which is associated with concentration inhomogeneities in the system of traps and the differences in characteristic diffusion lengths of free electrons and holes that are produced in ion tracks and collision cascades. Experimental data on radiation-induced luminescence (RIL) give evidence of the existence of three states of oxygen vacancies in Al₂O₃: an optically inactive (electrically neutral) vacancy, its excited state (known as F ⁺ center), and a negatively charged vacancy (F center). The formation of negatively charged regions under irradiation increases the intensity of the 415-nm band of F centers of RIL of Al₂O₃ single crystals. In Al₂O₃:Cr³⁺ ceramics, a radiation-induced negative charging of grain boundaries with respect to the bulk of grains takes place, which manifests itself as an increase in the intensity of the 690-nm band of RIL of Cr³⁺ ions, whereas the intensity of this band in Al₂O₃:Cr³⁺ single crystals remains unchanged. Using data on RIL, the local-charge density in grains of Al₂O₃:Cr³⁺ ceramics and the field produced by this charge are estimated.     

Electron spin resonance of aluminum-related color centers in α-TeO2:Al

ESR observations of aluminum-doped paratellurite (α-TeO₂:Al) single crystals after electron irradiation near room temperature indicate four radiation-induced electron-like defects with similarities to the intrinsic V_o^. center, which is also present. From measurements of angular variations and tellurium hyperfine interactions it is evident that the four centers represent a family of V_o^. centers perturbed by Al³⁺; that is, V_oAl_Te^x centers. The new centers are detected only in Al-doped crystals; however, the aluminum impurity ions appear to be too far from the V_o^. to yield ²⁷Al hyperfine interaction measurable by ESR.     

Structural and phase transition of α-Al2O3 powders obtained by co-precipitation method

Aluminium oxide has been synthesized by co-precipitation technique at different annealing temperature. Powder XRD confirms the formation of α-Al₂O₃ with rhombohedral crystal structure having lattice constant a = 4.76 Å and b = 12.99 Å by the Scherer formula, the average crystallite size is estimated to be 66 nm. The scanning electron microscope results expose the fact that the α-Al₂O₃ nanomaterials are seemingly porous in nature and highly agglomerated. Chemical composition of aluminium oxide is confirmed by energy dispersive spectroscopy. The molecular functional group is confirmed by FTIR. Optical absorption of α-Al₂O₃ has been studied in the UV–vis region and its direct band gap is estimated to be 5.97 eV. This study involves the structural and phase transition of Al₂O₃ and also indicates that α-Al₂O₃ has considerable properties, deserving further investigation for the energetic materials with excellent properties for the possibility of using thin-layer α-Al₂O₃ as a thermo luminescence material.     

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.     

Phase transformation and controllable size of γ-Al2O3 nanocrystals through Li doping using sol–gel method

The effects of Li, as the ‘additive’, on the structures of nano-sized Al₂O₃ were investigated. Li evidently increased the crystallite size of γ-Al₂O₃, whereas it showed evident effects on neither LiAl₂(OH)₇ precursors nor α-Al₂O₃ nanocrystals. α-Al₂O₃ and γ-Al₂O₃ co-existed in samples calcined at 1000 °C and 1100 °C, and the composition can be adjusted by altering Li concentration. When the calcination temperature reached 1200 °C, high purity of α-phase Al₂O₃ formed and Li showed no effects. In addition, mechanisms of doping Li on the structrure of Al₂O₃ were discussed.     

Blue long-lasting phosphorescence of Ti-doped BaZrO3 perovskites

Long-lasting phosphorescence was observed from Ti-doped BaZrO₃ perovskite synthesized by a solid-state reaction. The phosphorescence color is blue and the phosphorescence can still be seen with the naked eye in the dark for 30 min or more after stopping UV irradiation. By the measurements of electron spin resonance (ESR) spectra, it was confirmed that F⁺ center and Zr³⁺ exist in non-luminescent undoped BaZrO₃ and their concentrations decreased with the additive amount of TiO₂. It is expected that the luminescence center is F_A center composed of Ti³⁺ and F⁺ center.     

Electrical conductivity,diffusion of iron and the defect structure of α-Al2O3: Fe

A detailed defect model is developed for α-Al₂O₃:Fe accounting for conduction and diffusion at high temperatures as a function of iron concentration, oxygen pressure and temperature. This model involves single native and iron defects as well as triplets of (Fe_AlAl_iFe_Al), the latter dominating the defect structure at high Fe concentrations. Diffusion of Fe in A1₂O₃ is attributed to Fe_i^3.. The position of the Fe_Al' level is estimated.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al₂O₃ films was built and demonstrated. The main challenge of using the OSL from Al₂O₃ for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F⁺-centers (lifetime <7 ns) and the slow F-center emission of Al₂O₃:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al₂O₃:C,Mg, to take advantage of its greater F⁺-center emission compared to Al₂O₃:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.     

Optical properties of anodic aluminum oxide produced in a complex electrolyte

Features of the absorption, transmission, photoluminescence, and infrared (IR) spectra of anodic aluminum oxide (AAO) formed in a complex electrolyte and annealed at 800, 900, 1000, and 1300°C are investigated. The variations in the phase composition changes of the anodic aluminum oxide are reflected in the respective features of its optical properties. A decrease in the transmission coefficient in the visible range of the spectrum is shown for the phase series: amorphous AOA → γ-Al₂O₃, γ-, θ-, δ-Al₂O₃ mixture → α-Al₂O₃. It is established that the highest absorption coefficient is characteristic of α-Al₂O₃, and the amorphous samples are the most transparent in the visible range. An intensive luminescence band in the red region of the spectrum with maxima at 678 and 694 nm is found for α-Al₂O₃. The emergence of this band is explained by the presence of octahedrally coordinated Mn₊₄ and Cr₊₃ impurity ions in the structure. A luminescence band at 700–800 nm is observed for the mixture of low-temperature phases. Intense luminescence in the region 350–500 nm is found for amorphous AAO and γ-Al₂O₃.