Paramagnetic Hole Centers Research Articles

Creation and thermal annealing of structural defects in neutron-irradiated MgAl2O4 single crystals

Abstract Several novel hole-type defects (a hole localized at a regular oxygen ion near a negatively charged structural defect) have been revealed in fast neutron irradiated MgAl2O4 crystals using the EPR method. The pulse annealing of the EPR signal of these centers was compared to that of radiation induced optical absorption in the same crystals. Taking into account the determined models of V1, V2 and V22 paramagnetic centers, the tentative scenario of the thermal annealing process of neutron-induced defects (hole-type and complementary electron F-type ones) is proposed. In addition, one more paramagnetic hole center consisting of an Al|Mg as-grown antisite defect near an aluminum vacancy is created just by X-ray irradiation at room temperature.

Paramagnetic hole centres in natural zircon and zircon colouration

Paramagnetic defect centres and their relation to variable types of colouration were studied for eleven natural zircon crystals from three localities in North Carolina (USA), Massif Central (France), and Ural Mountains (Russia) using electron paramagnetic resonance (EPR) and optical absorption spectroscopy. The presence of tetragonal centres of trivalent rare-earth elements (REE 3+ ) and niobium (Nb 4+ ) irrespective of the colouration type and also in colourless grains suggests that these defects are not directly related to any specific zircon colouration. In contrast, the occurrence of two hole centres in crystals from North Carolina and the Massif Central correlates with the formation of specific broad bands in the absorption spectra. The appearance of pink colouration corresponds to the occurrence of a hole located at an oxygen nearest to Y 3+ on zirconium site. This defect displays a single line at g = 2.006 in the EPR spectra for external magnetic field orientations parallel to the tetragonal crystal axis c . The related broad band in the optical absorption spectra peaks at 340–350 nm and its low-energy wing extends into the visible range. Reddish colour results from an intense broad absorption feature around 510–515 nm caused by a hole located at an oxygen next nearest to Y 3+ substituting for Zr 4+ . A single line at g = 2.011 related to this “red” hole centre is observed in the EPR spectra in magnetic field orientation parallel c . The nature of the two paramagnetic centres was verified by their angular dependencies. Reddish-brownish and orange colour of zircon may appear for certain intensity relations between the two absorption bands with possible additional influence from charge transfer bands caused by Tb 4+ centres. Yellowish-brownish colouration of zircon from the Ural Mountains is essentially different in nature. Several other paramagnetic defects with spin S = 1/2 were found in zircon from this locality besides the “red” hole centre, Tb 4+ , and REE 3+ defects. There is a characteristic set of four bands in the optical absorption spectra. Additional heating and irradiation experiments are necessary for a correct assignment of paramagnetic defects to specific colour centres in this case.

Paramagnetic centers related to Al, Sc, In, and Nb impurities in KTiOAsO4crystals

Electron paramagnetic resonance (EPR) is applied to study Al-, Sc-, In-, and Nb-doped KTiOAsO4 (KTA) crystals. Paramagnetic hole centers O− are observed after ionizing irradiation of KTA crystals. These centers are, as a rule, unstable at room temperature and are slowly annealed for about two weeks. Oxygen ions are bridging two cations in KTA. Near the impurity, two p-orbitals of oxygen atoms participate in covalent bonding with cations, whereas the third p-orbital remains free and under the radiation effect captures the hole thus forming the paramagnetic center of Mn+-O−-M(n−1)+ (here Mn+ is the lattice cation and M(n−1)+ is the impurity cation of Al, In, Sc, or Nb). In the centers investigated the specific principal direction of the g-factor g ∼ 2 is normal to the Mn+-O−-M(n−1)+ plane, and the main value of gmax falls in this plane. The direction of the O−-M(n−1)+ bond is close to the selected direction of the hyperfine interaction with the impurity ion. The models of six hole centers and the found parameters of EPR spectra are discussed.

Irradiation-induced and postirradiation processes in fluoroaluminate glasses

We have measured the EPR and induced optical absorption spectra of several fluoroaluminate glasses prepared under different redox conditions and containing NH4HF2, EuF3, or Eu2O3 additions. Analysis of spectra taken after gamma irradiation to various doses and isothermal bleaching kinetics of individual absorption bands indicates that the color centers responsible for induced absorption in the UV spectral region are hole traps related to either oxygen (45000 and 42000 cm−1) or fluorine (37500 cm−1). In the visible range, we have revealed three absorption bands. We assume that the 17400 cm−1 absorption is due to an electron trap involving Y3+. Our data suggest some evidence that paramagnetic hole centers and electronic color centers may be formed in the same events.

Photoluminescent and paramagnetic centers in gamma irradiated porous silica

The photoluminescence and electron spin resonance properties of gamma irradiated (up to 500 kGy) porous silica are reported. By exciting at 5.6 eV a photoluminescence contribution can be detected before irradiation, peaked at about 4.1 eV. Gamma irradiation causes the generation of the E′ centers (about 1 × 10 14 defects cm −3) of paramagnetic hole centers and modifies the photoluminescence properties of the sample: the emission amplitude decreases and three contributions can be singled out at about 3.3, 3.8 and 4.4 eV.

Spin relaxation processes in the defect hole centres of Be doped MgO single crystals

The dependence of the width and intensity of the EPR spectrum lines of various paramagnetic hole centers in MgO:Be on the measurement temperature and applied microwave power has been measured. Using these data, the longitudinal and transverse relaxation times are estimated. With the aid of the found relaxation times, the temperature dependences of the EPR spectra of these centres are calculated. The validity of the obtained results is estimated by comparing the calculated temperature dependences of the EPR spectra with the measured ones. The mechanism of spin-lattice relaxation is analyzed using the models of the centres. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxygenic-hole centres in x-irradiated zircon: 10 K EPR studies

A surprisingly large number of paramagnetic hole centres in zircon have been observed and characterized by EPR. The field is reviewed and a model proposed based on some ten precisely measured centres. EPR data suggest that the trapped hole, produced by x-irradiation, is normally located in the non-bonding pz orbital of an O- ion and is stabilized by a nearby impurity cation, often, but not always, a +3 ion. Two previously unobserved oxygenic-hole centres are here reported. In one the stabilizing ion is shown from observed 89Y superhyperfine structure to be Y3+ and conforms to the above model. The second has the most anisotropic g values yet observed in an oxygenic-hole centre in zircon. Two g values are significantly below the free-electron value and a px ground state is inferred.

Ga 2+ hole centers and photostimulated luminescence in the x-ray storage phosphor RbBr:Ga+

It was previously shown that RbBr doped with Ga+ is an efficient x-ray storage phosphor with a figure of merit comparable to that of the commercially used BaFBr:Eu2+. The paramagnetic hole centers generated upon x-irradiation involved in the storage and read-out process were investigated by measuring the magnetic circular dichroism of the optical absorption (MCDA), the MCDA-detected electron paramagnetic resonance (MCDA-EPR) and the photostimulated luminescence (PSL). Two different Ga2+ hole centers with a hyperfine interaction with 69Ga of 69A = 8.2 GHz and 69A=6.0GHz, respectively, were found. No superhyperfine interaction was resolved. Therefore, the structural difference between the two Ga2+ centers could not be established from EPR. The PSL effect is correlated only to one of the two Ga2+ centers, namely that with A=8.2 GHz. The generation mechanism of the two hole centers is investigated. It is proposed that the PSL-active Ga2+ center is an isolated Ga2+ on a Rb+ site, whereas the other center is a Ga2+-cation vacancy complex.

Spectroscopic Study of KTiOAsO4 Single Crystals Doped with In, Sc, Fe

AbstractElectron paramagnetic resonance and optical absorption in the visible and infrared regions have been used to study flux‐grown KTiOAsO4 crystals doped with In, Sc, and Fe. An unstable broad absorption band and paramagnetic hole centres in KTA: In or Sc after γ‐irradiation are observed. The analysis of EPR data leads to a model wherein the holes are trapped on bridging oxygen ions between two titanium ions. The optical absorption band at 2.7 eV is ascribed to the light‐induced transfer of holes from one oxygen ion to another. Infrared absorption bands at 3584 and 3594 cm−1 are observed in all crystals and possible positions of the OH group in the structure are discussed.

Kinetics of electron-hole recombination in X-ray irradiated cubic stabilized zirconia

Electron paramagnetic resonance (EPR) and optical measurements have been performed to reveal photoinduced concentration changes of paramagnetic and optical defects in X-ray irradiated cubic stabilized zirconia (CSZ) single crystals. New results are reported showing that the photoinduced annihilation of radiation induced electron centers is not directly related to the decrease of paramagnetic hole centers. The interpretation of the experimental data suggests a more complex kinetics than previously proposed.

The function of paramagnetic iron species in amorphous materials formed by sol-gel method and conventional melting techniques

A 9 and 35 GHz electron paramagnetic resonance investigation of conventional Na 2OAl 2O 3SiO 2 and CaOAl 2O 3SiO 2 glasses has shown that: (a) a fraction of Fe 3+ acts as a network former, (b) precursors of non-stoichiometric FeO x phases are formed, (c) the ratio of the concentration of Fe 2+ ions to the concentration of Fe 3+ ions controls the formation and stabilization of radiation-induced paramagnetic hole centres. 9 GHz EPR of SiO 2 and xNa 2O·0.1Fe 2O 3·100SiO 2 ( x = 0, 0.1, 1.0) sols and gels showed that (i) Fe 3+ sites in the sols are less rhombicly distorted than in glasses (in the presence of a chelating agent such as acac an effect of changing Fe 3+/Na + ratio is not detectable) and (ii) in alcogels, ferric ions are observed in axially distorted sites; (iii) UV irradiation of frozen sols and gels leads to the formation of ·C 2H 5 and CH 3ĊHOH radicals, which provide information about the degree of hydrolyses of the samples; (iv) if sodium is added as NaNO 3, the UV irradiation produces ·NO 2 radicals; (v) the addition of 1 mol% nitrate is sufficient to remove residues of carbon compounds on tempering the gels.

Trapped-hole centers in alkaline-earth oxides

Alkaline-earth oxides frequently exhibit a visible coloration when stimulated with photons or ionizing radiation. The defects responsible for this coloration are often due to paramagnetic hole centers, which absorb over a broad spectrum in the visible and near-visible region. As a result, the defects can be detrimental to the use of these materials as hosts for tunable lasers. In this article, we present our current understanding of trapped-hole (or V-type) centers in the alkaline-earth oxides with emphasis on experimental research using optical and magnetic resonance techniques. Discussions are focused on the production mechanisms, the structure of both intrinsic and impurity-compensated defects, and their thermal stability.

Paramagnetic hole centres produced in germanium-doped crystalline quartz by X-irradiation at 4K

Paramagnetic resonance studies of SiO2:Ge crystals X-irradiated in a microwave cavity at 4K show a dominant hole centre stable below 35K. It is suggested that the hole is stabilised in an oxygen 2p orbital by a nearby germanium atom and that germanium is an amphoteric impurity in quartz. The relevance of the measurements to the mobility of hole polarons in crystalline quartz is discussed.

Optically Induced Localized Paramagnetic States in Amorphous Semiconductors

In addition to the previously reported paramagnetic hole center localized on the chalcogen, a conjugate optically induced electron center localized on an As atom is observed in glassy ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ and ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The chalcogen center is further identified as a hole localized in a nonbonding $p$ orbital. These optically induced centers are unique to the amorphous phase, and their density saturates at or below ${10}^{17}$ spins/${\mathrm{cm}}^{3}$.

Paramagnetic Hole Centers in CeO2

The ESR spectra of three hole centers have been observed in nominally pure cyrstals of Ce${\mathrm{O}}_{2}$ following uv irradiation at 77\ifmmode^\circ\else\textdegree\fi{}K. Designated ${V}_{\mathrm{I}}$, ${V}_{\mathrm{II}}$, and ${V}_{\mathrm{III}}$, the centers exhibit orthorhombic symmetry with principal axes ($x, y, z$) parallel to [$\overline{1}10$], [001], and [110], respectively. The $g$ values are: ${g}_{x}(\mathrm{I})=2.0311$, ${g}_{y}(\mathrm{I})=2.0065$, ${g}_{z}(\mathrm{I})=2.0330$; ${g}_{x}(\mathrm{II})=2.0175$, ${g}_{y}(\mathrm{II})=2.0054$, ${g}_{z}(\mathrm{II})=2.0317$; and ${g}_{x}(\mathrm{III})=2.0268$, ${g}_{y}(\mathrm{III})=2.0050$, ${g}_{z}(\mathrm{III})=2.0415$. The ${V}_{\mathrm{III}}$ center exhibits a hyperfine interaction with a nuclear spin $I=\frac{1}{2}$ for which ${A}_{x}=3.1$, ${A}_{y}=2.3$, and ${A}_{z}=2.7$ in units of ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The thermal bleaching of the centers has been correlated with the thermoluminescence of the same crystals measured by Land and Rodine. Models for these centers are porposed which involve an ${\mathrm{O}}_{2}^{3\ensuremath{-}}$ molecular ion near a stabilizing impurity ion.

Luminescence mechanism of tungstate phosphors

Luminescence and thermoluminescence of various tungstates have been observed. To investigate trapping phenomena comparing measurements on nominally pure and doped (10-3% Sm, 10-3% Pb, 10-4% Pb) single crystals of CaWO4 were performed. Decay times, temperature dependence of luminescence yields, thermoluminescence with spectral composition of the emitted glowlight, thermally stimulated conductivity and photoconductivity were measured. EPR-work was done on all investigated systems; correlations between all methods were found. A paramagnetic hole center (related to the Vk-center in the alkali halides) appears in all samples; its structure can be interpreted by discussing the EPR-spectra. This center is shown to be in the state of a “small polaron” and plays a dominant role in the energy storage and -transport in the tungstates. The results of the theory of polarons are verified by our experiments. The thermally activated hopping motion and the recombination mechanism are discussed. From all experiments for the whole class of tungstate and molybdate phosphors a unified luminescence model is suggested, that describes the transport properties correctly too. Interrelations with comparable mechanism in alkali halides are shown.

Irradiation Damage in SrF2and BaF2

The thermoluminescence of rare-earth-doped Sr${\mathrm{F}}_{2}$ and Ba${\mathrm{F}}_{2}$ irradiated with x rays at 77\ifmmode^\circ\else\textdegree\fi{}K is interpreted in terms of the appearance and disappearance of three paramagnetic hole centers. The ${V}_{K}$ center is similar to one previously reported in Ca${\mathrm{F}}_{2}$. The ${{V}_{K}}^{\ensuremath{'}}$ center appears to be a ${V}_{K}$ center trapped near a monovalent impurity ion. The third center, designated as ${V}_{H}$, is associated with an interstitial fluorine. Separate thermal activation of translational and rotational motion has been observed for the ${V}_{K}$ center in Ba${\mathrm{F}}_{2}$.

THERMOLUMINESCENCE AND PARAMAGNETIC HOLE CENTERS IN BaF2

Thermoluminescence of rare-earth doped BaF2, irradiated with x rays at 77°K, is interpreted in terms of the appearance and disappearance of three paramagnetic hole centers. One of the centers is associated with an interstitial fluorine.