Color Centers in Simple Oxides

Optical absorption spectra of MgO crystals irradiated with high energy (250 t o 450 keV) alkali ions (Li+, K+) present two bands of colloidal nature after thermal annealing of the crystal.Mie theory cannot represent all the features of this behaviour which are related to the very high dose (5 x 10l6 to lo1' ions/cm2) possible to get with such a technique (LIP to lo'" cm-3 implanted ions). The results are interpreted using Maxwell-Garnett theory applied to a granular metal film embedded in a dielectric host and it is possible to attribute the two observed absorption bands to plasma resonance and optical conduction absorption of the metal precipitate. Since the development' of nuclear reactors, the study of defects produced by irradiation damage became of great' interest and has been extensively developed in many kinds of materials. Insulating material like MgO is a good probe for this purpose.Among defects created by irradiation, and following the classification given by Sonder and Sibley [l], this paper is mai,ily interested in large defects like metal aggregates of implanted ions, leaving apart either electronic defects which involve changes in valence states or ionic defects which consists of displaced lattice ions.I n ion implanted crystals we are dealing with a spatial repartition of extra ions in a very thin layer close to t'he surface of the target crystal. The repartit'ion profile of the implanted ion can be deteriiiined by backscat'tering t'echniques [ 2 ] . Depending upon the dose, t'he concentration of foreign ions can be very high and under thermal annealing, these foreign ions can aggregate and form metal particles enibedded in the, host lattice. Optical absorption of such extrinsic "colloids" is generally analysed using Mie s theory [3]. Shape and/size of colloids can be studied by electron microscopy [4]. The Mie theory does not, represent the experimental result's in the case of very high doses. Therefore, direct use of the Maxwell-Garnett theory [5] is more appropriate to h a t ' the probletn optical properties of a discont'inuous metal film sandwiched wit'h a dielectric.The niainpurposesof this paper are to show that t'he Maxwell-Garnett theory is able to explain experimental absorption spectra of lithium and potassium precipitates in MgO crystals and to deduce some interesting aspects of the aggregat'ion processes in irradiated MgO.

Section: Resultsmentioning

confidence: 93%

Optical properties of metal aggregates in Li⁺ and K⁺ irradiated MgO crystals

Marichy

Chassagne

Durand

1979

“…Its amplitude largely exceeds that of Mn" lines at 20 K. The D line is difficult to saturate upon increasing the microwave power. Its short spin-lattice relaxation time Tl compared to that of Mn", and its positive g-shift are in favor of its assignment to a hole center [18,191, which can be considered as a S -ion at a sulfur lattice site adjacent to a cationic defect. This center could first result from the charge compensation of the substitution of a few Ca2+ ions (size r = 100 pm) by Na+ ions from the flux, which have approximately the same size (r = 102 pm) [20].…”

Section: General Features Of the Epr Spectramentioning

confidence: 97%

Multiple‐Quantum EPR Transitions of Mn^II in CaS: Mn^II Phosphor

Gourier

Simons

Vivien

et al. 1993

Multiple-quantum transitions in the electron paramagnetic resonance spectrum of naturally doped ( N 10 ppm) CaS : Mn" powder are reported. Considerable resolution enhancement is achieved because of the lack of strain broadening of the EPR lines, due to the very good crystallinity of the powder. Multiple-quantum transitions are identified by their linewidth, their saturation behavior, and their magnetic field position. Two-, three-, and five-quantum transitions were identified, but not the four-quantum transitions. A line at g = 1.983 is assigned to Cr"' impurities at low concentration, which also exhibits a two-quantum transition at high microwave power.Cet article traite de ]'etude de transitions RPE multi-quanta dans des poudres de CaS:Mn" naturellement dopees. Les spectres presentent une grande resolution due a l'absence d'elargissement des raies RPE sous l'effet de contraintes dues au desordre, grLce a la tris bonne cristallinite des poudres. Les transitions multi-quanta sont identifiees par leur largeur, leur comportement en saturation et leur position en champ magnetique. Des transitions a deux, trois et cinq quanta ont ete identifiees, mais pas de transitions a quatre quanta. Une raie a g = 1.983 est attribuee a des impuretes Cr"' en faible concentration, qui montrent aussi une transition a deux quanta a forte puissance microonde.

“…The g-factor is anisotropic and nearly axial with 911 = 2.004(6) and g 1 = 2.02(4), the x-axis being parallel to the crystallographic direction C. There is a slight anisotropy in the basal plane which is not considered here, and thus we report an average value for gL. The positive g-shift, its anisotropy, and the lack of hf interaction with A1 nuclei indicate that the defect is an unpaired electron-hole strongly localized a t an oxygen site [8]. Several kinds of oxygen centres have been recognized in oxides: 0-, O,, 0;-, and 0; centres 191:…”

Section: Electron Paramagnetic Resonancementioning

confidence: 90%

“…9 1 = g e -x * where A = -135 cm-l is the spin-orbit coupling constant for a free 0-ion [8], and A the energy separation between the (2ps, 2p,) and 2pz orbitals. We may propose two possible sites for this defect : (i) It could be first located a t an oxygen site adjacent to a cationic defect in the spinel block, an aluminum vacancy, or a divalent impurity a t an aluminum site, for example (V type centres).…”

Section: Electron Paramagnetic Resonancementioning

confidence: 99%

Colour centres in barium hexaaluminate (Phase I). Study by electron paramagnetic resonance, optical absorption, and thermally stimulated luminescence

Gourier

Gbehi

Visocekas³

et al. 1989

Colour centres produced by X-ray irradiation of barium hexaaluminate (phase I) with' p-alumina structure are studied by electron paramagnetic resonance, optical absorption, and thermally stimulated luminescence. It is shown that in addition to the F+ centres characteristic of p-alumina phases, this compound presents other colour centres such as F, 0-, and possibly V-type centres.The stability of these defects is investigated by means of thermal bleaching experiments and thermally stimulated luminescence. An alternative model to the generally accepted one is proposed, for the FC centres, together with a mechanism of defect formation.Les centres colo&s produits par irradiation X de l'hexaaluminate debarium (phase I) sont btudih par RPE, absorption UV-visible et luminescence stimulbe thermiquement. Nous montrons qu'en plus des centres F+ caractbristiques des phases de type alumine-p, d'autres centres sont produits tels que des centres F, 0-et peut etre des centres de type V. La stabilith de ces defauts est btudibe a l'aide d'expbriences de blanchiment thermique et de luminescence stimulbe thermiquement. Un autre modBle de centre F+ diff6rent de celui gbnhralement admis est proposb ainsi qu'un mhcanisme de formation des d6fauts.