B Henderson

EPR and optical absorption studies of radiation-produced defects in sodium beta -alumina

The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F+ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent 27Al(I=5/2) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), gperpendicular to =2.0032(2), A/sub ///=1.665(2) mT, Aperpendicular to =1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent 23Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to pz transition with mean energy EF=4.15(5) eV. The corresponding s to px,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.

Trapped hole centres in the alkaline earth oxides

Optical and ESR studies are reported of (Li)0 centres in MgO and SrO and V- centres in MgO and CaO. These defects undergo thermally induced reorientation at high temperature. ESR measurements under an applied uniaxial stress or electric field yield values of the elastic and electric dipole moments of these defects.

Optical detection of magnetic resonance in the vibronic sidebands of R and N lines in MgO:Cr3+

Measurements of ODMR in the excited states of Cr3+ ions in cubic and tetragonal sites in MgO have been used to elucidate the nature of the vibronic sidebands. A comparison of the experimental spectra for zero-phonon line and vibronic sideband with the theoretically calculated selection rules shows that interaction with T2u odd-parity modes of vibration is the dominant interaction giving rise to the vibronic sidebands.

Axial Cr3+ centres in MgO: EPR and fluorescence studies

The ESR, fluorescence and selective excitation spectra of Cr3+ in three different sites of tetragonal symmetry are reported. A comparison of the intensities of the different spectra allows a correlation of the ESR and fluorescence lines to three different tetragonal centres. From the measured splittings it has been possible to deduce the magnitudes of the tetragonal components of the crystalline field used in Macfarlanes theory (1967) of Cr3+ ions. Models involving slight changes in the mechanism of charge compensation are discussed.

Ion implantation in alkaline earth oxides

Abstract We report spectroscopic investigations of defects in alkaline earth oxides implanted with 400 keV Li+ and Na+ ions. Detailed comparisons of ESR, optical absorption and luminescence spectra confirm that the predominant point defect is the F+ centre and to a less extent the vacancy pair centre. Smaller concentrations of F2 and F2 2+ centres are also produced. V− centres and isolated anion vacancy centres are not produced by irradiation. Annealing studies demonstrate that colloidal centres grow during annealing above ∼500 K. The position and bandwidths of the colloidal optical absorption bands may be interpreted in terms of classical Maxwell-Garnett theory.

Optical detection of magnetic resonance in MgO:Cr3+. I. Octahedral and orthorhombic site symmetries

Optically detected magnetic resonance (ODMR) of Cr3+ ions in magnesium oxide has been monitored by detecting the fluorescence from Cr3+ ions in cubic and orthorhombic symmetry sites. Resonance is detected as a change in the circular polarisation of the R-line emission (cubic centre) and 770 nm emission band (orthorhombic centres) during microwave pumping. The simultaneous observation of both 4A2 ground state resonance and 2E excited state resonance for cubic centres is discussed in terms of the selection rules for circularly polarised light and cross relaxation between excited and unexcited ions. Failure to observe a resonance from the excited fluorescent 4T2 of the orthorhombic centres is explained in terms of the large zero-field splitting implied from level crossing data in the circular polarisation of the luminescence as a function of magnetic field. That one does observe a ground state resonance from the cubic Cr3+ centres as a change in the circular polarisation of the 770 nm luminescence from orthorhombic centres is evidence of energy transfer from cubic to orthorhombic centres. The observed resonances can be explained by a simple, qualitative extension to the selection rules for cubic centres.

Optical detection of magnetic resonance in MgO :Cr3 + : II. Cr3 + ions in tetragonal symmetry sites

Optical studies of magnetic resonance in the excited state of Cr3 + ions in tetra- gonal symmetry sites are reported. The observed spectra are discussed in term of the calcu- lated selection rules for circularly polarised fluorescence, and it is shown that the results are consistent with thermalisation in the excited spin states. The measured g tensor is that which is to be expected from a crystal-field split state in which the I2Egv) orbital lies lowest.

Exchange interactions between Cr3+ ions in magnesium oxide. II. Electron spin resonance spectra

For pt.I see ibid., vol.18, p.6397 (1985). Cr3+ ions in MgO may cluster in pairs with cation vacancies to form C.Mg VMg Cr.Mg neutral dimers, which behave magnetically as exchange-coupled pairs. The authors have identified two different neutral Cr3+ dimers in MgO using electron spin resonance (ESR). One is a linear aggregate parallel to (100) directions; the other has a right-angled configuration centred on the vacancy, with each of the impurity-vacancy interactions being in (100) directions. The ESR spectra have been interpreted using exact solutions of the spin Hamiltonian in which the magnetic ions are coupled by two linear terms Js1.s2+De(3(s1.n)(s2.n)-s1Os2). The exchange interactions are both antiferromagnetic with J-values of 0.004+or-0.001 cm-1 and 1.5+or-0.05 cm-1 for the linear and bent dimers respectively. These values reflect the variation of J with magnetic ion separation in the dimer.

Optical pumping cycle of exchange-coupled F+-centre pairs in MgO and CaO

In n-irradiated CaO and MgO crystals, triplet-state defects are produced in which two electrons are trapped in the O2- sites of a linear trivacancy configuration O2--Ca2+-O2- parallel to the (100) crystal direction. The electronic spins are coupled antiferromagnetically to give a ground-state singlet 1A1g for these F+-centre pairs, the first excited level 3A2u being thermally populated at room temperature. Measurements of the 3A2u state EPR spectrum at 1.4K under optical excitation demonstrates that optical spin alignment memory is induced during the optical pumping cycle.

EPR of Ag2+ in β-alumina

EPR measurements have been made on Ag2+ in sodium beta -alumina single crystals. Silver is introduced into the conduction plane as Ag+ by exchange with Na+ in amounts varying from 5 to 26%: Ag2+ is produced by X-ray irradiation at 300K. EPR results indicate that the Ag2+ occupies two sites of orthorhombic symmetry in the conduction plane. The measured g-values are gx=2.0230(2), gy=2.0678(15), gz=2.2730(5) and gx=2.015(4), gy=2.0580(15), gz=2.2335(4) where x, y, z are parallel to a*, c, a axes respectively. The hyperfine interaction with the 107Ag nuclei is unresolved and estimated to be less than 1 mT. No motional effects are observed. Models are discussed for the two sites which involve an excess of O2- ions in the conduction plane.