J.-M. Spaeth

Temperature dependence of the photostimulated luminescence of X-irradiate BaFBr:Eu2+

The temperature dependence of the luminescence from Eu 2+ activators in BaFBr:Eu2+ photostimulated between 10 and 300 K in the optical absorptions of F centres produced by room temperature X-irradiation, revealed two thermally-activated processes. The photostimulated luminescence (PSL) increases sharply in intensity when excited between 60 and 100 K. After the low-temperature PSL is exhausted by continuous F centre excitation, it can be almost entirely regained by warming the crystal to 300 K. This thermal cycle can be repeated several times. It is shown by EPR that no Eu3+ is formed upon X-irradiation at 300 K. These results are explained with a model for the PSL mechanism which involves a loose aggregate of Eu2+, F, and hole centres.

Electron spin resonance and electron-nuclear double-resonance investigation of a new Cr3+ defect on an Nb site in LiNbO3: Mg:Cr

For LiNbO3 crystals heavily doped with Mg and codoped with Cr a single isotropic ESR line with g=1.971+or-0.002 and a minimal linewidth of 1.8+or-0.2 mT has been observed. With ENDOR it could be shown unambiguously that the new defect is Cr3+ substituting for Nb. The 53Cr hyperfine and quadrupole constants are A/sub ///=50.8+or-0.2 MHz, Aperpendicular to =51.0+or-0.2 MHz and P=0.10+or-0.05 MHz. The crystal field at the chromium site is smaller by more than an order of magnitude than for previously reported Cr3+ centres in LiNbO3, indicating that those centres are not on the Nb site.

Investigation of Oxygen‐Related Luminescence Centres in AlN Ceramics

The structure of oxygen-related luminescence centres in nominally undoped and Y 2 O 3 doped AlN ceramics was investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g-values of 1.990 and 2.008 were assigned to a recombination between neighbouring donor and acceptor pairs. The two EPR lines at g = 1.987 and 2.003 detected via the recombination luminescence in the afterglow are thought to be due to a recombination between the same, but distant donor and acceptor pairs. The donor was previously speculated to be an electron trapped on an oxygen impurity which substitutes for a nitrogen on a regular lattice site. The defect structure of the acceptor was established by ENDOR to be a hole trapped on an O N -v Al complex (ON oxygen on a regular N site, v Al Al vacancy). The measured superhyperfine interaction is caused by two equivalent 27 Al nuclei both with a = ±29.6 MHz and one 27 Al nucleus with a = ±27.0 MHz.

The influence of oxide impurity on the generation by x-irradiation of F centres in BaFBr

Using magneto-optical and optically detected electron paramagnetic resonance methods, it is shown that in BaFBr single crystals grown using standard procedures the ubiquitous oxide impurity is primarily responsible for the X-irradiation-induced formation of F(Br-) centres. The bromide vacancy formed to charge compensate O2- is implicated in the direct trapping of electrons. F(F-) centres are also produced by irradiation, probably by an exciton collapse process requiring thermal activation. Cross relaxation measurements show that many of the F(F-) and F(Br-) centres are spatially correlated, more than expected on statistical grounds. The relevance of these results to the use of rare earth-activated BaFBr materials as storage phosphors is discussed.

EPR study of Er3+ in congruent LiNbO3

Erbium doped congruent LiNbO3 has been investigated with electron paramagnetic resonance (EPR). For the orientation of the magnetic fild parallel to the c-axis the previously observed characteristic hyperfine split EPR spectrum of Er3+ was measured. However, the angular dependence shows line splittings not reported before. They can be attributed to Er3+ defects on Li+ sites next to Li+ vacancies (VLi+), which tilt the g-tensor orientation away from the c-axis. Several differently oriented Er3+VLi+ complexes have been identified. The EPR spectrum can be explained assuming a statistical distribution of VLi+ in the neighborhood of Er3+VLi+.

Photostimulated luminescence from a fluorobromozirconate glass-ceramic and the effect of crystallite size and phase

We report a systematic study of the photoluminescence (PL), photostimulated luminescence (PSL) and thermostimulated luminescence (TSL) from europium-and bromine-doped fluorozirconate glass-ceramics. Eu2+ ions in the as-prepared glass show no PL, but after suitable thermal annealing hexagonal phase and orthorhombic phase barium bromide crystallites are precipitated and PL is observed from Eu2+ ions in these crystallites. Room temperature PSL is observed from the orthorhombic phase, with an efficiency which is up to 9% of the well known crystalline storage phosphor BaFBr:Eu2+. The emission is at 404 nm, and there is a maximum in the stimulation at 580 nm. We associate the PSL with an optically quenchable peak in the glow curve, which has an activation energy of 1.20 eV and attribute this feature to a perturbed F centre. Room temperature PSL from glass-ceramics containing predominantly the hexagonal phase of BaBr2 has a relative efficiency of less than 0.07%. The resultant trap depth determined from the glow curve is 0.60 eV, which suggests that this trap does not arise from a perturbed F centre. We find that the PSL efficiency systematically decreases with decreasing hexagonal BaBr2 nanocrystallite size and there is no detectable PSL from nanocrystallites smaller than 10 nm. However, below 100 K, the PSL efficiency for the hexagonal phase is nearly as high as for the orthorhombic phase and the concomitant electron traps are perturbed F centres. The measured low temperature emission was at 409 nm and an additional stimulation peak was observed at 700 nm. The PSL in this case is thermally bleached above about 150 K.

Structural phase changes in barium bromide nano-crystals in a fluorobromozirconate glass-ceramic x-ray storage phosphor

The structural changes in a fluorobromozirconate glass ceramic containing a ratio of 5% bromine to fluorine ions, following thermal annealing in the range 240-300 °C, are reported. The changes were monitored through x-ray diffraction, and the photoluminescence (PL) and electron paramagnetic resonance of Eu2+ dopant ions. In the range of 240-275 °C, the barium and bromine ions in the glass precipitate to form the metastable hexagonal phase of barium bromide. The Eu2+ PL spectrum comprises a narrow band at 410 nm, and a weaker broad band centred at 485 nm. The 410 nm band is assigned to two unresolved 4f65d1→4f7 emissions from Eu2+ ions at the two Ba2+ sites in this phase, whilst the 485 nm band is assigned to an impurity associated site or cluster. On annealing at 290 °C, the hexagonal phase transforms to the stable orthorhombic phase, and the PL spectrum comprises a single narrow band centred at 404 nm, assigned to a 4f65d1→4f7 emission from Eu2+ ions at the single Ba2+ site in the orthorhombic phase.

RbBr and CsBr doped with Eu2+ as new competitive X-ray storage phosphors

Abstract After X-irradiation of the alkali halides RbBr and CsBr doped with divalent Eu a significant photostimulated luminescence (PSL) effect was found. In both systems the electron trap centre is an F centre, the hole trap centre is still unknown. The Eu 2+ aggregation with cation vacancies plays an important role. Although all of the different Eu 2+ aggregates seem to show PSL effects, they have different PSL efficiencies, decay times and spectral characteristics. In the case of RbBr:Eu 2+ it was possible to investigate the different Eu 2+ -cation vacancy aggregate states by magnetic circular dichroism of the optical absorption (MCDA) and MCDA-detected electron paramagnetic resonance. In Eu 2+ -doped CsBr no paramagnetic Eu 2+ aggregate nor isolated Eu 2+ was found, in spite of intense and typical Eu 2+ -related PSL effects.

Sites of Fe3+ impurities in a congruent LiTaO3 crystal

With electron nuclear double resonance (ENDOR) the superhyperfine interactions of Fe3+ impurities with five shells of Li neighbours were measured in a congruent LiTaO3 crystal. From the analysis of the ENDOR spectra it is found that Fe3+ is on a substitutional Li+ site on the threefold symmetry axis of the crystal. From the shape of the ENDOR spectrum it is concluded that most Fe3+ impurities are on very perturbed sites.

A new fluorozirconate glass-ceramic X-ray storage phosphor

A new fluorozirconate glass-ceramic has been discovered that displays an X-ray storage phosphor effect with an efficiency of up to 8% of that observed in the established crystalline storage phosphor, BaFBr:Eu 2+ . The glass-ceramic is based on the well-known ZBLAN20 formulation, but the replacement of Na by Rb and Li, partial substitution of F by Br, as well as doping with Eu 2+ , results in the formation of RbBa 2 Br 5 :Eu 2+ microcrystallites. X-ray diffraction, photostimulated luminescence and EPR measurements on the as-made glass and the annealed glass-ceramic show that the X-ray storage phosphor effect arises from the RbBa 2 Br 5 :Eu 2+ microcrystallites.