Controlling disorder in the ZnGa2O4:Cr3+ persistent phosphor by Mg2+ substitution.

We have studied in this work the effect of increasing structural disorder on the persistent luminescence of a Cr3+ doped zinc gallate spinel. This disorder was introduced by progressive substitution of Zn2+ by Mg2+ ions, and was studied by photoluminescence, X-ray diffraction, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) spectroscopy. It was found that increasing the Mg/Zn substitution decreases the number of Cr3+ in undistorted sites and increases the number of Cr3+ with neighbouring antisite defects and with neighbouring Cr3+ ions (referred to as Cr clusters), which in turn decreases the intensity of persistent luminescence. Both XANES and EPR spectra could be simulated by a linear combination of Cr3+ spectra with three types of Cr3+ environments. The increasing disorder was found to be correlated with a decrease of the average Cr-O bond length and a decrease of crystal field strength experienced by Cr3+ ions.

Understanding the anomalous behavior of Vegard's law in Ce1-xMxO2 (M = Sn and Ti; 0 < x ≤ 0.5) solid solutions.

The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x ∼ 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, (119)Sn Mössbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce(4+) and Sn(4+). Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50% Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce1-xSnxO2 solid solutions.

Order and disorder around Cr(3+) in chromium doped persistent luminescent AB2O4 spinels.

The X-ray absorption near edge structure (XANES) spectroscopy technique is used to better understand the charging and decharging processes of the persistent luminescence in the Cr(3+)doped AB2O4 spinels (A = Zn, Mg and B = Ga and Al) with low photon energy excitation by visible light. Cr K edge XANES spectra have been simulated for different near neighbour environments around the Cr(3+) recombination centres and compared with the experimental curve. In the Cr(3+):ZnGa2O4 compound, the Cr(3+) local structure corresponds mostly to that of a normal spinel (∼70%), while the rest comprises of a distorted octahedral environment arising from cationic site inversion and a contribution from chromium clustering. This local structure is considerably different in Cr(3+):MgGa2O4 and Cr(3+):ZnAl2O4, where, for both cases, chromium clustering represents the main contribution. The strong correlation between the intensity of persistent luminescence and the percentage of Cr in clusters leads us to infer that the presence of Cr clusters is responsible for the decrease of the intensity of the visible light induced persistent luminescence in the Cr(3+) doped AB2O4 spinels.

The importance of inversion disorder in the visible light induced persistent luminescence in Cr³âº doped AB2O4 (A = Zn or Mg and B = Ga or Al).

Cr(3+) doped spinel compounds AB2O4 with A = Zn, Mg and B = Ga, Al exhibit a long, near infrared persistent luminescence when excited with UV or X-rays. In addition, the persistent luminescence of ZnGa2O4, and to a lesser extent MgGa2O4, can also be induced by visible light excitation via (4)A2→(4)T2 transition of Cr(3+), which makes these compounds suitable as biomarkers for in vivo optical imaging of small animals. We correlate this peculiar optical property with the presence of antisite defects, which are present in ZnGa2O4 and MgGa2O4. By using X-ray absorption fine structure (XAFS) spectroscopy, associated with electron paramagnetic resonance (EPR) and optical emission spectroscopy, it is shown that an increase in antisite defects concentration results in a decrease in the Cr-O bond length and the octahedral crystal field energy. A part of the defects occurs in the close environment of Cr(3+) ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl2O4, which exhibits the largest crystal field splitting of Cr(3+) and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa2O4 and MgGa2O4. These results highlight the importance of Cr(3+) ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr(3+) doped AB2O4 spinel compounds.

ZnGa2O4:Cr3+: a new red long-lasting phosphor with high brightness.

ZnGa2O4:Cr3+ is shown to be a new bright red UV excited long-lasting phosphor potentially suitable for in vivo imaging due to its 650 nm-750 nm emission range. Photoluminescence and X-ray excited radioluminescence show the 2E → 4A2 emission lines of both ideal Cr3+ and Cr3+ distorted by a neighboring antisite defect while long-lasting phosphorescence (LLP) and thermally stimulated luminescence (TSL) almost exclusively occur via distorted Cr3+. The most intense LLP is obtained with a nominal Zn deficiency and is related to a TSL peak at 335K. A mechanism for LLP and TSL is proposed, whereby the antisite defect responsible for the distortion at Cr3+ acts as a deep trap.