Abanti Nag

Role of B2O3 on the phase stability and long phosphorescence of SrAl2O4:Eu, Dy

Abstract The role of B2O3 addition on the long phosphorescence of SrAl2O4:Eu2+, Dy3+ has been investigated. B2O3 is just not an inert high temperature solvent (flux) to accelerate grain growth, according to SEM results. B2O3 has a substitutional effect, even at low concentrations, by way of incorporation of BO4 in the corner-shared AlO4 framework of the distorted ‘stuffed’ tridymite structure of SrAl2O4, which is discernible from the IR and solid-state MAS NMR spectral data. With increasing concentrations, B2O3 reacts with SrAl2O4 to form Sr4Al14O25 together with Sr-borate (SrB2O4) as the glassy phase, as evidenced by XRD and SEM studies. At high B2O3 contents, Sr4Al14O25 converts to SrAl2B2O7 (cubic and hexagonal), SrAl12O19 and Sr-borate (SrB4O7) glass. Sr4Al14O25:Eu2+, Dy3+ has also been independently synthesized to realize the blue emitting (λem≈490 nm) phosphor. The afterglow decay as well as thermoluminescence studies reveal that Sr4Al14O25:Eu, Dy exhibits equally long phosphorescence as that of SrAl2O4:Eu2+, Dy3+. In both cases, long phosphorescence is noticed only when BO4 is present along with Dy3+ and Eu2+. Here Dy3+, because of its higher charge density than Eu2+, prefers to occupy the Sr sites in the neighbourhood of BO4, as the effective charge on borate is more negative than that of AlO4. Thus, Dy3+ forms a substitutional defect complex with borate and acts as an acceptor-type defect center. These defects trap the hole generated by the excitation of Eu2+ ions and the subsequent thermal release of hole at room temperature followed by the recombination with electron resulting in the long persistent phosphorescence.

Photoluminescence of Sr2−xLnxCeO4+x/2(Ln = Eu, Sm or Yb) prepared by a wet chemical method

A wet chemical route is developed for the preparation of Sr2CeO4 denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO2.xH(2)O, 40<x<75) by the addition of ammonium carbonate. During calcination, CeO2.xH(2)O dehydroxylation is followed by the reaction with SrCO3 to form Sr2CeO4 with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr2CeO4 originates from the Ce4+-O2- charge-transfer (CT) transition resulting from the interaction of Ce4+ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce4+-O2- CT emission is studied by doping with Eu3+, Sm3+ or Yb3+ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce4+-O2- CT state to trivalent lanthanide ions (Ln(3+)) if the latter has CT excited states, leading to sensitizer-activator relation, through non-resonance process involving exchange interaction. Yb3+-substituted Sr2CeO4 does not show any line emission because the energy of Yb3+-O2- CT level is higher than that of the Ce4+-O2- CT level. Sr2-xEuxCeO4+x/2 shows white emission at xless than or equal to0.02 because of the dominant intensities of D-5(2)-F-7(0-3) transitions in blue-green region whereas the intensities of D-5(0)-F-7(0-3) transitions in orange-red regions dominate at concentrations xgreater than or equal to0.03 and give red emission. The appearance of all the emissions from D-5(2), D-5(1) and D-5(0) excited states to the F-7(0-3) ground multiplets of Eu3+ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu3+. White emission of Sr2-x SmxCeO4+x/2 at xless than or equal to0.02 is due the co-existence of Ce4+-O2- CT emission and (4)G(4)(5/2)-H-6(J) Sm3+ transitions whereas only the Sm3+ red emission prevails for xgreater than or equal to0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce4+. Quenching of Ce4+-O2- CT emission by other Ln(3+) is due to the ground state crossover arising out of the NNN interactions.

Role of interface states associated with transitional nanophase precipitates in the photoluminescence enhancement of SrTiO3∶Pr3+,Al3+

SrTiO3∶Pr3+,Al3+ phosphor samples with varying ratios of Sr/Ti/Al were prepared by the gel-carbonate method and the mechanism of enhancement of the red photoluminescence intensity therein was investigated. The photoluminescence (PL) spectra of SrTiO3∶Pr3+ show both 1D2 → 3H4 and 3P0 → 3H4 emission in the red and blue spectral regions, respectively, with comparable intensity. The emission intensity of 1D2 → 3H4 is drastically enhanced by the incorporation of Al3+ and excess Ti4+ in the compositional range Sr(Ti,Aly)O3+3y/2∶Pr3+ (0.2 ≤ y ≤ 0.4) and SrTi1+xAlyO3+z∶Pr3+ (0.2 ≤ x ≤ 0.5; 0.05 ≤ y ≤ 0.1; z = 2x + 3y/2) with the complete disappearance of the blue band. This cannot be explained by the simple point defect model as the EPR studies do not show any evidence for the presence of electron or hole centers. TEM investigations show the presence of exsolved nanophases of SrAl12O19 and/or TiO2 in the grain boundary region as well as grain interiors as lamellae which, in turn, form the solid-state defects, namely, dislocation networks, stacking faults and crystallographic shear planes whereby the framework of corner shared TiO6 octehedra changes over to edge-sharing TiO5–AlO5 strands as indicated from the 27Al MAS NMR studies. The presence of transitional nanophases and the associated defects modify the excitation–emission processes by way of formation of electronic sub-levels at 3.40 and 4.43 eV, leading to magnetic-dipole related red emission with enhanced intensity. This is evidenced by the fact that SrAl12O19∶Pr3+,Ti4+ shows bright red emission whereas SrAl12O19∶Pr3+ does not show red photoluminescence.

The light induced valence change of europium in Sr2SiO4∶ Eu involving transient crystal structure

Sr2SiO4 ∶ Eu3+ shows orange-red emission of Eu3+ substitutively present in two different Sr sites. The light-induced spectral changes from orange-red sharp line emission to yellow-white broad band are observed in Sr2SiO4 ∶ Eu at room temperature under irradiation with short UV or X-rays. The spectral changes are attributed to the optically assisted reduction of Eu3+ → Eu2+. The photoreduced Sr2SiO4 ∶ Eu shows emission containing contributions from both Eu2+ and Eu3+ in comparison to chemically reduced samples. This is explained on the basis of preferential reduction of Eu3+ present in Sr(1) sites under irradiation due to unsatisfied EuSr–O–Si bonds. The absence of photoactivity for Ba2SiO4 ∶ Eu3+ (space group = Pnam) as well as Ca2SiO4 ∶ Eu3+ (space group = P21/n) indicates that crystal structure plays an important role in the photoreduction of Sr2SiO4 ∶ Eu3+ because of the prevailing orientational as well as the positional disorder in the latter. Further, the orientationally disordered monoclinic random domains persist within the orthorhombic lattice of Sr2SiO4, resulting in the positionally disordered Sr atoms and orientationally disordered SiO4 tetrahedra. Electron paramagnetic resonance studies confirm the electron trapping by dynamically disordered (SiO4)4− under high energy photon illumination resulting in the formation of radical anion (SiO4)5−. The substitutional studies indicate that the [Eu3+ ← O2−] charge-transfer (CT) state is directly involved in the photoreduction process. The excitation of Sr2SiO4 ∶ Eu3+ produces the [Eu3+ ← O2−] CT state which relaxes and transfers electrons to SiO4 groups due to optically assisted rearrangement of local environment and mediates the electron transfer process to cause photoreduction of Eu3+ to Eu2+. The yellow emission is stable at room temperature and reverts to red on annealing at elevated temperature in Ar atmosphere due to thermally activated detrapping of charge carriers present at the defect centers which, in turn, convert Eu2+ to Eu3+. The thermally activated conversion of Eu2+ → Eu3+ in Sr2SiO4 is optically reversible, thereby resulting in a highly efficient material for application as an optical storage medium.