RESUMO
Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy; 0 ≤ x ≤ 0.2) phosphors were synthesized by a high-temperature solid-state reaction. Sc : R ratios for the NSP:xR samples were determined by ICP-MS, EDX-SEM and TEM-EDX measurements. An X-ray diffraction study revealed that solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.1. The luminescence properties of Na3Sc2(PO4)3 and Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) were studied in the range of 80-500 K. The highest R3+ luminescence intensity in Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) depending on R was found for x = 0.05 in the case of Dy and x = 0.1 in the case of Eu and Tb. The temperature behaviour of the R3+ emission intensity of Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) depends on R that replaces Sc. The decrease of the Eu3+ emission intensity depending on the transition energy by â¼26% and 18% at â¼420 K compared to TR allowed us to consider NSP:0.1Eu3+ as a suitable phosphor for pc-LEDs. The temperature dependence of the Dy3+ emission for NSP:0.05Dy3+ demonstrates a strong thermal quenching. Different temperature dependences of the Tb3+ emission intensity of NSP:0.1Tb3+ were found for two excitation bands at λex = 220 and 378 nm representing f-d and f-f intracentre transitions. No thermal quenching for f-f transitions takes place while the emission intensity for f-d transitions increases with a temperature rise from 80 to 500 K. The dielectric measurements for Na3Sc2(PO4)3 and Na3Sc1.9Eu0.1(PO4)3 were provided on ceramic pellets sintered under vacuum using a spark plasma sintering technique. Different dependences of conductivity were found for two samples. The calculated conductivity for Na3Sc1.9Eu0.1(PO4)3 with an R3Ìc structure (σbulk = 6.4 × 10-5 S cm-1 at 300 K, 1.14 × 10-3 S cm-1 at 360 K and 5.0 × 10-2 S cm-1 at 500 K) is higher than that for pure α-Na3Sc2(PO4)3 but lower than that for ß- and γ-Na3Sc2(PO4)3.
RESUMO
ß-Ca3(PO4)2-type phosphors Ca9-xMnxEu(PO4)7 have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca8MnEu(PO4)7 was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu3+ â Eu2+ in air was observed in Ca9-xMnxEu(PO4)7 according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu3+ shows partial reduction and coexistence of Eu2+/3+ states. It reflects in combination of a broad band from the Eu2+ 4f65d1 â 4f7 transition and a series of sharp lines attributed to 5D0 â 7FJ transitions of Eu3+. Eu2+/Eu3+ ions are redistributed among two crystal sites, M1 and M3, while Mn2+ fully occupies octahedral site M5 in Ca8MnEu(PO4)7. The main emission band was attributed to the 5D0 â 7F2 electric dipole transition of Eu3+ at 395 nm excitation. The abnormal quenching of Eu3+ emission was observed in Ca9-xMnxEu(PO4)7 phosphors with doping of the host by Mn2+ ions. The phenomena of abnormal reduction and quenching were discussed in detail.
RESUMO
The decay kinetics of Gd3Al2Ga3O12:Ce3+ single crystal luminescence were studied under dense laser excitation. It was shown that the decay times as well as the intensity of Ce3+ luminescence depend on the excitation density. The observed effects were ascribed to the interaction between excitons as well as to the features of energy transfer from the excitons to Ce3+. The numerical simulation of the experimental results was performed for justification of the proposed model.
RESUMO
A new series of Sr-based phosphates, Sr9-xMnxEu(PO4)7, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as strontiowhitlockite, which is a ß-Ca3(PO4)2 (or ß-TCP) structure. The concentration of Mn2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu3+ to Eu2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d-4f Eu2+ and 4f-4f Eu3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn2+ concentration due to quenching effect of Eu3+ by the Mn2+ levels. The temperature dependence of Eu2+ photoluminescence in a Sr9-xMnxEu(PO4)7 host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.
RESUMO
Na3.6Lu1.8-x(PO4)3:xEu3+ phosphors were synthesized by a high-temperature solid-state reaction. A powder X-ray diffraction study revealed that homogeneous solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.7. The Na3.6Lu1.8(PO4)3 structure was refined from the powder X-ray diffraction data and the cation distribution in the lattice sites of the NASICON-type structure was revealed. The refinement indicates structural disorder caused by the displacement of a part of Lu cations along the c axis inside the (Lu/Na)O6 octahedra that is confirmed by the broadened emission lines of Eu3+, which substitutes Lu cations. The highest Eu3+ luminescence intensity is found in Na3.6Lu1.8-x(PO4)3:xEu3+ for x = 0.5, whereas a further increase of the Eu3+ content leads to concentration quenching that is shown to occur due to the dipole-dipole interaction. An enhanced temperature stability of the Eu3+ emission was observed at the excitation energy of 3.23 eV. At this excitation energy, thermal quenching of the emission caused by the 7F0 â 5L7 transitions is compensated by the intensity increase of the emission related to the 7F1 â 5GJ transitions, which occurs due to the increase of the 7F1 level population, induced by a temperature rise.
