Luminescence emission from metastable Sm²⁺defects in Y PO₄:Ce,Sm

Abstract

Ce(3+) and Sm(3+) both form stable defect centres in Y PO(4), and their emission properties are well known. However, by irradiating co-doped Y PO(4):Ce,Sm with x-rays or UV light the charge states of the defects can be modified to become Ce(4+) and Sm(2+), which are metastable, and their behaviour acts as a model system for understanding carrier dynamics in charge storage phosphors. Here we report on the luminescence emission behaviour of the Sm(2+) defects that can be observed after x-irradiation. Under suitable excitation conditions, emission from both the stable Sm(3+) and metastable Sm(2+) can be monitored simultaneously. The Sm(2+) luminescence is found to be comprised of a series of narrow lines in the energy range 1.5-1.8 eV, identifiable as internal 4f-4f transitions, accompanied by a series of phonon replicas (phonon energy, 20.4 meV). The intensity of the metastable Sm(2+) emission increases in proportion to the x-irradiation time as their population is increased; under 1.92 eV laser stimulation, the PL is found to fully quench at temperatures above 150 K due to photo-thermal ionization of the defect.

Probing metastable Sm2+ and optically stimulated tunnelling emission in YPO4: Ce, Sm

“…1(a) shows the 4 G 5/2 → 6 H 5/2 ,7/2, 9/2, 11/2 transitions of Sm 3+ , excited in a non-dosed sample under 3.06 eV laser excitation; the separate spectral emission features of Sm 2+ and Sm 3+ are clearly distinguished. These results are in good agreement with those of Poolton et al (2012) confirming the reliability of our measurements.…”

“…The resolution of the spectral features obtained here allow for the first time a clear identification of the excited levels of Sm 2+ in YPO 4 host. These excitation peaks arise from internal 4f-4f transitions of Sm 2+ and, in combination with the previous work regarding analysing the emission lines (Poolton et al, 2012), we tentatively assign 7 F 0 → 5 D 0,1,2,3 at 1.837, 1.879, 1.898 and 1.977 eV respectively, and 7 F 1 → 5 D 0 at 1.785 eV. Low intensity phonon replicas (~20 meV) associated with each electronic transition are also observed.…”

“…The extensive knowledge accumulated over the last four to five decades in identifying the behaviour and energy levels of lanthanide dopants within the band gap of a host material gives us a clear idea about whether they will act as an electron or a hole trap on exposure to ionizing radiation (for summary, see Dorenbos et al, 2003Dorenbos et al, , 2011. In this respect, YPO 4 co-doped with Sm 3+ and Ce 3+ ions shows excellent promise as a model system for fundamental studies: on irradiation with X-rays, these defects form the metastable states Sm 2+ and Ce 4+ whose energy levels can be precisely characterised for controlled charge transfer (Dorenbos et al, 2003, Poolton et al, 2012.…”

“…Previous work to understand the luminescence charge transfer properties of YPO 4 :Ce 3+ , Sm 3+ have used synchrotron radiation (4-20 eV) and both optical and thermoluminescence spectroscopy (Pieterson et al, 2001;Poolton et al, 2010Poolton et al, , 2012Dorenbos et al, 2011;Mandowski and Bos, 2010;Bos et al, 2010). These studies have also allowed determination of the electron and hole trap energies within the band-gap.…”

“…As this involves excited-state tunnelling from Sm 2+ to Ce 4+ , the transitions were significantly broadened due to interference with 4f 5d (SmA, SmB) bands (which are the most effective conduit for the tunnelling), and only limited spectroscopic information was obtainable. By using a low flux, 1.92 eV laser stimulation at 10 K, Poolton et al (2012) were able to exclusively probe isolated Sm 2+ emission 4f-4f transition, and thereby determine the ground state energy levels in more detail. In the same study a 2.33 eV laser probe for the emission at 10 K confirmed the simultaneous presence of both stable Sm 3+ ions and metastable Sm 2+ ions in the YPO 4 host.…”