Electron and hole trapping in Eu- or Eu,Hf-doped LuPO4 and YPO4 tracked by EPR and TSL spectroscopy

Abstract

EPR spectroscopy in X- and Q-bands was employed to trace charge carrier trapping upon exposure to X-rays of LuPO₄:Eu, LuPO₄:Eu,Hf and YPO₄:Eu,Hf flux-grown single crystals, as well as LuPO₄:Eu sintered ceramics. These data were complemented with thermoluminescence (TL) studies on the same compositions. EPR spectra proved that all the single crystals contained Pb impurity incorporated from the flux. Upon exposure to X-rays the concentration of Pb³⁺ changed proving that Pb participated in carrier trapping. The ionizing radiation caused also partial conversion of Eu³⁺ into Eu²⁺ and in the (Eu,Hf) doubly doped crystals also Hf⁴⁺ was transformed into Hf³⁺ upon exposure to X-rays. In the LuPO₄:Eu sintered ceramics X-rays transformed Eu³⁺ into Eu²⁺ and the strong EPR signal of a hole trapped at O²⁻ in the vicinity of a defect (presumably Lu³⁺-vacancy) was recorded. Some contribution from the PO₄²⁻ molecular ion could also be observed. However, in crystals the EPR signal from the latter was much more intense compared to the former. In ceramics, the trapped hole was found to hop between the O²⁻ ions and only below ∼70 K it localized at just one of them. Complementary measurements of TL of the samples showed meaningful differences between the glow curves of the crystals and of the ceramics. The TL glow curve of the latter contained only one band peaking around 120 °C which is related to destruction of the trapped hole being mainly the O⁻ center. All single crystals showed multi-peak TL glow curves covering the range of 30–500 °C. They were found to result mainly from a hole released from the PO₄²⁻ molecular complex and to some extent also from the O⁻ center. The results relative to EPR and TL spectroscopy proved that in orthophosphates Eu³⁺ is an effective electron trapping center while holes may be trapped at impurity ions, like Pb²⁺, or localized at oxygen ions possibly stabilized by nearby defects, e.g. Lu-vacancies.