Electron Paramagnetic Resonance and Optical Studies of Thermoluminescence Processes in Mn-Doped YAlO3 Single Crystals

Hanka Przybylińska,Andrzej Suchocki,Vasyl Stasiv,Yaroslav Zhydachevskyy,Agnieszka Wołoś,Sergii Ubizskii,Agata Kaminska,Marek Berkowski,Aneta Grochot,Michał Glowacki

doi:10.1021/acs.jpcc.1c08997

Hanka Przybylińska, Andrzej Suchocki + Show 8 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c08997

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Abstract

A detailed electron paramagnetic resonance (EPR), optical absorption, luminescence, and thermoluminescence (TL) study of Mn-doped YAlO3 (YAP) single crystals was performed. The crystals were grown by the Czochralski method from stoichiometric (Y/Al = 1) and yttrium-rich (Y/Al = 1.04) melts and codoped with either Si or Hf ions. The EPR measurements revealed the presence of only one type of Mn2+ center, that is, isolated Mn ions occupying Y sites (MnY2+). It was found that only in yttrium-rich crystals, the MnY2+ ions undergo recharging to MnY3+ under ionizing irradiation, indicating that this process requires the availability of sufficiently deep electron traps. The initial charge state is fully restored only after subsequent warming above 600 K. The presented results demonstrate, moreover, that MnY3+ + e → MnY2+ recombination is not the most efficient excitation channel of the green 4T1 → 6A1 emission of MnY2+, possibly because of the huge energy difference between the recombination (>5.39 eV) and excitation (3 eV) energies. In contrast, energy transfer to MnY2+ proves to be dominant. A general model of trapping and recombination mechanisms responsible for TL of YAP:Mn crystals above room temperature is proposed. Besides MnY2+ ions and the defect-related electron and hole traps intrinsic to the YAP lattice, the model includes also unintentional dopants such as FeAl2+ acting as deep hole traps, as well as MnAl4+ and CrAl3+ ions acting both as deep hole and electron traps.

Highlights

Yttrium orthoaluminate (YAlO3), known as yttrium aluminum perovskite (YAP), is widely known as a host material for solid-state lasers and scintillators
Complex electron paramagnetic resonance (EPR), TL, photoluminescence, and absorption studies of nonstoichiometric YAP:Mn crystals codoped with Si and Hf, as well as stoichiometric YAP:Mn,Hf, allow us to propose a model of MnY2+(MnAl4+) TL processes consistent with all obtained results
We found in EPR that the energy level of MnY2+ must lie deeper than 5.4 eV below the YAP conduction band (CB) minimum, that is, less than 2.21 eV above the valence band (VB) maximum, since it is not ionized even under 230 nm excitation

Summary

Introduction

Yttrium orthoaluminate (YAlO3), known as yttrium aluminum perovskite (YAP), is widely known as a host material for solid-state lasers and scintillators. Manganese ions in YAP:Mn crystals can occur in different sites and charge states: as Mn2+ ions in strongly distorted dodecahedral coordination (Y3+ sites) or as Mn4+ ions in octahedral coordination (Al3+ sites).[1,5−8] The Mn4+ ions (3d3 electronic configuration), absorbing in the visible and UV regions, are sensitive to light exposure Such exposure, for example, with blue-green light gives an efficient photochromic effect manifesting itself in a blue-gray coloration of the crystal.[1,9,10] This coloration is caused by the absorption of MnAl5+ ions created as a result of the Mn4+ photoionization (MnAl4+ → MnAl5+ + e) process.[6,10,11] Photoionization under exposure to blue-green light takes place as a two-photon process.[11,12] The photoinduced absorption of MnAl5+ ions remains stable at room temperature and can be removed completely by the subsequent warming of the material at temperatures T ≥ 500 K.10. For example, with blue-green light gives an efficient photochromic effect manifesting itself in a blue-gray coloration of the crystal.[1,9,10] This coloration is caused by the absorption of MnAl5+ ions created as a result of the Mn4+ photoionization (MnAl4+ → MnAl5+ + e) process.[6,10,11] Photoionization under exposure to blue-green light takes place as a two-photon process.[11,12] The photoinduced absorption of MnAl5+ ions remains stable at room temperature and can be removed completely by the subsequent warming of the material at temperatures T ≥ 500 K.10 During warming, the characteristic red emission of MnAl4+ions near 710 nm is observed in thermal glow peaks at 400, 450, and 570 K when heated at a rate of 0.4 K/s10,13 Besides the photoinduced ones, metastable MnAl5+ ions have been observed in as-grown crystals,[11,13] especially those codoped with Ca2+ ions or crystals grown in a reducing atmosphere and subsequently annealed in air.[11]

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