Luminescence characteristics of magnesium aluminate spinel crystals of different stoichiometry

We are grateful to Drs E. Vasil’chenko and A. Maaroos for the help with experiments and useful discussions. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. In addition, the research leading to these results has received funding from the Estonian Research Council Institutional Research Funding IUT02-26.Magnesium aluminate spinel single crystals with different stoichiometry, MgAl2O4 (1:1 spinel) and MgO 2.5Al2O3 (1:2.5) were investigated using different optical methods (cathode-, photo- and thermally stimulated luminescence (TSL), optical absorption, "creation spectra" of TSL peaks and phosphorescence by VUV radiation). Low-temperature charge carrier traps and the position of intrinsic UV emission bands depend on the degree of stoichiometry. Antisite defects (ADs), Mg2+ or Al3+ located in a "wrong" cation site (Mg|A1 or Al|Mg) are the main as-grown structural defects, which serve also as efficient traps for electrons and holes as well as seeds for bound excitons. AD concentration is especially high in 1:2.5 spinel. There are several manifestations of ADs (electronic excitations near ADs) in the spectral region of 7-7.5 eV, slightly below the energy gap.Eesti Teadusagentuur IUT02-26; H2020 Euratom 633053; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Structural defect creation by swift heavy ions in CaF2 single crystals

Calcium fluoride single crystals belong to the most popular optical materials due to their transparency in a wide spectral region..

Low-temperature creation of Frenkel defects via hot electron-hole recombination in highly pure NaCl single crystals

The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value EFD, which is sufficient for the creation of Frenkel defects at low temperature

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

Different mechanisms of radiation damage of wide-gap metal oxides as well as a dual influence of impurity ions on the efficiency of radiation damage have been considered on the example of binary ionic MgO and complex ionic–covalent $Lu_{3}Al_{5}O_{12}$ single crystals. Particular emphasis has been placed on irradiation with $\sim$2 GeV heavy ions ($^{197}Au, ^{209}Bi, ^{238}U$, fluence of 10$^{12}$ ions/cm$^{2}$) providing extremely high density of electronic excitations within ion tracks. Besides knock-out mechanism for Frenkel pair formation, the additional mechanism through the collapse of mobile discrete breathers at certain lattice places (e.g., complex impurity centres) leads to the creation of complex defects that involve a large number of host atoms. The experimental manifestations of the radiation creation of intrinsic and impurity antisite defects (Lu|$_{Al}$ or Ce|$_{Al}$ – a heavy ion in a wrong cation site) have been detected in LuAG and $LuAG:Ce^{3+}$ single crystals. Light doping of LuAG causes a small enhancement of radiation resistance, while pair impurity centres (for instance, $Ce|_{Lu}–Ce|_{Al}$ or $Cr^{3+}–Cr^{3+}$ in MgO) are formed with a rise of impurity concentration. These complex impurity centres as well as radiation-induced intrinsic antisite defects (Lu|$_{Al}$ strongly interacting with Lu in a regular site) tentatively serve as the places for breathers collapse, thus decreasing the material resistance against dense irradiation

Excitation of different chromium centres by synchrotron radiation in MgO:Cr single crystals

The excitation spectra for the emissions of chromium-containing centres have been measured at 10 K using synchrotron radiation of 4–32 eV in MgO single crystals with different content of Cr$^{3+}$ (5–850 ppm) and Ca$^{2+}$ impurity ions. Both virgin crystals and the samples preliminarily irradiated with x-rays at 295 K have been studied. The role of complex chromium centres containing two Cr$^{3+}$ and a cation vacancy (sometimes nearby a Ca$^{2+}$ ion) on the luminescence processes and the transformation/creation of structural defects has been analysed. Such anharmonic complex centres could serve as the seeds for the creation of 3D defects that facilitate the cracking and brittle destruction of MgO crystals under their irradiation with ∼GeV heavy ions providing extremely high excitation density within cylindrical ion tracks