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CC BY-NC-ND ; Novel Er3 + and Yb3 + doped transparent oxyfluoride glass ceramics containing NaLaF4 nanocrystals were prepared from oxyfluoride glasses with general composition of Na2O-NaF-LaF3-Al2O3-SiO2. The crystallization processes, changes in morphology and upconversion luminescence were analyzed. The heat treatment of the precursor glasses promoted the formation of hexagonal NaLaF4 and an unidentified fluorite type compound. An enhancement of upconversion luminescence with the introduction of YbF3 in the glass ceramics was detected and indicated an efficient energy transfer from Yb3 + to Er3 +. The local environment of Er3 + in hexagonal NaLaF4 and the fluorite phase was analyzed using site-selective spectroscopy and compared with polycrystalline ceramics. The incorporation of Er3 + ions in three non-equivalent cationic sites in the crystalline lattice of hexagonal NaLaF4 nanocrystals was observed. The local environment of erbium ions in the fluorite phase was found to be similar to α-NaYF4 suggesting the formation of Er3 + and Yb3 + stabilized α-NaLaF4. --- / / / --- This is the pre-print version of the following article: G. Krieke, A. Sarakovskis, M. Springis, Upconversion luminescence of Er3 +/Yb3 + and their role in the stabilization of cubic NaLaF4 nanocrystals in transparent oxyfluoride glass ceramics, Journal of Non-Crystalline Solids (2018), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0022309317306075?via%3Dihub. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. CC BY-NC-ND ; IMIS2; University of Latvia Foundation; 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 CAMART². The authors wish to express gratitude to K. Smits for TEM measurements. This work was supported by National Research Program IMIS2 and Arnis Riekstins "MikroTik" donation. Donations are administered by the University of Latvia Foundation.

CC BY-NC-ND ; Novel Er3+ doped glass ceramics containing Ba4Lu3F17 nanocrystals were successfully prepared by heat treatment of melt-quenched glasses. Highly efficient upconversion luminescence was detected under near-infrared excitation and it was at least two orders of magnitude higher than that in the precursor glass. The activator content in the crystalline phase was estimated using polycrystalline Ba4Lu3F17 with different Er3+ content as the reference and it was found to be higher than in similar glass ceramics. The structure and thermal analysis of the glass ceramics revealed the formation of cubic and rhombohedrally distorted cubic modifications of Ba4Lu3F17. Phase transition between the two modifications was studied using site selective spectroscopy of Er3+ as a structural probe. Unusually efficient incorporation of Er3+ ions in the Ba4Lu3F17 nanocrystals combined with the low phonon energy of the fluoride crystals make this material a desirable host for upconversion luminescence. --- / / / --- This is the preprint version of the following article: G. Krieke, A. Sarakovskis, M. Springis, Cubic and rhombohedral Ba4Lu3F17:Er3+ in transparent glass ceramics: Crystallization and upconversion luminescence 200 (2018), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0022231317322056. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. CC BY-NC-ND ; National Research Program IMIS2; University of Latvia Foundation; 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 CAMART². The authors wish to express gratitude to R. Ignatans for Rietveld refinement. This work was supported by National Research Program IMIS2 and Arnis Riekstins "MikroTik" donation. Donations are administered by the University of Latvia Foundation.

In this study, novel transparent Er3+ doped glass ceramics were prepared from melt-quenched oxyfluoride glasses with general composition of Na2O-NaF-BaF2-YbF3-Al2O3-SiO2. The crystallization of fluorite (BaF2, BaF2-YbF3, NaF-BaF2-YbF3 and Na0.5-xYb0.5+xF2+2x) and distorted fluorite (rhombohedral Ba4Yb3F17 and tetragonal NaF-BaF2-YbF3) phases was analysed in glass ceramics with different BaF2 and YbF3 ratio. The phase composition and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Intense red upconversion luminescence (UCL) was detected under near-infrared excitation resulting from three photon upconversion followed by cross-relaxation between Er3+ and Yb3+ ions. The local environment of Er3+ ions in fluorite and distorted phases was analysed using site-selective spectroscopy. The Er3+ ions were found to act as nucleation centres in the glass ceramics containing BaF2. The phase transition from metastable fluorite to rhombohedrally and tetragonally distorted fluorite phases was detected using Er3+ ions as a probe. --- / / / --- This is the peer reviewed version of the following article: G. Krieke, A. Sarakovskis, M. Springis; Ordering of fluorite-type phases in erbium-doped oxyfluoride glass ceramics; Journal of the European Ceramic Society, 2018 38 (1), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0955221917305769. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. ; This work was supported by National Research Program IMIS2 and Arnis Riekstins "MikroTik" donation. Donations are administered by the University of Latvia Foundation; 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 CAMART²

CC BY-NC-ND ; Gadolinium doped SrF2 nanoparticles have been synthesized by the precipitation method and characterized by site-selective optical spectroscopy and electron paramagnetic resonance (EPR) spectroscopy methods at cryogenic temperatures. The type of Gd3+ centres present in SrF2 is strongly dependent on the heat treatment parameters – the chosen temperature and gas composition during the annealing. Gd3+ ions in SrF2 annealed at temperatures up to 400 °C are mainly incorporated into sites with cubic symmetry. Higher annealing temperatures enable charge compensators to locate in a closer vicinity of the Gd3+ ion lowering the site symmetry. Oxygen compensated Gd3+ centres form after heating in air atmosphere, whereas for samples heated at an identical temperature in inert atmosphere interstitial fluorine ions serve as charge compensators. A comparison with previous studies of transparent glass ceramics containing SrF2:Gd3+ shows similar patterns in gadolinium centre formation with the precipitated nanoparticles annealed in inert atmosphere here. The obtained results give a fundamental insight in trivalent defect formation mechanisms in nanostructured SrF2 systems. ---/ / / --- This is the preprint version of the following article: A. Antuzevics, M. Kemere, G. Krieke, Multisite formation in gadolinium doped SrF2 nanoparticles, Journal of Alloys and Compounds 762 (2018), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0925838818320164. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. CC BY-NC-ND ; Authors thank prof. Uldis Rogulis for valuable discussions and suggestions and Dr. Krisjanis Smits for TEM measurements. Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/2 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. 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 CAMART²

Novel Er3+ doped transparent glass ceramics containing hexagonal Na(Gd,Lu)F4 nanocrystals were prepared from oxyfluoride glasses. The distribution of rare earth ions in the crystalline and glassy phase has been analyzed by X-ray diffraction and erbium luminescence decay kinetics measurement. A strong deviation of rare earth ion content in fluoride nanocrystals in comparison to the base glass has been observed. Preferential incorporation of Gd3+ over Lu3+ ions in the fluoride lattice leads to the stabilization of hexagonal Na(Gd,Lu)F4 structure and prevents the formation of cubic fluorite type solid solutions. A considerable enhancement of upconversion luminescence correlates with the formation of hexagonal solid solutions and is attributed to an efficient energy transfer between Er3+ ions. --//-- This is the accepted version of the following article: G. Krieke, A. Sarakovskis, M. Springis, Upconversion luminescence of a transparent glass ceramics with hexagonal Na(Gd,Lu)F4 nanocrystals, Journal of Alloys and Compounds (2016), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0925838816332820?via%3Dihub. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. ; This work was supported by National Research Program IMIS2. G.K. gratefully acknowledges the financial support from the Foundation of University of Latvia – Arnis Riekstins "MikroTik" donation. 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 CAMART²

The authors are grateful to Dr. Andris Fedotovs for photography of the samples. This research is funded by the Latvian Council of Science, project "Novel transparent nanocomposite oxyfluoride materials for optical applications", project No. LZP-2018/1-0335. ; Assessment of activator distribution in lanthanide doped nanocomposites is an important and challenging task. Oxyfluoride glass ceramics have been chosen as a model system to characterize incorporation efficiency of Eu3+ ions in CaF2 nanocrystals using a combination of X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques. Lattice constant changes induced by size mismatch of Eu3+ and Ca2+ ions have been used to evaluate Eu3+ content in CaF2. The distortion of CaF2 lattice due to incorporation of europium ions can be detected from EPR investigations via Gd3+ paramagnetic probe spectra. It is shown that ratio of different symmetry CaF2:Gd3+ centres and linewidth of EPR transitions can be used to monitor europium content in the crystalline phase of glass ceramics. --- / / / --- This is the preprint version of the following article: A. Antuzevics, G. Krieke, E. Pavlovska, U. Rogulis, Eu3+ ion distribution in oxyfluoride glass nanocomposites, Journal of Non-Crystalline Solids, 522, 119548 (2019), DOI https://doi.org/10.1016/j.jnoncrysol.2019.119548, which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0022309319304193. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. This work is licensed under a CC BY-NC-ND 4.0 license. ; Latvian Council of Science LZP-2018/1-0335; 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 CAMART².

The authors wish to express gratitude to K. Smits for TEM measurements. This research is funded by the Latvian Council of Science, project "Novel transparent nanocomposite oxyfluoride materials for optical applications", project No. LZP-2018/1-0335. GK wishes to expresses gratitude to Arnis Riekstins "MikroTik" donation. Donations are administered by the University of Latvia Foundation. © 2019. This work is licensed under a CC BY-NC-ND 4.0 license. ; Transparent oxyfluoride glass ceramics containing hexagonal NaErF4 nanocrystals were synthesized by a heat treatment of a precursor glass obtained by the melt quenching technique. Combined X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis revealed the formation of single phase nanocrystals in the glass ceramics. The enhancement of intensity and spectral change of upconversion luminescence (UCL) confirmed the presence of Er3+ ions in the crystalline phase of the glass ceramics. Dominant energy transfer processes were identified using the rate equation formalism. Time-resolved site-selective spectroscopy studies at low temperatures were employed to elucidate the local structure of Er3+ ions in the glass ceramics and microcrystalline NaErF4. The origin of multi-site formation in hexagonal NaErF4 lattice is discussed. --- / / / --- This is the preprint of the following article: Guna Krieke; Andris Antuzevics, Maris Springis, Uldis Rogulis, Upconversion luminescence in transparent oxyfluoride glass ceramics containing hexagonal NaErF4, Journal of Alloys and Compounds Volume 798 (2019), which has been published at https://www.sciencedirect.com/science/article/abs/pii/S0925346717304731. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. © 2019. This work is licensed under a CC BY-NC-ND 4.0 license. ; Latvian Council of Science LZP-2018/1-0335; University of Latvia Foundation; 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 CAMART².

This is the peer reviewed version of the following article: G. Krieke, A. Sarakovskis, R. Ignatans, J. Gabrusenoks "Phase transitions and upconversion luminescence in oxyfluoride glass ceramics containing Ba4Gd3F17 nanocrystals", Journal of the European Ceramic Society, 2017, 37 (4), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0955221916306768 This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. ; Novel transparent Er3+ doped oxyfluoride glass-ceramics containing Ba4Gd3F17 nanocrystals were prepared by melt quenching followed by heat treatment of as-prepared glasses. The phase composition and microstructure were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Intense upconversion luminescence (UCL) was detected. Longer characteristic decay times and splitting of the luminescence bands compared to the precursor glass indicated the incorporation of erbium ions in the crystalline phase. The spectroscopic properties of glass ceramics were compared with single phase cubic and rhombohedral Ba4Gd3F17 ceramics. The unit cell parameters and atomic positions in the rhombohedral phase were calculated using Rietveld refinement. The local environment of Er3+ and the phonon energy of both polymorphs were analyzed using luminescence and Raman spectroscopy. In the glass ceramics, a phase transition from distorted metastable fluorite to ordered rhombohedral Ba4Gd3F17 was observed and resulted in the enhancement of the efficiency of UCL. --- /// --- This is the peer reviewed version of the following article: G. Krieke, A. Sarakovskis, R. Ignatans, J. Gabrusenoks "Phase transitions and upconversion luminescence in oxyfluoride glass ceramics containing Ba4Gd3F17 nanocrystals", Journal of the European Ceramic Society, 2017, 37 (4), which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S0955221916306768 This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. ; This work was supported by National Research Program IMIS2. This research is being implemented thanks to Arnis Riekstins "MikroTik" donation. Donations are administered by the University of Latvia Foundation. 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 CAMART²

Different compositions of europium doped aluminosilicate oxyfluoride glass ceramics prepared in air atmosphere have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy methods. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show presence of homogenously distributed SrF2 nanocrystals after the heat treatment of the precursor glass. Efficient Eu3+ incorporation in the high symmetry environment of glass ceramics is observed from the photoluminescence spectra. EPR spectra indicate Eu3+ → Eu2+ reduction upon precipitation of crystalline phases in the glass matrix. For composition abundant with Eu2+ in the glassy state such behaviour is not detected. Local structure around europium ions is discussed based on differences in chemical compositions. --- / / / --- This is the preprint of the following article: A. Antuzevics, M. Kemere, G. Krieke, R. Ignatans, Electron paramagnetic resonance and photoluminescence investigation of europium local structure in oxyfluoride glass ceramics containing SrF2 nanocrystals, Optical Materials 72 (2017), which has been published at https://www.sciencedirect.com/science/article/abs/pii/S0925346717304731. This article may be used for non-commercial purposes in accordance with Elsevier Terms and Conditions for Sharing and Self-Archiving. ; Authors thank prof. U. Rogulis for valuable discussions and suggestions and Dr. Krisjanis Smits for TEM measurements. Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/3 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. 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 CAMART²

This research is funded by the Latvian Council of Science, project "Novel transparent nanocomposite oxyfluoride materials for optical applications", project No. LZP-2018/1-0335. 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 CAMART2. ; LaOCl doped with 0–10 mol% Cr was synthesized by thermal decomposition of chlorides. X-ray diffraction (XRD) analysis revealed that incorporation of chromium results in a decrease of the lattice parameter a and a simultaneous increase of the lattice parameter c. The local structure of chromium ions was studied with X-ray photoelectron (XPS), X-ray absorption (XANES), multifrequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy techniques. It was determined that synthesis in oxidizing atmosphere promotes the incorporation of chromium ions predominantly in the 5+ oxidation state. Changes of chromium oxidation state and local environment occur after a subsequent treatment in reducing atmosphere. Spin-Hamiltonian (SH) parameters for a Cr5+ and two types of Cr3+ centers in LaOCl were determined from the EPR spectra simulations. ; Latvian Council of Science,project No. LZP-2018/1-0335; 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 CAMART2.

This study was supported by a grant from Latvian Council of Science (agreement No. LZP-2018/1-0214). ; Original results on the EPR and photoluminescence Cr3+ ions in natural magnesium aluminum spinel (MgAl2O4) are presented. The photoluminescence spectra of Cr3+ ions in natural MgAl2O4 have been measured before and after irradiated by fast neutrons. --- / / / --- This is the preprint version of the following article: N. Mironova-Ulmane, A. I. Popov, G. Krieke, A. Antuzevics, V. Skvortsova, E. Elsts, and A. Sarakovskis, Low-temperature studies of Cr3+ ions in natural and neutron-irradiated Mg-Al spinel, Low Temperature Physics,46, 1154 (2020), DOI https://doi.org/10.1063/10.0002467, which has been published in final form at https://aip.scitation.org/doi/10.1063/10.0002467. This article may be used for non-commercial purposes in accordance with American Institute of Physics terms and conditions for Sharing and Self-Archiving. ; Latvian Council of Science (agreement No. LZP-2018/1-0214); 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 CAMART².

This work was supported by the ERDF , European-Union Project No. 1.1.1.1/16/A/182 . ; This study presents a novel approach to produce phosphorescent coatings on metal surfaces. Strontium aluminates are the most popular modern phosphorescent materials exhibiting long afterglow at room temperature and a broad spectral distribution of luminescence in the visible range. However, despite a large amount of research done, methods for synthesis of such materials remain relatively energy inefficient and environmentally unfriendly. A long-afterglow luminescent coating containing SrAl2O4:Eu2+, Dy3+ is prepared by the plasma electrolytic oxidation on the surface of commercial aluminum alloy Al6082. During the electrical discharges in this process, the strontium aluminate is formed in a similar way to the solid-state reaction method. X-ray powder diffraction analysis confirms that the monoclinic SrAl2O4 phase is present in the coating. Optical properties of the obtained coating were analyzed with luminescence methods classically used for studies of luminophores. The performance of the coating was compared with commercially available strontium aluminate powder. The proposed method of coating synthesis may be of value for the development of energy-efficient and long-lasting automotive and public safety infrastructure. ; European Regional Development Fund 1.1.1.1/16/A/182; 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 CAMART²