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The results of X-ray diffraction studies of the Gd(Mg0.95−x,ZnxMn−0.05)B5O10 down-converting phosphor as a function of Mg–Zn composition are presented. The lattice parameters and unit cell volumes of GdMg0.95−xZnxMn0.05B5O10 pentaborates are examined. The relationships between the structure and optical properties of these materials are explicated based on the results of theoretical calculations of the energy structure. The effect of pressure on the luminescence of Mn2+ in this system was studied up to ca. 32 GPa. The observed quenching of Mn2+ luminescence is due to the crossing of the emitting 4T1g level with the non-emitting 2T2g state. This crossing sets a long-wavelength limit on the possibility of observing the emission of Mn2+ ions, which is around 850 nm. [ABSTRACT FROM AUTHOR]

Detailed investigations of the spectroscopic properties of Cr3+ ions in β-Ga2O3:0.05% Cr3+ single crystals grown by the floating zone technique have been performed in the temperature range 4.5–550 K. The luminescence of Cr3+ ions at low temperatures is due to narrow R-lines (2E → 4A2 transitions) and their vibronic sidebands, whereas the broad intense band (4T2 → 4A2 transition) dominates at temperatures above 200 K. The vibronic sidebands of R-lines with amplitude of less than 5% of the R1-line at a temperature of 4.5 K cover the region of 700–735 nm. The temperature dependences of the luminescence intensity and the decay time of Cr3+ ions indicate the same mechanism of quenching of R-line intensity and shortening of lifetime for 2E of Cr3+ ions. The temperature dependence of the R1-line decay time of β-Ga2O3:Cr3+ with maximal temperature coefficient |Δτ/ΔT| = 0.023 ms K−1 at 120 K and maximal specific sensivitity |(Δτ/ΔT)τ−1| = 0.017 K−1 at 160 K indicates an application potential of this phosphor for low-temperature fluorescence thermometry. [ABSTRACT FROM AUTHOR]

The MgGa2O4 ceramics co-doped with Mn2+ and Eu3+ ions were synthesized via a high-temperature solid-state reaction technique. The samples with various Eu3+ concentrations were characterised using high-resolution photoluminescence (PL) spectroscopy. The PL spectra show weak matrix emission in a blue spectral region with dominant excitation band around 380 nm. Manganese ions are highly excited deeply in UV region and exhibit emission band peaked at 502 nm. The Eu3+ ions show characteristic f-f excitation and emission lines. The energy transfer between host defects and activator ions was observed. Luminescence decay curves of Mn2+ and Eu3+ emission showed complex kinetics with both Eu3+-ion concentration and excitation wavelength changes. [ABSTRACT FROM AUTHOR]

Detailed investigations of the spectroscopic properties of Cr3+ ions in β-Ga2O3:0.05% Cr3+ single crystals grown by the floating zone technique have been performed in the temperature range 4.5–550 K. The luminescence of Cr3+ ions at low temperatures is due to narrow R-lines (2E → 4A2 transitions) and their vibronic sidebands, whereas the broad intense band (4T2 → 4A2 transition) dominates at temperatures above 200 K. The vibronic sidebands of R-lines with amplitude of less than 5% of the R1-line at a temperature of 4.5 K cover the region of 700–735 nm. The temperature dependences of the luminescence intensity and the decay time of Cr3+ ions indicate the same mechanism of quenching of R-line intensity and shortening of lifetime for 2E of Cr3+ ions. The temperature dependence of the R1-line decay time of β-Ga2O3:Cr3+ with maximal temperature coefficient |Δτ/ΔT| = 0.023 ms K−1 at 120 K and maximal specific sensivitity |(Δτ/ΔT)τ−1| = 0.017 K−1 at 160 K indicates an application potential of this phosphor for low-temperature fluorescence thermometry. [ABSTRACT FROM AUTHOR]

The appearance of the ultraviolet Bi3+-related emission band in the thermally stimulated luminescence (TSL) spectrum is observed around 465 K after selective irradiation of the YAlO 3 :Bi perovskite in the Bi3+-related absorption bands. The excitation spectrum of the TSL glow curve peak at 465 K, activation energies of its creation by photons of different energies, and the dependence of the TSL peak intensity on the irradiation duration are measured. The origin of the optically created electron centers and the mechanisms of photostimulated creation of the electron and hole centers under irradiation in the Bi3+-related absorption bands of YAlO 3 :Bi are discussed. The TSL glow curve peak at 465 K is suggested to appear as a result of electrons release from the electron centers intrinsic to the YAlO 3 lattice and their recombination with the hole Bi4+ centers. The same processes are shown to take place in the X-ray-irradiated YAlO 3 :Bi perovskite. The obtained results are important for possible applications of the investigated material in thermoluminescent dosimetry. • YAlO 3 :Bi3+ demonstrates UV thermally stimulated luminescence (TSL) with a main peak at around 465 K. • The TSL is created by the UV irradiation at about 6.1 eV. • X-ray irradiation produces the same TSL. • TSL appears as a result of electrons release from the electron traps intrinsic to YAlO 3 and their recombination with Bi4+ centers. [ABSTRACT FROM AUTHOR]

Characteristics of the intrinsic and Bi3+-related luminescence of YNbO 4 :Bi powders with different Bi contents are studied at 4.2–500 K by steady-state and time-resolved spectroscopy methods. The emission of YNbO 4 :Bi is suggested to be of an exciton origin. The main Bi3+-related emission band (2.53 eV) excited at 4.09 eV is ascribed to the radiative decay of an exciton localized around a single Bi3+ ion. A weak lower-energy (2.41 eV) emission is ascribed to an exciton localized around a dimer Bi3+ center. No emission arising from the 3P 1,0 → 1S 0 transitions of a Bi3+ ion is found. The structure and parameters of the exciton states, responsible for the luminescence of YNbO 4 :Bi, as well as the radiative and nonradiative processes ongoing in these states are clarified. From the dependence of the afterglow intensity on the irradiation energy, the band gap energy of YNbO 4 is estimated to be 5.3–5.4 eV. [ABSTRACT FROM AUTHOR]

The paper contains detailed crystal structure and photoluminescence studies of the Bi3+ and Bi3+-Eu3+ co-doped Gd 3 Ga 5 O 12 (GGG) garnet. The studied materials have been synthesized in the form of microcrystalline powders by the common solid-state reaction method. Precise crystal structure parameters were derived from the X-ray powder diffraction data. Photoluminescence studies were done by the steady-state and time-resolved techniques in the 4.2–400 K temperature range. The studied materials show no UV luminescence caused by electronic transitions of a Bi3+ ion. The observed luminescence in visible is interpreted by excitons localized around Bi3+ ions. In particular, two emission bands located at 2.55 eV (486 nm) and 2.05 eV (605 nm) at 4.2 K were revealed. The main emission band at 2.55 eV is attributed to the triplet relaxed state of excitons localized around single (regular) Bi3+ ions, whereas the weaker band at 2.05 eV is related to some associates of a single Bi3+ ion with a crystal lattice defect. The performed studies for Bi3+-Eu3+ co-doped samples indicate that the energy transfer from Bi3+ to Eu3+ is radiative and is caused by reabsorption of the Bi-related exciton emission by Eu3+ ions. The studied materials were also characterized as a single-phase converting phosphor for near-UV excited white LEDs. The possibility of the emission color tuning from blue to orange-red at UV excitation by the way of changing of Bi3+ and Eu3+ concentrations was shown. [ABSTRACT FROM AUTHOR]

Steady-state and time-resolved emission and excitation spectra and the luminescence decay kinetics are studied in the 4.2–500 K temperature range for the blue intrinsic emission and the yellow Bi3+ - related emission of YVO 4 :Bi, LuVO 4 :Bi, and GdVO 4 :Bi microcrystalline powders. It is suggested that the Bi3+ - related absorption (excitation) band, peaking around 3.75 eV, arises from the electronic transition from the ground 1S 0 state of a Bi3+ ion to its excited 3P 1 state located inside the conduction band (CB). The excitation in the Bi3+ - related absorption band is suggested to result in an electron release from the 3P 1 level of Bi3+ into the CB and its subsequent recombination with the hole remained at the Bi3+ ion. As a result, an exciton localized around the single Bi3+ ion is created. The radiative decay of this exciton is accompanied with the Bi3+ - related 2.12–2.19 eV emission. The origin, structure, and parameters of the exciton states, responsible for the luminescence of the Bi3+ - doped vanadates, and the processes taking place in these states are investigated and discussed. No ultraviolet emission arising from the 3P 1,0 - 1S 0 transition of Bi3+ ions is found. From the afterglow excitation spectra, the band gap energy in the investigated vanadates is estimated to be about 4.20–4.25 eV. [ABSTRACT FROM AUTHOR]

Abstract Spectral luminescence parameters of pure and Bi-doped YVO 4 microcrystals with different bismuth content (from 0.1 to 20 wt%) are studied in the 4.2–500 K temperature range. The broad photoluminescence band peaked at 570 nm is caused by localized exciton, which is formed by the electronic transition from bismuth to vanadium states. The temperature dependence of luminescence decay kinetics provides additional evidence of the localized exciton luminescence of Bi-doped YVO 4. Band structure calculations reveal location of bismuth 6s2 states above the top of valence band formed by 2p oxygen states by ∼0.4 eV. Highlights • YVO 4 :Bi microcrystals with different content of Bi were synthesized and studied. • Location of 6s2 Bi3+ states above the YVO 4 valence band by ∼0.4 eV was confirmed. • The broad emission at 570 nm was assigned to the exciton with hole localized on Bi3+ ion and electron on V5+ ion. • Several mechanisms of the localized exciton creation near Bi3+ were observed. [ABSTRACT FROM AUTHOR]

Abstract High pressure studies of monoclinic yttrium aluminum oxide single crystals (Y 4 Al 2 O 9 -YAM) doped with Ce3+ ions grown by micro-pulling-down method are reported. The results of absorption measurements in the mid-infrared prove the existence of four different sites in which Ce3+ ions substitute Y ions, in accordance with the crystallographic structure of YAM. The lowest 5d level of Ce3+ is located very close to the bottom of the conduction band for two of four Ce3+ related centers at ambient pressure, which results in strong temperature quenching of their luminescence. Two other Ce3+ centers do not emit at ambient pressure since their 5d levels are resonant with the conduction band. Application of high pressure above 11 GPa restores their luminescence. Consequences of such energy structure of various Ce3+ sites in YAM for the Dorenbos theory are discussed. The bandgap energy of YAM is found to be 5.93 eV, which is the smallest out of the three yttrium aluminum oxides. High-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements identify a phase transition occurring at pressures between 8 and 11 GPa, which has martensitic character. The second phase transition to another structure (likely with hexagonal symmetry) occurs at a pressure around 16 GPa. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Microcrystalline powders of Bi3+-doped Y 3 Al 5 O 12 and Lu 3 Al 5 O 12 garnets have been studied by the X-ray diffraction, steady-state and time-resolved photoluminescence, and thermally stimulated luminescence methods. The conclusion is made that the excitation spectra of the Bi3+-related visible exciton-like emission of these compounds, which are located in the 4–6 eV energy range, cannot be explained by the charge-transfer transitions between the energy levels of Bi3+ and the conduction or valence band of the host, whose energies exceed 6 eV. To explain the features observed, this emission is suggested to appear due to the photostimulated electron transfer from the ground state of a Bi3+ ion to defect levels located in the band gap, resulting in the creation of temporary pairs of electron centers and hole Bi4+ centers. A hole delocalization from the Bi4+ ion to the neighboring oxygen O2− ion is also suggested, resulting in the creation of the {O− - Bi3+}-type hole center. As a result of the electron recombination with this hole center, the exciton localized around the Bi3+ ion is formed whose radiative decay is accompanied with the visible emission. • Microcrystalline powders of Y 3 Al 5 O 12 :Bi and Lu 3 Al 5 O 12 :Bi are studied by XRD, PL,TSL. • harge-transfer transitions between Bi3+ and conduction or valence band do no occur. • No stable electron and hole centers are optically created in Bi3+-related absorption bands. • Mechanism of photostimulated creation of temporary electron and hole centers is proposed. • Mechanism of Bi3+-related visible exciton-like emission is proposed for such compounds. [ABSTRACT FROM AUTHOR]

An influence of the rare earth ions (La, Er) concentration on the luminescent properties of Ga 2 S 3 -La 2 S 3 -Er 2 S 3 glasses in the visible and near-infrared spectral region has been studied. The analysis of absorption spectra and luminescence allow understanding of the redistribution processes of radiation energy. The results suggest that favorable visible and near-IR luminescence of Er 3+ ions can be further modified in Ga 2 S 3 -La 2 S 3 -Er 2 S 3 chalcogenide glasses through the effect of doping concentration and modification of environment (rare earth ions of Lanthanum act as a network modifier) of the optically active ions by the matrices composition changes. [ABSTRACT FROM AUTHOR]

The work devoted to the investigation of a new luminescent and scintillation material based on the single crystalline films (SCFs) of Tm 3−x Lu x AG:Ce garnet; x = 0–1.5, grown by LPE method from PbO based flux. The best scintillation properties are achieved for SCFs of Tm 1.5 Lu 1.5 Al 5 O 12 :Ce composition. We have found that direct Tm → Ce and backside Ce → Tm energy transfer processes are observed in Tm 1.5 Lu 1.5 Al 5 O 12 :Ce. Due to elimination of traps in the 300–450 °C range, the relatively fast scintillation decay is realized in highly doped Tm 1.5 Lu 1.5 Al 5 O 12 :Ce SCFs. For this reason, Tm doping can be considered as a suitable way for improvement of the scintillation efficiency in other Ce 3+ doped garnet compounds. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) persistent luminescence (afterglow) and thermally stimulated luminescence observed in the 3.6–4.2 eV energy range are studied for microcrystalline powders of the Bi3+-doped Lu 3 Ga 5 O 12 , Y 3 Ga 5 O 12 , and Ca 3 Ga 2 Ge 3 O 12 garnets after their selective UV irradiation at 295 K in the 4–6 eV energy range. Emission and excitation spectra and decay kinetics of the persistent luminescence, as well as the luminescence intensity dependence on the irradiation duration, are measured at 295 K. No UV emission has been observed in the afterglow spectrum of the Gd 3 Ga 5 O 12 :Bi, Lu 3 Al 5 O 12 :Bi, and Y 3 Al 5 O 12 :Bi garnets. The processes resulting in the appearance of the UV persistent luminescence in the investigated Bi3+-doped garnets are discussed. The conditions are formulated which ensure the appearance of this phenomenon in Bi3+-doped compounds and, thus, could inspire the discovery of new effective UV persistent phosphors. • UV persistent luminescence is observed in Lu 3 Ga 5 O 12 :Bi, Y 3 Ga 5 O 12 :Bi, Ca 3 Ga 2 Ge 3 O 12 :Bi. • Afterglow emission and excitation spectra and decay kinetics are studied. • Most intense and long afterglow appears after irradiation in E > 4.8 eV energy range. • Processes resulting in UV persistent luminescence of Bi-doped compounds are discussed. • Conditions are formulated ensuring the appearance of this phenomenon. [ABSTRACT FROM AUTHOR]

Microcrystalline powders of Y 3 Ga 5 O 12 :Bi and Gd 3 Ga 5 O 12 :Bi are investigated by the X-ray diffraction, scanning electron microscopy, time-resolved photoluminescence, and thermally stimulated luminescence methods in a wide temperature range (4.2–500 K). The results are compared with those obtained recently for the Lu 3 Ga 5 O 12 :Bi powder. The presence of the ultraviolet (UV) emission of a Bi3+ ion and the visible (VIS) emission of an exciton localized around the Bi3+ ion is confirmed for all the investigated gallium garnets. The processes of photostimulated creation of electron and hole centers taking place under irradiation in the 4–6 eV energy range are investigated. The origin of the hole centers responsible for the UV and VIS electron recombination luminescence is clarified. Electron transitions between the energy levels of Bi3+ ions and the Bi3+ → Ga3+ oct , Bi3+ → Ga3+ tetr , and Bi3+ → Y3+ or Bi3+ → Gd3+ electron transfer transitions are found to appear in the photo- and thermoluminescence excitation spectra. The Bi3+ energy level positions in the band gap are determined. Their influence on the luminescence and photostimulated creation of electron and hole centers is discussed. • Y 3 Ga 5 O 12 :Bi and Gd 3 Ga 5 O 12 :Bi are studied by XRD, photoluminescence, TSL methods. • Luminescence of Bi3+ ions and excitons localized around Bi3+ ions is identified. • Bi3+.→Ga3+ oct , Bi3+→Ga3+ tetr , Bi3+→RE3+ electron transfer transitions are observed • Electron and Bi4+-, {O− - Bi3+}-type hole centers are created by UV irradiation. • Photoluminescence excitation and defects creation mechanisms are clarified. [ABSTRACT FROM AUTHOR]

Results of spectroscopic studies at ambient and high pressures of a LuAlO 3 :Ce3+ (LuAP:Ce) single crystalline film (SCF) as well as LuAP:Ce and YAlO 3 :Ce (YAP:Ce) single crystals are reported. Room temperature absorption measurements of the single crystals in the vacuum UV region allowed establishing the bandgap energies of 7.63 eV for YAP and 7.86 eV for LuAP, with an assumption of the direct band-gaps. Luminescence of Ce3+ in LuAP and YAP bulk crystals was measured as a function of temperature from 6 K up to 873 K. Temperature quenching of the Ce3+ luminescence in YAP:Ce was observed above 650 K, which is related to the location of the lowest Ce3+ 5d level at 1.27 eV below the conduction band minimum. No temperature quenching occurred in LuAP:Ce up to 873 K, mostly due to the lower energy of the 4f levels with respect to the valence band maximum. The barycenter energies and splittings of Ce3+ 5d states in YAP and LuAP at room temperature were precisely established. Theoretical calculations of the Ce3+ 5d states energy structure under pressure revealed a discrepancy between the obtained experimental results and the prediction of Dorenbos' theoretical model. The discrepancy can be removed if instead of the 5d state of the free Ce3+ ion the bandgap of the compound is taken as reference energy for the red-shift of the 5d level. This hypothesis also allows us to take into account the pressure-induced increase of the bandgap energy, expected for the studied compounds. Pressure dependences of LuAP:Ce luminescence spectra suggest that a certain type of phase transition occurs above 15 GPa. • Bandgap energies of YAP and LuAP from vacuum UV absorption spectra. • Position of Ce3+ levels in the band gap of YAP and LuAP. • High-pressure phase transition of LuAP, high-pressure luminescence of Ce3+ in YAP and LuAP. [ABSTRACT FROM AUTHOR]

Microcrystalline powders of LuNbO 4 :Bi3+, Eu3+ with the same nominal Bi3+ concentration (2 at.%) and different nominal Eu3+ concentrations (varying from 0 to 8 at.%) are synthesized and analyzed by the X-ray diffraction method. Photoluminescence characteristics of the co-doped LuNbO 4 :Bi, Eu powders are investigated by the steady-state and time-resolved luminescence spectroscopy methods in a wide temperature range (4.2–500 K) and compared with those of the undoped and bismuth-doped LuNbO 4. It is found that owing to the overlap of the intrinsic and Bi3+-related emission bands with the absorption bands of Eu3+, an effective non-radiative (resonance) energy transfer from both the host and the Bi3+-related exciton-like states to Eu3+ ions, resulting in the appearance of the luminescence in the 1.6–3.8 eV energy range, takes place in LuNbO 4 :Bi, Eu. The exchange interaction is found to be responsible for the energy transfer process. The possible application of LuNbO 4 :Bi, Eu in white light-emitting diodes is discussed. LuNbO 4 :Bi3+, Eu3+ powders with different Eu3+ concentration are synthesized. The powders are studied by XRD and time-resolved luminescence methods. Non-radiative energy transfer from Bi3+-related excitons to Eu3+ is revealed. Exchange interaction is responsible for the energy transfer in LuNbO 4 :Bi3+, Eu3+. The LuNbO 4 :Bi3+, Eu3+ compound can be suitable for application in WLEDs. [ABSTRACT FROM AUTHOR]

Photoluminescence characteristics of the undoped and Bi3+-doped lanthanide niobates are investigated by the steady-state and time-resolved spectroscopy methods in a wide temperature range of 4.2–500 K. The broad visible emission bands with the large Stokes shifts are found to arise from the triplet relaxed excited states characterized by a small (∼1 meV) spin-orbit splitting energy. This indicates the exciton-like origin of all the observed emission bands. The data allow to conclude that the Bi3+-related triplet relaxed excited state is located inside the conduction band. No ultraviolet emission caused by the radiative decay of the triplet relaxed excited state of a Bi3+ ion is observed. The dependences of the emission intensity on the Bi3+ content indicate that the complex visible emission band of the Bi3+-doped niobates consists of two main, the higher-energy and the lower-energy, components arising from the excitons localized around the single Bi3+ ions and the {Bi3+ - Bi3+} dimers, respectively. The processes resulting in the appearance of the exciton-like emission under photoexcitation in the Bi3+-related absorption bands are discussed. • LuNbO 4 :Bi and GdNbO 4 :Bi powders are synthesized and analyzed by XRD. • Intrinsic and Bi3+-related luminescence of Bi3+-doped lanthanide niobates is studied. • Triplet luminescence of Bi3+ ions is absent in YNbO 4 :Bi, LuNbO 4 :Bi and GdNbO 4 :Bi. • Emission of excitons localized around single and dimer Bi3+ centers is found. • Structure and parameters of the corresponding exciton-like states are determined. [ABSTRACT FROM AUTHOR]