Controlling the europium oxidation state in diopside through flux concentration.

This paper explores the connection between the H3BO3 flux concentration and the co-existence of Eu2+ and Eu3+ dopants within CaMgSi2O6 crystals (diopside). The samples were synthesised using a solid-state synthesis method under varying atmospheric conditions, including oxidative (air), neutral (N2), and reductive (H2/N2 mixture) environments. Additionally, some materials underwent chemical modification by partially substituting Si4+ with Al3+ ions acting as charge compensation defects stabilizing Eu3+ luminescence. Depending on the specific synthesis conditions, the materials predominantly displayed either the orange-red luminescence of Eu3+ (under oxidising conditions) or the blue luminescence of Eu2+; however, the comprehensive results confirmed the co-existence of Eu3+/Eu2+ luminescence in both cases. This work shows that varying flux concentrations added during synthesis significantly affect the relative strength of Eu2+ and Eu3+ emissions in a manner dependent on the synthesis atmosphere. The emission of Eu2+ increases with a higher flux concentration in materials synthesised under oxidative and neutral atmospheres independent of the chemical modification. In contrast, for materials obtained under a reductive atmosphere, the changes in the Eu3+ emission intensity depended on the presence or absence of Al3+ ions namely the increase of flux increased the Eu3+ intensity in the case of unmodified materials and decreased in the Al-modified ones. All observed effects were qualitatively explained considering the double role of the flux in the studied system, which besides facilitating the diffusion of chemical species during synthesis acts as a charge compensating agent by creating B'Si centres stabilizing Eu3+ emission.

Lanthanide ions (Eu3+, Er3+, Pr3+) as luminescence and charge carrier centers in Sr2TiO4.

A series of strontium orthotitanate (Sr2TiO4) samples doped with 2% of a mole of europium, praseodymium, and erbium were obtained using the solid-state synthesis method. The X-ray diffraction (XRD) technique confirms the phase purity of all samples and the lack of the influence of dopants at a given concentration on the structure of materials. The optical properties indicate, in the case of Sr2TiO4:Eu3+, two independent emission (PL) and excitation (PLE) spectra attributed to the Eu3+ ions at sites with different symmetries: low - excited at 360 nm and high - excited at 325 nm, while, for Sr2TiO4:Er3+ and Sr2TiO4:Pr3+, the emission spectra do not depend on the excitation wavelength. The measurements of X-ray photoemission spectroscopy (XPS) indicate the presence of only one type of charge compensation mechanism, which is based on the creation of strontium vacancies in all cases. This suggests that the different charge compensation mechanisms cannot easily explain the presence of Eu3+ at two non-equivalent crystal sites. The photocurrent excitation (PCE) spectroscopy investigations, that have not been reported in the literature so far, show that among all the studied dopants, only Pr3+ can promote the electrons to the conduction band and give rise to electron conductivity. The results collected from the PLE and PCE spectra allowed us to find the location of the ground states of lanthanides(II)/(III) in the studied matrix.

Energetic structure of Sm3+ luminescence centers in Sr2TiO4.

A luminescent material based on the strontium orthotitanate (Sr2TiO4) matrix doped with 1% of a mole of samarium was obtained using the typical solid-state synthesis method under a neutral atmosphere of nitrogen. The sample was investigated using powder X-ray diffraction (XRD) and several luminescence techniques to study the phase composition, luminescence properties as well as to determine the position of the energetic states of Sm3+ in relation to the valence and conduction bands of Sr2TiO4. The XRD result shows that the product of the synthesis is pure Sr2TiO4. From the PL spectra, it can be seen that the phosphor can be effectively excited at 409 and 342 nm to emit significantly different emission spectra. The luminescence obtained under 409 nm excitation is typical of Sm3+ in Sr2TiO4 and attributed to the nonsymmetrical luminescent center (A-center). In contrast, the luminescence obtained under excitation at 342 nm originates from the symmetrical center (B-center) and has not been reported in the literature so far. The presence of the two emission centers related to Sm3+ ions in the Sr2TiO4 matrix characterized by only one crystallographic site of Sr2+ was explained by considering the different ways of charge compensation: Sm3+ in the A-center via strontium vacancy (V''sr), and Sm3+ in the B-center via Ti3+ (Ti'Ti).

Spectroscopic properties and location of the Tb(3+) and Eu(3+) energy levels in Y2O2S under high hydrostatic pressure.

In this contribution, an extensive spectroscopic study of Y2O2S doped with Eu(3+) and Tb(3+) is presented. Steady-state luminescence and luminescence excitation spectra as well as the time-resolved spectra and luminescence kinetics were obtained at high hydrostatic pressures up to 240 kbar. It was found that pressure quenches the luminescence from the (5)D3 excited state of Tb(3+) and recovers additional luminescence related to transitions from the (5)D3 state of Eu(3+). These effects are related to the pressure-induced increases in the energies of the ground electronic manifold 4f(n) of Eu(3+) and Tb(3+) ions with respect to the band edges. Analysis of the emission and excitation spectra allowed the estimation of the energies of the ground states of all lanthanide (Ln) ions (Ln(3+) and Ln(2+)) with respect to the valence and conduction bands edges of the Y2O2S host. The bandgap energy and difference between energies of the ground states of Ln(2+) and Ln(3+) have been calculated as functions of pressure. The experimental high-pressure spectroscopy results allow the calculation of the absolute values (calculated with respect to the vacuum level) of the energies and pressure-induced shifts of the conduction and valence band edges and the ground states of Ln(3+) and Ln(2+) ions in Y2O2S.

Spectroscopic properties and location of the Ce(3+) energy levels in Y3Al2Ga3O12 and Y3Ga5O12 at ambient and high hydrostatic pressure.

In this study, we present the high pressure spectroscopy of Y3Al2Ga3O12 (YAGG) and Y3Ga5O12 (YGG) ceramics doped with Ce(3+) and Cr(3+). We have found that high hydrostatic pressure recovers the Ce(3+) luminescence in YGG. The pressure-induced shifts of the ground state and the 5d1 excited state of Ce(3+) with respect to the conduction band edge were estimated. Our experimental data allowed us to also obtain the shifts of the conduction and valence band edges, and the ground state and the 5d1 state of Ce(3+) ions have been estimated with respect to the vacuum level. It has been shown that simple equivalence between the external hydrostatic pressure and intrinsic chemical pressure related to different compositions of the isostructural matrices does not exist in garnet lattices.

Energy levels in CaWO4:Tb(3+) at high pressure.

The luminescence properties of Tb(3+) in CaWO4 crystals are investigated under a hydrostatic pressure of up to 200 kbar, i.e. across scheelite-to-fergusonite phase transition. It is shown that the typical blue ((5)D3) and green ((5)D4) emissions in this material are progressively quenched at room temperature as pressure is increased. This quenching is caused by a downshift of the charge transfer (or impurity trapped exciton) state that is formed between Tb(3+) and nearby W(6+) cations in conjunction with a pressure-induced increase of the lattice relaxation experienced by this excited state. An empirical model is introduced to calculate the evolution of the (Tb(3+)-W(6+)) charge transfer energy with pressure. Combined with the pressure dependence of the energy bandgap in CaWO4, the model allows locating the 4f levels of Tb(3+) relative to the fundamental host lattice for any pressure in the range 0-200 kbar.

Pressure effect on the zero-phonon line emission of Mn(4+) in K2SiF6.

In this work, effects of pressure and temperature on the luminescent properties of the K2SiF6:Mn(4+) system have been presented. At ambient pressure, the luminescent spectrum of Mn(4+) consists of several lines at 610-650 nm attributed to phonon repetitions of the (2)Eg → (4)A2g transition and does not contain the zero phonon line (ZPL). At pressure above 9 kbar, an additional line at about 624 nm occurs, which can be attributed to the ZPL of the (2)Eg → (4)A2g transition in the Mn(4+) ions. This change in the emission spectra is accompanied by shortening of the luminescence decay time. Further increasing pressure up to 220 kbar causes the red shift of all emission lines. Upon releasing pressure, all observed lines are going back to their previous positions. The ZPL remains visible even at ambient pressure. Taking into account XRD and Raman spectra at ambient pressure before and after compression-decompression, we have attributed these changes to pressure-induced local structure change of MnF6 (2-) octahedron.

Pressure dependence of the emission in CaF2 : Yb(2+).

We present a detailed spectroscopic investigation of CaF2 doped with Yb(2+) performed at high hydrostatic pressure which is applied in a diamond anvil cell. At ambient pressure and at temperatures lower than 175 K, the luminescence consists of a single broad band peaked at 18 500 cm(-1), attributed to the recombination of impurity-trapped excitons. Increasing pressure causes the luminescence to be observable at higher temperature. At a pressure of 72 kbar luminescence can be observed up to 275 K. The emission lineshape does not strongly depend on pressure below 85 kbar. However, at 85 kbar it is blue shifted to 21 630 cm(-1). This is attributed to the known phase transition of the CaF2 crystal from cubic to the orthorhombic phase. The absolute energy of the ground and 4f(13)5d states of Yb(2+) as well as the energy of the impurity-trapped exciton with respect to valence and conduction bands have been estimated. The results, are discussed in comparison with the pressure dependences observed for the luminescence of BaF2 : Eu(2+) and CaF2 : Eu(2+). The difference between the spectral properties of Eu(2+) and Yb(2+) is attributable to the fact that the ground and 4f(6)5d states of Eu(2+) are placed deeper in the CaF2 bandgap than the ground and excited 4f(13)5d states of Yb(2+), whereas the energies of the impurity-trapped exciton states for Yb(2+) and Eu(2+) with respect to the conduction band are approximately the same.

Luminescence properties of different Eu sites in LiMgPO4:Eu(2+), Eu(3+).

The effect of temperature on the luminescence properties of LiMgPO4 doped with Eu(3+) and Eu(2+) are presented. Depending on the excitation wavelength, luminescence spectra consist of two distinct broad emission bands peaking at 380 nm and 490 nm related to 4f(6)5d(1) â†’ 4f(7) ((8)S7/2) luminescence of Eu(2+) and to europium-trapped exciton, respectively, and/or several sharp lines between the 580 nm and 710 nm region, ascribed to the (5)D0 â†’ (7)FJ (J = 0, 1, 2, 3 and 4) transitions in Eu(3+). To explain all the features of the Eu(2+) and Eu(3+) luminescence we discussed the existence of two different Eu sites substituting for Li(+), with short and long distance compensation. The evident effect of increasing the intensity of the Eu(2+) luminescence with increasing temperature was observed. It was considered that the charge compensation mechanism for Eu(3+) and Li(+) as well as Eu(2+) replacing Li(+) in the LiMgPO4 is a long distance compensation that allows for the existence of some of the europium ions either as Eu(3+) at low temperature or as Eu(2+) at high temperature. We concluded that Eu(2+) in the Li(+) site with long distance compensation yields only 4f(6)5d(1) â†’ 4f(7) luminescence, whereas Eu(2+) in the Li(+) site with short distance compensation yields 4f(6)5d(1) â†’ 4f(7) luminescence and europium-trapped exciton emission.

Temperature evolution of the luminescence decay of Sr0.33Ba0.67Nb2O6 : Pr3+.

This article presents a spectroscopic investigation of Sr(0.33)Ba(0.67)(NbO2)3, doped with 1 mol% of Pr(3+). Photoluminescence and luminescence kinetics were measured at different temperatures at ambient (ferroelectric phase) and 76 kbar pressures (paraelectric phase). The photoluminescence spectrum is dominated by (1)D2 → (3)H4 transition of Pr(3+) in both phases. At ambient pressure when the system is excited with UV radiation, the intensity of dominant (1)D2 → (3)H4 emission evidently increases in the 200-293 K temperature range. This effect is attributed to enhancement of the excitation of the (1)D2 state through the praseodymium trapped exciton state, which at higher temperatures does not populate the higher lying (3)P0 state. Additionally, under UV radiation the material exhibits afterglow luminescence activated by temperature that can also have an impact on the increase of the (1)D2 emission. We propose that the afterglow luminescence is related to the existence of electron traps. At a pressure of 76 kbar the depth of the electron traps decreases in comparison to the ones observed at ambient pressure. However, the phase transition does not change the number of electron traps.

Time evolution of luminescence of Sr2SiO4:Eu²âº.

In this contribution, the photoluminescence, time-resolved luminescence and luminescence kinetics of α'-Sr2SiO4:Eu(2+) are studied. The luminescence of Sr2SiO4:Eu(2+) consists of two broad bands, peaked at 490 nm (blue-green) and 570 nm (yellow-orange), which originate from two luminescence centers, related to Eu(2+) in ten-coordinated SI and nine-coordinated SII sites, respectively. Based on spectroscopic data the energetic structure of Sr2SiO4:Eu(2+) has been developed, which includes the bands edges, energies of Eu(2+) in the SI and SII sites and energies of strontium and oxygen vacancies. To investigate the long-lasting luminescence phenomenon in Sr2SiO4:Eu(2+) the temperature influence on the time evolution of luminescence was analyzed. It has been found that the long-lasting luminescence is related to the Eu(2+) in SII site. The shallowest traps responsible for emission decaying within a few seconds are tentatively attributed to the [Eu(3+)(SII)-[Formula: see text]] centers. The depth of traps responsible for the long-lasting luminescence observed at room temperature has been estimated as equal 0.73 eV.

High pressure luminescence spectra of CaMoO4:Ln3+ (Ln = Pr, Tb).

Photoluminescence spectra and luminescence kinetics of pure CaMoO(4) and CaMoO(4) doped with Ln(3+) (Ln = Pr or Tb) are presented. The spectra were obtained at high hydrostatic pressure up to 240 kbar applied in a diamond anvil cell. At ambient pressure undoped and doped samples exhibit a broad band emission extending between 380 and 700 nm with a maximum at 520 nm attributed to the MoO(4)(2-) luminescence. CaMoO(4) doped with Pr(3+) or Tb(3+) additionally yields narrow emission lines related to f-f transitions. The undoped CaMoO(4) crystal was characterized by a strong MoO(4)(2-) emission up to 240 kbar. In the cases of CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), high hydrostatic pressure caused quenching of Pr(3+) and Tb(3+) emission, and this effect was accompanied by a strong shortening of the luminescence lifetime. In doped samples, CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), quenching of the emission band attributed to MoO(4)(2-) was also observed, and at pressure above 130 kbar this luminescence was totally quenched. The effects mentioned above were related to the influence of the praseodymium (terbium) trapped exciton PTE (ITE-impurity trapped exciton) on the efficiency of the Pr(3+) (Tb(3+)) and MoO(4)(2-) emissions.

High pressure and time-resolved luminescence spectra of Ca3Y2(SiO4)3 doped with Eu2+ and Eu3+.

Tricalcium diyttrium trisilicon oxide, Ca(3)Y(2)(SiO(4))(3) doped with Eu(2+) and Eu(3+) belongs to a very limited number of hosts able to accommodate both Eu(3+) and Eu(2+) ions, which might make it useful for white light emitting diodes (WLEDs) based on UV chip technology. In this contribution we present a detailed study of photoluminescence kinetics and high pressure spectroscopy of Eu(3+) and Eu(2+) doped Ca(3)Y(2)(SiO(4))(3). At ambient pressure and room temperature, under excitation with near-UV radiation, a broad emission band from 400 to 550 nm due to the 4f(6)5d(1)→4f(7)((8)S(7/2)) transition in Eu(2+) was observed, as well as several emission peaks in the region between 550 and 710 nm, ascribed to the (5)D(0)→ (7)F(J) (J = 0-4) transitions in Eu(3+). The bluish green luminescence related to Eu(2+) in the Ca(3)Y(2)(SiO(4))(3) exhibits a small red pressure-induced shift reaching -5.2 cm(-1)/kbar. The red shifts of the luminescence lines related to Eu(3+) ion emission vary from 0.15 to -0.54 cm(-1)/kbar. Time-resolved photoluminescence was measured at different temperatures and pressures. Luminescence decay traces were studied for the bluish green emission band of Eu(2+) and for the red emission peak due to the (5)D(0) → (7)F(2) transition of Eu(3+). Decay times slightly decreased with increasing pressure.

Luminescence of Gd2(WO4)3:Ln3+ at ambient and high hydrostatic pressure.

This paper presents a spectroscopic characterization of Gd(2)(WO(4))(3):Ln(3+) (Ln=Eu, Pr, Tb and Dy). The luminescence and luminescence kinetics were measured under pressures up to 250 kbar. It was found that pressure quenches the luminescence of Pr(3+) and Tb(3+), whereas the emission of Eu(3+) and Dy(3+) was stable up to 250 kbar. This effect was related to a decrease in the ionization energy of Pr(3+) and Tb(3+) caused by pressure induced increase in energies of the Ln(2+) and Ln(3+) ions with respect to the band edges. Analysis of emission and excitation spectra allowed us to estimate the energies of the ground states of Ln(3+) and Ln(2+) with respect to the valence and conduction band edges of the Gd(2)(WO(4))(3) host. Differences between energies of the ground states of Ln(2+) and Ln(3+) have also been calculated.

High pressure luminescence spectra of CaMoO4:Pr3+.

Steady state and time resolved luminescence measurements of CaMoO(4) doped with Pr(3+) as a function of hydrostatic pressure in the 1-175 kbar range are presented. It has been observed that with increasing pressure the spectral features shift towards lower energies, the decay times of both (3)P(0) and (1)D(2) emitting levels become shorter and the intensity of the (3)P(0) emission decreases to complete quenching at about 110 kbar, whereas that of the (1)D(2) emission increases in the 0-100 kbar range and then rapidly decreases when the pressure exceeds 127 kbar. A variation of the structure of the spectral manifolds indicates that a pressure induced phase transition of the host lattice occurs in the 80-100 kbar range. The quenching of the luminescence and the shortening of the decay times have been accounted for by means of a model that takes into account the role played by a praseodymium trapped exciton in the excited state dynamics of the investigated material.

Pressure-induced phase transition in LiLuF4:Pr3+ investigated by an optical technique.

The luminescence and luminescence kinetics of LiLuF(4) doped with 1.5 at.% of Pr(3+) obtained at high hydrostatic pressure changing from ambient to 220 kbar applied in a diamond anvil cell are presented. It has been shown that pressure causes shift of the emission lines toward the red with rates of the order of single cm(-1) kbar(-1). The pressure-induced phase transition from tetragonal to fergusonite structure for pressure above 100 kbar was observed. The crystal field calculations performed showed that this phase transition reduces the point symmetry of the Pr(3+) site from the S(4) to the C(2) point group.

New Eu²+ sites in KMgF3:Eu²+ crystal.

The luminescence properties of KMgF(3):Eu(2 + ) are investigated at different pressures in the temperature range 25-292 K. Five new Eu(2 + ) luminescence (NEL) lines due to the [Formula: see text] transition are identified at 362.49 nm (L(1)), 362.53 nm (L(2)), 360.72 nm (L(3)), 360.15 nm (L(4)) and 359.59 nm (L(5)) together with the line at 359.32 nm (L(0)) which is well known in KMgF(3):Eu(2 + ). The emission lines under excitation at 325 nm show a strong dependence on temperature. At 25 K the emission spectrum consists of only two sharp lines, L(1) and L(2). Three additional lines (L(3), L(4) and L(5)) begin to appear with increasing temperature. With a further increase in temperature from 150 to 292 K all the lines disappear except for the single sharp line at 359.32 nm (L(0)). The zero-phonon transition of line L(0) is accompanied by vibronic sidebands. A pressure shift of five NELs is estimated to be about - 0.6 cm( - 1) kbar( - 1) similarly to the shift of line L(0), while the lifetimes of the NELs are about 0.7 ms which is shorter than that (5.2 ms) of L(0) at 80 K. The new luminescence lines are attributed to the Eu(2 + ) ions occupying the K( + ) sites with fluorine vacancy (F( - ) center) complexes.

High pressure evolution of YVO(4):Pr(3+) luminescence.

Photoluminescence and time-resolved photoluminescence spectra of YVO(4) doped with Pr(3+) obtained at high hydrostatic pressure up to 76 kbar applied in a diamond anvil cell are presented. At pressures lower than 60 kbar the steady state emission spectra consist of sharp lines related to the [Formula: see text] transition in Pr(3+). At pressures above 68 kbar the Pr(3+) emission intensity decreases and the corresponding bands are replaced by a broad band peaking at 19 500 cm(-1) attributed to perturbed VO(4)(3-) host luminescence. The quenching of the [Formula: see text] emission has been attributed to nonradiative transition to the charge transfer exciton trapped at Pr(3+) ion. The recovering of the VO(4)(3-) host luminescence at high pressure has been attributed to energy transfer from a Pr(3+) trapped exciton (PTE) to the host YVO(4). The kinetics of such a process is analyzed using the model of PTE considered as a Pr(4+) + electron bound by the Coulomb potential at the delocalized Rydberg states.

Pressure evolution of LiBaF(3):Eu(2+) luminescence.

We investigated the single crystals of LiBaF(3) doped with Eu(2+) using high pressure spectroscopy, where high pressure was applied in a diamond anvil cell. Photoluminescence, time-resolved luminescence and luminescence kinetics at pressures from ambient to 200 kbar and at temperatures from 10 K to ambient were studied. At ambient conditions the luminescence spectra consisted of sharp lines peaked at 360 nm attributed to the [Formula: see text] transitions in the 4f(7) electronic configuration of Eu(2+) (the zero-phonon line and five single-phonon repetitions) and a broad band extending between 375 and 475 nm attributed to Eu(2+) trapped exciton recombination. When pressure was increased the Eu(2+) trapped exciton emission disappears and was replaced by the sharper band peaked at 355 nm, attributed to the [Formula: see text] transition in Eu(2+). To analyze the pressure dependence of luminescence spectra a model of impurity trapped excitons was developed. At temperatures lower than 50 K only the sharp lines related to [Formula: see text] transitions were observed for all pressures considered. Analysis of low temperature spectra allowed us to estimate the energies of the fifth phonon modes and the values of the Grüneisen parameters.

Temperature and pressure dependence of the luminescence of Eu(2+)-doped fluoride crystals Ba(x)Sr(1-x)F(2) (x = 0, 0.3, 0.5 and 1): experiment and model.

This paper summarizes experimental evidence of anomalous luminescence in Eu(2+)-doped fluoride crystals Ba(x)Sr(1-x)F(2) (x = 0, 0.3, 0.5 and 1). Luminescence, luminescence excitation spectra and luminescence kinetics obtained at ambient and high hydrostatic pressure at various temperatures are discussed. Hydrostatic pressure was shown to cause a redshift of normal [Formula: see text] emission and anomalous luminescence. The experimental data shows the existence of temperature- and pressure-induced spectral transformations where the anomalous luminescence is replaced by normal emission of Eu(2+) centers. We present a model that predicts a strong electron-lattice coupling of the trapped excitons as well as the pressure effect of the spectral transformation from anomalous to normal emission.