Exciton-like luminescence of Bi3+-doped yttrium niobate

Nuclear Inst. and Methods in Physics Research B 463 (2020) 7–15

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Exciton-like luminescence of Bi3+-doped yttrium niobate a

b,c

b

b

d

d,e

M. Baran , A. Kissabekova , A. Krasnikov , A. Lushchik , A. Suchocki , V. Tsiumra ⁎ L. Vasylechkof, S. Zazubovichb, , Ya. Zhydachevskyyd,f

,

T

a

Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland Institute of Physics, University of Tartu, W. Ostwaldi 1, 50411 Tartu, Estonia L.N. Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan d Institute of Physics, Polish AS, Al. Lotników 32/46, 02-668 Warsaw, Poland e Ivan Franko National University of Lviv, Kyryla and Mefodiya 8a, Lviv 79005, Ukraine f Lviv Polytechnic National University, Bandera 12, 79013 Lviv, Ukraine b c

ARTICLE INFO

ABSTRACT

Keywords: Luminescence Self-trapped and localized excitons Yttrium niobate Bi3+

Characteristics of the intrinsic and Bi3+-related luminescence of YNbO4:Bi powders with different Bi contents are studied at 4.2–500 K by steady-state and time-resolved spectroscopy methods. The emission of YNbO4: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 3P1,0 → 1S0 transitions of a Bi3+ ion is found. The structure and parameters of the exciton states, responsible for the luminescence of YNbO4: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 YNbO4 is estimated to be 5.3–5.4 eV.

1. Introduction For the first time, luminescence characteristics of Bi3+-doped lanthanide niobates were reported about 50 years ago [1,2]. The interest to these materials renewed in the recent years when it was found that the undoped and Bi3+-doped niobates have considerable potential for numerous applications. Indeed, besides an intense visible luminescence, these materials have high dielectric constant, low phonon frequencies, good photoelastic and nonlinear optical properties as well as an excellent chemical, mechanical, and thermal stability (see, e.g., Refs. [2–7] and references therein). Owing to that, YNbO4:Bi and GdNbO4:Bi were proposed for use in field emission displays [8-12], while YNbO4 and GdNbO4 co-doped with Bi3+ and different rare-earth ions (e.g., Tm3+, Dy3+, Yb3+, Eu3+, Nd3+) were found to be suitable for white light emitting diodes (WLED) [13–16] or solar cells [10,17–19]. The yttrium niobate YNbO4 exhibit fergusonite monoclinic structure, point symmetry at Y3+ site is C2 and CN = 8 [2,3,14,20]. From the data presented in Refs. [21,22], the band gap energy Eg in YNbO4 was estimated to be Eg ≈ 5.6 eV. The value of Eg = 5.3 eV was presented in Ref. [23] and Eg ≈ 5.45 eV, in Ref. [24]. However, much smaller values of Eg = 4.3–4.5 eV were reported in Refs. [7,11,25–29]. In the latter papers, the Eg values were estimated from the low-energy ⁎

edge of the absorbance spectrum or the excitation spectrum of the intrinsic luminescence (see Ref. [26] and references therein) ascribed to an electron transfer from the oxygen ion to the empty d-orbital of the central Nb5+ ion. The value of Eg = 4.68 eV was reported in Refs. [29,30]. Luminescence characteristics of the undoped and Bi3+-doped YNbO4 were studied mainly at room temperature (RT). The literature data on the positions of the intrinsic and Bi3+-related emission bands and the corresponding excitation bands in YNbO4 are presented in Table 1. In different papers, the positions of the intrinsic emission band vary from 3.02 to 3.13 eV and the positions of the corresponding excitation band, from 4.70 to 5.20 eV. The most detailed study of the structure of the excited states responsible for the intrinsic luminescence was carried out in Ref. [31]. It was found that in the undoped YNbO4, the decay of the 3.06 eV emission is single-exponential with τ = 380 μs at 1.5 K, 150 μs at 5 K, and 4.7 μs at RT. The decay kinetics was considered in three-level scheme where the presence of two very close (D = 0.74 meV, see Table 1) excited levels with strongly different radiative decay probabilities was suggested. The intrinsic emission intensity was found to be independent of temperature from 1.5 to 310 K (see also Ref. [2]).

Corresponding author. E-mail address: [email protected] (S. Zazubovich).

https://doi.org/10.1016/j.nimb.2019.11.023 Received 2 October 2019; Received in revised form 11 November 2019; Accepted 13 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.

Nuclear Inst. and Methods in Physics Research B 463 (2020) 7–15

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Table 1 Maxima positions of the emission (Eem) and excitation (Eexc) bands of the undoped and Bi3+-doped yttrium niobate, as well as the Stokes shifts (S), temperatures (Tq) where the emission intensity decreases twice, activation energies of the luminescence thermal quenching (Eq), luminescence decay times (τ), and energy distances (D) between the emitting and metastable minima of the corresponding triplet exciton states. The data obtained in this work at 4.2 K are shown in bold. Sample

Eexc, eV

Eem, eV

YNbO4

4.85 4.85 4.77 5.05 5.20 4.81 4.70

3.07 3.02 3.06 3.10

4.07 3.95 3.82 4.00 4.06 4.00 3.82

2.77 2.79 2.79 2.67

YNbO4:Bi

ex0Bi3+ ex0{Bi3+-Bi3+} STE

S, eV

Tq, K

Eq, eV

355

τ, μs (4.2 K)

~200

D, meV

0.74

3.03 3.13

4.00 3.95

2.82 2.72 2.76 2.70 2.79 2.80

4.09 4.06 4.96

2.53 2.41 2.89

Ref. [8] [13] [31] [9,30] [24] [27,33] [7] [13] [8] [34,37] [1] [13] [9,32,33] [14] [17] [18] [15] [32]

1.56 1.65 2.07

250 – 260

The position of the Bi3+-related emission band in YNbO4 varies in different papers from 2.67 to 2.82 eV and the position of the corresponding excitation band, from 3.82 to 4.07 eV (Table 1). An effective NbO43− → Bi3+ energy transfer was revealed in Refs. [15,20,32]. The Bi3+-related emission intensity in YNbO4:Bi was found to increase linearly or sublinearly with the increasing Bi content up to 0.5 at.% [33], 1 at.% [15,32] or 1.5–2 at.% [13,14]. The intensity decrease at higher Bi contents was explained by the concentration quenching. The explanations of the experimental results proposed in different papers were also different. In Ref. [21], the vacuum referred electron binding energies in the ground 1S0 state and in the lowest-energy excited 3P1 state of Bi3+ centers and the locations of the Bi3+ energy levels within the forbidden gap were determined for many Bi3+-doped compounds. In the compounds with a low lying conduction band (CB), including YNbO4:Bi, the 3P1 excited state of Bi3+ center was concluded to be located inside the CB (see also Ref. [22]). The lowest-energy excitation band and the emission of the compounds of such type were interpreted as a result of the metal-to-metal charge transfer (MMCT) transitions [21,22,34,35]. Namely, for YNbO4:Bi, the MMCT origin of the spectral bands was suggested in Refs. [1,9,34,36,37]. The lowestenergy absorption band was connected with the Bi3+(6s2) → Nb5+(3d0) charge transfer transitions and the emission band, with the Nb4+(3d1) → Bi4+(6s1) charge transfer transitions. However, in Refs. [8,14,17,18,33], the excitation and emission bands of YNbO4:Bi were ascribed to the electron transitions between the ground state (1S0) and the triplet excited state (3P1) of a single Bi3+ ion. In Refs. [1,9], it was concluded that the assignment of absorption and emission to the transitions between the 1S0 and 3P1 energy levels of Bi3+ ions is not very probable and that the spectral bands should arise from the transitions involving the Bi3+ ion as well as the host lattice. In Ref. [11], the absorption of YNbO4:Bi was ascribed to the charge transfer transitions from O2− 2p to the excited 6p levels of Bi3+. In Ref. [37], the experimental position of the lowest-energy Bi3+related excitation band in YNbO4:Bi was compared with the calculated energies of the 1S0 → 3P1 transitions of Bi3+ ions and the MMCT transitions. It was found that the calculated energies of the 1S0 → 3P1 transitions (3.91 eV) and the MMCT transitions (3.91 eV) are close to the maximum position of the lowest-energy excitation band in YNbO4:Bi obtained from experiments (3.82–4.07 eV, see Table 1) are close. This means that, in principle, the appearance of both these types of

0.22 – 0.25

33;0.37 46 180

0.78 0.77 0.63

this work this work this work

transitions could be expected in the absorption (and luminescence) spectra of YNbO4:Bi. As it is evident from the brief review of the literature data presented above, not only the interpretation of the experimental results but also the positions of the emission and excitation bands of the undoped and Bi3+-doped yttrium niobates reported in different papers and collected in Table 1 are different. No detailed study was carried out for the intrinsic and the Bi3+-related luminescence characteristics in a wide temperature range, including low temperatures, which could allow to determine the structure and parameters of the corresponding relaxed excited states (e.g., the probabilities of the radiative and non-radiative transitions, spin–orbit splitting energy, etc) and, thus, to make a justified conclusion on the luminescence origin in YNbO4:Bi. Therefore, in this work, we have carried out a detailed and systematic investigation of YNbO4:Bi powders with different Bi contents by the methods of the steady-state and time-resolved luminescence spectroscopy in the 4.2–500 K temperature range. Our aim was to clarify the origin of the luminescence centers and their absorption (excitation) and emission bands, to determine the structure and parameters of the relaxed excited states (RES) responsible for the intrinsic and Bi3+-related emission bands, and to investigate the processes taking place in the excited states of these centers. In the recent years, the interest to Bi3+-doped materials of various types increases drastically not only due to their possible new applications, but also owing to very interesting phenomena appearing under photoexcitation in the Bi3+-related absorption bands. Namely, a strong dependence of luminescence characteristics on the host material (in particular, on the position of Bi3+ energy levels with respect to the conduction and valence bands of the host) has been revealed (see, e.g., [21,22,34,36,37]). Therefore, the detailed spectroscopic study and comparison of various Bi3+-doped compounds should help to understand the mechanisms of these features as well as the reasons of different origin of the Bi3+-related luminescence centers in different materials. 2. Experimental The YNbO4:Bi microcrystalline powders with nominal bismuth content of 0.2, 1, and 5 at.% relative to yttrium were synthesized by the standard solid-state reaction method. The starting high-purity materials 8

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Nb2O5 (Aldrich, > 99.99%), Y2O3 (Aldrich, > 99.99%), and Bi2O3 (Aldrich, > 99.9%) were mixed in stoichiometric proportions. The high-energy ball-milling process with acetone as a medium and zirconia balls was used for mixing of the starting materials. Following the ballmilling process, the remaining medium was evaporated and the obtained powders were subsequently dried at 150 °C for 12 h in air atmosphere. Finally, all powders were grinded in an agate mortar and calcined in air for 12 h at 1250 °C. Before studies, the synthesized powders were once more grinded in an agate mortar. The chemical analysis of the synthesized powders was done in A.B. Blank department for analytical chemistry of STC “Institute for Single Crystals”, NAS of Ukraine. The concentration of Bi (relative to yttrium) inside the investigated powders was determined by the method of inductivelycoupled plasma atomic spectrometry (ICP-AES) using an iCAP 6300 Duo spectrometer (Thermo Scientific Corporation, USA) and found to be 0.21 ± 0.01, 1.02 ± 0.05, and 4.9 ± 0.2 at.%, respectively. For the analysis, the powders were dissolved in a condensed phosphoric acid. The calibration was performed using a series of standard solutions prepared from the special purity grade oxides. In such a way, the chemical analysis confirmed that the real content of Bi dopant reproduces the nominal content within the measurement error. For all the investigated YNbO4:Bi powders, the analysis of XRD patterns revealed pure monoclinic fergusonite-type structure. No traces of parasitic phases were detected. Crystal structure parameters of YNbO4:Bi were evaluated from XRD data by full profile Rietveld refinement technique. As a starting model for the refinement, the atomic positions in YNbO4 structure in standard setting of space group N15 C2/ c [38] were used. In the refinement procedure, the lattice parameters, atomic coordinates and displacement parameters of atoms were refined together with profile parameters and corrections for absorption and experimental sample shift. In all cases, a good agreement between the calculated and experimental XRD profiles was achieved. As an example, Fig. 1 illustrates the graphical results of the Rietveld refinement of the structure refinement of the YNbO4:Bi 1 at.% sample. Refined structural parameters of YNbO4:Bi are collected in Table 2. It was found that the lattice parameters and unit cell volume of the materials synthesized systematically increase with the increasing Bi content, which favor successful progressive incorporation of the Bi species into the YNbO4 structure. Based on these structural data, the metal–metal distances in the YNbO4:Bi series were also calculated and presented in Table 3. The experimental setup and the methods used for the photoluminescence, afterglow and thermally stimulated luminescence investigations were described in [39]. To separate strongly overlapping spectral bands, the emission spectra were measured under many

different excitations Eexc, and the excitation spectra were measured for many different emission energies Eem. The obtained spectra were carefully compared. As it was not possible to exclude completely the overlap of closely located broad emission bands, the best results, allowing to minimize the overlap, were presented in the figures where the chosen values of Eexc and Eem are indicated. 3. Experimental results and discussion 3.1. Photoluminescence characteristics under the steady-state excitation At 4.2 K, the emission spectrum of the YNbO4:Bi 0.2 at.% powder consists of four bands located at 2.89, 2.63, 2.53, and 2.41 eV (see Fig. 2 and Table 1). The broad 2.89 and 2.63 eV emissions with the full width at half maxima (FWHM) of 0.62 and 0.76 eV, respectively, are excited only in the band-to-band (Eexc > 5.5 eV) and exciton (around 4.96 eV) absorption regions (Fig. 2a). A weak 4.1 eV band appears in the excitation spectrum due to a strong overlap of the intrinsic and Bi3+-related emission bands. The 2.89 eV emission has been observed in the undoped YNbO4 as well (see Table 1). By analogy with the Bi3+doped vanadates studied in Ref. [39], we suggest that this emission arises from the self-trapped exciton (STE). As the Bi3+ concentration increases, the relative intensity of the 2.89 eV emission decreases, and it is practically absent in the YNbO4:Bi 5 at.% powder. This behaviour is characteristic just for the self-trapped exciton emission. The intensity of the weak 2.63 eV emission band is practically independent of the Bi3+ concentration which indicates its intrinsic origin. As it will be shown further, this emission is different from the yellow emission of YNbO4 reported in Ref. [31]. Probably, the 2.63 eV emission arises from tunneling recombination of optically created intrinsic electron and hole centers which appear under irradiation of YNbO4:Bi in the band-toband and exciton absorption region (see Section 3.4). Both the 2.53 and 2.41 eV emission bands arise from Bi3+-related luminescence centers. The most intense 2.53 eV emission is mainly excited in the Bi3+-related absorption band peaking at about 4.09 eV (Fig. 2b). The FWHM of this emission is 0.53 eV and the Stokes shift S = 1.56 eV. Due to a strong overlap of the 2.53 and 2.41 eV emission bands, the weak 2.41 eV emission (FWHM = 0.55 eV) appears most clearly only under excitation of the YNbO4:Bi 5 at.% powder at the lowenergy side of the Bi3+-related absorption band (around 3.6 eV). The excitation band maximum of this emission is shifted to lower energies (down to 4.06 eV, see Fig. 2c) with respect to the excitation band maximum of the dominating 2.53 eV emission. Temperature dependences of the maximum luminescence intensity – I(T), measured for the 2.89 eV (Fig. 3a, solid line) and 2.53 eV (Fig. 3b) emissions under excitation in the maxima of the corresponding excitation bands, are similar. Their intensity decreases twice at about Tq = 250–260 K. Thus, unlike the undoped YNbO4, where the intrinsic emission intensity was reported to be temperature-independent up to 310 K [2,31], the intensity of the 2.89 eV emission in the Bi3+-doped YNbO4 becomes thermally quenched at much lower temperature (Fig. 3a, solid line). This effect can be caused by the NbO43− → Bi3+ energy transfer reported in Refs. [15,20,32]. However, it is interesting to note that, unlike the Bi3+-doped vanadates studied in Ref. [39], no intensity redistribution is observed between the intrinsic (2.89 eV) and the Bi3+-related (2.53 eV) emission bands of YNbO4:Bi. The I(T) dependences presented in the ln I – 1/T coordinates (see the insets to Fig. 3) become linear in the T > 350 K temperature range. From these dependences, the activation energies Eq of the luminescence thermal quenching are determined. For the 2.89 and 2.53 eV emissions, the Eq values are 0.25 ± 0.01 and 0.22 ± 0.01 eV, respectively. The values of Tq and Eq are presented in Table 1. It should be noted that the I(T) dependences presented in Fig. 3a and 3b are slightly distorted due to the overlap of emission bands. It was not also possible to obtain the correct I(T) dependence for the weaker Bi3+-related 2.41 eV emission band due to its strong overlap

Fig. 1. Graphical results of Rietveld refinement of the YNbO4:Bi 1 at.% structure. Experimental XRD pattern (black dots) is shown in comparison with the calculated pattern (red line). The difference between the measured and calculated profiles is shown as a curve below the diagrams. Short vertical bars indicate the positions of diffraction maxima in fergusonite structure. Miller’s indices are given for the monoclinic YNbO4 structure, space group C2/c (ICSD N98-002–0335). 9

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Table 2 Crystallographic data for YNbO4:Bi series (space group N15 C2/c). Refined values of the lattice parameters, fractional atomic coordinates (x/a, y/b, z/c) and atomic displacement parameters (Biso/eq) are given. The digits in parentheses are standard deviations of the corresponding values. x/a

y/b

z/c

Biso/eq, Å2

Occupancy*

0.2 at.% Bi (RI = 0.047, RP = 0.169) Y,4e a = 7.0410(8) Å b = 10.942(1) Å Nb,4e O1,8f c = 5.2918(5) Å β = 134.090(4) Å O2,8f 3 V = 292.8(1) Å

0 0 0.241(2) 0.302(2)

0.6220(3) 0.1428(2) 0.0384(7) 0.2849(7)

1/4 1/4 0.335(2) 0.306(3)

0.71(4) 1.47(6) 0.9(2) 0.7(2)

0.998Y3++0.002Bi3+ Nb5+ O2− O2−

1 at.% Bi (RI = 0.026, RP = 0.125) Y,4e a = 7.0391(2) Å b = 10.9517(4) Å Nb,4e O1,8f c = 5.2987(1) Å β = 134.096(2) Å O2,8f 3 V = 293.36(5) Å

0 0 0.2434(12) 0.2972(13)

0.6220(2) 0.14371(9) 0.0401(5) 0.2854(5)

1/4 1/4 0.3379(14) 0.302(2)

0.47(3) 0.86(3) 0.84(12) 0.96(12)

0.99Y3++0.01Bi3+ Nb5+ O2− O2−

5 at.% Bi (RI = 0.039, RP = 0.158) a = 7.0464(3) Å Y,4e b = 10.9637(7) Å Nb,4e c = 5.3051(2) Å O1,8f β = 134.120(3) Å O2,8f 3 V = 294.22(6) Å

0 0 0.240(2) 0.296(2)

0.6212(2) 0.1435(2) 0.0417(7) 0.2861(6)

1/4 1/4 0.338(2) 0.304(3)

0.44(4) 0.64(5) 0.8(2) 0.8(2)

0.95Y3++0.05Bi3+ Nb5+ O2− O2−

Lattice parameters

Atoms, sites

*fixed according to nominal compositions Table 3 The nearest metal-metal distances in YNbO4:Bi. The shortest Nb-Nb distances are shown in bold. Atoms

Distances, Å 0.2 at. % Bi

1 at. % Bi

5 at. % Bi

Nb-2Nb

3.453(4)

3.442(2)

3.449(2)

Nb-2Y Nb-2Y Nb-2Y Nb-2Y Y-2Y Y-2Y

3.5280(4) 3.690(4) 3.8112(4) 3.852(4) 3.759(6) 3.779(6)

3.5276(3) 3.688(3) 3.8165(3) 3.860(3) 3.763(3) 3.780(3)

3.5317(3) 3.701(3) 3.8201(3) 3.854(3) 3.754(3) 3.797(3)

with the main 2.53 eV emission band. The increase of the 2.41 eV emission intensity with the increasing temperature observed under excitation at the low-energy slope of the excitation band, where this emission appears most clearly (with Eexc = 3.6 eV, see Fig. 3c), is most probably caused by the lower-energy shift and broadening of the excitation band. The intensity of the intrinsic 2.63 eV emission excited in the bandto-band transitions region is temperature-independent up to 500 K (see Fig. 3a, dashed line). 3.2. Luminescence decay kinetics and time-resolved emission and excitation spectra Decay curves measured at 4.2 K for all the emission bands of YNbO4:Bi under excitation with the xenon flash lamp with the pulse duration of about 1 μs (for more details, see [39]) are presented in Fig. 4. It is evident that in the decay kinetics of these emissions, two components, the fast and the slow, are observed. The decay times (τ) of the main (slow) component of the 2.89, 2.53, and 2.41 eV emissions are 180 μs (Fig. 4a, curve 1), 33 μs (Fig. 4b), and 46 μs (Fig. 4c), respectively. The decay kinetics of the intrinsic 2.63 eV emission is complicated and contains millisecond components as well (Fig. 4a, curve 2). The fast components (with the decay time shorter than 10 μs) could not been studied under excitation with the same xenon flash lamp. Therefore, the nanosecond light emitting diodes (LED) were used which allow the decay kinetics study in the ns time range (t < 7 μs). Under 264 nm (4.7 eV) LED excitation, the lightsum of the fast component of the 2.89 eV emission appears to be too weak for its detailed study. Besides,

Fig. 2. Steady-state emission (curves 1, 1′) and excitation (curve 2) spectra of the YNbO4:Bi 0.2 at.% powder measured at 4.2 K for (a) the intrinsic 2.89 eV (curves 1, 2) and 2.63 eV (curve 1′) emissions, (b) the Bi3+-related 2.53 eV emission, and (c) the Bi3+-related 2.41 eV emission. The emission (Eem) and excitation (Eexc) energies used at the spectra measurements are shown in the legends and indicated by arrows. These energies were chosen to minimize the overlap of the selected spectral band with the other bands. 10

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Fig. 4. Luminescence decay curves measured at 4.2 K for (a) the intrinsic 2.89 eV (curve 1) and 2.63 eV (curve 2) emissions, (b) the Bi3+-related 2.53 eV emission, and (c) the Bi3+-related 2.41 eV emission of the YNbO4:Bi 0.2 at.% powder under excitation with the xenon flash lamp with the pulse duration of about 1 μs (for more details, see [39]). The emission (Eem) and excitation (Eexc) energies used at the measurements are shown in the legends.

Fig. 3. Temperature dependences of the emission intensity in the YNbO4:Bi 0.2 at.% powder measured for (a) the intrinsic 2.89 eV (solid line) and 2.63 eV (dashed line) emissions, (b) the Bi3+ related 2.53 eV emission, and (c) the Bi3+related 2.41 eV emission. In the insets, the I(T) dependences are presented in the lnI – 1/T coordinates to determine the activation energies Eq for the luminescence thermal quenching The emission (Eem) and excitation (Eexc) energies used at the measurements are shown in the legends.

(Fig. 7b) spectra of the Bi3+-related 2.41 eV emission with respect to the spectra of the 2.53 eV emission most clearly appears in the sample with the largest Bi content (5 at.%). Due to a strong overlap of the emission bands in YNbO4:Bi, the time-resolved spectra allow to separate better the emission bands of different origin, to determine the correct maxima positions of the emission bands and to choose the optimum energy Eexc for their selective excitation. Temperature dependences of the main component decay time of the 2.89 eV, 2.53, and 2.41 eV emissions are similar (Fig. 8). As the temperature increases up to about 20 K, the decay times of the slower component of all the emissions (τSC) decrease drastically (see also Fig. 5c, filled circles). The decay time of the fast component also decreases (Fig. 5b, filled circles). The reduction of its lightsum (Fig. 5b, empty circles) is accompanied with the lightsum enhancement of the slow component (Fig. 5c, empty circles). In the 30–80 K range, the decay times are practically independent of temperature, and at 79 K, their values are about 27–40 μs (Fig. 8a), 4.5 μs (Fig. 8b), and 6.3 μs (Fig. 8c), respectively. At higher temperatures, the shortening of the decay time is caused by the luminescence thermal quenching (compare Figs. 8 and 3). It is interesting to note that the decay time values do not reach the constant values with the decreasing temperature at least down to 4.2 K. For the intrinsic 2.89 eV emission, the temperature dependence presented in Fig. 8a is similar to that reported in Ref. [31] where the decay time of 380 μs was obtained at 1.5 K. The decay kinetics data presented in Fig. 5b, c and 8 indicate that the intrinsic 2.89 eV emission and both the Bi3+-related emissions (2.53 eV and 2.41 eV) of YNbO4:Bi are of similar origin and can be considered in a three-level scheme where two closely located excited levels have strongly different radiative decay probabilities. For the

the Bi3+-related 2.53 eV emission is also excited by this LED. Under 305 nm (4.06 eV) LED excitation, the decay curve obtained for the 2.53 eV emission is presented in Fig. 5a. It is evident that besides the slow component with τSC = 33 μs (Fig. 4b), the decay kinetics of this emission contains also the fast component with τFC = 0.37 μs. The emission spectra of the fast and the slow decay components coincide which indicates that both these decay components arise from the same 2.53 eV emission. An additional component with the decay time of about 2.8 μs could arise from the weaker 2.41 eV emission. Unfortunately, the fast decay kinetics of the 2.41 eV emission was not measured due to the absence of the suitable LED. Time-resolved emission and excitation spectra are measured at 79 K in the 8 μs – 2 ms time range for different emission energies (Eem) under excitation with different excitation energies (Eexc). In Fig. 6, the timeresolved emission spectra are presented for selected time moments where the considered emission band appears more clearly. It is evident that the intrinsic 2.89 eV emission, more clearly appearing in the sample with the smallest Bi content, is excited only in the band-to-band and exciton absorption region (Fig. 7a, filled squares). The slow 2.63 eV intrinsic emission is mainly excited in the same energy range (empty circles). The appearance of this emission under lower-energy excitation is most probably caused by its overlap with the Bi3+-related emission bands. The lower-energy shift of the emission (Fig. 6b) and excitation 11

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Fig. 6. Time-resolved emission spectra measured at 79 K (a) for the YNbO4:Bi 0.2 at.% powder under excitation in the exciton (filled circles), band-to-band (empty circles) and Bi3+-related (filled squares) absorption regions and (b) for the YNbO4:Bi 5 at.% powder under excitation in different ranges of the Bi3+related absorption band. The excitation energies (Eexc) used at the spectra measurements and the chosen time moments are shown in the legends.

Fig. 5. (a) Decay curve obtained for the 2.53 eV emission of the YNbO4:Bi 0.2 at.% powder under 305 nm (4.06 eV) nanosecond LED excitation. Approximate temperature dependences of the decay times (filled circles) and light sums (empty circles) of (a) the fast (FC) and (b) the slow (SC) decay components of the 2.53 eV emission measured in the 4.2–20 K temperature range.

intrinsic emission of YNbO4, such consideration was proposed in Ref. [31]. Temperature dependence of the slower component decay time is caused by the thermally stimulated transitions from the lower-energy metastable level to the higher-energy emitting level of the triplet RES. From the dependences of the decay time on the reciprocal temperature in the 4.2–25 K temperature range presented in the insets to Fig. 8, the energy distances (D) between the upper and the lower excited state levels can be estimated. The approximate D values obtained for the RES responsible for the 2.89 eV, 2.53, and 2.41 eV emissions, are 0.63 ± 0.02 meV, 0.78 ± 0.02, and 0.77 ± 0.02 meV, respectively. The value of D = 0.74 meV was obtained in Ref. [31] for the intrinsic emission of YNbO4. Thus, the spin-orbit splitting energies of the corresponding triplet RES are also close. The fast component of the luminescence decay arises from the radiative decay of the higher-energy emitting level. The temperature dependences of the luminescence decay time in YNbO4:Bi presented in Fig. 8 and the corresponding D values are similar to those reported earlier for the triplet luminescence of the excitons localized around Bi3+ ions in garnets, orthosilicates, tungstates, and orthovanadates (see, e.g., Refs. [39-45]). Therefore, we suggest that the 2.89 eV, 2.53, and 2.41 eV emissions investigated in this work are also of exciton origin. Indeed, owing to a strong exciton-phonon interaction and a weak spin–orbit interaction, the FWHM and S values of the triplet emission of the localized exciton are relatively large, but the spin–orbit

Fig. 7. Time-resolved excitation spectra measured at 79 K (a) for the intrinsic 2.89 eV (filled squares) and 2.63 eV (empty circles) emissions and for the Bi3+related 2.53 eV emission (filled circles) of the YNbO4:Bi 0.2 at.% powder, and (b) for the Bi3+-related 2.53 eV (filled circles) and 2.41 eV (empty circles) emission bands of the YNbO4:Bi 5 at.% powder. The emission energies (Eem) used at the excitation spectra measurements and the chosen time moments are shown in the legends.

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data presented above (especially temperature dependences of the luminescence decay time) clearly indicate that the Bi3+-related emission bands of YNbO4 cannot arise from the 3P1,0 → 1S0 electronic transitions of a Bi3+ ion and are of an exciton-like origin. No other emission which could arise from the triplet RES of Bi3+ ions is found in YNbO4:Bi. This means that the lowest-energy triplet excited state of a Bi3+ ion in YNbO4:Bi is located inside the conduction band. 3.3. Concentration dependence of the luminescence intensity In the recent years, many papers appeared where the Bi3+-related emission bands in various compounds were connected with dimer Bi centers (see, e.g., Refs. [43,46–50] and references therein). However, in many cases this conclusion was made without correct measurements of the concentration dependences of the luminescence intensity. To clarify if the Bi3+-related emission bands in YNbO4:Bi are connected with single or dimer Bi3+-related centers, the dependence of the emission intensity on the Bi content was investigated. For single impurity centers, the dependence of the absorption coefficient on the impurity content should be linear, while for dimer impurity centers, the superlinear dependence should be observed. The luminescence intensity is proportional to the absorption coefficient only under excitation in the energy range where the optical density does not exceed 0.5. Therefore, the correct dependence of the emission intensity on the Bi content in YNbO4:Bi could be obtained only under excitation in the low-energy region of the Bi3+-related absorption band (e.g., around 3.6 eV) where the optical density is surely low. As it is evident from Fig. 9, the concentration dependences of the 2.53 eV and 2.41 eV emissions intensity are strongly different. Under excitation at 85 K in the absorption band maximum (Eexc ≈ 4.0 eV), the maximum intensity of the 2.53 eV emission increases with the increasing Bi content (CBi), reaching the saturation at CBi > 5 at.% (dashed line). The divergence of the concentration dependence from the linear and the appearance of the saturation are most probably caused by too high optical density around 4.0 eV. As the 2.53 eV emission is observed in the samples with a low Bi content and in these samples its intensity has been found to increase linearly or sublinearly with the increasing Bi content (up to 0.5 at.% [33], 1 at.% [15,32] or 1.5–2 at.% [13,14]), we suggest that this emission arises from an exciton localized around a single Bi3+ ion (ex0Bi3+). However, under excitation at the lower-energy side of the Bi3+-related absorption band (Eexc = 3.6 eV), where the optical density is low, the dependence of the 2.41 eV emission intensity on the Bi content is superlinear (solid line). Similar concentration dependence is obtained for the emission intensity measured at Eem = 2.1 eV under Eexc = 3.6 eV. The superlinear concentration dependence of the emission intensity allows us to suggest that the 2.41 eV emission arises from

Fig. 8. Temperature dependences of the slow component decay time measured for (a) the intrinsic 2.89 eV emission and for the Bi3+-related (b) 2.53 eV and (c) 2.41 eV emissions. In the insets, the corresponding decay time dependences on the reciprocal temperature. The emission (Eem) and excitation (Eexc) energies used at the measurements are shown in the legends.

splitting energy of the triplet exciton state is very small. Due to that, the experimentally determined D values are usually of the order of 0.1–1 meV, i.e. they are by 1–2 orders of magnitude smaller as compared with the D values (50–110 meV, see, e.g., Refs. [42,43]) obtained for the triplet RES of Bi3+ ions in all the compounds investigated up to now. It should be noted that in this work, the exciton-like origin of the luminescence of Bi3+-doped niobates is confirmed by the first time. Unlike the localized exciton emission, the triplet emission of Bi3+ ions is characterized by relatively small values of the Stokes shifts and FWHM. The decay time of the slow (ms) decay component of this emission is temperature-independent from 4.2 up to 60–110 K due to a large spin-orbit splitting energy D of the corresponding triplet RES (see, e.g., Figs. 1 and 2 in Ref. [42]. This is caused by the fact that a free Bi3+ ion has the largest spin-orbit interaction energy (2.102 eV) among all the ns2 ions. In the above-mentioned temperature range, the slow component arises from the radiative decay of the metastable level of the triplet state. Thermally stimulated transitions from the metastable to emitting level of the triplet state, resulting in the shortening of the slow component decay time, start in this case at much higher temperature (T > 60–110 K) as compared with the excitons localized around Bi3+ ions (at T < 1.5 K in YNbO4, see Ref. [31]). No such emission is observed in YNbO4:Bi. Thus, a strong difference of the triplet RES parameters characteristic for excitons and Bi3+ ions in any material results in their strongly different luminescence characteristics which allow to make a justified conclusion on the origin of the corresponding luminescence center. The

Fig. 9. Dependences of the maximum luminescence intensity on the Bi content inside the powders measured for the 2.41 eV emission under excitation with Eexc = 3.6 eV (solid line) and for the 2.53 eV emission under excitation in the maximum of the Bi3+-related absorption band (dashed line). 13

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should correspond to the absorption edge of the YNbO4 host, i.e., the low-energy edge of the exciton absorption band. The data presented in Fig. 10 indicate that the recombining close electron-hole pairs can be created also under irradiation in the exciton absorption region of YNbO4 (around 4.95 eV) but with much lower efficiency. 4. Conclusions The detailed and systematic investigation of the intrinsic and the Bi3+-related luminescence in microcrystalline YNbO4:Bi powders with different Bi content carried out by the methods of the steady-state and time-resolved luminescence spectroscopy in the 4.2–500 K temperature range allows us to clarify the origin of the corresponding luminescence centers and their absorption (excitation) and emission bands, the structure and parameters of their excited states, and radiative and nonradiative processes ongoing in these states. Two Bi3+-related emission bands are observed in YNbO4:Bi. The analysis of temperature dependences of their decay time allows us to make the conclusion on their exciton-like origin. The most intense emission band peaking at about 2.53 eV and excited around 4.09 eV is ascribed to the radiative decay of an exciton localized around a single Bi3+ ion. The weaker 2.41 eV emission with the superlinear intensity dependence versus impurity concentration is ascribed to an exciton localized around a dimer Bi3+ center. No ultraviolet emission arising from the 3P1,0 → 1S0 transition of a Bi3+ ion is found. This fact as well as the exciton-like origin of the Bi3+-related emission bands indicate that the triplet excited level of Bi3+ ion is located inside the conduction band of YNbO4. Two intrinsic emission bands are also observed in YNbO4:Bi under excitation in the host absorption region. The 2.89 eV emission is ascribed to the radiative decay of the self-trapped exciton, while the broad 2.63 eV emission is suggested to arise from the delayed electronhole recombination processes in YNbO4. After irradiation of the YNbO4:Bi powders in the exciton and bandto-band transitions region at 85 K, a weak afterglow and thermally stimulated luminescence appear. From the dependence of the afterglow and thermally stimulated luminescence intensity on the irradiation energy, the band gap energy in YNbO4 is estimated to be about 5.3–5.4 eV.

Fig. 10. Dependences of the afterglow (solid line) and TSL (dashed line) intensity on the irradiation energy measured for the YNbO4:Bi 1 at.% powder irradiated at 85 K for tirr = 10 min and tirr = 2 h, respectively. In the inset (a), the dependence of the afterglow intensity on time. In the inset (b), the TSL glow curve measured after irradiation of the sample at 85 K in the 5.0–5.6 eV energy range.

an exciton localized around a dimer Bi3+ center (ex0{Bi3+ - Bi3+}). The emission of this type has been earlier observed in Bi3+-doped aluminum garnets (see, e.g., [42,43]). Despite the negligible overlap of the emission and absorption (excitation) bands due to the large Stokes shift of the Bi3+-related emission bands in YNbO4:Bi, the intensity reduction at high Bi contents was explained in Refs. [13–15,32,33] by the concentration quenching. However, it is not excluded that the strongly different and relatively small optimum CBi values obtained in these works are caused by the fact that the concentration dependences of the luminescence intensity were measured under excitation in the energy range with too high optical density. 3.4. Afterglow and thermally stimulated luminescence in the UV irradiated YNbO4:Bi powders After irradiation of YNbO4:Bi in the Eirr > 4.4 eV energy region at 85 K, a fast afterglow appears. It completely decays in 15–20 s after the moment t = 0 when the irradiation is switched off (see, e.g., the inset (a) to Fig. 10). The afterglow intensity is constant and reaches maximum at Eirr > 5.4 eV, while decreases with the decreasing irradiation energy (Fig. 10, solid line). No afterglow appears after irradiation in the Bi3+-related absorption band. This means that no close electron-hole pairs are optically created in this energy range, despite the fact that the lowest-energy excited level of Bi3+ is located inside the CB. The TSL is also absent even after intense and prolonged irradiation of the samples at 85 K. This indicates that the stable electron and hole centers are not optically created in this energy range as well. The weak TSL peak appears around 160 K (see the inset (b) to Fig. 10) only after irradiation with Eirr > 4.6 eV (Fig. 10, dashed line). From the dependence of the afterglow and TSL intensity on the irradiation energy presented in Fig. 10, the band gap energy Eg in YNbO4 can be estimated. Indeed, the afterglow and TSL can appear as a result of optical creation of free electrons and holes, and the minimum energy, where the free charge carriers can be created, is the energy difference between the top of the valence band and the bottom of the conduction band, i.e., the band gap energy Eg. As it was mentioned in Introduction, strongly different Eg values were obtained for YNbO4 in different papers. The data obtained in this work indicate that the values of Eg > 5.3 eV reported in Refs. [21–24] should be more correct. Much smaller energy values of 4.3–4.5 eV reported in Refs. [7,11,25–29]

Acknowledgments The work was supported by the ERDF funding in Estonia granted to the Center of Excellence TK141 “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics“ (project No. 2014-2020.4.01.15-0011), by the Polish National Science Center (project 2015/17/B/ST5/01658), Poland, by the Ministry of Education and Science of Ukraine (project DB/Feryt, N 0118U000264), Ukraine, and by the European Union within the European Regional Development Fund through the Innovative Economy grant (POIG.01.01.02-00-108/09). The research of the visiting PhD student A. Kissabekova for this article was conducted with the support from the European Regional Development Fund, Programme “Supporting internationalisation of higher education, mobility and new generations Dora Plus”. References [1] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217–222. [2] G. Blasse, A. Bril, J. Lumin. 3 (1970) 109–131. [3] J. Hou, R. Zhou, J. Zhang, Z. Wang, Z. Zhang, Z. Ding, J. Phys. Chem. C 121 (2017) 14787–14794. [4] Y. Lü, X. Tang, L. Yan, K. Li, X. Liu, M. Shang, Ch. Li, J. Lin, J. Phys. Chem. C 117 (2013) (1980) 21972–21982. [5] X. Liu, C. Chen, S. Li, Y. Dai, H. Guo, X. Tang, Y. Xie, L. Yan, Inorg. Chem. 55 (2016) 10383–10396.

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