Luminescence of Borates with Yttrium and Lutetium ... - Springer Link

ISSN 10637834, Physics of the Solid State, 2013, Vol. 55, No. 1, pp. 150–159. © Pleiades Publishing, Ltd., 2013. Original Russian Text © D.A. Spassky, V.S. Levushkina, V.V. Mikhailin, B.I. Zadneprovski, M.S. Tret’yakova, 2013, published in Fizika Tverdogo Tela, 2013, Vol. 55, No. 1, pp. 134–142.

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Luminescence of Borates with Yttrium and Lutetium Cations D. A. Spasskya, V. S. Levushkinab, *, V. V. Mikhailina, b, B. I. Zadneprovskic, and M. S. Tret’yakovac a

Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia b Moscow State University, Moscow, 119991 Russia * email: [email protected] c Central Scientific Research Institute of Chemistry and Mechanics, ul. Nagatinskaya 16A, Moscow, 115487 Russia Received April 9, 2012

Abstract—This paper reports on the results of an investigation into the luminescence properties of yttrium and lutetium borates, as well as the Y0.35Lu0.65BO3 solid solution, under excitation with the synchrotron radi ation in the Xray and ultraviolet spectral ranges. It has been shown that there exists an intrinsic luminescence band in the ultraviolet spectral range 260–270 nm due to the luminescence of selftrapped excitons. It has been found that the kinetic characteristics of this band depend on the density of the exciting synchrotron radi ation. A number of luminescence bands have been observed in the longwavelength range due to the presence of defects in the crystal structure of borates. It has also been shown that the energy transfer to impurity centers has a recombination nature and can also occur through impact ionization of defects. It has been revealed that, for the solid solution, the excitation efficiency of the luminescence of defects increases under interband exci tation, which can be associated with the limited separation of the components of an electron–hole pair as a result of shortrange order disturbance in the structure of the solid solution. DOI: 10.1134/S1063783413010319

1. INTRODUCTION Borate compounds doped with rareearth elements are efficient converters of highenergy radiation to vis ible light. A number of borates have already been used in plasma displays, fluorescent tubes, and scintillation detectors. In particular, lutetium borate doped with cerium ions is a promising material for the use in scin tillation detectors, and yttrium gadolinium borate doped with europium has been widely used as a phos phor in plasma displays. Despite the fact that borates exhibit a highinten sity luminescence due to doping with rareearth ions, the intrinsic luminescence is also of interest for the investigation. It has been shown that, in a number of oxides, the intrinsic luminescence itself and the intrin sic luminescence intensity affect the efficiency of exci tation energy transfer to impurity luminescence cen ters associated with cerium ions. For example, it has been demonstrated that, in lutetium orthosilicate and lutetium pyrosilicate crystals doped with cerium ions, the temperature quenching of intrinsic luminescence leads to a simultaneous enhancement in the lumines cence of cerium ions [1, 2]. It has also been found that the absorption bands of the cerium impurity in ScBO3 : Ce are overlapped with the intrinsic lumines cence bands of the borate [3]. This leads to quenching of the intrinsic luminescence and to a change in the luminescence decay time, which is associated with the excitation energy transfer from the regular lattice of

the crystal to impurity luminescence centers. Korzhik and Trower [4] assumed that the scintillation light yield, in some cases, can be estimated from the intrin sic luminescence itself and the intrinsic luminescence intensity, because intrinsic luminescence centers are intermediate states in the energy transfer to lumines cence centers of cerium. Therefore, investigation of the intrinsic luminescence properties in a series of borates is of interest for the design and synthesis of new scintillation materials. In the spectra of undoped borates, there usually exist a number of luminescence bands under excita tion in the ultraviolet (UV), vacuum ultraviolet (VUV), and Xray ranges [5–8]. One or several intrin sic luminescence bands are observed in the UV range, whereas the bands observed in the longwavelength range are assigned to the luminescence of defects. The intrinsic luminescence, as a rule, is attributed to the luminescence of selftrapped excitons (STEs). The lit erature also provides data on the luminescence of undoped borates of yttrium and lutetium. For exam ple, Ivanenko [9] showed that undoped yttrium borate YBO3 at a temperature of 80 K exhibits luminescence with a maximum at 4.7 eV and a weak luminescence band in the visible range (2–3 eV). Ivanenko assumed that UV luminescence centers, in this case, are intra 5–

anion excitons in the BO 4 complexes. Xray lumi nescence of undoped lutetium borate LuBO3 was

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investigated at room temperature by Duan et al. [10]. These authors observed a broad luminescence band with a maximum at 360 nm (3.44 eV). They suggested that this band is associated with the intrinsic lumines cence, although a definite conclusion about the nature of luminescence centers was not drawn. In the spectra of LuBO3 : Ce under Xray excitation, in addition to the Ce3+ luminescence, Mansuy et al. [11] observed a weak luminescence band in the range 200–350 nm, which was assigned to the STE luminescence. Thus, as far as we know, the luminescence properties of undoped borates of yttrium and lutetium have not been adequately investigated and the results available in the literature are rather contradictory. In this work, we have investigated the luminescence properties of nominally undoped borates with cations of yttrium YBO3 and lutetium LuBO3, as well as the solid solution of yttrium and lutetium borates, namely, Y0.35Lu0.65BO3. 2. EXPERIMENTAL TECHNIQUE AND SAMPLE PREPARATION Yttrium and lutetium borates are insulators with a band gap of greater than 6 eV. Therefore, the excita tion of intrinsic luminescence requires the use of exci tation sources with an intense radiation in the vacuum ultraviolet region. Synchrotron radiation is the most convenient source of excitation for the use in studying the luminescence properties of widebandgap insula tors owing to both the highintensity continuous radi ation over a wide spectral range from the infrared region to the Xray region and the time structure of synchrotron radiation. The luminescence characteristics of the borates were measured using synchrotron radiation in the VUV and Xray ranges at the DORIS III storage ring of the German Synchrotron Research Centre (DESY) (Hamburg, Germany). The spectral and kinetic char acteristics of the luminescence under excitation in the range from 3.7 to 25.0 eV were measured at the SUPERLUMI experimental station installed in the synchrotron radiation channel I [12]. The lumines cence measurements were carried out using an ARC “Spectra Pro 300i” monochromator operating in the spectrograph and CCD detector modes (Princeton Instruments). The spectral region of luminescence measurements ranged from 200 to 1000 nm. The mea sured spectra were normalized to the function of the spectral sensitivity of the detection system. For the decomposition of the luminescence spectra into Gaussian components, the spectra were converted from nanoscale units of measurement into energy units. In this conversion, the intensities were multi plied by the square of the corresponding wavelength so that the area under the curve remained constant. The luminescence measurements under excitation in the soft Xray range were performed on a facility PHYSICS OF THE SOLID STATE

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installed at the BW3 undulatorradiation beamline at the DORIS III storage ring. The excitation photon flux density at the sample reached 1012 photons s–1 [13]. The secondary monochromator of the facility was optimized for the UV spectral region, and the rotation angle of the diffraction grating of the mono chromator permitted measurements of the lumines cence spectra in the range from 110 to 500 nm. The timeresolved luminescence spectra were recorded using the timecorrelated singlephoton counting technique. Luminescence was measured in the “fast” and “slow” time windows, which ranged from 0 to 6.5 ns and from 40 to 80 ns, respectively, relative to the maximum of the excitation pulse of synchrotron radi ation. The synchrotron radiation flux density at the sample was controlled by varying the width of the undulator gap and checked against the electric current through a gold mesh mounted on the path of the syn chrotron radiation. The luminescence spectra were not normalized to the function of the instrumental sensitivity of the detection system of the facility installed at the BW3 beamline. The measurements were carried out in the temperature range from 10 to 300 K. Samples of the YBO3, Lu0.65Y0.35BO3, and LuBO3 borates were synthesized using the sol–gel technology. The external morphology of crystalline particles is almost identical for all the synthesized samples. According to the analysis of the granulometric compo sition of the prepared powders on a Shimadzu SALD 2201 laser diffraction analyzer, the dominant size of particles of all the compositions is approximately equal to 500 nm. As follows from the Xray diffraction data, the YBO3 borate crystallizes in the structural type of vaterite. The presence of lutetium cations in the structure of the sample leads to a natural transition from the monophase composition to a composition that is characteristic of the LuBO3 borate and includes two phases isostructural with the phases of vaterite and calcite. The relative fraction of the calcite phase in the samples is relatively small and, according to the per formed analysis, is equal to 5% in LuBO3 and reaches 14% in Lu0.65Y0.35BO3. 3. EXPERIMENTAL RESULTS 3.1. Luminescence under HighEnergy Excitation The luminescence spectra of the YBO3, LuBO3, and Lu0.65Y0.35BO3 borates at the excitation energy Eex = 130 eV and temperature T = 300 K exhibit a broad luminescence band with a maximum in the UV spectral region (Fig. 1a). A narrow luminescence peak at 313 nm is associated with the presence of uncon trollable Gd3+ impurities in the samples. The position of the maximum of the broad luminescence band depends on the sample, and the maximum itself is observed at 290 nm for the YBO3 borate and at 275 nm for the LuBO3 and Lu0.65Y0.35BO3 borates. The use of

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Fig. 2. Kinetics of the luminescence decay of (1) YBO3, (2) Y0.35Lu0.65BO3, and (3) LuBO3 borates for the param eters Eex = 130 eV, λem = 270 nm, and T = 300 K. The inset shows the kinetics of the luminescence decay of LuBO3 for two excitation photon flux densities at the sample: (1) ~1012 and (2) ~1011 photons s–1.

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Fig. 1. Luminescence spectra of (1) YBO3, (2) Y0.35Lu0.65BO3, and (3) LuBO3 borates for the excitation energy Eex = 130 eV at temperatures T = (a) 300 and (b) 10 K. Inset (a) shows (1, 2) timeresolved lumines cence spectra of YBO3 measured in (1) the “fast” (0– 6.4 ns) time window and (2) the “slow” (40–80 ns) time window and (3) difference of these spectra at T = 300 K. Inset (b) shows the degradation of the luminescence inten sity in (1) YBO3, (2) Y0.35Lu0.65BO3, and (3) LuBO3 for the excitation energy Eex = 130 eV, emission wavelength λem = 260 nm, and temperature T = 10 K.

timeresolved spectroscopy made it possible to reveal that the luminescence band is nonelementary; i.e., the position of the peak depends on the time delay in the measurement of the luminescence spectrum after the synchrotron radiation pulse (see inset to Fig. 1a). The difference in the luminescence spectra measured in the fast and slow time windows is an elementary band with a maximum at 4.75 eV. The parameters of the bands obtained from the difference in the spectra for each of the samples are presented in the table. A shift in the maximum of the nonelementary band from 290 nm in YBO3 to 275 nm in LuBO3 and Lu0.65Y0.35BO3 is caused by an increase in the relative

contribution of the shortwavelength band to the luminescence spectrum. The kinetics of the luminescence decay is mea sured at T = 300 K and consists of a fast component with a decay time of the order of 10–8 s and a slow component that forms the pedestal (Fig. 2). The decay time of the fast component increases upon changing over from the yttrium borate to the lutetium borate. A change in the photon flux of the synchrotron radiation at the sample also leads to a variation in the lumines cence decay time. A decrease in the photon flux by one order of magnitude results in an increase in the lumi nescence decay time. The table presents the lumines cence decay times obtained for the studied borates from the kinetics of the luminescence decay at differ ent excitation densities. The profile of the lumines cence spectra with a variation in the density of photons incident on the sample remains unchanged. The slow component of the kinetics of the luminescence decay, apparently, is a characteristic of the longwavelength band with a maximum at 290 nm. Indeed, the highest level of the pedestal for the fast component of the kinetics of the luminescence decay is observed for the YBO3 sample, in which the longwavelength band has the largest relative contribution. Most probably, since the pedestal makes a significant contribution to the kinetics of the luminescence decay for the YBO3 sam ple, no change in the decay time of the fast component is observed with a variation in the density of the excit ing radiation. As the temperature decreases to 10 K, the lumines cence spectrum shifts toward the shortwavelength

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range and consists of a single band (Fig. 1b). The pro file of the band does not depend on the time delay in the measurement of the luminescence spectrum after the synchrotron radiation pulse. The position of the maximum of this band is well consistent with the band obtained from the difference between the fast and slow components at the temperature T = 300 K (see inset to Fig. 1a). The luminescence intensity at T = 10 K is increased by a factor of approximately ten as com pared to T = 300 K. In this case, there is a gradual decrease in the luminescence intensity upon long term exposure of the sample to soft Xrays (see inset to Fig. 1b). The most pronounced degradation of the luminescence intensity occurs in the solid solution. During the irradiation for 600 s, the luminescence intensity is reduced by 25%. 3.2. Luminescence under UV and VUV Excitation The use of UV and VUV synchrotron radiations makes it possible to selectively excite luminescence bands due to different luminescence centers. The luminescence spectra of the studied borates under this excitation are shown in Fig. 3. At a low temperature T = 10 K, the spectrum of each sample contains at least two luminescence bands with maxima in the UV and visible spectral regions (Fig. 3). The UV luminescence band in the spectrum of the YBO3 borate has a maximum at 264 nm. Upon substitution of lutetium cations for yttrium cations, the maximum of the luminescence band is shifted toward the longwavelength range up to 270 nm. The UV luminescence band can be approximated by a sin gle Gaussian. The parameters of the approximation of the luminescence spectrum for each of the samples are presented in the table. Upon substitution of lutetium for yttrium, the intensity of the UV luminescence gradually decreases under excitation at the maximum of the lowenergy peak. The UV luminescence excitation spectra of the studied borates are shown in Fig. 4a. The spectra are normalized to the intensity of the first excitation peak. In the region of the fundamental absorption edge from 7.5 to 7.8 eV, we observe a threshold increase in the intensity and, then, a welldefined peak. The maxima of the peaks for the lutetium borate and the yttrium lutetium borate coincide (at 8.35 eV). In the lumines cence excitation spectrum of the yttrium borate, the first peak is shifted toward the lowenergy range and observed at 7.7 eV; moreover, this peak is significantly narrowed. As the excitation energy increases to 14.0 eV, the intensity of the luminescence excitation spectra gradually decreases without pronounced structural features. At excitation energies above 17.0 eV, the intensity of the luminescence excitation spectrum again begins to increase. The maximum of the longwavelength lumines cence band is located in the visible region of the spec trum and, depending on the sample, is shifted from PHYSICS OF THE SOLID STATE

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Decay times τ of the STE luminescence and parameters of the STE luminescence bands

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Position of the maximum of the FWHM, STE luminescence eV band, eV*

4.75 (RT, Xray) 4.71 (LHT, Xray) 4.61 (LHT, UV) 7.14 4.61 (RT, Xray) (8.72)** 4.67 (LHT, Xray) 4.52 (LHT, UV) 11.09 4.65 (RT, Xray) (17.89) 4.63 (LHT, Xray) 4.48 (LHT, UV)

0.80 0.82 0.89 0.82 0.79 0.92 0.81 0.83 0.95

*Given in parentheses are the measurement temperature (RT = 300 K or LHT = 10 K) and the excitation type ( Xray radiation or UV radiation). **Given in parentheses is the value obtained for the excitation density decreased by a factor of 10.

425 to 450 nm (see Fig. 3b). The spectrum of the Lu0.65Y0.35BO3 sample also contains an additional broad luminescence band with the maximum shifted toward the shortwavelength range up to 350 nm and a number of narrow luminescence bands in the wave length range from 590 to 620 nm, which can be asso ciated with the emission of uncontrollable europium impurities. The excitation spectra for longwavelength lumi nescence bands are shown in Fig. 4b. The excitation spectra of different samples were measured under sim ilar experimental conditions; consequently, these spectra contain information about the relative lumi nescence intensity of the samples. The longwave length luminescence is most effectively excited in the transparent region of the borates. In the energy range from 4.5 to 8.1 eV, there is a nonelementary broad peak with shoulders on the highenergy and low energy wings with excitation energies of 5.2 and 7.3 eV. The maximum of the broad peak is observed in the range from 5.7 to 6.6 eV, and its position depends on the compound. The excitation spectra of the lumines cence bands with maxima at 430 and 350 nm for the Lu0.65Y0.35BO3 sample almost coincide. This can be associated with the strong spectral overlap of the lumi nescence bands; as a result, the excitation spectra of the two aforementioned bands contribute to each other. However, the excitation spectrum for the long wavelength band exhibits an additional peak at 5.2 eV, which is not observed in the excitation spectrum of the luminescence band at 350 nm. It is under excitation in this peak that the selective excitation of the lumines cence band at 450 nm becomes possible.

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Fig. 3. (a) UV luminescence spectra measured at the tem perature T = 10 K for (1) YBO3, Eex = 7.75 eV; (2) Lu0.65Y0.35BO3, Eex = 8.30 eV; and (3) LuBO3, Eex = 8.30 eV. The inset shows the temperature dependences of the intensity of (1–3) intrinsic luminescence of the borates for (1) YBO3, Eex = 7.75 eV; (2) Lu0.65Y0.35BO3, Eex = 8.40 eV; and (3) LuBO3, Eex = 8.40 eV and (4) lumines cence due to defects of the crystal structure for LuBO3, Eex = 6.50 eV. The spectra are normalized in intensity to unity. (b) Luminescence spectra measured at T = 10 K in the visible range for (1) YBO3, Eex = 5.9 eV; (2, 4) Lu0.65Y0.35BO3, Eex = (2) 5.2 eV and (4) 6.5 eV; and (3) LuBO3, Eex = 6.5 eV. The spectra are normalized in intensity to unity.

Fig. 4. (a) Luminescence excitation spectra of (1) YBO3, (2) Lu0.65Y0.35BO3, and (3) LuBO3 borates measured in the intrinsic emission band at the wavelength λem = 270 nm and temperature T = 10 K. Curve 4 shows the ratio of the luminescence excitation spectra at wavelengths λem = 450 and 270 nm for Lu0.65Y0.35BO3. (b) Excitation spectra of the luminescence due to defects in the crystal lattice for (1) YBO3, (2) Lu0.65Y0.35BO3, and (3) LuBO3 measured in the intrinsic emission band at λem = 450 nm and T = 10 K. The inset shows the luminescence excitation spectra of Lu0.65Y0.35BO3 measured at wavelengths λem = (1) 450 and (2) 350 nm.

The relative intensity of the luminescence in the visible range gradually decreases in the series YBO3, Lu0.65Y0.35BO3, and LuBO3 under excitation of the luminescence in the first peak of the excitation spec trum (5.7–6.6 eV). The luminescence intensity signif icantly decreases under excitation in the region of the fundamental absorption edge. An increase in the intensity of the excitation spectrum begins to occur only from 14 eV. It should be noted that, under excita

tion in the region of the fundamental absorption edge, the relative intensity of the longwavelength lumines cence has the maximum value for the solid solution. The luminescence of all three samples undergoes temperature quenching due to heating from 10 to 300 K (see inset to Fig. 3a). The luminescence inten sity is reduced by half for LuBO3 at a temperature of 146 K, for Lu0.65Y0.35BO3 at 154 K, and for YBO3 at 176 K. The luminescence quenching curve for LuBO3


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We note the main differences in the luminescence and luminescence excitation spectra of the borates that arise when the samples are heated from 10 to 300 K. Instead of the UV luminescence band at 264 nm in the spectrum of YBO3 at 300 K, there arises an addi tional luminescence band with a maximum at 290 nm (see inset to Fig. 5). The excitation spectrum of this band is shown in Fig. 5. As can be seen, this spectrum is significantly different from the excitation spectrum of the UV luminescence band at 264 nm (Fig. 4a). The luminescence band at 290 nm is excited in the trans parent region of the yttrium borate, and the excitation peaks are antisymbatic to the peaks observed in the excitation spectrum of the longwavelength lumines cence band at 450 nm (T = 300 K). In the region of the fundamental absorption edge, the excitation spectra of the two aforementioned bands coincide, and their intensity gradually increases with an increase in the excitation energy. Another difference with an increase in the temper ature to 300 K is observed in the excitation spectra of UV luminescence for the lutetium borate and yttrium lutetium borate. The excitation threshold is shifted toward the lowenergy range by 0.2–0.3 eV in the region of the fundamental absorption edge and by No. 1

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Fig. 6. UV luminescence excitation spectra (λem = 270 nm) of the LuBO3 borate at temperatures T = 10 (curve 1) and 300 K (curve 2). The spectra are normalized to the intensity of the first excitation peak.

and Lu0.65Y0.35BO3 is flatter than that for YBO3. As a result, the UV luminescence in YBO3 at T = 300 K, in contrast to that in LuBO3 and Lu0.65Y0.35BO3, is com pletely quenched. The luminescence in the visible range also undergoes temperature quenching, as is seen in LuBO3. The shape of this curve is almost iden tical to that of the UV luminescence.

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Fig. 5. Luminescence excitation spectra of the YBO3 borate at the temperature T = 300 K for emission wave lengths λem = (1) 430 and (2) 290 nm. The inset shows the luminescence spectrum at the excitation energy Eex = 9 eV.


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4. DISCUSSION OF THE RESULTS The experimental results have demonstrated that the UV luminescence spectra of the yttrium borate, lutetium borate, and yttrium lutetium borate at T = 300 K contain two overlapping luminescence bands with maxima at 260–270 and 290 nm. The first band is characterized by a fast component of the kinetics of the luminescence decay and becomes dominant due to the cooling of the sample to 10 K. We believe that it is this band which corresponds to the intrinsic lumines cence of the yttrium and lutetium borates and origi nates from the STE luminescence. This assumption is confirmed by the following experimental data. The lowenergy luminescence excitation threshold is 7.5 eV. This value is in satisfactory agreement with the existing estimates of the band gap of yttrium borate in the range 7.0–7.1 eV [14, 15]. The energy transfer to intrinsic luminescence centers begins to occur in the region of the fundamental absorption edge, which, in oxides, is usually described by the Urbach rule [16]. From the Urbach rule, it follows that, with an increase in the temperature, the fundamental absorption edge shifts toward the lowenergy range due to the enhancement of the electron–phonon interaction. The lowenergy threshold in the excitation spectrum of the UV luminescence also shifts with the increase of temperature toward the highenergy range (Fig. 6) fol lowing the shift of the fundamental absorption edge. The shift equal 0.2 eV is characteristic of the excitation spectra of the intrinsic luminescence in complex oxides (see, for example, [17]).

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The shift of the fundamental absorption edge with an increase in the temperature also manifests itself in a shift of the threshold of increase in the intensity of the UV luminescence excitation spectrum at Eex > 17 eV. The observed increase in the intensity of the spectrum is associated with the multiplication effect of elec tronic excitations, when the energy of an excited elec tron (or hole) is sufficient to create one more elec tron–hole pair. The additional e–h pair is excited through the Auger relaxation of the primary excited electron or hole. The threshold of increase in the intensity of the spectrum in the region of photon mul tiplication is shifted toward the lowenergy range by 1 eV when the temperature is changed from 10 to 300 K, which exceeds the shift of the spectrum in the region of the fundamental absorption edge (0.2 eV). This is explained by the increase in the probability of relaxation of highenergy electrons through the inter action with phonons with an increase in the tempera ture. As a result of this interaction with phonons, high energy electrons in the conduction band lose energy and reside below the multiplication threshold of elec tronic excitations. Thus, the UV luminescence is excited at the funda mental absorption edge and is not excited in the trans parent region of the crystal, which is also characteris tic precisely of the intrinsic luminescence. Intrinsic luminescence of complex oxides is often the STE luminescence. The results obtained from the analysis of the luminescence excitation spectra and decay kinetics count in favor of this mechanism of luminescence in the yttrium and lutetium borates. The behavior of the excitation spectrum in the region of the fundamental absorption edge, namely, a decrease in the luminescence intensity after the first peak, is characteristic of exciton energy transfer to intrinsic luminescence centers. The STE lumines cence has a maximum intensity in the region of the direct creation of excitons. The position of the first peak in the excitation spectrum of the UV lumines cence at 7.75 eV, apparently, corresponds to the direct creation of excitons in the YBO3 borate. As will be shown below, the energy at the maximum of this peak does not exceed the band gap (Eg). An increase in the excitation energy leads to the formation of separated electron–hole pairs in the crystal. The average dis tance between components of an electron–hole pair increases with an increase in the excitation energy. In this case, the probability that the separated electrons and holes will be bound into an exciton decreases, which, in turn, leads to a decrease in the intensity of the luminescence excitation spectra of the borates. The kinetics of the UV luminescence decay depends on the density of incident Xray photons. As the density of the synchrotron radiation increases by one order of magnitude (from 1011 to 1012 photons s–1), there occurs an acceleration of the initial stage of the luminescence decay. Similar effects of the acceleration

of the decay kinetics of the STE luminescence were observed previously in widebandgap insulators, in particular, in other complex oxides, for example, in cadmium tungstate under excitation by higher har monics of a titanium : sapphire laser [18], as well as in zinc and lead tungstates under excitation by the fourth harmonic of a highpower YAG : Nd laser [19, 20]. This effect is associated with the fact that, at a high density of the exciting radiation, the concentration of excitons in the initial stage of relaxation is so large that the distance between them does not exceed the dipole–dipole interaction radius. Consequently, the excitons interact with each other, which leads to a nonradiative relaxation of one of these excitons and to an energy transfer to another exciton through the Auger process. As a result of the nonradiative quench ing of a part of the excitons, in the initial stage of energy relaxation the kinetics of the luminescence decay accelerates and deviates from the exponential law. For cadmium tungstate, the distance at which the excitons must be located to ensure this process was estimated at approximately 2.1 nm. A change in the luminescence decay time with a variation in the exci tation density is characteristic precisely of the exciton luminescence. The observation of this effect in borates further confirms the hypothesis of the excitonic nature of the UV luminescence band. Note that we also inves tigated the dependence of the kinetics of Ce3+ lumi nescence decay in the yttrium and lutetium borates on the density of incident synchrotron radiation. In this case, we did not observe any changes in the kinetics of the luminescence decay, because the luminescence of cerium has a recombination nature rather than the excitonic nature. It should also be noted that the decay times of the STE luminescence reach a few nanosec onds. This indicates that excitons in the studied borates have a singlet nature. In order to answer the question as to where the self trapping of excitons occurs, it is necessary to have information on the crystal structure and the structure of energy bands. Borates belong to the class of ternary oxides that contain anionic groups with strong intra atomic covalent bonds. The crystal structure of vater ite consists of triangles formed by vertexshared BO4 tetrahedra, so that the oxygen atoms located in vertices of the tetrahedra are involved in the formation of two BO4 complexes simultaneously. Therefore, it is rea sonable to assume that the nature of the UV lumines cence band is associated with the emission of excitons 5– localized in the BO 4 complexes. Previously, it was shown for a number of borates that the selftrapping of excitons occurs precisely in the oxyanion complex [21–23]. An important argument in favor of this con clusion is provided by the data obtained from the cal culations of the structure of the energy bands, accord ing to which the conduction band bottom and the valence band top, as a rule, are formed from the elec tronic states of this complex.


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The energy band structure of yttrium and lutetium borates was calculated in [14, 24]. In the YBO3 borate, the valence band is separated into one wide subband and two narrow subbands. The upper part of the valence band predominantly consists of the 2p elec tronic states of oxygen, which are weakly hybridized with the electronic states of boron. At the bottom of the conduction band in the yttrium borate, there is a narrow splitoff subband that is predominantly formed by the 4d electronic states of yttrium. The main part of the conduction band, which is formed by the elec tronic states of yttrium with some contribution of the electronic states of boron, is located at higher energies. With an increase in the energy, the degree of hybridiza tion of the electronic states of yttrium and boron increases. In the lutetium borate, which crystallizes in the structural type of vaterite, the 4f electronic states of lutetium lie deep in the valence band. Near the bottom of the conduction band, the 5d electronic states of lutetium dominate; however, they do not form a split off subband. The nonbonding states of boron appear in the conduction band at energies above 10 eV. There fore, the electronic states of boron do not contribute significantly to the formation of both the conduction band bottom and the valence band top. Hence, it can be assumed that the hole component of the exciton in the yttrium and lutetium borates is localized at the 2p states of oxygen, whereas the electronic component is localized at the d states of the cation. It should be noted that the d states of yttrium at the bottom of the conduction band are localized to a greater extent than the states of lutetium and form a narrow splitoff sub band at the bottom of the conduction band. It is this difference that can be responsible for the narrower profile of the first peak in the excitation spectrum of the STE luminescence in YBO3 as compared to LuBO3. The presence of a localized subband in YBO3 can also be responsible for the more intense STE lumi nescence in YBO3 as compared to LuBO3 under exci tation in the region of the direct creation of excitons, as well as for the shorter luminescence decay times. The solid solution is characterized by intermediate values of both the intensity of the STE luminescence and the luminescence decay time, which, apparently, is associated with the gradual changes in these quanti ties upon substitution of lutetium cations for yttrium cations. It should also be noted that, under Xray exci tation, the intensity of the STE luminescence in the solid solution is lower than that in LuBO3, which is explained by a higher degree of degradation of the luminescence in the solid solution under irradiation (see Fig. 1b). The higher degree of degradation of the luminescence in the solid solution is associated with the formation of defects in the crystal structure and can be due to the existence of a relatively high concen tration of the calcite phase. PHYSICS OF THE SOLID STATE

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It was shown using the timeresolved spectroscopy under Xray excitation that the UV luminescence band of the studied borates at room temperature is nonelementary. Apart from the STE luminescence band, there is an additional band at ~290 nm with a slow kinetics of the luminescence decay (τ > 10–6 s). Let us now analyze the excitation spectrum of the luminescence band at 290 nm for YBO3, where the STE luminescence at 300 K is completely quenched. In contrast to the STE luminescence, this band is effectively excited in the transparent region of the borates, and the excitation spectrum exhibits peaks at energies of 5.55, 6.25, and 7.30 eV. This suggests that the band under consideration is attributed to radiative transitions in defects of the crystal structure. The luminescence intensity significantly decreases when the excitation energy reaches the fundamental absorp tion edge. In this region, there occurs a direct creation of excitons that provide energy transfer to the compet ing band of the STE luminescence (quenched in YBO3 at 300 K). A further increase in the excitation energy leads to a gradual increase in the intensity of the lumi nescence excited at Eex > 8.3 eV. This behavior of the excitation spectrum is characteristic of the recombi nationtype energy transfer to luminescence centers, when charge carriers are successively captured into a luminescence center [25]. Thus, an increase in the luminescence intensity at an excitation energy Eex > 8.3 eV indicates the appearance of free charge carriers. This value is an estimate of the band gap in YBO3. It should be noted that this estimate is significantly higher than the estimates obtained previously, i.e., in the range 7.0–7.1 eV [14, 15]. Moreover, in [14], the estimate of the band gap was obtained from the results of theoretical calculations of the energy band struc ture, which, as a rule, give underestimated values of Eg. An analysis of the excitation spectra of the lumi nescence band at 290 nm in the lutetium borate and solid solution is difficult to perform because of the sig nificant contribution from the excitation spectrum of the STE luminescence. Indeed, the relative contribu tion from the STE luminescence to the UV lumines cence significantly increases in the LuBO3 and Lu0.65Y0.35BO3 borates under Xray excitation at 300 K (Fig. 1a). The observed behavior is consistent with the temperature dependence of the intensity of the STE luminescence under UV excitation. As follows from this dependence, the STE luminescence in the yttrium borate is most susceptible to temperature quenching. Therefore, we cannot correctly determine the band gap of the lutetium borate; however, a similar behavior of the excitation spectra of the STE luminescence sug gests that the band gap Eg in the lutetium borate and solid solution will not substantially differ from the value obtained for the YBO3 borate. Note that, in our previous work [26], the energy gap for LuBO3 : Ce was estimated as Eg > 7.75 eV. This estimate does not con tradict the results obtained in the present work.

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The luminescence in the visible spectral range is also excited in the transparent region and can be attributed to the luminescence from defects of the crystal structure, by analogy with the luminescence band at 290 nm. From the reported data, it is difficult to draw an unambiguous conclusion about the nature of defects responsible for the luminescence bands in the region of 290 nm and in the visible range. It can be assumed that the nature of defects responsible for the aforementioned bands is different. This follows from the antisymbatic behavior of the excitation spectra of these luminescence bands in the transparent region of the YBO3 borate. Another feature of the luminescence excitation spectrum in the visible range is its behavior in the region of the fundamental absorption edge at low temperatures (Fig. 4b). We note a significant increase in the luminescence intensity even at an exci tation Eex > 14 eV. An increase in the luminescence intensity is observed at excitation energies that are considerably higher than Eg; however, the lowenergy threshold of increase in the luminescence intensity is lower than the multiplication threshold of electronic excitations, i.e., 17 eV, a value obtained from analyz ing the excitation spectrum of the STE luminescence. According to [27], the multiplication of electronic excitations can occur through the Auger relaxation of a highenergy electron (or hole) with the creation of a secondary electron–hole pair, or a secondary exciton, or an excited center associated with defects in the crys tal structure. The excitation energy at which the lumi nescence intensity begins to increase, i.e., 14 eV, is suf ficient only for the third of the aforementioned pro cesses. Therefore, an increase in the intensity of the luminescence excitation spectrum in the visible range is associated with the multiplication of electronic exci tations through the resonance energy transfer to defects from highenergy electrons (impact ionization of defects). The ratio of the luminescence excitation spectra of defects and STEs is presented in Fig. 4a (curve 4). This comparison of the excitation spectra makes it possible to separate the processes of multipli cation of electronic excitations caused by different mechanisms and to get rid of distortions of the excita tion spectra due to both the nearsurface loss and the loss by reflection [27, 28]. The ratio of the excitation spectra of the longwavelength luminescence and STE luminescence bands is a smooth curve without pro nounced features inherent in luminescence excitation spectra. An increase in the luminescence intensity begins at 14 eV, when the multiplication of electronic excitations through the impact ionization of defects becomes possible. This process reaches a maximum relative efficiency at 17.5 eV. The multiplication of electronic excitations also becomes possible due to the creation of secondary electron–hole pairs. This man ifests itself as a gradual decrease in the luminescence intensity in the range 18–21 eV. In the solid solution, there can arise additional defects as compared to YBO3 and LuBO3. Indeed, in

the spectrum of Lu0.65Y0.35BO3, an additional lumines cence band appears at 350 nm, which indicates the presence of additional defects in the crystal structure of the solid solution. The luminescence intensity in the visible range decreases in the series YBO3, Lu0.65Y0.35BO3, and LuBO3 under direct excitation of defects, as in the case with the decrease in the intensity of the STE lumines cence. However, under interband excitation, the intensity of the luminescence band for Lu0.65Y0.35BO3 in the visible range is significantly higher than that for YBO3 and LuBO3 (see Fig. 4b). Thus, in the solid solu tion, there is a relatively high probability that sepa rated electrons and holes created under interband excitation will be captured by recombinationtype luminescence centers. This can be associated with the disturbance of shortrange order in the structure of the solid solution. In the solid solutions, there can arise clusters with a predominant concentration of one of the cations, i.e., either yttrium or lutetium. This leads to the formation of a potential barrier at the bound aries of the clusters and limits the distance between charge carriers. As a consequence, an increase in the luminescence intensity can be observed precisely under interband excitation. The disturbance of short range order in the structure of the solid solutions was revealed earlier for perovskites. This effect manifests itself as an increase in the luminescence intensity under interband excitation and also as an increase in the scintillation light yield [29]. Owing to the manifes tation of this effect in LuxY1 – xBO3 solid solutions, they can be considered as potential matrices for scin tillation materials with an increased light yield as com pared to the yttrium and lutetium borates. 5. CONCLUSIONS The luminescence properties of the yttrium and lutetium borates, as well as the Lu0.65Y0.35BO3 solid solution, have been investigated using the VUV and X ray synchrotron radiations. It has been shown that the UV luminescence at a wavelength of 260 nm is associ ated with the STE luminescence, which has the elec tronic component localized on the d states of the cat ion. The band gap of the YBO3 borate is estimated at 8.3 eV. It has been found that the UV luminescence band at 290 nm and also the luminescence band in the visible range are attributed to different centers formed on defects of the crystal structure. The conclusion has been drawn about the manifestation of the effect of shortrange order disturbance in the structure of the solid solution. Therefore, the LuxY1 – xBO3 solid solu tions can be considered as potential matrices for scin tillation materials.


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ACKNOWLEDGMENTS We would like to thank A. Kotlov for his assistance in performing the measurements at the SUPERLUMI experimental station. This study was supported by the Ministry of Educa tion and Science of the Russian Federation within the framework of the Russian Federal Targeted Program “Scientific and Scientific–Pedagogical Human Resources for the Innovative Russia in 2009–2013” (state contract no. 02.740.11.0546), the Russian Foundation for Basic Research (project no. 1102 01506a), and the BMBF Project RUS 10/037. REFERENCES 1. L. Pidol, B. Viana, A. KahnNarari, A. Galtayries, A. Bessiere, and P. Dorenbos, J. Appl. Phys. 95 (12), 7731 (2004). 2. D. W. Cooke, B. L. Bennett, R. E. Muenchausen, J.K. Lee, and M. A. Nastasi, J. Lumin. 106, 125 (2004). 3. S. P. Feofilov, Y. Zhou, J. Y. Jeong, D. A. Keszler, and R. S. Meltzer, J. Lumin. 125, 80 (2007). 4. M. V. Korzhik and W. P. Trower, Appl. Phys. Lett. 66 (18), 2327 (1995). 5. M. V. Korzhik, Physics of Scintillators on a Base of Oxide Single Crystals (Belarusian State University, Minsk, 2003). 6. I. N. Ogorodnikov, A. V. Kruzhalov, E. A. Radzhabov, and L. I. Isaeko, Phys. Solid State 41 (2), 197 (1999). 7. I. V. Berezovskaya, N. P. Efryushina, A. S. Voloshi novski, G. B. Stryganyuk, P. V. Pir, and V. P. Dotsenko, Radiat. Meas. 42, 878 (2007). 8. I. N. Ogorodnikov, V. A. Pustovarov, and M. Kirm, Phys. Solid State 46 (5), 842 (2004). 9. L. V. Ivanenko, Candidate’s Dissertation (Stavropol, 2004). 10. C. Duan, J. Yuan, and J. Zhao, J. Solid State Chem. 178 (12), 3698 (2005). 11. C. Mansuy, J. M. Nedelec, C. Dujardin, and R. Mahiou, J. Sol–Gel Sci. Technol. 32 (1–3), 253 (2004). 12. G. Zimmerer, Radiat. Meas. 42 (4–5), 859 (2007). 13. C. Larsson, A. Beutler, O. Bjorneholm, F. Federmann, U. Hahn, A. Rieck, S. Verbin, and T. Moller, Nucl. Instrum. Methods Phys. Res., Sect. A 337, 603 (1994).


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Translated by O. BorovikRomanova