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Time-resolved luminescence spectroscopy of structurally disordered K3WO3F3 crystals
Optical Materials 58 (2016) 285e289
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Time-resolved luminescence spectroscopy of structurally disordered K3WO3F3 crystals S.I. Omelkov a, *, D.A. Spassky a, b, V.A. Pustovarov c, A.V. Kozlov c, L.I. Isaenko d, e a
Institute of Physics, University of Tartu, W. Ostwaldi 1, 50411 Tartu, Estonia Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia c Ural Federal University, Mira St. 19, 620002 Yekaterinburg, Russia d Institute of Geology and Mineralogy SB RAS, Russkaya St. 43, 630058 Novosibirsk, Russia e Novosibirsk National Research University, Pirogova St. 2, 630090 Novosibirsk, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2016 Received in revised form 11 May 2016 Accepted 19 May 2016
Three emission centers of exciton-like origin, with distinct relaxation time, emission and excitation spectra were revealed in K3WO3F3 and described taking into account its structural disordering. Lowtemperature monoclinic phase of K3WO3F3 features few anion sites with mixed oxygen/fluorine occupancy per [WO3F3] octahedron. Therefore, different kinds of distorted octahedra form, providing different luminescence centers. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers. The simultaneous thermal quenching of them above ~200 K was qualitatively explained involving dynamic structural disorder of the compound. The energy transfer mechanism between centers was found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was discovered and the role of shallow charge carrier traps in the low-temperature persistent luminescence was revealed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Potassium fluorotungstate Oxyfluorides Time-resolved luminescence Intrinsic luminescence
1. Introduction Complex metal oxyfluorides such as K3WO3F3 are attractive compounds for developing new noncentrosymmetric crystals having ferroelectric and ferroelastic properties. This is achieved due to the strong distortion of metal-(O,F) polyhedra in crystal lattice because of different ionicity of metal-O and metal-F bonds. The detailed structures of different phases of K3WO3F3 are described in Ref. [1]. During past years, ferroelectric phase transitions of K3WO3F3 have been extensively studied (see Ref. [2] and references therein). Two phase transitions were revealed at T1 ¼ 452 K and T2 ¼ 414 K. In the high-temperature cubic phase (space group Fm3m), all anion sites are crystallographically equivalent having equal probability to be occupied by fluorine or oxygen. On cooling below T1, the ordering initially appears along the z-axis, providing one site devoted exclusively to oxygen and one to fluorine. The [WO3F3]3 polyhedra become distorted, reducing lattice symmetry to tetragonal (space group I4mm). The room-temperature monoclinic phase (space group Cm) observed at temperatures below T2
* Corresponding author. E-mail address: [email protected] (S.I. Omelkov). http://dx.doi.org/10.1016/j.optmat.2016.05.032 0925-3467/© 2016 Elsevier B.V. All rights reserved.
orders the lattice along the y-axis, yielding even more distortion to the octahedra. In Ref. [3] the ordering along the y-axis is reported to be full, like along z-axis, with 100% oxygen occupancy for one of the sites and 100% fluorine occupancy for the other. In Ref. [2], however, the occupancies along the y-axis are shown to be less than 100%, meaning partial ordering. NMR studies [4] reveal that the disorder is dynamic, i.e. mixed occupancy sites change from fluorine to oxygen with characteristic frequencies depending on the temperature. According to the lattice dynamics model developed in Ref. [2], the full disorder along x-axis remains down to T / 0, meaning the dynamic disorder frequency become very low when approaching absolute zero. A variety of nonequivalent anionic sites in a monoclinic phase leads to the formation of differently distorted [WO3F3]3 octahedra. This distortion was studied by methods such as XRD [5,6], Raman [7] and NMR [4] spectroscopy revealing structural and dynamic properties. However, most tungstates feature intrinsic luminescence associated with [WO6] or [WO4] polyhedra, so the presence of crystallographically non-equivalent polyhedra in K3WO3F3 should have the distinct influence on luminescence properties of the material. Therefore, the luminescence spectroscopy can be a sensitive method to study the character of lattice distortion of oxyfluorides at different temperatures. The current study attempts to apply this approach to K3WO3F3 in
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the forms of single crystals and polycrystalline ceramics. 2. Experimental 2.1. Spectroscopy The photoluminescence (PL) and PL excitation spectra were recorded using laboratory setup in Tartu. The continuous D2 lamp and DMR-2 primary and secondary monochromators were used to record the steady-state spectra in the range of 1.5e5.7 eV (Figs. 2 and 7). For the time-resolved measurements, the Perkin-Elmer FX-1151 ms lamp was used with DMR-2 primary and ARC SpectraPro-2300i secondary monochromators. In both cases, the luminescence was detected by an H8259-02 photon counting head. The pulse repetition rate of the microsecond lamp is 200 Hz, and the time resolution of the detection method is 100 ns. The decay kinetics was recorded for each point of the spectrum and then fit by a convolution of a lamp excitation pulse with the multi-exponential decay function. The obtained spectra of initial amplitudes were then multiplied by corresponding decay constants to obtain spectra of partial light yield (LY) of PL components. The excitation spectra were then corrected by the equal number of incident photons using sodium salicylate as a standard. 2.2. Samples Two types of samples were studied: one in the form of ceramics and another d single crystal. The growth and characterization of samples are thoroughly described in Ref. [3]. In addition to that, the samples were characterized also by micro-Raman technique. The Raman scattering microspectroscopy was performed using a Renishaw micro-Raman setup, where the 488 nm laser is used as excitation source. The sample is illuminated and the scattered light registered through Leica confocal microscope. The spatial resolution of the setup is 1 mm. The Raman spectra were recorded from ~10 various micron-sized spots on each sample (see Fig. 1), and no differences were found in the line structure of all recorded curves. The comparison with the Raman scattering spectra of K3WO3F3 published in Ref. [7], however, revealed that all our curves contain one weak band at ~960 cm1, which has not been reported in the cited work. It can indicate the presence of trace quantities of some different tungstate or fluorotungstate phase in the sample. Its intensity relative to the main peak at 920 cm1 is 3%. This peak cannot be attributed to K2WO4 residual phase possibly created during crystal growth, because this compound has its main Raman peak at 924 cm1 [8]. On the other hand, the 488 nm laser may be already
Fig. 1. The Raman scattering spectra of K3WO3F3 ceramics (1), single crystal (2) and the spectrum from Ref. [7] (3). Curves are plotted in logarithmic scale to stress the weak line at 960 cm1.
blue enough to cause some luminescence in the sample. The absolute photon energy value of the peak with the Raman shift of 960 cm1 is 2.42 eV (512 nm).
3. Results and discussion During this study, only a few notable differences in the main PL features of the ceramic and single-crystalline samples were discovered. Unless specified otherwise, the data presented is obtained on a ceramic sample but single crystal has shown no differences. In the cases when the results on the samples differ, it is mentioned explicitly. The most intense emission band in the studied compound (Fig. 2) manifests itself around 2.4 eV. Its shape does not change with temperature, but the maximum is shifted towards higher energies when increasing excitation energy. The excitation spectra of this band have the main peak at 4.2 eV at T ¼ 89 K which shifts to 4.0 eV at T ¼ 295 K. The other features of excitation spectrum depend also on the emission energy monitored. This behavior guides to a conclusion that this PL band is complex, comprising of several elementary bands. Time-resolved PL spectroscopy reveals the complexity of 2.4 eV band in detail (Fig. 3). Three PL components with different decay times were found. The first component (labeled a) has the decay constant ta ¼ 230 ms at T ¼ 89 K and is peaking at 2.3 eV. The decay constants of second and third components (b and c) depend on excitation energy. At Eexc ¼ 4.1 eV for component b tb ¼ 98 ms , and component c is not visible. At Eexc ¼ 4.6 eV tb ¼ 80 ms while exact value of tc is ambiguous due to the low partial intensity of component c, but it lies in the range 12e25 ms. Both PL components b and c are peaking at 2.5 eV. In ceramic sample, there is one more component manifested below 180 K with decay time t[ 5 ms. We hereby call it a “persistent” component of luminescence. Its PL band is peaking at about 2.8 eV, which is higher than that of faster components a,b and c. The excitation spectra of all components are different (Fig. 4). The main a component is excited almost only in 4.2 eV peak, showing just some trace emission above 4.6 eV. The b component is excited both in 4.2 eV peak and with less efficiency at higher energies while c component is not excited below 4.5 eV at all. The decrease of a decay constant with rising excitation energy is shown in Fig. 4, right. The “persistent” component is efficiently excited
Fig. 2. The normalized PL emission (1,2,3) and excitation (4,5,6,7) spectra of K3WO3F3, obtained using D2 lamp and DMR-2 secondary mono. T ¼ 89 K (1,2,3,6,7) and 295 K (4,5). Excitation energies: 4.17 eV (1), 4.75 eV (2), 5.3 eV (3). Emission energies: 2.75 eV (4,6) and 1.9 eV (5,7).
S.I. Omelkov et al. / Optical Materials 58 (2016) 285e289
Fig. 3. The spectra of partial light yield of PL components a (red), b (blue) and c (black) of K3WO3F3 at Eexc ¼ 4.1 eV (solid lines), Eexc ¼ 4.6 eV (dashed) and Eexc ¼ 5.4 eV (dotted). The decay constant of corresponding components (in ms) is indicated. On the right panel the spectra are normalized for better visibility. T ¼ 89 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The excitation spectra of PL components a (red), b (blue), c (black) and persistent component (gray dots, normalized arbitrarily) of K3WO3F3 (left), and the dependencies of their decay time on excitation energies (right), recorded monitoring 2.35 eV emission. T ¼ 89 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
only above 5.0 eV. We discuss this component in detail in the end of this section. The complex PL band at 2.4 eV in K3WO3F3 shows all the characteristic features of the intrinsic emission, associated with [WO6] octahedra in tungstates. The excitation spectrum of this emission is also very typical. The peak around 4.1 eV arises due to creation of exciton-like states of [WO3F3]3. This peak is shifted with temperature, following the change in the bandgap. The band-to-band transitions are formed by O / W charge transfer transitions. The different components of this complex PL band, clearly manifested in the spectra of ceramic sample, can arise due to disorder in the crystal lattice of K3WO3F3. There are crystallographic sites in the compound with mixed occupancy, hosting either fluorine or oxygen atoms. Obviously, the differences in the site occupancy in the octahedra will lead to their different distortion and different energy structures of their ground and excited states. In turn, different octahedra will have different PL peak positions and radiative transition probabilities, providing few distinct PL components. It is interesting to discuss the number of observed components. As mentioned above, in Ref. [3] and earlier research the roomtemperature phase is reported to have only two sites with mixed anion occupancy (along the x-axis), therefore allowing only two different types of octahedra. In Ref. [2], however, the occupancies along the y-axis are shown to be also partly mixed, leaving two most probable configurations and up to two least probable. In our study, we see three PL centers, two of which (a and b) have a higher yield and longer decay constants, and the remaining one (c) has a lower yield and significantly faster decay. The latter could mean that PL center c is less probable to occur and has a shorter lifetime. Therefore, our observations favor the structural data of [2]. All three components show the temperature quenching above 200 K, which is well described by the Mott law with the same activation energy 0.21 ± 0.02 eV (Fig. 5, right). It should be mentioned that the quenching curve is astonishingly similar to a temperature dependence of the second moment of the NMR
287
Fig. 5. The temperature dependence of decay constants of PL components a (red), b (blue) and c (black) of ceramic sample at Eexc ¼ 4.1 eV (solid lines), Eexc ¼ 4.6 eV (dashed) and Eexc ¼ 5.4 eV (dotted), recorded monitoring 2.35 eV emission. The circles represent approximations (see text for details). Big crosses (marked NMR) reproduce the temperature dependence of the second moment of the NMR spectrum of 19F in polycrystalline K3WO3F3 [4], scaled arbitrarily. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
spectrum of 19F in polycrystalline K3WO3F3 [4]. The NMR data show that at temperatures higher than ~380 K, anionic octahedron [WO3F3]3 isotropically reorients at frequencies above 104 Hz, i.e. the octahedral group is dynamically disordered. With some assumptions, the quenching can be tentatively described using these data. If the reorientations destroy the excited state of octahedra, it leads to luminescence quenching if excited state lifetime time is longer than the time between reorientations (less than 100 ms at 380 K). This disordering, however, seems to “freeze” at lower temperatures, so that the frequency of reorientations decreases providing enough lifetime for an excited octahedron to decay radiatively. The activation energy of the Mott approximation in this model might be the characteristic thermal energy required to reorient the [WO3F3]3 octahedron. When lowering the temperature below 25 K, the decay times of components a and b increase. This increase is close to an exponential law with characteristic temperature 4.5 ± 0.5 K. It is not possible to tell whether the c component undergoes the same increase due to its low partial intensity. Such low-temperature behavior is common for some tungstates as their first excited state has triplet nature, and split due to spin-orbit interactions. The lifetimes of these sub-levels are different, and the effective lifetime depends on the sub-level occupancy, which in turn depends on temperature [9]. Let us discuss, what is the measure of the number of initially excited PL centers after the excitation pulse. There are three parameters describing the decay process: initial amplitude, decay constant and total light yield (LY), which is proportional to the product of the first two. In the region 25e200 K, where the decay constant does not change, both amplitude and LY are proportional to the number of initially excited PL centers. If the excitation mechanism is not affected by temperature, and the change of decay constant is only due to intracenter processes, like quenching and changes in triplet sub-level occupancy, the number of initially excited PL centers will remain the same throughout all the temperature range. In that case, below 25 K the LY does not change and is proportional to the number of initially excited PL centers. Above 200 K LY decreases due to the quenching, but the intensity just after the excitation pulse remain the same. Therefore, above 200 K the amplitude is proportional to the number of initially excited PL centers. Fig. 6 shows the temperature dependence of abovediscussed values, which are proportional to the number of initially excited PL centers. It is clearly seen that for the center b excited at 5.0 eV this number does not change significantly throughout the whole studied temperature range. However, while excited at 4.1 eV, the number of excited centers a gradually decreases with temperature while a number of centers b increases. In this connection, we can describe the temperature
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Fig. 6. The temperature dependence of amplitudes (points) and LY (lines) of the PL components a (red, Eexc ¼ 4.1 eV) and b (blue, Eexc ¼ 4.1 eV; green, Eexc ¼ 5.0 eV) of K3WO3F3 recorded monitoring 2.35 eV emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dependence of excitation efficiency of components a and b (Fig. 6) by diffusion of self-trapped excitons, the probability of which gradually increases with temperature. After hopping to the new place in a crystal lattice, the self-trapped exciton can find different site occupancy, and its energy will be changed. From this assumption, we can tell that the PL center b is more stable (or, less mobile) than the center a, as increase in the centers mobility lead to increase of its excitation efficiency. When excited at Eexc ¼ 5.0 eV, where centers a are not created, the increase in the centers mobility does not change the excitation efficiency of center b. Apart from the main complex band at 2.5 eV, the K3WO3F3 compound exhibits another emission band at 3.0e3.2 eV (Fig. 7). This band is much more intense in a single crystal but is manifested in ceramics as well. The intensity of this band does not depend on temperature. It has excitation spectrum, which is completely different from any of the 2.5 eV band (Fig. 7, curves 3,4). Its dominant peak is at 3.9 eV, which is lower than the excitonic peak, and does not move with temperature. On this basis this emission can be ascribed to a defect center, its first excitation band at 3.5 eV is a direct excitation, and the dominating peak at 3.9 eV is due to the creation of bound excitons. Another notable process in K3WO3F3 is a “persistent” component of luminescence (with decay time t[5 ms), found in ceramic sample (see Figs. 3,4) at temperatures below 180 K (Fig. 8, curve 2). It reveals the presence of traps, which can delay the electron-hole energy transfer mechanism. The traps are populated only if the charge carriers created in the excitation process have high enough mobility, therefore the energies well above bandgap (>5.0 eV) are needed to excite this component efficiently. The TSL curve recorded after x-ray irradiation has the main peak structure at 120e150 K
Fig. 8. The TSL curve of K3WO3F3 ceramics, excited by 20 keV x-ray at 90 K and obtained by linear heating with the speed of 20 K/min (1), and the temperature dependence of the persistent component of PL at Eexc ¼ 5.4 eV, Eem ¼ 2.35 eV (2).
(Fig. 8), revealing the thermal release of the charge carriers. Consequently, at higher temperatures the trapping efficiency is lower and the intensity of the persistent component drops drastically. 4. Conclusions The intrinsic luminescence of tungstates is usually ascribed to the radiative relaxation of exciton-like excitations localized on WO6 octahedra or WO4 tetrahedra. In K3WO3F3 there are anion sites with mixed oxygen/fluorine occupancy. Therefore, different octahedra form with different distortion. Three emission centers of excitonlike origin, with distinct relaxation time, emission and excitation spectra were revealed and assigned to different types of such octahedra. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers, proving itself as a sensitive method to study the character of lattice distortion of oxyfluorides at different temperatures. The energy transfer mechanism between these PL centers is found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was found and the role of shallow charge carrier traps in the low-temperature persistent luminescence was revealed. Acknowledgments This work has been partly supported by Estonian Ministry of Education and Research (project IUT2-26), European Social Fund (“Mobilitas programme”, grants MJD219 and MTT83), the Ministry of Education and Science of the Russian Federation (the basic part of the government mandate), Center of Excellence “Radiation and Nuclear Technologies” (Competitiveness Enhancement Program of Ural Federal University). References
Fig. 7. The normalized PL excitation and emission spectra of K3WO3F3 single crystal with D2 lamp excitation and DMR-2 secondary. Emission spectra: Eexc ¼ 3.57 eV (1), 3.4 eV (2). Excitation spectra: Eem ¼ 3.25 eV (3) and 3.0 eV (4). Red curves (1,3) recorded at T ¼ 295 K, blue (2,4) d at T ¼ 85 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[1] M.S. Molokeev, S.V. Misyul’, V.D. Fokina, A.G. Kocharova, K.S. Aleksandrov, Structure transformations during phase transitions in the K3WO3F3 oxyfluoride, Phys. Solid State 53 (4) (2011) 834e839. [2] M.P. Ivliev, S.V. Misyul, M.S. Molokeev, V.P. Sakhnenko, ‘pseudo-proper’ ferroelectric phase transitions in oxyfluoride K3WO3F3, Phase Transit. 87 (6) (2014) 592e602. [3] V. Atuchin, L. Isaenko, V. Kesler, Z. Lin, M. Molokeev, A. Yelisseyev, S. Zhurkov, Exploration on anion ordering, optical properties and electronic structure in K3WO3F3 elpasolite, J. Solid State Chem. 187 (2012) 159e164. [4] A. Krylov, S. Sofronova, E. Kolesnikova, Y. Ivanov, A. Sukhovsky, S. Goryainov, A. Ivanenko, N. Shestakov, A. Kocharova, A. Vtyurin, Experimental and theoretical methods to study structural phase transition mechanisms in K3WO3F3 oxyfluoride, J. Solid State Chem. 218 (2014) 32e37. [5] V. Atuchin, T. Gavrilova, V. Kesler, M. Molokeev, K. Aleksandrov, Low-temperature synthesis and structural properties of ferroelectric K3WO3F3 elpasolite, Chem. Phys. Lett. 493 (1e3) (2010) 83e86, http://dx.doi.org/10.1016/
S.I. Omelkov et al. / Optical Materials 58 (2016) 285e289 j.cplett.2010.05.023. [6] M. Molokeev, A. Vasiliev, A. Kocharova, Crystal structures of room- and lowtemperature phases in oxyfluoride (NH4)2KWO3F3, Powder Diffr. 22 (3) (2007) 227e230 cited By 6. [7] A.A. Ekimov, A.S. Krylov, A.N. Vtyurin, A.A. Ivanenko, N.P. Shestakov, A.G. Kocharova, Vibrational spectroscopy studies of temperature phase transitions in K3WO3F3, Ferroelectrics 401 (2010) 168e172.
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[8] C. Luz-Lima, J. Batista, P. Freire, G. de Sousa, F. dos Santos, J.M. Filho, B. Viana, G. Saraiva, Temperature-dependent raman spectroscopy studies of phase transformations in the K[2]WO[4] and the MgMoO4 crystals, Vib. Spectrosc. 65 (0) (2013) 58e65, http://dx.doi.org/10.1016/j.vibspec.2012.11.016. [9] R. Laasner, V. Nagirnyi, S. Vielhauer, M. Kirm, D. Spassky, V. Sirutkaitis, R. Grigonis, A.N. Vasil’ev, Cation influence on exciton localization in homologue scheelites, J. Phys. Condens. Matter 27 (38) (2015) 385501.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Time-resolved luminescence spectroscopy of structurally disordered K3WO3F3 crystals S.I. Omelkov a, *, D.A. Spassky a, b, V.A. Pustovarov c, A.V. Kozlov c, L.I. Isaenko d, e a
Institute of Physics, University of Tartu, W. Ostwaldi 1, 50411 Tartu, Estonia Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia c Ural Federal University, Mira St. 19, 620002 Yekaterinburg, Russia d Institute of Geology and Mineralogy SB RAS, Russkaya St. 43, 630058 Novosibirsk, Russia e Novosibirsk National Research University, Pirogova St. 2, 630090 Novosibirsk, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2016 Received in revised form 11 May 2016 Accepted 19 May 2016
Three emission centers of exciton-like origin, with distinct relaxation time, emission and excitation spectra were revealed in K3WO3F3 and described taking into account its structural disordering. Lowtemperature monoclinic phase of K3WO3F3 features few anion sites with mixed oxygen/fluorine occupancy per [WO3F3] octahedron. Therefore, different kinds of distorted octahedra form, providing different luminescence centers. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers. The simultaneous thermal quenching of them above ~200 K was qualitatively explained involving dynamic structural disorder of the compound. The energy transfer mechanism between centers was found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was discovered and the role of shallow charge carrier traps in the low-temperature persistent luminescence was revealed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Potassium fluorotungstate Oxyfluorides Time-resolved luminescence Intrinsic luminescence
1. Introduction Complex metal oxyfluorides such as K3WO3F3 are attractive compounds for developing new noncentrosymmetric crystals having ferroelectric and ferroelastic properties. This is achieved due to the strong distortion of metal-(O,F) polyhedra in crystal lattice because of different ionicity of metal-O and metal-F bonds. The detailed structures of different phases of K3WO3F3 are described in Ref. [1]. During past years, ferroelectric phase transitions of K3WO3F3 have been extensively studied (see Ref. [2] and references therein). Two phase transitions were revealed at T1 ¼ 452 K and T2 ¼ 414 K. In the high-temperature cubic phase (space group Fm3m), all anion sites are crystallographically equivalent having equal probability to be occupied by fluorine or oxygen. On cooling below T1, the ordering initially appears along the z-axis, providing one site devoted exclusively to oxygen and one to fluorine. The [WO3F3]3 polyhedra become distorted, reducing lattice symmetry to tetragonal (space group I4mm). The room-temperature monoclinic phase (space group Cm) observed at temperatures below T2
* Corresponding author. E-mail address: [email protected] (S.I. Omelkov). http://dx.doi.org/10.1016/j.optmat.2016.05.032 0925-3467/© 2016 Elsevier B.V. All rights reserved.
orders the lattice along the y-axis, yielding even more distortion to the octahedra. In Ref. [3] the ordering along the y-axis is reported to be full, like along z-axis, with 100% oxygen occupancy for one of the sites and 100% fluorine occupancy for the other. In Ref. [2], however, the occupancies along the y-axis are shown to be less than 100%, meaning partial ordering. NMR studies [4] reveal that the disorder is dynamic, i.e. mixed occupancy sites change from fluorine to oxygen with characteristic frequencies depending on the temperature. According to the lattice dynamics model developed in Ref. [2], the full disorder along x-axis remains down to T / 0, meaning the dynamic disorder frequency become very low when approaching absolute zero. A variety of nonequivalent anionic sites in a monoclinic phase leads to the formation of differently distorted [WO3F3]3 octahedra. This distortion was studied by methods such as XRD [5,6], Raman [7] and NMR [4] spectroscopy revealing structural and dynamic properties. However, most tungstates feature intrinsic luminescence associated with [WO6] or [WO4] polyhedra, so the presence of crystallographically non-equivalent polyhedra in K3WO3F3 should have the distinct influence on luminescence properties of the material. Therefore, the luminescence spectroscopy can be a sensitive method to study the character of lattice distortion of oxyfluorides at different temperatures. The current study attempts to apply this approach to K3WO3F3 in
286
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the forms of single crystals and polycrystalline ceramics. 2. Experimental 2.1. Spectroscopy The photoluminescence (PL) and PL excitation spectra were recorded using laboratory setup in Tartu. The continuous D2 lamp and DMR-2 primary and secondary monochromators were used to record the steady-state spectra in the range of 1.5e5.7 eV (Figs. 2 and 7). For the time-resolved measurements, the Perkin-Elmer FX-1151 ms lamp was used with DMR-2 primary and ARC SpectraPro-2300i secondary monochromators. In both cases, the luminescence was detected by an H8259-02 photon counting head. The pulse repetition rate of the microsecond lamp is 200 Hz, and the time resolution of the detection method is 100 ns. The decay kinetics was recorded for each point of the spectrum and then fit by a convolution of a lamp excitation pulse with the multi-exponential decay function. The obtained spectra of initial amplitudes were then multiplied by corresponding decay constants to obtain spectra of partial light yield (LY) of PL components. The excitation spectra were then corrected by the equal number of incident photons using sodium salicylate as a standard. 2.2. Samples Two types of samples were studied: one in the form of ceramics and another d single crystal. The growth and characterization of samples are thoroughly described in Ref. [3]. In addition to that, the samples were characterized also by micro-Raman technique. The Raman scattering microspectroscopy was performed using a Renishaw micro-Raman setup, where the 488 nm laser is used as excitation source. The sample is illuminated and the scattered light registered through Leica confocal microscope. The spatial resolution of the setup is 1 mm. The Raman spectra were recorded from ~10 various micron-sized spots on each sample (see Fig. 1), and no differences were found in the line structure of all recorded curves. The comparison with the Raman scattering spectra of K3WO3F3 published in Ref. [7], however, revealed that all our curves contain one weak band at ~960 cm1, which has not been reported in the cited work. It can indicate the presence of trace quantities of some different tungstate or fluorotungstate phase in the sample. Its intensity relative to the main peak at 920 cm1 is 3%. This peak cannot be attributed to K2WO4 residual phase possibly created during crystal growth, because this compound has its main Raman peak at 924 cm1 [8]. On the other hand, the 488 nm laser may be already
Fig. 1. The Raman scattering spectra of K3WO3F3 ceramics (1), single crystal (2) and the spectrum from Ref. [7] (3). Curves are plotted in logarithmic scale to stress the weak line at 960 cm1.
blue enough to cause some luminescence in the sample. The absolute photon energy value of the peak with the Raman shift of 960 cm1 is 2.42 eV (512 nm).
3. Results and discussion During this study, only a few notable differences in the main PL features of the ceramic and single-crystalline samples were discovered. Unless specified otherwise, the data presented is obtained on a ceramic sample but single crystal has shown no differences. In the cases when the results on the samples differ, it is mentioned explicitly. The most intense emission band in the studied compound (Fig. 2) manifests itself around 2.4 eV. Its shape does not change with temperature, but the maximum is shifted towards higher energies when increasing excitation energy. The excitation spectra of this band have the main peak at 4.2 eV at T ¼ 89 K which shifts to 4.0 eV at T ¼ 295 K. The other features of excitation spectrum depend also on the emission energy monitored. This behavior guides to a conclusion that this PL band is complex, comprising of several elementary bands. Time-resolved PL spectroscopy reveals the complexity of 2.4 eV band in detail (Fig. 3). Three PL components with different decay times were found. The first component (labeled a) has the decay constant ta ¼ 230 ms at T ¼ 89 K and is peaking at 2.3 eV. The decay constants of second and third components (b and c) depend on excitation energy. At Eexc ¼ 4.1 eV for component b tb ¼ 98 ms , and component c is not visible. At Eexc ¼ 4.6 eV tb ¼ 80 ms while exact value of tc is ambiguous due to the low partial intensity of component c, but it lies in the range 12e25 ms. Both PL components b and c are peaking at 2.5 eV. In ceramic sample, there is one more component manifested below 180 K with decay time t[ 5 ms. We hereby call it a “persistent” component of luminescence. Its PL band is peaking at about 2.8 eV, which is higher than that of faster components a,b and c. The excitation spectra of all components are different (Fig. 4). The main a component is excited almost only in 4.2 eV peak, showing just some trace emission above 4.6 eV. The b component is excited both in 4.2 eV peak and with less efficiency at higher energies while c component is not excited below 4.5 eV at all. The decrease of a decay constant with rising excitation energy is shown in Fig. 4, right. The “persistent” component is efficiently excited
Fig. 2. The normalized PL emission (1,2,3) and excitation (4,5,6,7) spectra of K3WO3F3, obtained using D2 lamp and DMR-2 secondary mono. T ¼ 89 K (1,2,3,6,7) and 295 K (4,5). Excitation energies: 4.17 eV (1), 4.75 eV (2), 5.3 eV (3). Emission energies: 2.75 eV (4,6) and 1.9 eV (5,7).
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Fig. 3. The spectra of partial light yield of PL components a (red), b (blue) and c (black) of K3WO3F3 at Eexc ¼ 4.1 eV (solid lines), Eexc ¼ 4.6 eV (dashed) and Eexc ¼ 5.4 eV (dotted). The decay constant of corresponding components (in ms) is indicated. On the right panel the spectra are normalized for better visibility. T ¼ 89 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The excitation spectra of PL components a (red), b (blue), c (black) and persistent component (gray dots, normalized arbitrarily) of K3WO3F3 (left), and the dependencies of their decay time on excitation energies (right), recorded monitoring 2.35 eV emission. T ¼ 89 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
only above 5.0 eV. We discuss this component in detail in the end of this section. The complex PL band at 2.4 eV in K3WO3F3 shows all the characteristic features of the intrinsic emission, associated with [WO6] octahedra in tungstates. The excitation spectrum of this emission is also very typical. The peak around 4.1 eV arises due to creation of exciton-like states of [WO3F3]3. This peak is shifted with temperature, following the change in the bandgap. The band-to-band transitions are formed by O / W charge transfer transitions. The different components of this complex PL band, clearly manifested in the spectra of ceramic sample, can arise due to disorder in the crystal lattice of K3WO3F3. There are crystallographic sites in the compound with mixed occupancy, hosting either fluorine or oxygen atoms. Obviously, the differences in the site occupancy in the octahedra will lead to their different distortion and different energy structures of their ground and excited states. In turn, different octahedra will have different PL peak positions and radiative transition probabilities, providing few distinct PL components. It is interesting to discuss the number of observed components. As mentioned above, in Ref. [3] and earlier research the roomtemperature phase is reported to have only two sites with mixed anion occupancy (along the x-axis), therefore allowing only two different types of octahedra. In Ref. [2], however, the occupancies along the y-axis are shown to be also partly mixed, leaving two most probable configurations and up to two least probable. In our study, we see three PL centers, two of which (a and b) have a higher yield and longer decay constants, and the remaining one (c) has a lower yield and significantly faster decay. The latter could mean that PL center c is less probable to occur and has a shorter lifetime. Therefore, our observations favor the structural data of [2]. All three components show the temperature quenching above 200 K, which is well described by the Mott law with the same activation energy 0.21 ± 0.02 eV (Fig. 5, right). It should be mentioned that the quenching curve is astonishingly similar to a temperature dependence of the second moment of the NMR
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Fig. 5. The temperature dependence of decay constants of PL components a (red), b (blue) and c (black) of ceramic sample at Eexc ¼ 4.1 eV (solid lines), Eexc ¼ 4.6 eV (dashed) and Eexc ¼ 5.4 eV (dotted), recorded monitoring 2.35 eV emission. The circles represent approximations (see text for details). Big crosses (marked NMR) reproduce the temperature dependence of the second moment of the NMR spectrum of 19F in polycrystalline K3WO3F3 [4], scaled arbitrarily. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
spectrum of 19F in polycrystalline K3WO3F3 [4]. The NMR data show that at temperatures higher than ~380 K, anionic octahedron [WO3F3]3 isotropically reorients at frequencies above 104 Hz, i.e. the octahedral group is dynamically disordered. With some assumptions, the quenching can be tentatively described using these data. If the reorientations destroy the excited state of octahedra, it leads to luminescence quenching if excited state lifetime time is longer than the time between reorientations (less than 100 ms at 380 K). This disordering, however, seems to “freeze” at lower temperatures, so that the frequency of reorientations decreases providing enough lifetime for an excited octahedron to decay radiatively. The activation energy of the Mott approximation in this model might be the characteristic thermal energy required to reorient the [WO3F3]3 octahedron. When lowering the temperature below 25 K, the decay times of components a and b increase. This increase is close to an exponential law with characteristic temperature 4.5 ± 0.5 K. It is not possible to tell whether the c component undergoes the same increase due to its low partial intensity. Such low-temperature behavior is common for some tungstates as their first excited state has triplet nature, and split due to spin-orbit interactions. The lifetimes of these sub-levels are different, and the effective lifetime depends on the sub-level occupancy, which in turn depends on temperature [9]. Let us discuss, what is the measure of the number of initially excited PL centers after the excitation pulse. There are three parameters describing the decay process: initial amplitude, decay constant and total light yield (LY), which is proportional to the product of the first two. In the region 25e200 K, where the decay constant does not change, both amplitude and LY are proportional to the number of initially excited PL centers. If the excitation mechanism is not affected by temperature, and the change of decay constant is only due to intracenter processes, like quenching and changes in triplet sub-level occupancy, the number of initially excited PL centers will remain the same throughout all the temperature range. In that case, below 25 K the LY does not change and is proportional to the number of initially excited PL centers. Above 200 K LY decreases due to the quenching, but the intensity just after the excitation pulse remain the same. Therefore, above 200 K the amplitude is proportional to the number of initially excited PL centers. Fig. 6 shows the temperature dependence of abovediscussed values, which are proportional to the number of initially excited PL centers. It is clearly seen that for the center b excited at 5.0 eV this number does not change significantly throughout the whole studied temperature range. However, while excited at 4.1 eV, the number of excited centers a gradually decreases with temperature while a number of centers b increases. In this connection, we can describe the temperature
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Fig. 6. The temperature dependence of amplitudes (points) and LY (lines) of the PL components a (red, Eexc ¼ 4.1 eV) and b (blue, Eexc ¼ 4.1 eV; green, Eexc ¼ 5.0 eV) of K3WO3F3 recorded monitoring 2.35 eV emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dependence of excitation efficiency of components a and b (Fig. 6) by diffusion of self-trapped excitons, the probability of which gradually increases with temperature. After hopping to the new place in a crystal lattice, the self-trapped exciton can find different site occupancy, and its energy will be changed. From this assumption, we can tell that the PL center b is more stable (or, less mobile) than the center a, as increase in the centers mobility lead to increase of its excitation efficiency. When excited at Eexc ¼ 5.0 eV, where centers a are not created, the increase in the centers mobility does not change the excitation efficiency of center b. Apart from the main complex band at 2.5 eV, the K3WO3F3 compound exhibits another emission band at 3.0e3.2 eV (Fig. 7). This band is much more intense in a single crystal but is manifested in ceramics as well. The intensity of this band does not depend on temperature. It has excitation spectrum, which is completely different from any of the 2.5 eV band (Fig. 7, curves 3,4). Its dominant peak is at 3.9 eV, which is lower than the excitonic peak, and does not move with temperature. On this basis this emission can be ascribed to a defect center, its first excitation band at 3.5 eV is a direct excitation, and the dominating peak at 3.9 eV is due to the creation of bound excitons. Another notable process in K3WO3F3 is a “persistent” component of luminescence (with decay time t[5 ms), found in ceramic sample (see Figs. 3,4) at temperatures below 180 K (Fig. 8, curve 2). It reveals the presence of traps, which can delay the electron-hole energy transfer mechanism. The traps are populated only if the charge carriers created in the excitation process have high enough mobility, therefore the energies well above bandgap (>5.0 eV) are needed to excite this component efficiently. The TSL curve recorded after x-ray irradiation has the main peak structure at 120e150 K
Fig. 8. The TSL curve of K3WO3F3 ceramics, excited by 20 keV x-ray at 90 K and obtained by linear heating with the speed of 20 K/min (1), and the temperature dependence of the persistent component of PL at Eexc ¼ 5.4 eV, Eem ¼ 2.35 eV (2).
(Fig. 8), revealing the thermal release of the charge carriers. Consequently, at higher temperatures the trapping efficiency is lower and the intensity of the persistent component drops drastically. 4. Conclusions The intrinsic luminescence of tungstates is usually ascribed to the radiative relaxation of exciton-like excitations localized on WO6 octahedra or WO4 tetrahedra. In K3WO3F3 there are anion sites with mixed oxygen/fluorine occupancy. Therefore, different octahedra form with different distortion. Three emission centers of excitonlike origin, with distinct relaxation time, emission and excitation spectra were revealed and assigned to different types of such octahedra. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers, proving itself as a sensitive method to study the character of lattice distortion of oxyfluorides at different temperatures. The energy transfer mechanism between these PL centers is found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was found and the role of shallow charge carrier traps in the low-temperature persistent luminescence was revealed. Acknowledgments This work has been partly supported by Estonian Ministry of Education and Research (project IUT2-26), European Social Fund (“Mobilitas programme”, grants MJD219 and MTT83), the Ministry of Education and Science of the Russian Federation (the basic part of the government mandate), Center of Excellence “Radiation and Nuclear Technologies” (Competitiveness Enhancement Program of Ural Federal University). References
Fig. 7. The normalized PL excitation and emission spectra of K3WO3F3 single crystal with D2 lamp excitation and DMR-2 secondary. Emission spectra: Eexc ¼ 3.57 eV (1), 3.4 eV (2). Excitation spectra: Eem ¼ 3.25 eV (3) and 3.0 eV (4). Red curves (1,3) recorded at T ¼ 295 K, blue (2,4) d at T ¼ 85 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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