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Excited states of molybdenum oxyanion in scheelite and wolframite structures
Radiation Measurements 42 (2007) 767 – 770 www.elsevier.com/locate/radmeas
Excited states of molybdenum oxyanion in scheelite and wolframite structures A. Kotlov a , L. Jönsson b , H. Kraus c , V. Mikhailik c , V. Nagirnyi a,∗ , G. Svensson d , B.I. Zadneprovski e a Institute of Physics, University of Tartu, 142 Riia Str., 51014 Tartu, Estonia b Department of Physics, Lund University, Professorsgatan 1, 22100 Lund, Sweden c Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK d Department of Inorganic Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden e All-Russia Research Institute of Mineral Materials Synthesis, Irkutskaya Str. 1, 601600 Alexandrov, Vladimir Region, Russia
Received 19 December 2006; accepted 1 February 2007
Abstract Scheelite CdMoO4 and wolframite CdWO4 :Mo (0.04–0.4 wt%) single crystals were studied using the time-resolved spectroscopy under UV excitation over a temperature range of 1.85–300 K. The threshold energies for the creation of free charge carriers were measured using the method of photostimulated luminescence. The decay kinetics of the main emission in CdMoO4 and the molybdenum-related emission in CdWO4 :Mo was studied and the parameters of the triplet excited states of molybdenum-related oxyanions were calculated. © 2007 Elsevier Ltd. All rights reserved. Keywords: CdMoO4 ; Luminescence; Decay kinetics; Triplet states
1. Introduction Tungstate crystals are prone to be contaminated by molybdenum. In ZnWO4 and CdWO4 scintillators, the presence of this impurity has been shown to cause additional bands of red luminescence peaking at 1.78 and 1.82 eV, respectively (Földvári et al., 1990; Nagirnyi et al., 2004; Garces et al., 2003). The relatively slow decay of these bands deteriorates the performance of the scintillator. CdWO4 :Mo has wolframite structure with MoO6 impurity complexes replacing regular WO6 oxyanions. CdMoO4 crystals have scheelite structure, and MoO4 complexes should be responsible for the excitonic emission. The latter system is not much studied. Some preliminary results on emission and excitation spectra of CdMoO4 have been reported by Mikhailik et al. (2005). By comparing the luminescence properties of molybdenum centres in wolframites and scheelites one could estimate how much they are affected by the
∗ Corresponding author. Tel.: +372 7428946; fax: +372 7383033.
E-mail address: vetal@fi.tartu.ee (V. Nagirnyi). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.02.009
symmetry of the luminescence centre. The present work is also aimed to investigate the effect of the crystal symmetry on the structure of the excited states of molybdenum oxyanion centres by studying the decay kinetics of molybdenum-related emissions. 2. Experimental methods The CdWO4 :Mo crystals studied here were grown at Chalmers University of Technology, Göteborg from a hightemperature solution using the slow cooling technique. The starting material was prepared from a mixture of CdO, MoO3 and WO3 in a Na2 WO4 solvent. The microprobe analysis showed Mo contents of 0.04, 0.1 and 0.4 wt% in the samples chosen for the study. The concentration of Na in the crystals grown did not exceed 1.3 at%. The CdMoO4 single crystal was grown using the Czochralski technique. The methods for measuring excitation spectra of photostimulated recombination luminescence, steady-state emission and excitation spectra as well as the emission decay kinetics have been described elsewhere (Nagirnyi et al., 2004).
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Fig. 1. Normalized emission spectra of a CdMoO4 crystal (d) and CdWO4 :Mo crystals containing 0.04 wt% (a), 0.1 wt% (b) and 0.4 wt% (c) of molybdenum under excitation with Eexc = 3.8 eV (1 in a, b, c), 3.6 eV (1 in d) and 5.5 eV (2). T = 4.2 K.
Fig. 2. Normalized excitation spectra of 1.82 eV (1 in a, b, c), 1.95 eV (1 in d), 2.46 eV (2 in a, b, c) and 2.19 eV emissions (2 in d) for CdWO4 :Mo crystals containing 0.04 wt% (a), 0.1 wt% (b) and 0.4 wt% (c) of molybdenum and for CdMoO4 (d). Curves 3 show the excitation spectra of recombination luminescence at 2.46 eV (a, b, c) and 2.19 eV (d) under stimulation by 1.8 eV photons. T = 4.2 K.
3. Experimental results and discussion 3.1. Emission and excitation spectra The emission spectrum of CdWO4 :Mo (0.04 wt%) contains two main bands peaked at 2.46 and 1.82 eV (Fig. 1). The 2.46 eV emission has been ascribed to the radiative decay of self-trapped excitons localized at the WO6 complex oxyanions (Lammers et al., 1981). The 1.82 eV emission is connected with MoO6 impurity oxyanion complexes (Garces et al., 2003). It can be excited near the fundamental absorption edge of CdWO4 in a band peaking at about 3.8 eV (Fig. 2a). The high-energy part of this band is distorted due to the matrix absorption. An analogous emission peaking at 1.75 eV has been investigated in ZnWO4 :Mo crystals using the method of time-resolved spectroscopy (Nagirnyi et al., 2004). Thus, the position of the luminescence band near 1.8 eV is characteristic of the molybdenum oxyanion complex embedded in the wolframite crystalline structure. On the other hand, the emission band at 2.19 eV in CdMoO4 crystals with scheelite structure is associated with MoO4 complexes (Fig. 1d). This emission band is fundamental; its shape is the same under excitation near the absorption edge at 3.6 eV and in the fundamental absorption region at 5 eV
(curves 1 and 2, respectively). This conclusion is supported also by the decay kinetics (see below). The excitation spectrum of the 2.19 eV emission is shown in Fig. 2d, curve 2. The onset of the spectrum is at 3.4 eV, the intensity reaches its maximum at 3.6 eV and then changes very little as the excitation energy increases up to 5 eV. Only traces of the red emission peaking near 1.95 eV can be observed for excitation below 3.4 eV. The excitation spectrum of this defect-related emission is shown by curve 1 in Fig. 2d. As the concentration of Mo in CdWO4 increases, the position of the red emission band observed under excitation by 3.8 eV photons remains effectively unchanged (curves 1 in Fig. 1a, b, c). However, its relative intensity substantially increases with molybdenum concentration under excitation in the fundamental absorption region at 5.5 eV (curves 2). This increase is accompanied by a reduction of the intensity of the intrinsic excitonic emission at 2.46 eV. This result is in good agreement with the results of Ivanov et al. (2006), who detected the same effect in CdWO4 :Mo crystals under X-ray irradiation. In addition to the low-energy shift of the excitation edge of the Mo-related band observed at increasing molybdenum con-
A. Kotlov et al. / Radiation Measurements 42 (2007) 767 – 770
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Fig. 3. The spectra of decay times (1, 2) and light sums (1 , 2 ) of the fast and slow decay components of the 1.82 eV emission of CdWO4 :Mo (0.04 wt%) at Eexc = 3.8 eV (a) and of the 2.19 eV emission of CdMoO4 (b) at Eexc = 3.6 eV. T = 4.2 K.
Fig. 4. Temperature dependences of decay times (1, 2) and light sums (1 , 2 ) of the fast and slow decay components of the 1.82 eV emission of CdWO4 :Mo (0.04 wt%) (a) and CdWO4 :Mo (0.1 wt%) (b) at Eexc = 3.8 eV and of the 2.19 eV emission of CdMoO4 at Eexc = 3.6 eV (c).
centration (curves 1 in Fig. 2a, b, c) the edge of the excitation spectrum of the 2.46 eV emission moves gradually from 3.9 to 4.2 eV (curves 2). This is clear evidence for the competition in light absorption between molybdenum and tungsten related oxyanions in the region of excitonic absorption in CdWO4 (Nagirnyi et al., 2003). The influence of the Mo impurity in the region of the electron–hole continuum is not so obvious. We estimated the threshold for the creation of free electrons and holes by measuring excitation spectra of the photostimulated recombination luminescence in CdWO4 :Mo (0.04 and 0.4 wt%) and CdMoO4 (Fig. 2, curves 3). The onset of the spectrum in CdWO4 :Mo (0.04 wt%) is at the same energy (5 eV) as in undoped CdWO4 (Nagirnyi et al., 2002), while the recombination luminescence is suppressed below 5.5 eV in CdWO4 :Mo (0.4 wt%) probably due to the efficient trapping of electrons by Mo-related oxyanions. The threshold energy for electron–hole pair creation in CdMoO4 was measured to be 4 eV, which is 1 eV less than in CdWO4 . This indicates that the upper oxyanionic excited states play a significant role in the electronic processes in the crystals under investigations.
than those of corresponding components of the emission of MoO6 complexes in CdWO4 :Mo. In contrast, the characteristics of the decay kinetics in the latter system are very similar to those of Mo luminescence centres in another crystal with wolframite structure, such as ZnWO4 (Nagirnyi et al., 2004). It should be mentioned that the decay probabilities of the forbidden radiative transitions from triplet states are extremely sensitive to the interactions that allow these transitions, such as exchange, spin–orbit, vibronic interaction, and hence they are sensitive to the crystal symmetry and the structure of the luminescence centre. Therefore, the fact that the decay times remain nearly exactly the same over the whole emission band both in CdWO4 :Mo and CdMoO4 , undoubtedly proves that the corresponding bands are elementary. The temperature dependences of the decay times () and the light sums (S) observed for Mo-related emissions (Fig. 4) are characteristics of triplet states. The (T ) dependence of the slow component exhibits a trend to form a plateau at temperatures below 2 K (less obvious for CdMoO4 due to the smaller triplet splitting) and a high-temperature plateau at 20–150 K, when the radiative and the metastable levels of the triplet are in complete thermal equilibrium. In the intermediate region (1.85–20 K), the redistribution of the light sums of the non-equilibrium fast and the slow components can be observed. The temperature dependences of (T ) and S(T ) are slightly different for CdMoO4 and CdWO4 :Mo. The decay times are generally shorter for the former, its fast component disappears at lower temperature, and the thermal quenching of the Mo-related emission takes place at 250 K in CdMoO4 and at 300 K in CdWO4 :Mo. Based on the procedure published by Hizhnyakov et al. (1983), we calculated the triplet state parameters for Mo centres in the
3.2. Emission decay kinetics The decay kinetics of Mo-related emission in CdWO4 :Mo and CdMoO4 suggests a triplet nature of the corresponding excited states. At 4.2 K two decay components are observed in the emission, which cannot be spectrally resolved. To illustrate this, the spectra of decay times () and light sums (S) of both components are shown for CdWO4 :Mo and CdMoO4 in Fig. 3. It is remarkable that the decay times of each component of the MoO4 complex emission in CdWO4 are substantially shorter
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Table 1 Parameters of the triplet relaxed state of the crystals studied
(s−1 )
2 1 (s−1 )
p0 (s−1 )
(eV)
CdWO4 :Mo (0.04 wt%)
CdWO4 :Mo (0.1 wt%)
CdWO4 :Mo (0.4 wt%)
CdMoO4
2600 590 760 1.2e − 3
3000 250 330 5.3e − 4
3000 240 280 4.5e − 4
8000 40 20 6.9e − 6
examined crystals. Table 1 lists these parameters, where 1 is the radiative decay probability of the metastable level, 2 is that of the radiative level, p0 is the probability of spontaneous nonradiative transition from the radiative to metastable level and is the energy splitting of the triplet state. It is interesting, that 2 is noticeably larger, while 1 and are smaller in CdMoO4 compared with these in CdWO4 :Mo. This is indicative of the larger exchange interaction and the smaller spin–orbit interaction in MoO4 centres in CdMoO4 . As the concentration of Mo in CdWO4 :Mo increases the parameters tend to approach those in CdMoO4 . 4. Conclusion The emission bands at 1.82 eV in CdWO4 :Mo and 2.19 eV in CdMoO4 are caused by the radiative decay of the triplet excited states of MoO6 and MoO4 oxyanions, respectively. The parameters of the triplet states in both systems are different, reflecting the differences in exchange, spin–orbit and vibronic interactions caused by different luminescence centre structures. The Mo impurity substantially reduces the light yield of CdWO4 crystal due to dissipative recharging processes in the defect centres. Acknowledgements This work was supported by the Estonian Science Foundation (Grant 6652) and the European Community—Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”).
References Földvári, I., Kappers, L.A., Gilliam, O.R., Hamilton, D.S., Lyu, Li-Ji., Cravero, I., Schmidt, F., 1990. The luminescence of molybdenum in ZnWO4 single crystals. J. Phys. Chem. Solids 51, 953–956. Garces, N.Y., Chirila, M.M., Murphy, H.J., Foise, J.W., Thomas, E.A., Wicks, C., Grencewicz, K., Halliburton, L.E., Giles, N.C., 2003. Absorption, luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO4 . J. Phys. Chem. Solids 64, 1195–1200. Hizhnyakov, V., Liidja, G., Nagirnyi, V., Soovik, T., Zazubovich, S., 1983. Effect of magnetic field on the decay kinetics and polarization of the AT emission of KCl: Ga at 0.38 K. Phys Status Solidi (b) 120, 105–115. Ivanov, S.N., Kolobanov, V.N., Mikhailin, V.V., Rzhevskaya, O.V., Spassky, D.A., Zadneprovski, B.I., Jönsson, L., Svensson, G., 2006. Luminescence of the set of tungstate crystals with wolframite structural type. In: Gektin, A., Grinyov, B. (Eds.), Proceedings of the 8th International Conference on Inorganic Scintillators and their Use in Scientific and Industrial Applications. Kharkov, pp. 375–377. Lammers, M.J.J., Blasse, G., Robertson, Ds., 1981. The luminescence of cadmium tungstate (CdWO4 ). Phys. Status Solidi (a) 63, 569–572. Mikhailik, V.B., Kraus, H., Wahl, D., Mykhaylyk, M.S., 2005. Studies of electronic excitations in MgMoO4 , CaMoO4 and CdMoO4 crystals using VUV synchrotron radiation. Phys Status Solidi (b) 242, R17–R19. Nagirnyi, V., Feldbach, E., Jönsson, L., Kirm, M., Kotlov, A., Lushchik, A., Nefedov, V.A., Zadneprovski, B.I., 2002. Energy transfer in ZnWO4 and CdWO4 scintillators. Nucl. Instrum. Methods Phys. Res. A 486, 395–398. Nagirnyi, V., Kirm, M., Kotlov, A., Lushchik, A., Jönsson, L., 2003. Separation of excitonic and electron–hole processes in tungstates of heavy metals. J. Lumin 102–103C, 597–603. Nagirnyi, V., Jönsson, L., Kirm, M., Kotlov, A., Lushchik, A., Martinson, I., Watterich, A., Zadneprovski, B.I., 2004. Luminescence study of pure and Fe or Mo doped ZnWO4 crystals. Radiat. Meas 38, 519–522.
Excited states of molybdenum oxyanion in scheelite and wolframite structures A. Kotlov a , L. Jönsson b , H. Kraus c , V. Mikhailik c , V. Nagirnyi a,∗ , G. Svensson d , B.I. Zadneprovski e a Institute of Physics, University of Tartu, 142 Riia Str., 51014 Tartu, Estonia b Department of Physics, Lund University, Professorsgatan 1, 22100 Lund, Sweden c Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK d Department of Inorganic Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden e All-Russia Research Institute of Mineral Materials Synthesis, Irkutskaya Str. 1, 601600 Alexandrov, Vladimir Region, Russia
Received 19 December 2006; accepted 1 February 2007
Abstract Scheelite CdMoO4 and wolframite CdWO4 :Mo (0.04–0.4 wt%) single crystals were studied using the time-resolved spectroscopy under UV excitation over a temperature range of 1.85–300 K. The threshold energies for the creation of free charge carriers were measured using the method of photostimulated luminescence. The decay kinetics of the main emission in CdMoO4 and the molybdenum-related emission in CdWO4 :Mo was studied and the parameters of the triplet excited states of molybdenum-related oxyanions were calculated. © 2007 Elsevier Ltd. All rights reserved. Keywords: CdMoO4 ; Luminescence; Decay kinetics; Triplet states
1. Introduction Tungstate crystals are prone to be contaminated by molybdenum. In ZnWO4 and CdWO4 scintillators, the presence of this impurity has been shown to cause additional bands of red luminescence peaking at 1.78 and 1.82 eV, respectively (Földvári et al., 1990; Nagirnyi et al., 2004; Garces et al., 2003). The relatively slow decay of these bands deteriorates the performance of the scintillator. CdWO4 :Mo has wolframite structure with MoO6 impurity complexes replacing regular WO6 oxyanions. CdMoO4 crystals have scheelite structure, and MoO4 complexes should be responsible for the excitonic emission. The latter system is not much studied. Some preliminary results on emission and excitation spectra of CdMoO4 have been reported by Mikhailik et al. (2005). By comparing the luminescence properties of molybdenum centres in wolframites and scheelites one could estimate how much they are affected by the
∗ Corresponding author. Tel.: +372 7428946; fax: +372 7383033.
E-mail address: vetal@fi.tartu.ee (V. Nagirnyi). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.02.009
symmetry of the luminescence centre. The present work is also aimed to investigate the effect of the crystal symmetry on the structure of the excited states of molybdenum oxyanion centres by studying the decay kinetics of molybdenum-related emissions. 2. Experimental methods The CdWO4 :Mo crystals studied here were grown at Chalmers University of Technology, Göteborg from a hightemperature solution using the slow cooling technique. The starting material was prepared from a mixture of CdO, MoO3 and WO3 in a Na2 WO4 solvent. The microprobe analysis showed Mo contents of 0.04, 0.1 and 0.4 wt% in the samples chosen for the study. The concentration of Na in the crystals grown did not exceed 1.3 at%. The CdMoO4 single crystal was grown using the Czochralski technique. The methods for measuring excitation spectra of photostimulated recombination luminescence, steady-state emission and excitation spectra as well as the emission decay kinetics have been described elsewhere (Nagirnyi et al., 2004).
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Fig. 1. Normalized emission spectra of a CdMoO4 crystal (d) and CdWO4 :Mo crystals containing 0.04 wt% (a), 0.1 wt% (b) and 0.4 wt% (c) of molybdenum under excitation with Eexc = 3.8 eV (1 in a, b, c), 3.6 eV (1 in d) and 5.5 eV (2). T = 4.2 K.
Fig. 2. Normalized excitation spectra of 1.82 eV (1 in a, b, c), 1.95 eV (1 in d), 2.46 eV (2 in a, b, c) and 2.19 eV emissions (2 in d) for CdWO4 :Mo crystals containing 0.04 wt% (a), 0.1 wt% (b) and 0.4 wt% (c) of molybdenum and for CdMoO4 (d). Curves 3 show the excitation spectra of recombination luminescence at 2.46 eV (a, b, c) and 2.19 eV (d) under stimulation by 1.8 eV photons. T = 4.2 K.
3. Experimental results and discussion 3.1. Emission and excitation spectra The emission spectrum of CdWO4 :Mo (0.04 wt%) contains two main bands peaked at 2.46 and 1.82 eV (Fig. 1). The 2.46 eV emission has been ascribed to the radiative decay of self-trapped excitons localized at the WO6 complex oxyanions (Lammers et al., 1981). The 1.82 eV emission is connected with MoO6 impurity oxyanion complexes (Garces et al., 2003). It can be excited near the fundamental absorption edge of CdWO4 in a band peaking at about 3.8 eV (Fig. 2a). The high-energy part of this band is distorted due to the matrix absorption. An analogous emission peaking at 1.75 eV has been investigated in ZnWO4 :Mo crystals using the method of time-resolved spectroscopy (Nagirnyi et al., 2004). Thus, the position of the luminescence band near 1.8 eV is characteristic of the molybdenum oxyanion complex embedded in the wolframite crystalline structure. On the other hand, the emission band at 2.19 eV in CdMoO4 crystals with scheelite structure is associated with MoO4 complexes (Fig. 1d). This emission band is fundamental; its shape is the same under excitation near the absorption edge at 3.6 eV and in the fundamental absorption region at 5 eV
(curves 1 and 2, respectively). This conclusion is supported also by the decay kinetics (see below). The excitation spectrum of the 2.19 eV emission is shown in Fig. 2d, curve 2. The onset of the spectrum is at 3.4 eV, the intensity reaches its maximum at 3.6 eV and then changes very little as the excitation energy increases up to 5 eV. Only traces of the red emission peaking near 1.95 eV can be observed for excitation below 3.4 eV. The excitation spectrum of this defect-related emission is shown by curve 1 in Fig. 2d. As the concentration of Mo in CdWO4 increases, the position of the red emission band observed under excitation by 3.8 eV photons remains effectively unchanged (curves 1 in Fig. 1a, b, c). However, its relative intensity substantially increases with molybdenum concentration under excitation in the fundamental absorption region at 5.5 eV (curves 2). This increase is accompanied by a reduction of the intensity of the intrinsic excitonic emission at 2.46 eV. This result is in good agreement with the results of Ivanov et al. (2006), who detected the same effect in CdWO4 :Mo crystals under X-ray irradiation. In addition to the low-energy shift of the excitation edge of the Mo-related band observed at increasing molybdenum con-
A. Kotlov et al. / Radiation Measurements 42 (2007) 767 – 770
769
Fig. 3. The spectra of decay times (1, 2) and light sums (1 , 2 ) of the fast and slow decay components of the 1.82 eV emission of CdWO4 :Mo (0.04 wt%) at Eexc = 3.8 eV (a) and of the 2.19 eV emission of CdMoO4 (b) at Eexc = 3.6 eV. T = 4.2 K.
Fig. 4. Temperature dependences of decay times (1, 2) and light sums (1 , 2 ) of the fast and slow decay components of the 1.82 eV emission of CdWO4 :Mo (0.04 wt%) (a) and CdWO4 :Mo (0.1 wt%) (b) at Eexc = 3.8 eV and of the 2.19 eV emission of CdMoO4 at Eexc = 3.6 eV (c).
centration (curves 1 in Fig. 2a, b, c) the edge of the excitation spectrum of the 2.46 eV emission moves gradually from 3.9 to 4.2 eV (curves 2). This is clear evidence for the competition in light absorption between molybdenum and tungsten related oxyanions in the region of excitonic absorption in CdWO4 (Nagirnyi et al., 2003). The influence of the Mo impurity in the region of the electron–hole continuum is not so obvious. We estimated the threshold for the creation of free electrons and holes by measuring excitation spectra of the photostimulated recombination luminescence in CdWO4 :Mo (0.04 and 0.4 wt%) and CdMoO4 (Fig. 2, curves 3). The onset of the spectrum in CdWO4 :Mo (0.04 wt%) is at the same energy (5 eV) as in undoped CdWO4 (Nagirnyi et al., 2002), while the recombination luminescence is suppressed below 5.5 eV in CdWO4 :Mo (0.4 wt%) probably due to the efficient trapping of electrons by Mo-related oxyanions. The threshold energy for electron–hole pair creation in CdMoO4 was measured to be 4 eV, which is 1 eV less than in CdWO4 . This indicates that the upper oxyanionic excited states play a significant role in the electronic processes in the crystals under investigations.
than those of corresponding components of the emission of MoO6 complexes in CdWO4 :Mo. In contrast, the characteristics of the decay kinetics in the latter system are very similar to those of Mo luminescence centres in another crystal with wolframite structure, such as ZnWO4 (Nagirnyi et al., 2004). It should be mentioned that the decay probabilities of the forbidden radiative transitions from triplet states are extremely sensitive to the interactions that allow these transitions, such as exchange, spin–orbit, vibronic interaction, and hence they are sensitive to the crystal symmetry and the structure of the luminescence centre. Therefore, the fact that the decay times remain nearly exactly the same over the whole emission band both in CdWO4 :Mo and CdMoO4 , undoubtedly proves that the corresponding bands are elementary. The temperature dependences of the decay times () and the light sums (S) observed for Mo-related emissions (Fig. 4) are characteristics of triplet states. The (T ) dependence of the slow component exhibits a trend to form a plateau at temperatures below 2 K (less obvious for CdMoO4 due to the smaller triplet splitting) and a high-temperature plateau at 20–150 K, when the radiative and the metastable levels of the triplet are in complete thermal equilibrium. In the intermediate region (1.85–20 K), the redistribution of the light sums of the non-equilibrium fast and the slow components can be observed. The temperature dependences of (T ) and S(T ) are slightly different for CdMoO4 and CdWO4 :Mo. The decay times are generally shorter for the former, its fast component disappears at lower temperature, and the thermal quenching of the Mo-related emission takes place at 250 K in CdMoO4 and at 300 K in CdWO4 :Mo. Based on the procedure published by Hizhnyakov et al. (1983), we calculated the triplet state parameters for Mo centres in the
3.2. Emission decay kinetics The decay kinetics of Mo-related emission in CdWO4 :Mo and CdMoO4 suggests a triplet nature of the corresponding excited states. At 4.2 K two decay components are observed in the emission, which cannot be spectrally resolved. To illustrate this, the spectra of decay times () and light sums (S) of both components are shown for CdWO4 :Mo and CdMoO4 in Fig. 3. It is remarkable that the decay times of each component of the MoO4 complex emission in CdWO4 are substantially shorter
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Table 1 Parameters of the triplet relaxed state of the crystals studied
(s−1 )
2 1 (s−1 )
p0 (s−1 )
(eV)
CdWO4 :Mo (0.04 wt%)
CdWO4 :Mo (0.1 wt%)
CdWO4 :Mo (0.4 wt%)
CdMoO4
2600 590 760 1.2e − 3
3000 250 330 5.3e − 4
3000 240 280 4.5e − 4
8000 40 20 6.9e − 6
examined crystals. Table 1 lists these parameters, where 1 is the radiative decay probability of the metastable level, 2 is that of the radiative level, p0 is the probability of spontaneous nonradiative transition from the radiative to metastable level and is the energy splitting of the triplet state. It is interesting, that 2 is noticeably larger, while 1 and are smaller in CdMoO4 compared with these in CdWO4 :Mo. This is indicative of the larger exchange interaction and the smaller spin–orbit interaction in MoO4 centres in CdMoO4 . As the concentration of Mo in CdWO4 :Mo increases the parameters tend to approach those in CdMoO4 . 4. Conclusion The emission bands at 1.82 eV in CdWO4 :Mo and 2.19 eV in CdMoO4 are caused by the radiative decay of the triplet excited states of MoO6 and MoO4 oxyanions, respectively. The parameters of the triplet states in both systems are different, reflecting the differences in exchange, spin–orbit and vibronic interactions caused by different luminescence centre structures. The Mo impurity substantially reduces the light yield of CdWO4 crystal due to dissipative recharging processes in the defect centres. Acknowledgements This work was supported by the Estonian Science Foundation (Grant 6652) and the European Community—Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”).
References Földvári, I., Kappers, L.A., Gilliam, O.R., Hamilton, D.S., Lyu, Li-Ji., Cravero, I., Schmidt, F., 1990. The luminescence of molybdenum in ZnWO4 single crystals. J. Phys. Chem. Solids 51, 953–956. Garces, N.Y., Chirila, M.M., Murphy, H.J., Foise, J.W., Thomas, E.A., Wicks, C., Grencewicz, K., Halliburton, L.E., Giles, N.C., 2003. Absorption, luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO4 . J. Phys. Chem. Solids 64, 1195–1200. Hizhnyakov, V., Liidja, G., Nagirnyi, V., Soovik, T., Zazubovich, S., 1983. Effect of magnetic field on the decay kinetics and polarization of the AT emission of KCl: Ga at 0.38 K. Phys Status Solidi (b) 120, 105–115. Ivanov, S.N., Kolobanov, V.N., Mikhailin, V.V., Rzhevskaya, O.V., Spassky, D.A., Zadneprovski, B.I., Jönsson, L., Svensson, G., 2006. Luminescence of the set of tungstate crystals with wolframite structural type. In: Gektin, A., Grinyov, B. (Eds.), Proceedings of the 8th International Conference on Inorganic Scintillators and their Use in Scientific and Industrial Applications. Kharkov, pp. 375–377. Lammers, M.J.J., Blasse, G., Robertson, Ds., 1981. The luminescence of cadmium tungstate (CdWO4 ). Phys. Status Solidi (a) 63, 569–572. Mikhailik, V.B., Kraus, H., Wahl, D., Mykhaylyk, M.S., 2005. Studies of electronic excitations in MgMoO4 , CaMoO4 and CdMoO4 crystals using VUV synchrotron radiation. Phys Status Solidi (b) 242, R17–R19. Nagirnyi, V., Feldbach, E., Jönsson, L., Kirm, M., Kotlov, A., Lushchik, A., Nefedov, V.A., Zadneprovski, B.I., 2002. Energy transfer in ZnWO4 and CdWO4 scintillators. Nucl. Instrum. Methods Phys. Res. A 486, 395–398. Nagirnyi, V., Kirm, M., Kotlov, A., Lushchik, A., Jönsson, L., 2003. Separation of excitonic and electron–hole processes in tungstates of heavy metals. J. Lumin 102–103C, 597–603. Nagirnyi, V., Jönsson, L., Kirm, M., Kotlov, A., Lushchik, A., Martinson, I., Watterich, A., Zadneprovski, B.I., 2004. Luminescence study of pure and Fe or Mo doped ZnWO4 crystals. Radiat. Meas 38, 519–522.