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The luminescence of molybdenum in ZnWO4 single crystals
J. Phys. Chem.
Solids Vol. 51. No. 8. pp. 953-956, 1990
0022.3697pa
Printed in Great Britain.
8
THE LUMINESCENCE OF MOLYBDENUM SINGLE CRYSTALS I. FbLDv,&t
13.00 + 0.00
1990 Pergamoo Press
pk
IN ZnWO,
L. A. KAPPERS,$ 0. R. GILLIAM,~ D. S. HA,WLTON,~ Lr-JI Lw,$ I. f&AvERot and F. SCHMIDTt
TResearch Laboratory for Crystal Physics, Hungarian Academy of Sciences, Buda6rsi tit 45, H-1112 Budapest, Hungary funiversity of Connecticut, Physics Department, Institute of LMaterials Science, Storm. CT 06269-3046. U.S.A. (Received
20 November
1989; accepted
7 February
1990)
Abstract-Zinc tungstate (ZnWO, ) single crystals have an intrinsic luminescence at 480 nm which is utilized in their application in scintillator devices. A broad component of luminescence which peaks at about 635 nm at 77 K is shown to result from molybdenum impurity ions. The molybdenum luminescence can be excited in the transparent optical range of ZnWO, close to its absorption edge. The molybdenum emission has a resolvable doublet structure and a decay time of 145 ps at 300 K and 800 ps at 77 K, which is long compared with the intrinsic luminescence decay. KeyworrLr: ZnWO, : MO crystals, luminescence, optical, absorption, scintillator materials.
lNTRODUCTION Zinc tungstate is a potential scintillator material with a high density (7.8 g cmm3) and is a strong absorber of X-rays. Its high scintillation efficiency (35% of the quantum yield of NaI : TI [l]) and low after-glow
have led to practical applications in computerassisted tomography. Zinc tungstate possesses an intrinsic luminescence which has been studied in detail [2-A. Among the dopants previously investigated in ZnWO,, chromium has a characteristic luminescence (8,9], while others, like iron, absorb the intrinsic luminescence [ 1, lo]. In the present work we have investigated the longer wavelength luminescence band of ZnWO, which has been detected by several authors as a yellow shoulder of the intrinsic blue luminescence [2,3,7, lo]. This shoulder could be separated from the main luminescence component by its characteristic excitation curve, and was interpreted previously as another intrinsic luminescence band [7j. However, the relative intensity of the blue and yellow components reported by various authors was different, and considerable variation in the relative intensity was also noted in our crystals prepared from different raw materials. Therefore, we suspected that an impurity with chemical properties like tungsten was responsible for the yellow luminescence. This luminescence could be enhanced in our experiments by adding molybdenum to the ZnWO, crystals, which provided the opportunity to investigate the yellow luminescence in more detail. EXPERIMENTAL In earlier research we concentrated on improvement of the quality of ZnWO, single crystals [ 11.This PCS II/S-E
effort involved the trial of different sources of tungsten and zinc starting materials and the use of various chemical purification methods. Molybdenum has chemical properties similar to those of tungsten, and its removal was beyond the scope of this research. Thus the actual MO concentration in our undoped crystals was dependent upon the raw materials and chemical procedures used. To enhance the influence of MO, doped crystals were also grown adding lo-’ mol mol-’ MOO, to the melt. This molybdenum content of the crystals was determined by atomic absorption spectroscopy (AAS) by the procedure described in [l]. All of the crystals for this research were grown in air, using a platinum crucible and an automated Czochralski technique, under approximately the same growth conditions as those described earlier [l]. The pulling velocity (2.5 mm h-‘) was appropriate to provide quasi-equilibrium conditions for solid-liquid segregation of impurities. The segregation coefficient for MO in ZnWO,, according to our observations, was close to unity. Consequently the variation of MO concentration along the growth axis was negligible. Crystal luminescence was induced by photoexcitation using either the 325~nm line of a Liconix 4230NB He-Cd laser or a xenon lamp combined with a Jarrell-Ash 1/4-m monochromator. The emission spectra were measured with a Pacific 1/2-m spectrometer and an RCA 4832 photomultiplier tube, and they were corrected for the spectral response of the instruments. The absorption spectra of the ZnWO, samples were measured on a Perkin-Elmer Lambda 4 spectrophotometer. For luminescence and absorption measurements, oriented crystal samples were used, which were
I.
954
F6LDVh
et
al.
polished on (100) and (001) faces and cleaved along the (010) plane. The typical optical surfaces were 10 x IZmm, with thicknesses ranging from 0.5 to 1Omm. For the room temperature lifetime measurements, the third harmonic (355 nm) of a Quanta Ray DCR- 1 Nd : YAG laser, with a IO-ns duration pulse, constituted the excitation source. For the 77-K excitation, the 330~nm laser light was produced by a frequencydoubled tunable dye laser. The lifetime data were obtained using a LeCroy 3500 transient digitizer and signal averager. Wavelength
(nm)
Fig. 2. Excitation and absorption spectra for ZnWO, crysRESULTS AND DISCU~I~N Our measurements confirmed the findings that photoexcitation along the intrinsic absorption edge and at larger photon energies lead to the well-known blue intrinsic luminescence peaking at about 480 nm in nominaliy pure ZnWOa crystals [2,3, lo], Upon scanning the exciting light towards longer wavelength, a yellow emission which peaks at about 600 nm (at 300 K) became more enhanced, while the intrinsic luminescence decreased. For measurements at 77 K, the yellow emission was more intense for excitation at shorter wavelengths as a consequence of the absorption-edge shift toward the UV with reduced temperature. The intensity of the yellow luminescence was strongly dependent on the raw materials used for crystal growth and was considerably larger for MO-doped crystals. A plot of the emission intensity at 635 nm (77 K) vs the MO concentration is shown in Fig. I, which indicates that the yellow emission is correlated with MO concentration. The excitation spectrum of the MO emission in ZnWO,, measured at 77 K, is shown in curve A of Fig. 2. The corresponding absorption spectrum of the MO-doped crystal is depicted in Fig. 2 (curve B). In addition, Fig. 2 shows the absorption spectra of
MOConcentration(~mole/molel Fig. I. The intensity of the 63%nm luminescence at 77 K vs the MO concentration in ZnWO, single crystals. The excita-
tion wavelength was 335 nm. Data points are labeled (Of for doped and (+) for undoped crystals.
tals. (A) Excitation spectrum of the 635~run luminescence at 77 K in Mo-doped ZnWO,. (B) Absorption of a 0.5 mm thick MO-doped ZnWO, sample at 77 K. (C) Absorption of a 0.5 mm thick undoped ZnWO, crystal at 77 K. (D) The
absorption of the undoped crystal at 300 K. nominally pure ZnWQo at 77 K (curve C) and at 300 K (curve D). The maximum of the excitation curve for the yellow MO luminescence occurs at 336 nm at 77 K. The MO absorption is close to the intrinsic absorption edge in zinc tungstate. The difference spectrum of MO-doped and pure samples (B minus C) has a maximum of approx. 315 nm (77 K}. The variation between the absorption (I3 minus C) and the excitation spectrum (A) for the MO-doped crystal is due to the intrinsic absorption of the host, which drastically reduces the effective excitation intensity at short wavelengths. The 300-K absorption edge of the pure material (curve D in Fig. 2) covers a much larger part of the MO abso~tion. Thus the peak of the 300-K excitation spectrum, which occurs at 338 nm, less reliably reflects the absorption spectrum of molybdenum. The emission spectrum of the MO-doped ZnWO,, excited at 77 K by 335-nm light, is shown in Fig. 3. Under these conditions the maximum emission is at 635 nm, while the intrinsic blue luminescence is negligible. The emission curve was fit to two Gaussian bands, whose characteristics are presented in Table 1. The shape of the emission spectrum is similar to that obtained for excitation at 300K by 350~nm light, well-removed from the effective excitation region of the intrinsic blue luminescence. The maximum of the 300-K emission is situated at 605 nm, and the position of the resolved main component is at 582 nm, close to the results published by Ovechkin et al. [7]. They have resolved two additional bands at 652 and 738 nm using their different lifetime and temperature dependence. The 652~nm peak may correspond to the shoulder in the emission spectrum of our MO-doped crystal. The role of molybdenum in the isomorphic CdW04 crystal has been studied in detail. A characteristic absorption band was found at 380 nm [l l] and interpreted as due to MO impu~ty 1121. Yellow emission, similar to that found in ZnWOl, was also
Luminescence of molybdenum in ZnWO, single crystals Wavelength
(nm)
955
emission at 77 K is shown in the insert of Fig. 3. Similar curves were obtained at 300 K, and for the other component of this luminescence. The maximum emission intensity was observed for E parallel to the [OOl] direction, the minimum for E parallel to [loo]. A smaller intensity maximum appeared for E parallel to [OlO]. The axis of the MOO, octahedron (apart from the distorted symmetry in the ZnWO, lattice) is parallel to [1 111.In this arrangement the yz and xz crystal field directions are parallel to the [OOl] and [OlO] crystallographic axes, respectively. In the octahedral approximation, t18-+tti and t,,+t& transitions are expected to be involved in the luminescence. The former transition corresponds to the irreducible representation of the yz and xz directions. TWO approximations have been introduced for the Wavenumber (cm-i) symmetry reduction in ZnWO,. In both Du [8,9] Fig. 3. The emission spectrum of ZnWO, : MO (solid curve) and C,,, [16, 17] symmetries, the degeneracy of yz excited at 77 K and 335 nm. The dashed curves indicate the and xz directions is removed. The observed experitwo Gaussian components. The polarization dependence of mental polarization directions for the main compothe emission intensity for the main component is illustrated in the inset. In the upper curve the emission is propagating nent of the yellow emission are in agreement in the [OIO] direction, in the lower curve in the [OOI] with these simplified symmetry requirements. The direction. interpretation of the second component (shoulder) of the MO luminescence is more difficult. Spinorbit interaction, symmetry reduction and vibration observed in CdWO,. The conclusion was, however, interaction may lead to the appearance of this that the yellow luminescence in CdW04 was intrinsic band. [7, 131.This conclusion was based on its much shorter In accordance with our expectation, the intrinsic decay time compared with the luminescence decay blue luminescence of ZnWO, itself has a doublet times in molybdates [13, 141. In addition, no correcharacter. The figures in Refs 2,5 and 7 contain some lation was revealed between MO content and the indication of this doublet but that evidence is not yellow component of the CdW04 luminescence. The conclusive. In our experiments using polarization doublet structure of the luminescence in tungstates techniques, the doublet structure of the intrinsic and molybdates has been observed by several luminescence in ZnWO, is clearly shown. In contrast authors; e.g. see Refs 7 and 13-15. A configurationto the MO luminescence, the relative intensities of the coordinate model was developed for CaWO, luminesintrinsic emission bands peaking at 450 and 510 nm cence [15], and its modified version was used for for 300 K were significantly different in the different ZnWO, and CdWO, [7,13]. In the zinc and cadmium polarization directions. The intensity ratio &/I,,, is tungstates, perturbed oxygen octahedrons represent 0.7 along [loo], 1.6 along [OlO] and 1.9 along [OOl]. the crystal field around the tungsten cations. This The doublet separation of the intrinsic luminescence model will also be used for a qualitative description is about 2600 cm-‘, which is about 50% larger than of the MO luminescence in ZnWO,, assuming Mo6+ that of the MO-emission band. substitution for W6+. The time kinetics of the yellow luminescence in The polarization properties of the MO lumiZnWO,: MO could be measured separately from nescence in ZnWO, were investigated at 77 and those of the intrinsic emission. A typical 300-K decay 300 K. The angular dependence of the polarization curve for the MO yellow luminescence is shown in direction for the main component of the yellow Fig. 4. At temperatures of 300 and 77 K, the decay curves were exponential over two decades. Within Table 1. The spectroscopic parameters of the molybdenum measured error limits the lifetimes for both comluminescence in ZnWO, ponents of the MO luminescence were the same. 77 K 300K The extremely long decay time at 77 K (800 ps) is identical to that of the r5 component in Ref. 7. They Excitation maximum (nm) 336 338 Emission maximum (nm) found more complicated decay kinetics for ZnWO,, 635 605 band position (nm) 629,700 582,645 but in their measurements the overlapping of the band halfwidths (cm-‘) 3500.3300 4300,350o intrinsic and the yellow luminescence was unavoidEmission decay time @s) 800 145 able. Note that the decay time of the MO luminesIntensity ratio of the bands l/O.33 l/O.38 cence at 300 K (145 ps) is about six times longer (Gaussian components) Polarization dependence than that of the intrinsic blue emission. This value maximum emission E parallel to [OOI] is close to the 100 ps observed for CaMoO, crystals minimum emission E parallel to [lOO] 1141.
I. F~LDV~I
er al.
high sampling rates, even for very low concentrations of molybdenum. Acknowledgements-This
research was supported jointly by the University of Connecticut Research Foundation, the U.S. National Science Foundation under Grant No. INT-8617352 and the Academic Research Foundation OTKA of Hungary. The authors wish to express their gratitude to Profs R. Voszka, R. H. Bartram and Mrs A. Peter for helpful discussions, and Mrs Zs. Toth for technical help in crystal growth.
REFERENCES I. Fiildvlri I., Peter A., Keszthelyi-Landori
Fig. 4. The decay for the main component of the yellow luminescence in a ZnWO, : MO single crystal measured at room temperature. Excitation was at 355 nm and the luminescence was measured at 582 nm. CONCLUSIONS Contrary to the previously reported finding (71, our experiments show that the yellow luminescence in ZnWO, is not intrinsic, but is associated with MO impurities. The absorption of the Mo6+ ions is close to the intrinsic absorption edge of the ZnWO, host material, and therefore excitation of the MO emission is effective only in a narrow wavelength range. As the excitation is scanned to shorter wavelength, the intrinsic luminescence becomes dominant before the 315nm maximum (at 77 K) of the MO absorption, even in heavily doped samples (lo-’ mol mol-‘). At room temperature the blue emission is already dominant at 335 nm. The concentration of MO in commercial ZnWO, crystals, or in raw materials for crystal growth, is typically less than IO-‘mol mol-I. If we extrapolate from the photoexcitation experiments, this MO content does not significantly influence the room temperature scintillation properties of the crystals. However, the long decay time of the MO luminescence might be detrimental in some applications with
S., Capelletti R., Craver0 I. and Schmidt F., J. Cry& Growth 79,714 (1986). 2. Leonov Yu. S., Opr. Spektrosk. 12, 255 (1962). 3. Oi T., Takagi K. and Fukazawa T., Appl. Phys. Len. 36, 278
(1980).
4. Born P. J., Robertson D. S., Smith P. W., Hames G., Reed J. and Telfor J., J. Laminesc. 24/25, 131 (1981). 5. Grassmann H., Moser H. G. and Lorentz E., J. Luminesc. 33, 109 (1985). 6. Zhu Y. C., Lu J. G., Shao Y. Y., Sun H. S., Li J., Wang S. Y., Dong B. Z., Zheng Z. P. and Zhou Y. D., Nucl. Instrum. &feth. Phys. Res. A-244, 579 (1986). 7. Ovechkin A. E., Ryzhikov V. D., Tamulaitis G. and Zukauskas A.. Phvs Status Solidi /a) 103. 285 (1987). 8. Peterman K. and- Huber G., J. iuminesc. 31j32, 71 (1984). 9. Kolbe W., Peterman K. and Huber G., IEEE J. Quunturn Elecfron. 21, 1596 (1985). 10. Takagi K., Oi T. and Fukazawa T., J. Crysfal Growth 52, 580 (1981). II. Robertson D. S., Young I. M. and Telfor J. R., J. Mater. Sci. 14, 2967 (1979). 12. Oeder R., Scharmann A. and Schwabe D., J. Crystal Growth 49. 349 (1980).
13. Lammers M. J. J:, Blasse G. and Robertson D. S., Phys. Status Solidi (a) 63, 569 (1981).
14. Groenink J. A., Hakfoort C. and Blasse G., Phys. Status Solidi (a) 54, 329 (1979).
15. Treadaway M. J. and Powell R. C., J. them. Phys. 61, 4003 (1974). 16. Borromei R., Ingletto G., Oleari L. and Day P., J. them. Sot. Furuday Trans. 77, 2249 (1981). 17. Borromei R., Ingletto G., Oleari L. and Day P., J. them. Sot. Faraday Trans. 78, 1705 (1982).
Solids Vol. 51. No. 8. pp. 953-956, 1990
0022.3697pa
Printed in Great Britain.
8
THE LUMINESCENCE OF MOLYBDENUM SINGLE CRYSTALS I. FbLDv,&t
13.00 + 0.00
1990 Pergamoo Press
pk
IN ZnWO,
L. A. KAPPERS,$ 0. R. GILLIAM,~ D. S. HA,WLTON,~ Lr-JI Lw,$ I. f&AvERot and F. SCHMIDTt
TResearch Laboratory for Crystal Physics, Hungarian Academy of Sciences, Buda6rsi tit 45, H-1112 Budapest, Hungary funiversity of Connecticut, Physics Department, Institute of LMaterials Science, Storm. CT 06269-3046. U.S.A. (Received
20 November
1989; accepted
7 February
1990)
Abstract-Zinc tungstate (ZnWO, ) single crystals have an intrinsic luminescence at 480 nm which is utilized in their application in scintillator devices. A broad component of luminescence which peaks at about 635 nm at 77 K is shown to result from molybdenum impurity ions. The molybdenum luminescence can be excited in the transparent optical range of ZnWO, close to its absorption edge. The molybdenum emission has a resolvable doublet structure and a decay time of 145 ps at 300 K and 800 ps at 77 K, which is long compared with the intrinsic luminescence decay. KeyworrLr: ZnWO, : MO crystals, luminescence, optical, absorption, scintillator materials.
lNTRODUCTION Zinc tungstate is a potential scintillator material with a high density (7.8 g cmm3) and is a strong absorber of X-rays. Its high scintillation efficiency (35% of the quantum yield of NaI : TI [l]) and low after-glow
have led to practical applications in computerassisted tomography. Zinc tungstate possesses an intrinsic luminescence which has been studied in detail [2-A. Among the dopants previously investigated in ZnWO,, chromium has a characteristic luminescence (8,9], while others, like iron, absorb the intrinsic luminescence [ 1, lo]. In the present work we have investigated the longer wavelength luminescence band of ZnWO, which has been detected by several authors as a yellow shoulder of the intrinsic blue luminescence [2,3,7, lo]. This shoulder could be separated from the main luminescence component by its characteristic excitation curve, and was interpreted previously as another intrinsic luminescence band [7j. However, the relative intensity of the blue and yellow components reported by various authors was different, and considerable variation in the relative intensity was also noted in our crystals prepared from different raw materials. Therefore, we suspected that an impurity with chemical properties like tungsten was responsible for the yellow luminescence. This luminescence could be enhanced in our experiments by adding molybdenum to the ZnWO, crystals, which provided the opportunity to investigate the yellow luminescence in more detail. EXPERIMENTAL In earlier research we concentrated on improvement of the quality of ZnWO, single crystals [ 11.This PCS II/S-E
effort involved the trial of different sources of tungsten and zinc starting materials and the use of various chemical purification methods. Molybdenum has chemical properties similar to those of tungsten, and its removal was beyond the scope of this research. Thus the actual MO concentration in our undoped crystals was dependent upon the raw materials and chemical procedures used. To enhance the influence of MO, doped crystals were also grown adding lo-’ mol mol-’ MOO, to the melt. This molybdenum content of the crystals was determined by atomic absorption spectroscopy (AAS) by the procedure described in [l]. All of the crystals for this research were grown in air, using a platinum crucible and an automated Czochralski technique, under approximately the same growth conditions as those described earlier [l]. The pulling velocity (2.5 mm h-‘) was appropriate to provide quasi-equilibrium conditions for solid-liquid segregation of impurities. The segregation coefficient for MO in ZnWO,, according to our observations, was close to unity. Consequently the variation of MO concentration along the growth axis was negligible. Crystal luminescence was induced by photoexcitation using either the 325~nm line of a Liconix 4230NB He-Cd laser or a xenon lamp combined with a Jarrell-Ash 1/4-m monochromator. The emission spectra were measured with a Pacific 1/2-m spectrometer and an RCA 4832 photomultiplier tube, and they were corrected for the spectral response of the instruments. The absorption spectra of the ZnWO, samples were measured on a Perkin-Elmer Lambda 4 spectrophotometer. For luminescence and absorption measurements, oriented crystal samples were used, which were
I.
954
F6LDVh
et
al.
polished on (100) and (001) faces and cleaved along the (010) plane. The typical optical surfaces were 10 x IZmm, with thicknesses ranging from 0.5 to 1Omm. For the room temperature lifetime measurements, the third harmonic (355 nm) of a Quanta Ray DCR- 1 Nd : YAG laser, with a IO-ns duration pulse, constituted the excitation source. For the 77-K excitation, the 330~nm laser light was produced by a frequencydoubled tunable dye laser. The lifetime data were obtained using a LeCroy 3500 transient digitizer and signal averager. Wavelength
(nm)
Fig. 2. Excitation and absorption spectra for ZnWO, crysRESULTS AND DISCU~I~N Our measurements confirmed the findings that photoexcitation along the intrinsic absorption edge and at larger photon energies lead to the well-known blue intrinsic luminescence peaking at about 480 nm in nominaliy pure ZnWOa crystals [2,3, lo], Upon scanning the exciting light towards longer wavelength, a yellow emission which peaks at about 600 nm (at 300 K) became more enhanced, while the intrinsic luminescence decreased. For measurements at 77 K, the yellow emission was more intense for excitation at shorter wavelengths as a consequence of the absorption-edge shift toward the UV with reduced temperature. The intensity of the yellow luminescence was strongly dependent on the raw materials used for crystal growth and was considerably larger for MO-doped crystals. A plot of the emission intensity at 635 nm (77 K) vs the MO concentration is shown in Fig. I, which indicates that the yellow emission is correlated with MO concentration. The excitation spectrum of the MO emission in ZnWO,, measured at 77 K, is shown in curve A of Fig. 2. The corresponding absorption spectrum of the MO-doped crystal is depicted in Fig. 2 (curve B). In addition, Fig. 2 shows the absorption spectra of
MOConcentration(~mole/molel Fig. I. The intensity of the 63%nm luminescence at 77 K vs the MO concentration in ZnWO, single crystals. The excita-
tion wavelength was 335 nm. Data points are labeled (Of for doped and (+) for undoped crystals.
tals. (A) Excitation spectrum of the 635~run luminescence at 77 K in Mo-doped ZnWO,. (B) Absorption of a 0.5 mm thick MO-doped ZnWO, sample at 77 K. (C) Absorption of a 0.5 mm thick undoped ZnWO, crystal at 77 K. (D) The
absorption of the undoped crystal at 300 K. nominally pure ZnWQo at 77 K (curve C) and at 300 K (curve D). The maximum of the excitation curve for the yellow MO luminescence occurs at 336 nm at 77 K. The MO absorption is close to the intrinsic absorption edge in zinc tungstate. The difference spectrum of MO-doped and pure samples (B minus C) has a maximum of approx. 315 nm (77 K}. The variation between the absorption (I3 minus C) and the excitation spectrum (A) for the MO-doped crystal is due to the intrinsic absorption of the host, which drastically reduces the effective excitation intensity at short wavelengths. The 300-K absorption edge of the pure material (curve D in Fig. 2) covers a much larger part of the MO abso~tion. Thus the peak of the 300-K excitation spectrum, which occurs at 338 nm, less reliably reflects the absorption spectrum of molybdenum. The emission spectrum of the MO-doped ZnWO,, excited at 77 K by 335-nm light, is shown in Fig. 3. Under these conditions the maximum emission is at 635 nm, while the intrinsic blue luminescence is negligible. The emission curve was fit to two Gaussian bands, whose characteristics are presented in Table 1. The shape of the emission spectrum is similar to that obtained for excitation at 300K by 350~nm light, well-removed from the effective excitation region of the intrinsic blue luminescence. The maximum of the 300-K emission is situated at 605 nm, and the position of the resolved main component is at 582 nm, close to the results published by Ovechkin et al. [7]. They have resolved two additional bands at 652 and 738 nm using their different lifetime and temperature dependence. The 652~nm peak may correspond to the shoulder in the emission spectrum of our MO-doped crystal. The role of molybdenum in the isomorphic CdW04 crystal has been studied in detail. A characteristic absorption band was found at 380 nm [l l] and interpreted as due to MO impu~ty 1121. Yellow emission, similar to that found in ZnWOl, was also
Luminescence of molybdenum in ZnWO, single crystals Wavelength
(nm)
955
emission at 77 K is shown in the insert of Fig. 3. Similar curves were obtained at 300 K, and for the other component of this luminescence. The maximum emission intensity was observed for E parallel to the [OOl] direction, the minimum for E parallel to [loo]. A smaller intensity maximum appeared for E parallel to [OlO]. The axis of the MOO, octahedron (apart from the distorted symmetry in the ZnWO, lattice) is parallel to [1 111.In this arrangement the yz and xz crystal field directions are parallel to the [OOl] and [OlO] crystallographic axes, respectively. In the octahedral approximation, t18-+tti and t,,+t& transitions are expected to be involved in the luminescence. The former transition corresponds to the irreducible representation of the yz and xz directions. TWO approximations have been introduced for the Wavenumber (cm-i) symmetry reduction in ZnWO,. In both Du [8,9] Fig. 3. The emission spectrum of ZnWO, : MO (solid curve) and C,,, [16, 17] symmetries, the degeneracy of yz excited at 77 K and 335 nm. The dashed curves indicate the and xz directions is removed. The observed experitwo Gaussian components. The polarization dependence of mental polarization directions for the main compothe emission intensity for the main component is illustrated in the inset. In the upper curve the emission is propagating nent of the yellow emission are in agreement in the [OIO] direction, in the lower curve in the [OOI] with these simplified symmetry requirements. The direction. interpretation of the second component (shoulder) of the MO luminescence is more difficult. Spinorbit interaction, symmetry reduction and vibration observed in CdWO,. The conclusion was, however, interaction may lead to the appearance of this that the yellow luminescence in CdW04 was intrinsic band. [7, 131.This conclusion was based on its much shorter In accordance with our expectation, the intrinsic decay time compared with the luminescence decay blue luminescence of ZnWO, itself has a doublet times in molybdates [13, 141. In addition, no correcharacter. The figures in Refs 2,5 and 7 contain some lation was revealed between MO content and the indication of this doublet but that evidence is not yellow component of the CdW04 luminescence. The conclusive. In our experiments using polarization doublet structure of the luminescence in tungstates techniques, the doublet structure of the intrinsic and molybdates has been observed by several luminescence in ZnWO, is clearly shown. In contrast authors; e.g. see Refs 7 and 13-15. A configurationto the MO luminescence, the relative intensities of the coordinate model was developed for CaWO, luminesintrinsic emission bands peaking at 450 and 510 nm cence [15], and its modified version was used for for 300 K were significantly different in the different ZnWO, and CdWO, [7,13]. In the zinc and cadmium polarization directions. The intensity ratio &/I,,, is tungstates, perturbed oxygen octahedrons represent 0.7 along [loo], 1.6 along [OlO] and 1.9 along [OOl]. the crystal field around the tungsten cations. This The doublet separation of the intrinsic luminescence model will also be used for a qualitative description is about 2600 cm-‘, which is about 50% larger than of the MO luminescence in ZnWO,, assuming Mo6+ that of the MO-emission band. substitution for W6+. The time kinetics of the yellow luminescence in The polarization properties of the MO lumiZnWO,: MO could be measured separately from nescence in ZnWO, were investigated at 77 and those of the intrinsic emission. A typical 300-K decay 300 K. The angular dependence of the polarization curve for the MO yellow luminescence is shown in direction for the main component of the yellow Fig. 4. At temperatures of 300 and 77 K, the decay curves were exponential over two decades. Within Table 1. The spectroscopic parameters of the molybdenum measured error limits the lifetimes for both comluminescence in ZnWO, ponents of the MO luminescence were the same. 77 K 300K The extremely long decay time at 77 K (800 ps) is identical to that of the r5 component in Ref. 7. They Excitation maximum (nm) 336 338 Emission maximum (nm) found more complicated decay kinetics for ZnWO,, 635 605 band position (nm) 629,700 582,645 but in their measurements the overlapping of the band halfwidths (cm-‘) 3500.3300 4300,350o intrinsic and the yellow luminescence was unavoidEmission decay time @s) 800 145 able. Note that the decay time of the MO luminesIntensity ratio of the bands l/O.33 l/O.38 cence at 300 K (145 ps) is about six times longer (Gaussian components) Polarization dependence than that of the intrinsic blue emission. This value maximum emission E parallel to [OOI] is close to the 100 ps observed for CaMoO, crystals minimum emission E parallel to [lOO] 1141.
I. F~LDV~I
er al.
high sampling rates, even for very low concentrations of molybdenum. Acknowledgements-This
research was supported jointly by the University of Connecticut Research Foundation, the U.S. National Science Foundation under Grant No. INT-8617352 and the Academic Research Foundation OTKA of Hungary. The authors wish to express their gratitude to Profs R. Voszka, R. H. Bartram and Mrs A. Peter for helpful discussions, and Mrs Zs. Toth for technical help in crystal growth.
REFERENCES I. Fiildvlri I., Peter A., Keszthelyi-Landori
Fig. 4. The decay for the main component of the yellow luminescence in a ZnWO, : MO single crystal measured at room temperature. Excitation was at 355 nm and the luminescence was measured at 582 nm. CONCLUSIONS Contrary to the previously reported finding (71, our experiments show that the yellow luminescence in ZnWO, is not intrinsic, but is associated with MO impurities. The absorption of the Mo6+ ions is close to the intrinsic absorption edge of the ZnWO, host material, and therefore excitation of the MO emission is effective only in a narrow wavelength range. As the excitation is scanned to shorter wavelength, the intrinsic luminescence becomes dominant before the 315nm maximum (at 77 K) of the MO absorption, even in heavily doped samples (lo-’ mol mol-‘). At room temperature the blue emission is already dominant at 335 nm. The concentration of MO in commercial ZnWO, crystals, or in raw materials for crystal growth, is typically less than IO-‘mol mol-I. If we extrapolate from the photoexcitation experiments, this MO content does not significantly influence the room temperature scintillation properties of the crystals. However, the long decay time of the MO luminescence might be detrimental in some applications with
S., Capelletti R., Craver0 I. and Schmidt F., J. Cry& Growth 79,714 (1986). 2. Leonov Yu. S., Opr. Spektrosk. 12, 255 (1962). 3. Oi T., Takagi K. and Fukazawa T., Appl. Phys. Len. 36, 278
(1980).
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