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Luminescence in terbium activated fluorozirconate glasses
Journal of Luminescence 23 (1981) 261—268 North-Holland Publishing Company
261
LUMINESCENCE IN TERBIUM ACTIVATED FLUOROZIRCONATE GLASSES P.B. PERRY, M.W. SHAFER and I.F. CHANG IBM Thomas J. Watson Research Center, Yorktown heights, NY 10598, USA Received 22 February 198 I
High efficiency luminescence has been observed in fluorozirconate glasses doped with rare-earth ions. In particular terbium-activated fluorozirconate glasses ~ZrF 4.—BaF2 —ThF4 ThF~at a temperature of 2 K show radiation 3efficiencies comparable to examined. that of ZnSThe at + from 0.1% to 15% was temperature andofconcentration 300 K. A range concentrationdependences doping with of Ththe luminescence from these samples are reported. Lifetime data as a function of rare-earth concentration at room temperature are also presented.
1. Infroduction Studies of the physical properties of various types of glasses have been quite extensive and span a long period of time [1—4].Great strides have been made in the understanding of the physical properties of glasses, as well as the response of such glasses to various probes. Nonetheless there still exists a gap in the theoretical understanding of the intrinsic properties of these systems. This is in part due to the complexity of the structure of glasses. Experimentally, however, one can probe the glass systems by chemically inserting impurities and monitoring those physical properties which are known to have a crystal field dependence. The luminescence radiation of a rare-earth doped glass in one such quantity. In this work we measure the luminescence of terbium doped fluorozirconate glasses under visible optical, 2537 A Hg line and electron excitation. We have selected a series of glasses which are based on ZrF 4 for this study. These glasses are unique in that they contain no oxygen and presumably have no tetrahedrally coordinated cations. Furthermore, for this system there has been little or no systematic examination of the optically excited luminescence, and the decay time as a function of terbium concentration and temperature has not been measured. Prior studies have been made on silicate [5J, phosphate [6], and borate [7] phospor glasses, in an effort to better understand the physical properties of 3~to study because of the large luminesthese materials. We observed have selected cence enhancement in theTb visible. 0022-2313/81 /0000—0000/$02.50 © 1981 North-Holland
262
P. B. Perry et a!.
/
Luminescence in Tb activated fluorozirconate glasses
2. Experimental The glasses were prepared by a modification of the techniques previously described [8—10].A glass frit of 0.62ZrF4 + 0.3lBaF2 + 0.O7ThF4 was mixed with the appropriate amount of ThF3 and heated with an excess of NH3HF from 500°Cto 900°Cover a period of two hours. After holding the temperature at 900°Cfor 20 mm the glass melt was poured into a mold heated to about 290°C; it was then cooled to room 3 ~ slowly as quoted in this paper,temperature. is defined inThus termsthe of concentration x of the Tb (ThF 3)5 and (0.62ZrF4 + 0.3lBaF2 + 0.07ThF4)1~~. In this work values of x 0, 0.5, 1.0, 2.0, 2.5, 5.0, 10, and 15% were used in the preparation of the samples. Optical flats, approximately 5 mm X 4 mm X 2 mm suitable for both luminescence and optical absorption measurements, were made by the mechanical polishing of the crystals. The samples were mounted in a Janus dewar and the temperature maintained to within 0.1 K with the aid of an SI Model 3700 temperature controller. Data were taken at temperatures from below the lambda point to 300 K for all concentrations. The excitation lines selected were 4880 A provided by an argon ion laser and 2537 A provided by a low pressure mercury lamp, equipped with an appropriate filtering assembly. The luminescence radiation from the samples was filtered using a Corning low pass filter to remove the radiation contribution due to Rayleigh scattering and stray reflections of light originating from the excitation source. The light was then chopped and detected using phase-locking techniques with a PAR Synchro-Het Model 186 lock-in amplifier. The light was dispersed and detected with a Spex ~ m spectrometer and a thermoelectrically cooled PMT respectively. The absorption spectra were measured using a double beam Cary I7DX spectrometer from the near infrared (3.0~tm)to the UV (2000 A), and the data were transmitted via an interface to a computer for on-line analysis. The slit width selected for the luminescence measurements was 2 A, and, for the absorption measurements, the slit width was dynamically controlled from a minimum of 2 A to a maximum of 5 A. The cathodoluminescence of these glass samples were measured with a Pritchard photometer while the samples were excited by 10 kV electrons in a demountable electron beam system. These lifetime measurements were all taken at room temperature for the concentrations specified above.
3. Results and discussion A typical emission spectrum of a 5% terbium doped sample excited by 2537 A radiation at 2 K is shown in fig. 1. The luminescence was taken at this temperature, which is just below the lambda point, to avoid intensity fluctua-
P.B. Perry et al.
50 40
/
Luminescence in Tb activated fluorozirconate glasses
I
263
I
T = 2K concentration = 5% Tb A 0 =2537A slt=4A X6
X6 eg
30
U
f
hi
20
a bcd
i
J\~~ ~
10 0
2000 3000 4000 5000 6000 7000 WAVELENGTH (Angstroms) 3 + sample illuminated with 2537 A mercury Fig. I. Steady state luminescence spectrum of a 5% Th light at 2 K.
tions due to helium bubbling in the dewar. For clarity the spectrum has been amplified by a factor of 6, and offset by 5 arbitrary units along the abscissa. The original spectrum is shown as a dotted curve. The emission peaks in this spectrum have been labelled by the letters a—f. Fig. 2 shows the intensity of the 5D 7F 4— 5 transition (i.e., the dominant peak g at 5396 A) for various terbium concentrations. It is this strong green emission that makes this material of interest. The photoluminescence efficiency of this
ioo
i
I
T=2K
I
I
A=5396A
:°4°,/”~”~
0
2
4
6
8
10 12
14 16 3’ PERCENT CONCENTRATION OF 5D Th Fig. 2. The concentration dependence of the 7F 4 —~ 5transition at 2 K.
P.B. Perry et a!.
264
/
Luminescence in Tb activated fluorozirconate glasses
line at 2 K in the 10% sample is found to be within a factor of 2 of the efficiency at room temperature of our typical 0.5% Mn in ZnS standard used in electroluminescence devices. This makes it one of the most efficient of the phosphor glasses. One notes that as terbium concentration is increased so does the intensity of the transition up to approximately 10% ThF3, after which it begins to decrease. The reduction in the green emission when .x > — 10% is due to the onset of a resonance concentration quenching. It is well known that if the concentration of activator ions exceeds the ‘critical concentration’, in this case 10%, the probability for energy transfer becomes greater than that for emission. We therefore get a reduction in the radiated emission, as the excitation energy repeatedly goes from one activator ion to another. Thehigher peak 7F~to the at 5396 level 5DA is part of a multiplet transition from the initial state 4 wherej = 0, 1,... ,6. The highest peak in the multiplet, i.e. at 5396 A, corresponds to the] = 5 final state. The assignment of these levels are based on observations of similar transitions in other glass and crystalline systems and a knowledge of the optical transitions of elemental terbium. The dependence of the intensity of this 5396 A peak is summarized in fig. 3. The peaks labelled in fig. 1 are tabulated in table 1 along with the peak location, the linewidth of the transition, and the normalized peak intensity of 5D each transition. We see that the linewidth of the 4 —*~D5 transition is —~50 A in these glasses compared with —~285 A and 170 A for the same transition in the metaphosphate [6] and silicate [5] glasses respectively. There is also a shift towards lower energy of this transition when compared with the other systems. The location of this transition in this system is at 5396 A, whereas it is located at 5430 A in the other glasses. ‘—j
.~
.d
100
I
I
I
T=2K
80
3~
.4
B—~-1% A—~5%Tb C—2%
D—-5% E—10%
U’J
E
5350
5400
F—15%
5450
5500
5550
Wavelength (Angstroms) Fig. 3. The maximum peak intensity of Th3+ doped fluorozirconate glasses as a function of concentration (x0.5, 1,2,5, 10, 15%).
P. B. Perry et a!. / Luminescence in Tb activated fluorozirconate glasses
265
Table I Luminescence peak characteristics
A0(A)
Peak
~A(A)
‘~/‘~
±2A b c d e gf
3790 4123 4350 4882 5143 5396
78 58.1 79 130 135 50
0.0100 0.0125 0.0075 0.1700 0.0555 1.0000
h
5800 6210 6530 6650 6825
120 72 83 77.1 63.9
0.0800 0.0640 0.0082 0.0102 0.0050
k 1
Transition 5D 7F 5D3 —~7F6 5D3 —~ 7F5 5D3 —~7F 4 5D4 —~7F6 ~1)4 3 —~ —~7F 5D 7F5 5D4 —~7F 4 5D4 —~ 7F3 5D4 —~7F 2 1 5D4 —s 7F 4 —~ 0
Both the narrow linewidth and the position of this peak can be attributed to the uniqueness of the fluorozirconate system. The relatively narrow linewidth is undoubtedly due both to the increased of our the data fluorozirconates to 3 + sites. ionicity Although do not allowand us to the uniquea coordination uniformity of number the Th for the Th3~ion, from a size argument alone, postulate it would be expected to prefer a coordination from 6—9, although 12 cannot be completely ruled out. Nevertheless, it appears that, whatever the coordination number is, a large fraction of the Th3~ions are likely to be located in the not-too-distorted polyhedra it forms. 50
I
I
40. in
defect center
3o.
/
_____________
2.0 1.8 1.6 ______
1.6
%.~ o~
2.3
3.0
i~o
WAVELENGTh (microns) Fig. 4. The absorption spectrum of a 2.5% Th3~sample taken at 300 K. The insert shows the intra-configurational transitions 7Fk ~ The insert shows the high energy end of the spectrum and the terbium independent peak.
P. B. Perry et a!.
266
/
Luminescence in Tb activated fluorozirconate glasses
The lowest wavelength peak labelled a has not been included in table I since it does not have its origin in the presence of terbium. Luminescence spectra taken on the undoped host material shows this peak as its most prominent feature. Although the origin of this transition is not clear, we believe it is associated with a center formed by a complex defect. The optical absorption spectrum at 300 K of a 2.5% Tb3~sample is shown in fig. 4. The inset is the contribution of the host material alluded to above. We see a well-defined peak in the region just below the absorption edge of the glass. This peak persists in the absence of terbium. More interesting is the structure in the region between 1.6 ~tm and 3.2 ~tm, also shown in fig. 4. This transition is the multiplet 7Fk —*7F~,where k, n=0,1,...,6. This is the first time, we believe, that this multip let for the Tb3 + ion has been resolved when the host is a glass. The separation of the levels at 300 K, ~k~n’ was found to correlate with the luminescence emission peak locations of the 5D 4 F~. transitions, which confirms that the absorption 7F~subsystem. data are the result of . transitions taking place within intra-configuration The the results of cathodoluminescence decay time measurements of the 5D 4 ~F5transition are shown as a function of concentration in fig. 5. Comparison 3 doped borate and silicate glasses are also shown. It is seen, as with 1% Tb reported by others [5], that the decay rate is concentration dependent and is greater for the fluorozirconate glasses than for the borate and silicate. If we use —*~
—*
+
102
I
I .~
b~ate(1% Tbh
+1.0%Tb x 2.5%Tb~ ~ 7.5%Tb:’ o 15%Tb
10
0
0
‘o
N
PLOF10%T~’
A
‘No ‘N
4
A
5
6
Decay Time (millisec). Fig. 5. Cathodoluminescence decay curves of various concentration of Th3~doped fluorozirconate glasses. The photoluminescence decay characteristics of silicate (1%). borate (1%) and fluorozirconate (10%) glasses are included.
P.B. Perry et a!.
/
Luminescence in Tb activated fluorozirconate glasses
267
Table 2 Cathodoluminescence efficiency of Th doped glasses as a function of Th concentration 3+ conc. CL efficiency Th (%) 1.0 0.27 2.5 0.31 7.5 0.42 15.0 1.38
the argument, as others have [5],that the lifetime of the 5D in 3~is proportional to how tightly it is bound to the 4host,~F5transition we find some Th inconsistencies. This binding energy can be correlated to the ionic field strength, F = z/r2 (z = charge, r = ionic radius) of the network forming ions of the host. When we do this we find that F for the borate> silicate> fluorozirconate glasses. Thus, the borate glasses should have shorter decay times than the fluorozirconate— a fact that is not seen experimentally. On the other hand, as we mentioned previously, the line width of the 5D 4 transition is appreciably narrower in the fluorozirconate glasses than for the borates or silicates. A possible reason for this, in addition to some narrowing 3~ due to the increased ionicity, is for the the uniformity occupied byglass Th is In other words the environment Tb3 ionofinthe thesites fluorozirconate more crystal-like than in either the borate or silicate. Should this be the case, then it would be expected to have the more rapid decay characteristic of crystals. In fact, the~decaytimes observed here, 1.5—3 msec, agree with that seen for Th3 in a number of crystalline oxide hosts [11]. For some phosphor applications, a long decay time in the emission radiation at 5396 A is desirable. The emission decay rate in these glasses is found to fall off monotonically with increasing Th concentration, and on the other hand the cathodoluminesceñce intensity increases with increasing Th content. The photoluminescence decay curve of a 10% Th3+ fluorozirconate glass is also included in fig. 5 for comparison. All the curves fall into the envelope defined by the dashed lines, although the 10% sample has a photoluminescence decay rate which is closer to that of the cathodoluminescence of the 1% sample. The cathodoluminescence efficiency of these glasses, measured against a willemite crystal [12], yields efficiencies on the order o 1% as shown in table 2. Hence a tradeoff between the efficiency and persistence may be made in the selection of the Th content. —~
—~
+
+
4. Summary We have investigated Th3 doped fluorozirconate glasses by examining the luminescence spectra as a function of temperature and rare-earth ion con+
PB. Perry et a!. / Luminescence in Tb activated fluorozirconate glasses
268
centration. We have also studied the radiation decay time of the 5396 A transition as a function of concentration using the excitation of a 10 kV e-beam. The optical transitions in these glasses show little or no shift in frequency as we vary the concentration of the terbium ions. We have been able to observe and fully resolve the intra-configuration transitions 7Fk where k, n =0,l,...,6. These levels have hitherfore not been resolved in such systems, and they provide complimentary information as to the location in energy space of final states of the 5D 7F~transitions. The narrow line-widths 4 observed in all the transitions considered are evidence for the presence for Tb3 sites of uniform symmetry. Finally, this system exhibits an increasing cathodoluminescence efficiency in the visible with increasing Th concentration up to a large concentration range (>10%). Therefore this system may be useful for device applications. —~
—.*
+
Acknowledgments The authors wish to thank R.E. Fern, J.S. Wilson for their technical assistance, R. Figat for preparing the samples, and Y. Thefaine for some photoluminescence decay measurements.
References [I] [2] [3] [4] [5]
W.H. Zachariasen, J. Amer. Chem. Soc. 54 (1932) 3841. CR. Kurkjian and E.A. Sigety, Phys. Cheni. Glasses 9 (1968) 73. R.L. Mozzi and BE. Warren, J. Appl. Crystallogr. 3 (1970) 251. A.J. Bennett and L.M. Roth, Phys. Rev. B 4 (1971) 2686. R. Reisfeld, A. Honigbaum, G. Michaeli, L. Hard and M. Ish-Shalom, Israel J. Chem. 7 (1969) 613. [6] T. Takahashi and 0. Yamada, J. Electroochem. Soc. 126, no. 12 (1979) 2206. [7] I.J. Brandstadter, R. Reisfeld and S. Larach, Sol. St. Comm. 11(1972)1235. [8] M. Poulain, M. Poulain, J. Lucas and P. Brun, Mat. Res. Bull. 10 (1979) 243. [9] M. Poulain, M. Chanthanasinh and J. Lucas, Mat. Res. Bull. 12 (1977) 151. [10] MW. Shafer and P. Perry, Mat. Res. Bull. 14 (1979) 899. [II] AD. Pearson, G.E. Peterson and W.R. Northover, J. AppI. Phys. 37 (1966) 729. [12] s. Larach and A.E. Hardy, Proc. S.I.D. vol. 16 (1975) 21.
261
LUMINESCENCE IN TERBIUM ACTIVATED FLUOROZIRCONATE GLASSES P.B. PERRY, M.W. SHAFER and I.F. CHANG IBM Thomas J. Watson Research Center, Yorktown heights, NY 10598, USA Received 22 February 198 I
High efficiency luminescence has been observed in fluorozirconate glasses doped with rare-earth ions. In particular terbium-activated fluorozirconate glasses ~ZrF 4.—BaF2 —ThF4 ThF~at a temperature of 2 K show radiation 3efficiencies comparable to examined. that of ZnSThe at + from 0.1% to 15% was temperature andofconcentration 300 K. A range concentrationdependences doping with of Ththe luminescence from these samples are reported. Lifetime data as a function of rare-earth concentration at room temperature are also presented.
1. Infroduction Studies of the physical properties of various types of glasses have been quite extensive and span a long period of time [1—4].Great strides have been made in the understanding of the physical properties of glasses, as well as the response of such glasses to various probes. Nonetheless there still exists a gap in the theoretical understanding of the intrinsic properties of these systems. This is in part due to the complexity of the structure of glasses. Experimentally, however, one can probe the glass systems by chemically inserting impurities and monitoring those physical properties which are known to have a crystal field dependence. The luminescence radiation of a rare-earth doped glass in one such quantity. In this work we measure the luminescence of terbium doped fluorozirconate glasses under visible optical, 2537 A Hg line and electron excitation. We have selected a series of glasses which are based on ZrF 4 for this study. These glasses are unique in that they contain no oxygen and presumably have no tetrahedrally coordinated cations. Furthermore, for this system there has been little or no systematic examination of the optically excited luminescence, and the decay time as a function of terbium concentration and temperature has not been measured. Prior studies have been made on silicate [5J, phosphate [6], and borate [7] phospor glasses, in an effort to better understand the physical properties of 3~to study because of the large luminesthese materials. We observed have selected cence enhancement in theTb visible. 0022-2313/81 /0000—0000/$02.50 © 1981 North-Holland
262
P. B. Perry et a!.
/
Luminescence in Tb activated fluorozirconate glasses
2. Experimental The glasses were prepared by a modification of the techniques previously described [8—10].A glass frit of 0.62ZrF4 + 0.3lBaF2 + 0.O7ThF4 was mixed with the appropriate amount of ThF3 and heated with an excess of NH3HF from 500°Cto 900°Cover a period of two hours. After holding the temperature at 900°Cfor 20 mm the glass melt was poured into a mold heated to about 290°C; it was then cooled to room 3 ~ slowly as quoted in this paper,temperature. is defined inThus termsthe of concentration x of the Tb (ThF 3)5 and (0.62ZrF4 + 0.3lBaF2 + 0.07ThF4)1~~. In this work values of x 0, 0.5, 1.0, 2.0, 2.5, 5.0, 10, and 15% were used in the preparation of the samples. Optical flats, approximately 5 mm X 4 mm X 2 mm suitable for both luminescence and optical absorption measurements, were made by the mechanical polishing of the crystals. The samples were mounted in a Janus dewar and the temperature maintained to within 0.1 K with the aid of an SI Model 3700 temperature controller. Data were taken at temperatures from below the lambda point to 300 K for all concentrations. The excitation lines selected were 4880 A provided by an argon ion laser and 2537 A provided by a low pressure mercury lamp, equipped with an appropriate filtering assembly. The luminescence radiation from the samples was filtered using a Corning low pass filter to remove the radiation contribution due to Rayleigh scattering and stray reflections of light originating from the excitation source. The light was then chopped and detected using phase-locking techniques with a PAR Synchro-Het Model 186 lock-in amplifier. The light was dispersed and detected with a Spex ~ m spectrometer and a thermoelectrically cooled PMT respectively. The absorption spectra were measured using a double beam Cary I7DX spectrometer from the near infrared (3.0~tm)to the UV (2000 A), and the data were transmitted via an interface to a computer for on-line analysis. The slit width selected for the luminescence measurements was 2 A, and, for the absorption measurements, the slit width was dynamically controlled from a minimum of 2 A to a maximum of 5 A. The cathodoluminescence of these glass samples were measured with a Pritchard photometer while the samples were excited by 10 kV electrons in a demountable electron beam system. These lifetime measurements were all taken at room temperature for the concentrations specified above.
3. Results and discussion A typical emission spectrum of a 5% terbium doped sample excited by 2537 A radiation at 2 K is shown in fig. 1. The luminescence was taken at this temperature, which is just below the lambda point, to avoid intensity fluctua-
P.B. Perry et al.
50 40
/
Luminescence in Tb activated fluorozirconate glasses
I
263
I
T = 2K concentration = 5% Tb A 0 =2537A slt=4A X6
X6 eg
30
U
f
hi
20
a bcd
i
J\~~ ~
10 0
2000 3000 4000 5000 6000 7000 WAVELENGTH (Angstroms) 3 + sample illuminated with 2537 A mercury Fig. I. Steady state luminescence spectrum of a 5% Th light at 2 K.
tions due to helium bubbling in the dewar. For clarity the spectrum has been amplified by a factor of 6, and offset by 5 arbitrary units along the abscissa. The original spectrum is shown as a dotted curve. The emission peaks in this spectrum have been labelled by the letters a—f. Fig. 2 shows the intensity of the 5D 7F 4— 5 transition (i.e., the dominant peak g at 5396 A) for various terbium concentrations. It is this strong green emission that makes this material of interest. The photoluminescence efficiency of this
ioo
i
I
T=2K
I
I
A=5396A
:°4°,/”~”~
0
2
4
6
8
10 12
14 16 3’ PERCENT CONCENTRATION OF 5D Th Fig. 2. The concentration dependence of the 7F 4 —~ 5transition at 2 K.
P.B. Perry et a!.
264
/
Luminescence in Tb activated fluorozirconate glasses
line at 2 K in the 10% sample is found to be within a factor of 2 of the efficiency at room temperature of our typical 0.5% Mn in ZnS standard used in electroluminescence devices. This makes it one of the most efficient of the phosphor glasses. One notes that as terbium concentration is increased so does the intensity of the transition up to approximately 10% ThF3, after which it begins to decrease. The reduction in the green emission when .x > — 10% is due to the onset of a resonance concentration quenching. It is well known that if the concentration of activator ions exceeds the ‘critical concentration’, in this case 10%, the probability for energy transfer becomes greater than that for emission. We therefore get a reduction in the radiated emission, as the excitation energy repeatedly goes from one activator ion to another. Thehigher peak 7F~to the at 5396 level 5DA is part of a multiplet transition from the initial state 4 wherej = 0, 1,... ,6. The highest peak in the multiplet, i.e. at 5396 A, corresponds to the] = 5 final state. The assignment of these levels are based on observations of similar transitions in other glass and crystalline systems and a knowledge of the optical transitions of elemental terbium. The dependence of the intensity of this 5396 A peak is summarized in fig. 3. The peaks labelled in fig. 1 are tabulated in table 1 along with the peak location, the linewidth of the transition, and the normalized peak intensity of 5D each transition. We see that the linewidth of the 4 —*~D5 transition is —~50 A in these glasses compared with —~285 A and 170 A for the same transition in the metaphosphate [6] and silicate [5] glasses respectively. There is also a shift towards lower energy of this transition when compared with the other systems. The location of this transition in this system is at 5396 A, whereas it is located at 5430 A in the other glasses. ‘—j
.~
.d
100
I
I
I
T=2K
80
3~
.4
B—~-1% A—~5%Tb C—2%
D—-5% E—10%
U’J
E
5350
5400
F—15%
5450
5500
5550
Wavelength (Angstroms) Fig. 3. The maximum peak intensity of Th3+ doped fluorozirconate glasses as a function of concentration (x0.5, 1,2,5, 10, 15%).
P. B. Perry et a!. / Luminescence in Tb activated fluorozirconate glasses
265
Table I Luminescence peak characteristics
A0(A)
Peak
~A(A)
‘~/‘~
±2A b c d e gf
3790 4123 4350 4882 5143 5396
78 58.1 79 130 135 50
0.0100 0.0125 0.0075 0.1700 0.0555 1.0000
h
5800 6210 6530 6650 6825
120 72 83 77.1 63.9
0.0800 0.0640 0.0082 0.0102 0.0050
k 1
Transition 5D 7F 5D3 —~7F6 5D3 —~ 7F5 5D3 —~7F 4 5D4 —~7F6 ~1)4 3 —~ —~7F 5D 7F5 5D4 —~7F 4 5D4 —~ 7F3 5D4 —~7F 2 1 5D4 —s 7F 4 —~ 0
Both the narrow linewidth and the position of this peak can be attributed to the uniqueness of the fluorozirconate system. The relatively narrow linewidth is undoubtedly due both to the increased of our the data fluorozirconates to 3 + sites. ionicity Although do not allowand us to the uniquea coordination uniformity of number the Th for the Th3~ion, from a size argument alone, postulate it would be expected to prefer a coordination from 6—9, although 12 cannot be completely ruled out. Nevertheless, it appears that, whatever the coordination number is, a large fraction of the Th3~ions are likely to be located in the not-too-distorted polyhedra it forms. 50
I
I
40. in
defect center
3o.
/
_____________
2.0 1.8 1.6 ______
1.6
%.~ o~
2.3
3.0
i~o
WAVELENGTh (microns) Fig. 4. The absorption spectrum of a 2.5% Th3~sample taken at 300 K. The insert shows the intra-configurational transitions 7Fk ~ The insert shows the high energy end of the spectrum and the terbium independent peak.
P. B. Perry et a!.
266
/
Luminescence in Tb activated fluorozirconate glasses
The lowest wavelength peak labelled a has not been included in table I since it does not have its origin in the presence of terbium. Luminescence spectra taken on the undoped host material shows this peak as its most prominent feature. Although the origin of this transition is not clear, we believe it is associated with a center formed by a complex defect. The optical absorption spectrum at 300 K of a 2.5% Tb3~sample is shown in fig. 4. The inset is the contribution of the host material alluded to above. We see a well-defined peak in the region just below the absorption edge of the glass. This peak persists in the absence of terbium. More interesting is the structure in the region between 1.6 ~tm and 3.2 ~tm, also shown in fig. 4. This transition is the multiplet 7Fk —*7F~,where k, n=0,1,...,6. This is the first time, we believe, that this multip let for the Tb3 + ion has been resolved when the host is a glass. The separation of the levels at 300 K, ~k~n’ was found to correlate with the luminescence emission peak locations of the 5D 4 F~. transitions, which confirms that the absorption 7F~subsystem. data are the result of . transitions taking place within intra-configuration The the results of cathodoluminescence decay time measurements of the 5D 4 ~F5transition are shown as a function of concentration in fig. 5. Comparison 3 doped borate and silicate glasses are also shown. It is seen, as with 1% Tb reported by others [5], that the decay rate is concentration dependent and is greater for the fluorozirconate glasses than for the borate and silicate. If we use —*~
—*
+
102
I
I .~
b~ate(1% Tbh
+1.0%Tb x 2.5%Tb~ ~ 7.5%Tb:’ o 15%Tb
10
0
0
‘o
N
PLOF10%T~’
A
‘No ‘N
4
A
5
6
Decay Time (millisec). Fig. 5. Cathodoluminescence decay curves of various concentration of Th3~doped fluorozirconate glasses. The photoluminescence decay characteristics of silicate (1%). borate (1%) and fluorozirconate (10%) glasses are included.
P.B. Perry et a!.
/
Luminescence in Tb activated fluorozirconate glasses
267
Table 2 Cathodoluminescence efficiency of Th doped glasses as a function of Th concentration 3+ conc. CL efficiency Th (%) 1.0 0.27 2.5 0.31 7.5 0.42 15.0 1.38
the argument, as others have [5],that the lifetime of the 5D in 3~is proportional to how tightly it is bound to the 4host,~F5transition we find some Th inconsistencies. This binding energy can be correlated to the ionic field strength, F = z/r2 (z = charge, r = ionic radius) of the network forming ions of the host. When we do this we find that F for the borate> silicate> fluorozirconate glasses. Thus, the borate glasses should have shorter decay times than the fluorozirconate— a fact that is not seen experimentally. On the other hand, as we mentioned previously, the line width of the 5D 4 transition is appreciably narrower in the fluorozirconate glasses than for the borates or silicates. A possible reason for this, in addition to some narrowing 3~ due to the increased ionicity, is for the the uniformity occupied byglass Th is In other words the environment Tb3 ionofinthe thesites fluorozirconate more crystal-like than in either the borate or silicate. Should this be the case, then it would be expected to have the more rapid decay characteristic of crystals. In fact, the~decaytimes observed here, 1.5—3 msec, agree with that seen for Th3 in a number of crystalline oxide hosts [11]. For some phosphor applications, a long decay time in the emission radiation at 5396 A is desirable. The emission decay rate in these glasses is found to fall off monotonically with increasing Th concentration, and on the other hand the cathodoluminesceñce intensity increases with increasing Th content. The photoluminescence decay curve of a 10% Th3+ fluorozirconate glass is also included in fig. 5 for comparison. All the curves fall into the envelope defined by the dashed lines, although the 10% sample has a photoluminescence decay rate which is closer to that of the cathodoluminescence of the 1% sample. The cathodoluminescence efficiency of these glasses, measured against a willemite crystal [12], yields efficiencies on the order o 1% as shown in table 2. Hence a tradeoff between the efficiency and persistence may be made in the selection of the Th content. —~
—~
+
+
4. Summary We have investigated Th3 doped fluorozirconate glasses by examining the luminescence spectra as a function of temperature and rare-earth ion con+
PB. Perry et a!. / Luminescence in Tb activated fluorozirconate glasses
268
centration. We have also studied the radiation decay time of the 5396 A transition as a function of concentration using the excitation of a 10 kV e-beam. The optical transitions in these glasses show little or no shift in frequency as we vary the concentration of the terbium ions. We have been able to observe and fully resolve the intra-configuration transitions 7Fk where k, n =0,l,...,6. These levels have hitherfore not been resolved in such systems, and they provide complimentary information as to the location in energy space of final states of the 5D 7F~transitions. The narrow line-widths 4 observed in all the transitions considered are evidence for the presence for Tb3 sites of uniform symmetry. Finally, this system exhibits an increasing cathodoluminescence efficiency in the visible with increasing Th concentration up to a large concentration range (>10%). Therefore this system may be useful for device applications. —~
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+
Acknowledgments The authors wish to thank R.E. Fern, J.S. Wilson for their technical assistance, R. Figat for preparing the samples, and Y. Thefaine for some photoluminescence decay measurements.
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