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Luminescence decay of lead tungstate and lead molybdate
Volume
47A, number
1
PHYSICS
LUMINESCENCE
DECAY
LETTERS
25 February
OF LEAD TUNGSTATE
1974
AND LEAD MOLYBDATE
W. Van LOO and D.J. WOLTERINK Solid State Chemistry Department,
Physical Laboratory, Received
The temperature attempt to identify
State University, Utrecht, The Netherlands
15 January
1974
dependence of the luminescence decay of PbWO4 and PbMo04 has been investigated as part of an the nature of the unknown emitting centres. This dependence is found to be anomalous.
Kroger [l] has reported for the first time a rather extensive investigation on the luminescence properties of both PbW04 and PbMo04. In both compounds Kroger observed a green emission band. When excited at higher photon energy PbW04 showed a blue emission. In the case of PbMo04 Maksakov et al. [2]. Have found a decay time of lob5 set at liquid-nitrogen temperature. We have started to investigate these compounds because of their anomalous optical behaviour with respect to the alkaline-earth tungstates andmolybdates [ 11. In the course of our study on the optical properties of PbW04 and labMoO [3] we have determined the decay times of these emissions in the temperature range from 1.5 up to the quenching temperature. The measurement of the decay of the green emissions was performed on Czochralski-grown single crystals; those of the blue emission on a powder. The green emission was excited by a nitrogen flash lamp and the blue PbWO4 emission by a deuterium flash lamp (T.R.W.). The time dependence of the total emission was measured with a photomultiplier (Philips 56 TUVP), whose output was displayed on an oscilloscope (Tektronix 564B), using a sampling plug in (3876) and a random sampling sweep plug in (3T2). Averaging and storing of the sigrial was performed by a waveformeductor (PAR-THD) 9). In this way the intensity could be measured on a range of almost two decades. The intensity of the emission does not decay pure exponentially, but can be described by the sum of two exponent&&. The two decay times were determined by means of a computer fitting procedure. Figs. 1 and 2 show the temperature dependence of the green emission decay. The uncertainty in the decay time is estimated to be + 10%. The analogy of the temperature dependent behaviour of the decay in both compounds is striking.
5c
,
4(I-.
a 313t L
21D-
1' o-
i 0
1 50
100
150
IO T(K)
Fig. 1. Decay times T of the green PbMo04 emission as function of the absolute temperature. Excitation: flash lamp (300-400 nm.).
plotted nitrogen
The decay of the blue emission of PbW04 could also be described by the sum of two components, but we were not able to analyse the data at each temperature 83
Volume 47A, number 1
PHYSICS LETTERS
25 February 1974
15
-7
I
\<
50
I loo
I 150
I
200 T(K)
50
100
200
150 T(K)
Fig. 2. Decay times r of the green PbW04 emission plotted as a function of the absolute temperature. Excitation: nitrogen flash lamp (300-400 nm.).
(rr and 72 very close). Therefore we approximated the decay curve to be pure exponential. Results are shown in fig. 3; the uncertainty in the decay times is f 20%. The temperature dependence of the blue PbWO4 decay is similar to that observed in CaW04 by Beard et al. [4]. In all cases the thermal quenching region of the decay times coincide with the thermal quenching region of the emission intensity. Below this region the emission intensity remains constant within a few percent, whereas the decay times show a strong variation with temperature. Especially striking is the minimum of the decay times in the low temperature region. At the moment we have no explanation for this exceptional behaviour. For temperatures below the thermal quenching region a temperature dependent decay time generally implies the competition of at least two different states. When a decay is built up of two components, as in our case, the excited state can be described by at 84
Fig. 3. Decay times r of the blue PbWO4 emission plotted as a function of the absolute temperature.Excitation: deuterium flas lamp (200-320 nm.).
least three different states, but probably more. This implies that the emitting state has a much more complicated nature than is generally assumed. Further investigations are in progress and more details will be published elsewhere [3]. The investigations were performed as a part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie” (F.O.M.) with financial support from the “Nederlandse Organisatie voor Zuiverwetenschappelijk Onderzoek” (Z.W.O.).
References [ 1] F.A. Krdger, Some aspects of the luminescence of solids, (Elsevier, Amsterdam, 1948) chapter III. [2] B.I. Maksakov, A.M. Morozov and N.G. Romanova, Opt. and Spectr. 14 (1963) 166. [ 31 W. van Loo, to be published. [4] C.B. Beard, W.H. Kelly and M.L. Mallory, J. Appl. Phys. 33 (1962) 144.
47A, number
1
PHYSICS
LUMINESCENCE
DECAY
LETTERS
25 February
OF LEAD TUNGSTATE
1974
AND LEAD MOLYBDATE
W. Van LOO and D.J. WOLTERINK Solid State Chemistry Department,
Physical Laboratory, Received
The temperature attempt to identify
State University, Utrecht, The Netherlands
15 January
1974
dependence of the luminescence decay of PbWO4 and PbMo04 has been investigated as part of an the nature of the unknown emitting centres. This dependence is found to be anomalous.
Kroger [l] has reported for the first time a rather extensive investigation on the luminescence properties of both PbW04 and PbMo04. In both compounds Kroger observed a green emission band. When excited at higher photon energy PbW04 showed a blue emission. In the case of PbMo04 Maksakov et al. [2]. Have found a decay time of lob5 set at liquid-nitrogen temperature. We have started to investigate these compounds because of their anomalous optical behaviour with respect to the alkaline-earth tungstates andmolybdates [ 11. In the course of our study on the optical properties of PbW04 and labMoO [3] we have determined the decay times of these emissions in the temperature range from 1.5 up to the quenching temperature. The measurement of the decay of the green emissions was performed on Czochralski-grown single crystals; those of the blue emission on a powder. The green emission was excited by a nitrogen flash lamp and the blue PbWO4 emission by a deuterium flash lamp (T.R.W.). The time dependence of the total emission was measured with a photomultiplier (Philips 56 TUVP), whose output was displayed on an oscilloscope (Tektronix 564B), using a sampling plug in (3876) and a random sampling sweep plug in (3T2). Averaging and storing of the sigrial was performed by a waveformeductor (PAR-THD) 9). In this way the intensity could be measured on a range of almost two decades. The intensity of the emission does not decay pure exponentially, but can be described by the sum of two exponent&&. The two decay times were determined by means of a computer fitting procedure. Figs. 1 and 2 show the temperature dependence of the green emission decay. The uncertainty in the decay time is estimated to be + 10%. The analogy of the temperature dependent behaviour of the decay in both compounds is striking.
5c
,
4(I-.
a 313t L
21D-
1' o-
i 0
1 50
100
150
IO T(K)
Fig. 1. Decay times T of the green PbMo04 emission as function of the absolute temperature. Excitation: flash lamp (300-400 nm.).
plotted nitrogen
The decay of the blue emission of PbW04 could also be described by the sum of two components, but we were not able to analyse the data at each temperature 83
Volume 47A, number 1
PHYSICS LETTERS
25 February 1974
15
-7
I
\<
50
I loo
I 150
I
200 T(K)
50
100
200
150 T(K)
Fig. 2. Decay times r of the green PbW04 emission plotted as a function of the absolute temperature. Excitation: nitrogen flash lamp (300-400 nm.).
(rr and 72 very close). Therefore we approximated the decay curve to be pure exponential. Results are shown in fig. 3; the uncertainty in the decay times is f 20%. The temperature dependence of the blue PbWO4 decay is similar to that observed in CaW04 by Beard et al. [4]. In all cases the thermal quenching region of the decay times coincide with the thermal quenching region of the emission intensity. Below this region the emission intensity remains constant within a few percent, whereas the decay times show a strong variation with temperature. Especially striking is the minimum of the decay times in the low temperature region. At the moment we have no explanation for this exceptional behaviour. For temperatures below the thermal quenching region a temperature dependent decay time generally implies the competition of at least two different states. When a decay is built up of two components, as in our case, the excited state can be described by at 84
Fig. 3. Decay times r of the blue PbWO4 emission plotted as a function of the absolute temperature.Excitation: deuterium flas lamp (200-320 nm.).
least three different states, but probably more. This implies that the emitting state has a much more complicated nature than is generally assumed. Further investigations are in progress and more details will be published elsewhere [3]. The investigations were performed as a part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie” (F.O.M.) with financial support from the “Nederlandse Organisatie voor Zuiverwetenschappelijk Onderzoek” (Z.W.O.).
References [ 1] F.A. Krdger, Some aspects of the luminescence of solids, (Elsevier, Amsterdam, 1948) chapter III. [2] B.I. Maksakov, A.M. Morozov and N.G. Romanova, Opt. and Spectr. 14 (1963) 166. [ 31 W. van Loo, to be published. [4] C.B. Beard, W.H. Kelly and M.L. Mallory, J. Appl. Phys. 33 (1962) 144.