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Residual stress effect on luminescence of CeBr3 and its application as a pressure-memory material
Optical Materials 33 (2011) 1800–1802
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Residual stress effect on luminescence of CeBr3 and its application as a pressure-memory material T. Kobayashi ⇑, N. Hirosaki National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 24 February 2011 Received in revised form 7 June 2011 Accepted 17 June 2011 Available online 16 July 2011 Keywords: Residual stress Compaction Photoluminescence CeBr3
a b s t r a c t Photoluminescence spectra of CeBr3 powder compacts with various compaction pressures have been measured under ambient conditions and they are found significantly redshifted compared with those of the original powder by nearly 20 nm (1440 cm 1) at the compaction pressure of 1.0 GPa. The spectral shift increases extremely rapidly with the compaction pressure and then plateaus at 1.0 GPa. The observed residual stress effect of the CeBr3 powder on the luminescence spectra is much larger than that of the EuCl2 powder reported previously. The luminescence peak wavelength of the compact can be easily tuned between 360 and 380 nm by simply changing the compaction pressure applied to the CeBr3 powder. With a very large residual stress effect of the CeBr3 powder on the photoluminescence spectra, this material would make a good pressure-memory material which may be used as a kind of pressure sensor in high-pressure experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The 4f–5d electronic transitions of rare-earth (RE) ions are electric-dipole and spin allowed transitions and observed as very strong transitions in the electronic spectra, whereas the 4f–4f (intra-4f) transitions are forbidden transitions and observed as much less intense spectra [1]. Therefore, compounds of RE ions with very intense electronic spectra arising from the 4f–5d transitions are often used as high luminance phosphor materials [2]. Luminescence spectra of RE ions corresponding to the 4f–4f transitions exhibit many sharp lines with long luminescence lifetimes in the visible region. However, the visible or near-visible luminescence spectra arising from the 5d ? 4f transition of RE ions are observed only in limited RE ions and they appear as broad band spectra. Those ions are mainly Eu2+ as a divalent ion and Ce3+ as a trivalent ion. This is qualitatively understood by looking at the 4f–5d electronic transition energies of RE ions [3]. The 4f–5d electronic transition energies for the divalent and trivalent RE ions increase with the atomic number and the transition energy of Ce3+ is the smallest in the trivalent RE ions, which is already in the ultraviolet region. The luminescence peak wavelength of Ce3+ compounds varies from the ultraviolet to the visible region depending on the environment of the Ce3+ ion [2]. Although the actual 4f–5d transition energies of RE ions are a bit more complicated than described above, the luminescence peak wavelengths of other RE compounds are mostly in the ultraviolet or infrared regions [4]. ⇑ Corresponding author. Tel.: +81 29 860 4419; fax: +81 29 860 4693. E-mail address: [email protected] (T. Kobayashi). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.016
Ce3+ compounds and Eu2+ compounds are unique and similar to each other in the sense that they are among the few RE ions which show very intense luminescence spectra in the visible region and that their peak wavelengths vary over a wide range depending on the material [2]. The 5d electrons of RE ions are strongly influenced by their environments, whereas the 4f electrons are not because the 4f electrons are shielded by the surrounding 5s and 5p electrons. Therefore, the 4f–5d electronic transition energies of RE ions can be easily changed by external fields such as pressure. Ce3+ compounds are thus expected to reveal a large pressure effect as Eu2+ compounds [5–7]. In our previous paper on EuCl2 [8], we reported that EuCl2 powder shows a strong residual stress effect on its luminescence spectra and suggested its potential as a stress sensor material. It is expected that some other RE ionic compounds may exhibit properties similar to those of the EuCl2 powder. In this paper, we report a much larger residual stress effect of CeBr3 powder on its luminescence spectra and its possible application as a pressure-memory material. 2. Experimental procedure A CeBr3 powder sample with a purity of 99.9% was obtained from Sigma–Aldrich and made into a disk-shaped powder compact (10 mm in diameter and 100 lm in thickness) using a quench hardened steel mold and a hydraulic (oil) press. Before compaction the powder was ground lightly with a mortar grinder to make sure that the powder did not contain large grains. The luminescence spectrum of the sample did not change by this grinding procedure. The
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380
375
Peak position (nm)
compaction pressure was changed between 0.075 and 1.44 GPa. After the powder compact was released to the ambient pressure, it was confined in a cell with a quartz window for the photoluminescence measurement. Since CeBr3 powder is hygroscopic this compaction procedure was performed quickly in a dry atmosphere and the powder compact was kept from direct contact with air as much as possible during the experiment. All the photoluminescence spectra of the compacts were measured under ambient conditions using a Multi Channel Photo Detector (OTSUKA MCPD-7000) with 280 nm excitation. The peak wavelengths were determined by curve fitting. The compaction pressures were calculated using the readings of the hydraulic press and the sample diameters measured. Compaction pressures thus determined may contain relatively large uncertainties (up to about ±14%), part of the reason being the difficulty in compressing powder materials homogeneously by a hydraulic press.
370
365
360 0.0
3. Results and discussion
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Compaction pressure (GPa) Photoluminescence spectra of Ce3+ are shown in Fig. 1 for CeBr3 powder compacts with various compaction pressures between 0 and 1.44 GPa. The dotted spectrum with the highest intensity is for the original CeBr3 powder without compaction. It consists of two components, one is the main component at 360 nm and the other is observed as a shoulder at 385 nm. The electron configuration of Ce3+ ion is 4f15s25p6 and the ground electronic state is 2 F which splits into two spin–orbit states, namely 2F5/2 and 2F7/2. 2 F5/2 is lower in energy than 2F7/2 by 2100 cm 1 [3]. The observed two components in the photoluminescence spectrum are attributed to the transitions from the 5d orbital to these two spin–orbit states. It is seen in Fig. 1 that the peak wavelength of the spectrum increases with the compaction pressure and the luminescence intensity decreases. It was found that the CeBr3 powder undergoes a change in color from white to gray under compaction, which is considered to be the main reason for the observed decrease in luminescence intensity. The peak wavelengths are plotted against the compaction pressures in Fig. 2 which indicates that the peak wavelength increases very rapidly and almost linearly with the compaction pressure up to 0.4 GPa, then the peak shift rate slows down and plateaus above 1.0 GPa. It is understood that the maximum residual stress is reached at the compaction pressure of 1.0 GPa and no further
Emission intensity (arb. unit)
(a)
(b) (c) (d)
(e)
300
350
400
450
500
Wavelength (nm) Fig. 1. Photoluminescence spectra of CeBr3 powder compacts for different compaction pressures measured under ambient conditions; (a) uncompacted powder, (b) 0.075 GPa, (c) 0.15 GPa, (d) 0.34 GPa and (e) 1.44 GPa.
Fig. 2. Luminescence peak wavelength vs. compaction pressure plot for CeBr3 powder compacts. The slope is about 2700 cm 1/GPa in the pressure range of 0– 0.45 GPa. Compaction pressure values contain large uncertainties of about ±14%.
increase in the residual stress is gained at higher compaction pressures. The peak wavelength of the uncompacted powder is at 360.2 nm and that of the most shifted spectrum for the compaction pressure of 1.44 GPa is at 380.0 nm. A very large wavelength shift of 20 nm (1450 cm 1) was observed because of the residual stress. It is also possible to adjust the peak wavelength of the luminescence spectrum over this 20 nm range by simply changing the compaction pressure between 0 and 1.0 GPa. It should be noted here that the compaction pressure is different from the residual stress. The residual stress in the powder compacts should not exceed the compaction pressure and may be significantly less than the compaction pressure even in the lower compaction pressure region. It is desirable to be able to plot the peak wavelength or the spectral shift against the residual stress, but estimating the residual stress in the powder compacts is very difficult. It is also very important to note here that the compaction data in Fig. 2 may not represent the correct data for different CeBr3 powder samples. As was mentioned in Introduction, the luminescence spectra of Ce3+ corresponding to the 5d ? 4f transition change sensitively with the environment of the Ce3+ ion. The CeBr3 powder is very hygroscopic and, as the CeBr3 powder absorbs moisture, the luminescence intensity decreases and the peak wavelength changes. It is desirable that the sample powder is completely dry and free from any strains, but in reality the conditions of samples differ from one sample to another and thus the peak wavelength vs. compaction pressure data as in Fig. 2 can be slightly different depending on the sample. In Fig. 3, the peak shifts are plotted against the compaction pressures, and the EuCl2 data [8] are also shown in the figure for comparison. We reported in our previous paper that the EuCl2 powder exhibited a very large redshift of the luminescence spectrum caused by the residual stress. It is seen, in Fig. 3, that a much larger redshift is observed for the CeBr3 powder. Using the data in the lower compression pressure range (0–0.45 GPa) where the peak shift is almost linear with the compression pressure, the pressure shift of the CeBr3 powder is calculated to be 38 nm/GPa ( 2700 cm 1/GPa) which is about five times larger than that of the EuCl2 powder (8.8 nm/GPa or 520 cm 1/GPa). An extremely large pressure shift has been found for the CeBr3 powder in the lower pressure region of up to 0.45 GPa. It should be noted here, however, that the obtained pressure shift is not the real pressure shift for the CeBr3 powder because it was calculated using the
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T. Kobayashi, N. Hirosaki / Optical Materials 33 (2011) 1800–1802
20
CeBr3
Peak shift (nm)
15
10
EuCl2 5
0 0.0
0.5
1.0
1.5
Compaction pressure (GPa) Fig. 3. Luminescence peak shifts against compaction pressure for CeBr3 powder compacts and EuCl2 powder compacts. The EuCl2 data were taken from Ref. [8]. The original peak wavelengths for the uncompacted powder samples of CeBr3 and EuCl2 are 360.2 nm and 408.5 nm, respectively.
compaction pressure. A more accurate pressure shift can be determined if we know the residual stress, but it is difficult to measure the residual stress as described earlier and thus we have not made an attempt to measure the residual stress as yet. The residual stress should not exceed the compaction pressure and thus the actual pressure shift should be even larger than 38 nm/GPa. Our preliminary measurements on luminescence of CeCl3 and EuBr2 compacts show that they exhibit very small residual stress effect in contrast to CeBr3 and EuCl2. This is a puzzling result and may be associated with the nature of the residual stress observed in RE halide materials. As can be seen in Fig. 3, the CeBr3 powder is much more sensitive to the compaction pressure than the EuCl2 powder. The peak shift increases very rapidly with the compaction pressure up to 1.0 GPa and then plateaus, which suggests that the CeBr3 powder can be used as a good pressure-memory material for the pressure range of 0–1.0 GPa; namely, the highest pressure the CeBr3 powder has experienced can be derived from the spectral shift of the recovered CeBr3 powder. If one has spectral shift data for the CeBr3 powder such as those shown in Fig. 2, it is possible to estimate the pressure the CeBr3 powder has experienced because the compaction pressure can be considered to be the highest pressure the CeBr3 powder has experienced. Particularly for cases where in situ pressure measurements are difficult, by including a small amount of CeBr3 powder in a sample material that will be subjected to a high pressure, it is possible to determine the highest pressure applied to the sample by measuring the spectral shift of the CeBr3 powder in the recovered sample. There are several important technical points to note in order to use the CeBr3 powder as a pressure-memory material. The CeBr3 powder is very
hygroscopic and thus it must be kept under a dry condition during the entire process. The residual stress effect on the CeBr3 powder is very large and spectral profiles of the CeBr3 powder obtained from chemical makers may differ slightly from one batch to another. Therefore, for each sample of CeBr3 powder, it is necessary to obtain calibration data as shown in Fig. 2 before the CeBr3 powder can be used as a pressure-memory material. It is most desirable to determine precise calibration curves under the same conditions as the sample is actually placed and compressed. Another important point is thermal annealing effect on the luminescence spectra of the CeBr3 powder compacts. As in the case of the EuCl2 powder compacts [8], the peak wavelength moves back gradually to the shorter wavelength as thermal annealing proceeds. We adopted an annealing temperature of 120 °C and within a day the luminescence spectrum of the CeBr3 powder compact shifted back completely, indicating that thermal annealing effect on the CeBr3 powder compacts is much stronger than that on the EuCl2 powder compacts. The CeBr3 powder cannot be used as pressure-memory material at high temperatures. 4. Conclusions Photoluminescence spectra of the CeBr3 powder compacts measured at ambient conditions showed very large redshifts compared with those of the original powder due to residual stress. An extremely large shift of nearly 20 nm (1450 cm 1) at the compaction pressure of 1.0 GPa was observed, which is much larger than that of the EuCl2 powder reported previously. By utilizing the effect of an unusually large residual stress on the luminescence spectra, the CeBr3 powder appears to make a very good pressure-memory material for cases where in situ pressure measurements are difficult. It can record the maximum pressure it has experienced as the peak shift of the luminescence spectrum. The applicable pressure range is roughly 0–1.0 GPa, but for the lower pressure region (0–0.45 GPa), the CeBr3 powder is much more effectively used as a pressure-memory material than the EuCl2 powder because of its unusually large spectral shifts against compaction. Acknowledgment We are grateful to Ms. Kazuko Nakajima for her help in photoluminescence measurements. References [1] S. Hufner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978 (Chapter 9). [2] W.M. Yen, S. Shionoya, H. Yamamoto (Eds.), Phosphor Handbook, second ed., CRC Press, 2007. [3] G.H. Dieke, H.M. Crosswhite, Appl. Opt. 7 (1963) 675. [4] J. Sugar, J. Reader, J. Chem. Phys. 59 (1973) 2083. [5] C.E. Tyner, H.G. Drickamer, J. Chem. Phys. 67 (1977) 4116. [6] Th. Troster, S. Schweizer, M. Secu, J.-M. Spaeth, J. Lumin. 99 (2002) 343. [7] D.B. Gatch, D.M. Boye, Y.R. Shen, M. Grinberg, Y.M. Yen, R.S. Meltzer, Phys. Rev. B 74 (2006) 19517. [8] T. Kobayashi, T. Sekine, Opt. Mater. 32 (2010) 1227.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Residual stress effect on luminescence of CeBr3 and its application as a pressure-memory material T. Kobayashi ⇑, N. Hirosaki National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 24 February 2011 Received in revised form 7 June 2011 Accepted 17 June 2011 Available online 16 July 2011 Keywords: Residual stress Compaction Photoluminescence CeBr3
a b s t r a c t Photoluminescence spectra of CeBr3 powder compacts with various compaction pressures have been measured under ambient conditions and they are found significantly redshifted compared with those of the original powder by nearly 20 nm (1440 cm 1) at the compaction pressure of 1.0 GPa. The spectral shift increases extremely rapidly with the compaction pressure and then plateaus at 1.0 GPa. The observed residual stress effect of the CeBr3 powder on the luminescence spectra is much larger than that of the EuCl2 powder reported previously. The luminescence peak wavelength of the compact can be easily tuned between 360 and 380 nm by simply changing the compaction pressure applied to the CeBr3 powder. With a very large residual stress effect of the CeBr3 powder on the photoluminescence spectra, this material would make a good pressure-memory material which may be used as a kind of pressure sensor in high-pressure experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The 4f–5d electronic transitions of rare-earth (RE) ions are electric-dipole and spin allowed transitions and observed as very strong transitions in the electronic spectra, whereas the 4f–4f (intra-4f) transitions are forbidden transitions and observed as much less intense spectra [1]. Therefore, compounds of RE ions with very intense electronic spectra arising from the 4f–5d transitions are often used as high luminance phosphor materials [2]. Luminescence spectra of RE ions corresponding to the 4f–4f transitions exhibit many sharp lines with long luminescence lifetimes in the visible region. However, the visible or near-visible luminescence spectra arising from the 5d ? 4f transition of RE ions are observed only in limited RE ions and they appear as broad band spectra. Those ions are mainly Eu2+ as a divalent ion and Ce3+ as a trivalent ion. This is qualitatively understood by looking at the 4f–5d electronic transition energies of RE ions [3]. The 4f–5d electronic transition energies for the divalent and trivalent RE ions increase with the atomic number and the transition energy of Ce3+ is the smallest in the trivalent RE ions, which is already in the ultraviolet region. The luminescence peak wavelength of Ce3+ compounds varies from the ultraviolet to the visible region depending on the environment of the Ce3+ ion [2]. Although the actual 4f–5d transition energies of RE ions are a bit more complicated than described above, the luminescence peak wavelengths of other RE compounds are mostly in the ultraviolet or infrared regions [4]. ⇑ Corresponding author. Tel.: +81 29 860 4419; fax: +81 29 860 4693. E-mail address: [email protected] (T. Kobayashi). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.06.016
Ce3+ compounds and Eu2+ compounds are unique and similar to each other in the sense that they are among the few RE ions which show very intense luminescence spectra in the visible region and that their peak wavelengths vary over a wide range depending on the material [2]. The 5d electrons of RE ions are strongly influenced by their environments, whereas the 4f electrons are not because the 4f electrons are shielded by the surrounding 5s and 5p electrons. Therefore, the 4f–5d electronic transition energies of RE ions can be easily changed by external fields such as pressure. Ce3+ compounds are thus expected to reveal a large pressure effect as Eu2+ compounds [5–7]. In our previous paper on EuCl2 [8], we reported that EuCl2 powder shows a strong residual stress effect on its luminescence spectra and suggested its potential as a stress sensor material. It is expected that some other RE ionic compounds may exhibit properties similar to those of the EuCl2 powder. In this paper, we report a much larger residual stress effect of CeBr3 powder on its luminescence spectra and its possible application as a pressure-memory material. 2. Experimental procedure A CeBr3 powder sample with a purity of 99.9% was obtained from Sigma–Aldrich and made into a disk-shaped powder compact (10 mm in diameter and 100 lm in thickness) using a quench hardened steel mold and a hydraulic (oil) press. Before compaction the powder was ground lightly with a mortar grinder to make sure that the powder did not contain large grains. The luminescence spectrum of the sample did not change by this grinding procedure. The
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T. Kobayashi, N. Hirosaki / Optical Materials 33 (2011) 1800–1802
380
375
Peak position (nm)
compaction pressure was changed between 0.075 and 1.44 GPa. After the powder compact was released to the ambient pressure, it was confined in a cell with a quartz window for the photoluminescence measurement. Since CeBr3 powder is hygroscopic this compaction procedure was performed quickly in a dry atmosphere and the powder compact was kept from direct contact with air as much as possible during the experiment. All the photoluminescence spectra of the compacts were measured under ambient conditions using a Multi Channel Photo Detector (OTSUKA MCPD-7000) with 280 nm excitation. The peak wavelengths were determined by curve fitting. The compaction pressures were calculated using the readings of the hydraulic press and the sample diameters measured. Compaction pressures thus determined may contain relatively large uncertainties (up to about ±14%), part of the reason being the difficulty in compressing powder materials homogeneously by a hydraulic press.
370
365
360 0.0
3. Results and discussion
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Compaction pressure (GPa) Photoluminescence spectra of Ce3+ are shown in Fig. 1 for CeBr3 powder compacts with various compaction pressures between 0 and 1.44 GPa. The dotted spectrum with the highest intensity is for the original CeBr3 powder without compaction. It consists of two components, one is the main component at 360 nm and the other is observed as a shoulder at 385 nm. The electron configuration of Ce3+ ion is 4f15s25p6 and the ground electronic state is 2 F which splits into two spin–orbit states, namely 2F5/2 and 2F7/2. 2 F5/2 is lower in energy than 2F7/2 by 2100 cm 1 [3]. The observed two components in the photoluminescence spectrum are attributed to the transitions from the 5d orbital to these two spin–orbit states. It is seen in Fig. 1 that the peak wavelength of the spectrum increases with the compaction pressure and the luminescence intensity decreases. It was found that the CeBr3 powder undergoes a change in color from white to gray under compaction, which is considered to be the main reason for the observed decrease in luminescence intensity. The peak wavelengths are plotted against the compaction pressures in Fig. 2 which indicates that the peak wavelength increases very rapidly and almost linearly with the compaction pressure up to 0.4 GPa, then the peak shift rate slows down and plateaus above 1.0 GPa. It is understood that the maximum residual stress is reached at the compaction pressure of 1.0 GPa and no further
Emission intensity (arb. unit)
(a)
(b) (c) (d)
(e)
300
350
400
450
500
Wavelength (nm) Fig. 1. Photoluminescence spectra of CeBr3 powder compacts for different compaction pressures measured under ambient conditions; (a) uncompacted powder, (b) 0.075 GPa, (c) 0.15 GPa, (d) 0.34 GPa and (e) 1.44 GPa.
Fig. 2. Luminescence peak wavelength vs. compaction pressure plot for CeBr3 powder compacts. The slope is about 2700 cm 1/GPa in the pressure range of 0– 0.45 GPa. Compaction pressure values contain large uncertainties of about ±14%.
increase in the residual stress is gained at higher compaction pressures. The peak wavelength of the uncompacted powder is at 360.2 nm and that of the most shifted spectrum for the compaction pressure of 1.44 GPa is at 380.0 nm. A very large wavelength shift of 20 nm (1450 cm 1) was observed because of the residual stress. It is also possible to adjust the peak wavelength of the luminescence spectrum over this 20 nm range by simply changing the compaction pressure between 0 and 1.0 GPa. It should be noted here that the compaction pressure is different from the residual stress. The residual stress in the powder compacts should not exceed the compaction pressure and may be significantly less than the compaction pressure even in the lower compaction pressure region. It is desirable to be able to plot the peak wavelength or the spectral shift against the residual stress, but estimating the residual stress in the powder compacts is very difficult. It is also very important to note here that the compaction data in Fig. 2 may not represent the correct data for different CeBr3 powder samples. As was mentioned in Introduction, the luminescence spectra of Ce3+ corresponding to the 5d ? 4f transition change sensitively with the environment of the Ce3+ ion. The CeBr3 powder is very hygroscopic and, as the CeBr3 powder absorbs moisture, the luminescence intensity decreases and the peak wavelength changes. It is desirable that the sample powder is completely dry and free from any strains, but in reality the conditions of samples differ from one sample to another and thus the peak wavelength vs. compaction pressure data as in Fig. 2 can be slightly different depending on the sample. In Fig. 3, the peak shifts are plotted against the compaction pressures, and the EuCl2 data [8] are also shown in the figure for comparison. We reported in our previous paper that the EuCl2 powder exhibited a very large redshift of the luminescence spectrum caused by the residual stress. It is seen, in Fig. 3, that a much larger redshift is observed for the CeBr3 powder. Using the data in the lower compression pressure range (0–0.45 GPa) where the peak shift is almost linear with the compression pressure, the pressure shift of the CeBr3 powder is calculated to be 38 nm/GPa ( 2700 cm 1/GPa) which is about five times larger than that of the EuCl2 powder (8.8 nm/GPa or 520 cm 1/GPa). An extremely large pressure shift has been found for the CeBr3 powder in the lower pressure region of up to 0.45 GPa. It should be noted here, however, that the obtained pressure shift is not the real pressure shift for the CeBr3 powder because it was calculated using the
1802
T. Kobayashi, N. Hirosaki / Optical Materials 33 (2011) 1800–1802
20
CeBr3
Peak shift (nm)
15
10
EuCl2 5
0 0.0
0.5
1.0
1.5
Compaction pressure (GPa) Fig. 3. Luminescence peak shifts against compaction pressure for CeBr3 powder compacts and EuCl2 powder compacts. The EuCl2 data were taken from Ref. [8]. The original peak wavelengths for the uncompacted powder samples of CeBr3 and EuCl2 are 360.2 nm and 408.5 nm, respectively.
compaction pressure. A more accurate pressure shift can be determined if we know the residual stress, but it is difficult to measure the residual stress as described earlier and thus we have not made an attempt to measure the residual stress as yet. The residual stress should not exceed the compaction pressure and thus the actual pressure shift should be even larger than 38 nm/GPa. Our preliminary measurements on luminescence of CeCl3 and EuBr2 compacts show that they exhibit very small residual stress effect in contrast to CeBr3 and EuCl2. This is a puzzling result and may be associated with the nature of the residual stress observed in RE halide materials. As can be seen in Fig. 3, the CeBr3 powder is much more sensitive to the compaction pressure than the EuCl2 powder. The peak shift increases very rapidly with the compaction pressure up to 1.0 GPa and then plateaus, which suggests that the CeBr3 powder can be used as a good pressure-memory material for the pressure range of 0–1.0 GPa; namely, the highest pressure the CeBr3 powder has experienced can be derived from the spectral shift of the recovered CeBr3 powder. If one has spectral shift data for the CeBr3 powder such as those shown in Fig. 2, it is possible to estimate the pressure the CeBr3 powder has experienced because the compaction pressure can be considered to be the highest pressure the CeBr3 powder has experienced. Particularly for cases where in situ pressure measurements are difficult, by including a small amount of CeBr3 powder in a sample material that will be subjected to a high pressure, it is possible to determine the highest pressure applied to the sample by measuring the spectral shift of the CeBr3 powder in the recovered sample. There are several important technical points to note in order to use the CeBr3 powder as a pressure-memory material. The CeBr3 powder is very
hygroscopic and thus it must be kept under a dry condition during the entire process. The residual stress effect on the CeBr3 powder is very large and spectral profiles of the CeBr3 powder obtained from chemical makers may differ slightly from one batch to another. Therefore, for each sample of CeBr3 powder, it is necessary to obtain calibration data as shown in Fig. 2 before the CeBr3 powder can be used as a pressure-memory material. It is most desirable to determine precise calibration curves under the same conditions as the sample is actually placed and compressed. Another important point is thermal annealing effect on the luminescence spectra of the CeBr3 powder compacts. As in the case of the EuCl2 powder compacts [8], the peak wavelength moves back gradually to the shorter wavelength as thermal annealing proceeds. We adopted an annealing temperature of 120 °C and within a day the luminescence spectrum of the CeBr3 powder compact shifted back completely, indicating that thermal annealing effect on the CeBr3 powder compacts is much stronger than that on the EuCl2 powder compacts. The CeBr3 powder cannot be used as pressure-memory material at high temperatures. 4. Conclusions Photoluminescence spectra of the CeBr3 powder compacts measured at ambient conditions showed very large redshifts compared with those of the original powder due to residual stress. An extremely large shift of nearly 20 nm (1450 cm 1) at the compaction pressure of 1.0 GPa was observed, which is much larger than that of the EuCl2 powder reported previously. By utilizing the effect of an unusually large residual stress on the luminescence spectra, the CeBr3 powder appears to make a very good pressure-memory material for cases where in situ pressure measurements are difficult. It can record the maximum pressure it has experienced as the peak shift of the luminescence spectrum. The applicable pressure range is roughly 0–1.0 GPa, but for the lower pressure region (0–0.45 GPa), the CeBr3 powder is much more effectively used as a pressure-memory material than the EuCl2 powder because of its unusually large spectral shifts against compaction. Acknowledgment We are grateful to Ms. Kazuko Nakajima for her help in photoluminescence measurements. References [1] S. Hufner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978 (Chapter 9). [2] W.M. Yen, S. Shionoya, H. Yamamoto (Eds.), Phosphor Handbook, second ed., CRC Press, 2007. [3] G.H. Dieke, H.M. Crosswhite, Appl. Opt. 7 (1963) 675. [4] J. Sugar, J. Reader, J. Chem. Phys. 59 (1973) 2083. [5] C.E. Tyner, H.G. Drickamer, J. Chem. Phys. 67 (1977) 4116. [6] Th. Troster, S. Schweizer, M. Secu, J.-M. Spaeth, J. Lumin. 99 (2002) 343. [7] D.B. Gatch, D.M. Boye, Y.R. Shen, M. Grinberg, Y.M. Yen, R.S. Meltzer, Phys. Rev. B 74 (2006) 19517. [8] T. Kobayashi, T. Sekine, Opt. Mater. 32 (2010) 1227.