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Thermoluminescence spectra of rare earth doped magnesium orthosilicate
Journal of Alloys and Compounds 797 (2019) 1338e1347
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Thermoluminescence spectra of rare earth doped magnesium orthosilicate Y. Zhao a, Y. Wang a, *, H. Jin a, L. Yin a, X. Wu a, Y. Ma b, P.D. Townsend c a
School of Science, China University of Geosciences, Beijing, 100083, China Beijng Center for Deceases Prevention and Control, Beijing, 100013, China c Physics Building, University of Sussex, Brighton, BN1 9QH, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2019 Received in revised form 9 May 2019 Accepted 13 May 2019 Available online 16 May 2019
Thermoluminescence (TL) spectra of undoped and rare earth doped magnesium orthosilicate, Mg2SiO4: RE3þ (RE ¼ Yb, Tm, Er, Dy, Tb, Eu, Sm, Nd, Pr, Ce) samples, were recorded from room temperature to 450 C. The rare earth dopants mostly suppress signals from the host material. The glow peak temperatures are a function of rare earth size relative to the magnesium ions. The patterns differ between glow peaks, and this is interpreted as evidence for rare earth ions on the two possible magnesium lattice sites. Further, the peak TL temperatures can vary between the transition wavelengths of the same dopant, which is consistent with close association of the rare earth and charge trapping sites. Overall, this large body of spectrally resolved thermoluminescence data indicate systematic changes with the size of the dopants. Powder formation introduces lattice damage which lowers the peak temperatures relative to single crystals, and inherent temperature sensitive spectral complexity of the data emphasis that energy analyses of polychromatic signals are inappropriate. © 2019 Elsevier B.V. All rights reserved.
Keywords: Rare earth Thermoluminescence Mg2SiO4:RE
1. Introduction Silicates are an attractive class of materials among inorganic phosphors with a wide range of applications due to desirable properties such as water resistance, chemical stability and transparency of visible light. Luminescence efficiency in inorganic phosphors doped with trivalent rare earth cations has advantages, both in terms of overall efficiency, and in matching the spectra to specific types of detector. At the more fundamental level, the dopants and their emission spectra, can offer indications on the models for luminescence sites. Among the silicate range of materials, the Mg2SiO4 (forsterite) host, doped with rare earth ions, exhibit interesting applications such as a long-lasting phosphor, X-ray imaging, light emitting display (LED), environmental monitoring, etc. Further, luminescent materials, doped with Eu3þ ions are widely studied because of their high efficiency and their CIE chromaticity coordinates. Eu3þ doped phosphors are effectively excited by near-UV or blue light, and as a result it emits a strong red color which attributes to the 4fe4f transitions [1e5], additionally RE3þ (RE ¼ Eu, Sm, Tb, Dy) doped
* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jallcom.2019.05.173 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Mg2SiO4 exhibit a range of colors [6e9]. Because of its high sensitivity, magnesium orthosilicate doped with terbium (Mg2SiO4: Tb) has been studied since 1971 as a radiation dosimeter [10,11]. The material is mostly formed of relatively light elements and its effective atomic number is approximately 11, so that it is useful in environmental and personnel monitoring [12]. Despite the diversity of applications, the details of how the rare earth ions are incorporated into the host lattice of magnesium orthosilicate are far less clear, and over time there have been a variety of ever more complex models. It is assumed that lattice defects are directly linked and stabilized by the rare-earth impurities. This is an ideal situation as trapped charges can directly couple to the emission site and hence offer a high efficiency TL material, and the spectra may reveal characteristics of the defect sites. The details of trapping and recombination sites are elusive as many factors need to be considered. The wealth of experience gained from defect and TL studies in the last 70 years has revealed patterns which are worth recalling in order to consider if optimal materials processing has been used. This experience is based on a wide range of host lattices that have been activated with rare earth (RE) ions [13e16].
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2. Key features of the prior TL literature A summary of the key factors seen in a diversity of materials can indicate how thermoluminescence might be produced more efficiently, but the requirements are not always compatible with commercial production of the material. In general, dopants and lattice imperfections are involved, often in close association, and these define the temperature, spectra and efficiency of the TL responses. A brief list of factors (a to e), with examples and consequences will now follow. Unfortunately, some are experimentally overlooked, or not always considered, unless there is spectral recording of the TL. (a) Lattice defects and distortions are inevitable from doping, but many imperfections have extremely long-range interactions that can reduce TL efficiency. For example, surfaces, dislocations and grain boundaries will all have extensive strain fields that reduce luminescence efficiency. Comparisons of TL from single crystals and crushed and powdered material demonstrate this quite dramatically. An early comparison of calcite TL from crystals, before and after crushing into powder, showed lower intensity, changes in spectra and the number of glow peaks, and the TL peak temperatures decreased [17]. For example, a major peak at 230 C dropped below 200 C, plus other changes in the number and relative intensities of the peaks. Similarly, luminescence from dislocation free layers of Nd:YAG were 50% more intense than normal crystals, but destruction of the crystallinity dropped the signals by more than 200 times and shifted the emission wavelengths [18]. (b) Alternative growth techniques can alter the defect structures, for example, in successful attempts to grow stoichiometric yttrium orthovanadate, YVO4, but in terms of TL spectra and temperatures, two growth methods (Czochralski and top seeded solution growth methods) generated totally different TL signals, because of subtle differences in their intrinsic defect structures [19]. (c) The physical size of dopants may fairly efficiently replace host lattice sites but they will always differ in size, even if they have the same valency. In rare earth doped LaF3 the RE ions occupy the La site, but their TL peak temperatures scale with the ionic size of the dopants [14], and it appears that to minimize the local strain they associate into groups [20]. The pattern is repeated in many materials as diverse as bismuth germanate [15], calcite [21], calcium sulphate [22] etc. In extreme cases the lattice accommodates the dopants as nanoparticles of dopant inclusion phases [23,24]. Other TL examples indicate that rare earth dopants may even modify the overall lattice structure (i.e. not just in the immediate neighbourhood of the dopant) [25e27]. (d) The pairing and clustering of the dopants can be critically dependent on the thermal treatments, both for separation whilst at high temperature, and retention via thermal quenching. The TL patterns of CaF2 can be totally altered by such thermal treatments [28]. An extreme thermal example is to use laser pulse anneals, as were applied for enhancements to lifetime and efficiency in both Er implanted sapphire, and Eu implanted silica [29], where both properties were enhanced by 5e8 times relative to furnace treatments. In general, all such processing can modify wavelength, line width, lifetime and relative intensity of the rare earth transitions [30] and the TL [31]. (e) Because rare earth dopants and trapping sites may be associated in a single complex the charge transfer can be via tunneling or barrier crossing, and then the temperature of
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the TL peaks will differ between luminescence transitions of the RE ion. These variations have been seen in spectral measurements of many materials [32] (and will be further identified in this current data). Such examples clearly indicate that dissociation of defect clusters can raise luminescence efficiency and should be considered in enhancing the performance of dosimetry materials, and the preceding patterns (a to e) have been observed in a wide range of host materials. The current data were obtained in an exploration of rare earth doped magnesium orthosilicate. A series of rare earth ions were selected because they are the commonly used ones to have different colors so that the result are valuable for the studies. Besides, their ion sizes vary and they could give us a general idea on how the ion size alters peak temperatures of the TL process. A particular interest with this material is that the magnesium is incorporated at two different sites, thus the TL behavior may differ depending on which Mg site the RE ions occupy. With such inherent complexity for the rare earth doped materials, accurate and unequivocal interpretation of the details of the TL sites is unlikely. Nevertheless, for applications, progress becomes a mixture of modelling and empirical data, and thus a broad understanding of the size effects is valuable. 3. Experimental Mg2SiO4: RE used here for this study of TL has been synthesized by a high temperature solid state reaction technique. The raw material of Mg(OH)2$4MgCO3$5H2O and SiO2, with the addition of rare earth oxides were mixed according to the stoichiometric ratio. The mixtures were then heated in the air at 1650 C for an hour. The concentration of rare earth ions is 1 wt% for all the Mg2SiO4: RE samples. The sintered materials were then ground and sieved through a 200-mesh sieve into a fine powder form for the experiment. Three dimensional thermoluminescence spectral plots (intensity versus wavelength and temperature) were recorded through a TOSL-3DS spectrometer made by Guangzhou Ruidi Aisheng Technology Co. Ltd. All the Mg2SiO4: RE samples (RE ¼ Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yb) were exposed to X-rays with the irradiation dose of 120 mGy for the TL measurements. The temperature range is from room temperature to 450 C with a heating rate of 3 C/s. 4. Results and comments Before considering deliberately doped material it was necessary to monitor the TL of the “pure” starting material. This sample was produced with the same origin material as rare earth doped samples, and the same preparation condition but longer sintering time (2 h). In Fig. 1 are presented isometric and contour maps for the magnesium orthosilicate powder. Several glow peaks are apparent which have the dominant emission at different wavelengths. The spectral pattern is broadly similar to the data obtained with single crystal samples [33] where the emission spectra are centered near 630 nm (i.e. as for the powder material). However, the crystalline data did not record any low temperature signal near 460 nm. Matching their crystal TL, with three main glow peaks, to the powder data requires a minor adjustment as, at a heating rate of 1 C/min, the crystal data show peaks near 160, 260 and 390 C. The powder data are all for a higher heating rate of 3 C/s, so the equivalent crystal data should appear nearer 165, 265 and 395 C. In spectral terms there is a match between an emission band for “pure” powder and crystal data near 625 nm. The powder peak temperatures are lower at ~112, 244 and 329 C (plus a possible
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Fig. 2. Thermoluminescence from another nominally pure powder sample of magnesium orthosilicate which has a strong low temperature signal at short wavelengths.
Fig. 1. Thermoluminescence from a nominally pure powder sample of magnesium orthosilicate. (N.B. The long wavelength high temperature signal is primarily from black body radiation, but there is an indication of signal near 625 nm above 425 C).
feature above 425 C). For this example, there are a second set of strong peaks nearer 710 nm which maximize near ~120 and 240 C. Whilst a 625 nm region emission is seen in all the “pure” powder samples (and some of the doped ones) other features have been noted, and a further example is shown in Fig. 2 from another pure powder sample with definite peaks near 112, 244 and 329 C. The sample was produced in the same starting material but a different heating procedure. Once again, the 625 nm band is present, plus a stronger short wavelength low temperature band, but the 710 nm emission is absent. A simplistic integration of the signals of Mg2SiO4 samples doped with different rare earths is shown in Fig. 3. Note however that these signals have been corrected for the wavelength sensitivity of the equipment and so will differ significantly for recordings of polychromatic or filtered light. The emission spectra of all the RE samples are shown in Fig. 4. TL spectra and contour maps of Mg2SiO4 samples doped with different rare earths are shown in Fig. 5 to Fig. 14. Note that some appear to contain the 625 nm emission characteristic of the undoped material, in addition to TL at RE line transitions. Comments on each set of data follow the figures. There are two broad band emission zones with peaks near
467 nm and 615 nm as shown in Fig. 5. The emission at 467 nm and the other bands may be from Ce3þ (5D0-7F2) as the charge state of cerium can vary with the local environments. The glow peak temperatures are roughly at 124, 227, 245 and 323 C for the shorter wavelengths. At 615 nm the values are ~118, 221, and 347 C. Note that cerium is the largest dopant ion used, and thus will provide most lattice distortion. The two TL patterns may be the result of Ce on the two possible Mg sites. As shown in Fig. 6, the TL spectra of Pr doped Mg2SiO4 are in the red. There are at least five main emission lines which include those at 612 nm, 650 nm and 718 nm from the transition of 3P0 to 3H6, 3F2 and 3F4 separately. Peak values are at ~112, 245, 317 C. At longer wavelengths there is a peak near 323 C. As shown in Fig. 7, the TL spectra from Nd peak near 124, 200, 257 and 353 C at shorter wavelengths but slightly differently, at ~202 and 245 C in the red end of the spectra. All the emission bands are characteristic Sm3þ, being at 579 nm, 610 nm，660 nm and 715 nm as shown in Fig. 8, which are assigned to the transitions of 4G5/2 to 6H5/2, 7/2, 9/2, 11/2, plus there are infrared lines at 787 nm, 926 nm and 959 nm. There are two very intense glow peak temperatures which are at 116 C and 229 C, plus a possible unresolved peak above 300 C, but beyond ~900 nm there is evidence for a peak above 400 C. The emission intensity of the Eu doped material is far more intense than for the previous samples, as shown in Fig. 9, this is immediately obvious by noting that the black body signal is
Y. Zhao et al. / Journal of Alloys and Compounds 797 (2019) 1338e1347
Fig. 3. Integrated thermoluminescence signals for the rare earth doped powders.
Fig. 4. Integrated spectra from different RE doped Mg2SiO4 powder samples.
minimal by comparison with the Eu emission bands. This is expected, as Eu as a preferred dopant for this dosimeter. The main peak of the TL is around 610 nm which is assigned as the transition of 5D0-7F2. The others are at 597 nm, 660 nm, 694 nm and 711 nm. There is no sign of Eu2þ since the sample was prepared in the air
Fig. 5. Thermoluminescence spectra of Mg2SiO4: Ce.
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Fig. 6. Thermoluminescence spectra of Mg2SiO4: Pr.
Fig. 7. Thermoluminescence spectra of Mg2SiO4: Nd.
atmosphere. The peak temperature for all the emission lines are at 112 and 239 C. Tb is also an excellent dopant for the TL with many lines extending into the blue end of the spectrum. In this sense it is a more favourable dopant than Eu as most photomultiplier detectors are more efficient at the blue end of the spectrum. Similar to the Eu doped Mg2SiO4, Mg2SiO4: Tb response is also high, as noted by the minimal blackbody signal. All the emission lines are the characteristic lines of Tb3þ. The most intense signal is at 196 C, but there are higher peaks near 323 and 415 C, as shown in Fig. 10. The data shown here are somewhat unusual, as in most examples of TL with Dy dopants there are often just the 4 Dy lines of very similar intensity, whereas here the 582 nm line is considerably stronger than the others, as shown in Fig. 11. The second oddity is that the Dy line signals seem to be accompanied by a weak broad band intrinsic emission TL centered around 625 nm. Dy TL peaks occur near 100, 166 and 263 C, whereas the intrinsic peaks around 625 nm are near 190, 281 and > 430 C. A possible reason is that there is a resonance coupling between an intrinsic and the adjacent Dy emission line. The TL spectra of Er doped Mg2SiO4 powder has only one broad band emission centered near 625 nm as shown in Fig. 12, but progressively moving to shorter wavelengths at higher temperatures. The response of this sample to the irradiation is very low compared to the other samples. The peak temperature of the glow curves are at 196, 269 and above 430 C. There is thus a suspicion that the signal might arise from intrinsic features.
As shown in Fig. 13, the TL spectra of Mg2SiO4:Tm show the normal Tm3þ lines. There is one obvious Tm glow peak at ~94 C, and near 625 nm there is a wavelength dependent signal at 103, 270 and > 430 C which might well be intrinsic. The Yb3þ lines are in the infrared region at 977 nm and 1000 nm as shown in Fig. 14. Whilst the TL extends over a wide temperature range, the only resolvable peaks are near 118 C but unresolved peaks are approximately near 184, 281 and above 350 C. Overall, the patterns of TL from the various dopants lack an obvious visual pattern, and some samples have both host and dopant signals. A integration of the signals of differently doped phosphor is shown in Fig. 15. Since the signals vary with doping, two sets of the samples are presented as Fig. 15a and b. A tabular listing (Table I) of component peaks in terms of the ionic radii of the RE ions suggests that there may be 5 or 6 common peaks that alter in temperature with ionic size. Values for undoped “pure” powder and a single crystal [33] are included. Some smaller changes between glow peak temperature with wavelength have not been listed. A cautionary note is that not all peaks are clearly resolved (as for Yb). Secondly, although literature discussions are in terms of ionic radii the more important parameter is the volume of the dopant ion relative to the available space, and moving from a magnesium ion to cerium implies a volume increase of ~5 times. This impresses considerable lattice distortions, which in turn alters the lattice parameters around the defect zone, and probably extends changes over many neighbouring lattice and imperfection sites. As noted earlier, even for LaF3 doped with other rare earth
Fig. 10. Thermoluminescence spectra of Mg2SiO4: Tb.
Fig. 8. Thermoluminescence spectra of Mg2SiO4: Sm.
Fig. 11. Thermoluminescence spectra of Mg2SiO4: Dy. Fig. 9. Thermoluminescence spectra of Mg2SiO4: Eu.
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Fig. 13. Thermoluminescence spectra of Mg2SiO4: Tm. Fig. 12. Thermoluminescence spectra of Mg2SiO4:Er.
ions, where there are only small distortions, the TL peaks alter with ionic size and to minimize lattice strains there is evidence for dopant clustering (in that case with a group of at least three RE ions). Thus, one may expect even greater site complexity for the very large RE ions entering one or other (or both) of the two Mg sites since all the RE ions are much larger than the Mg sites. The Mg ions occupy two crystallographically distinct positions, which have inversion (i) and mirror (m) symmetry and are designated as Mg(I) and Mg(II) respectively, and Si tetrahedra have mirror symmetry. The salient feature of the structure is that all of the polyhedra are obviously distorted. With such a complex table a visual display, as in Fig. 16, may help to indicate how the data are interlinked.
5. Discussion The initial consideration must be to ask why the TL peaks occur at different temperatures in the single crystal data [33] compared with the various undoped powder samples listed here (e.g. Figs. 1 and 2 and Table 1). In part this might be differences in purity since the current aim is to develop improved dosimeters and so the starting material may be less pure than that used in the crystalline data. However, as already mentioned, earlier data for CaCO3 (in both calcite and aragonite crystalline versions [17]) offered very different TL curves, and spectra, between the original crystals and crushed powder variants of the same pieces of material. In each
case the glow peak temperatures were reduced by up to 40 C, with changes in their relative intensities, and sensitivity to storage time between irradiation and measurement. Etching to remove surface damage of the powder did not alter the effect. Therefore, it appeared that the crushing had generated dislocations, grain boundaries and other localized defects which in turn weaken the lattices, and so reduce the thermal energy required for charge transfer and TL. For the current material, magnesium orthosilicate, the differences between single crystal TL with 3 peaks, and a range of lower peaks in different powder samples, follows the same pattern. To emphasise this on Fig. 16 the crystalline and powder TL peaks are bracketed into three groups to suggest that the lower temperature powder peaks are actually variants of the same crystalline example in which peak values have variously been depressed by as much as 60e100 . On the plot, in Celsius, this appears to be a large percentage change but in the more appropriate temperature scale in Kelvin, then these decreases are around just 10% (i.e. both a modest change and comparable with the effects or RE dopants in LaF3). Two caveats are that crushing may produce a mixture of original and a variety of distorted sites, plus there are two Mg site. Hence there may be more than one signal from the powder samples for nominally equivalent defects. The arrows A, B and C indicate how the temperature changes may group the crystalline and powder TL sets of signals. Adding rare earth dopants will be a further complication to the overall data presented in Fig. 16. In the case of cerium doped
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Fig. 14. Thermoluminescence spectra of Mg2SiO4: Yb.
powder, it is very clear (Fig. 5) that there are two sets of TL peak temperatures emitting at different wavelengths, which could support the view that the Ce is occupying two different Mg sites. As seen from Table 1 wavelength peak sensitivity is noted for many other RE dopants, and expected from the suggestion of differences between the two Mg sites. A far greater problem is that since the undoped powder peaks differ between samples, these effects may equally imply variations between differently prepared sets of samples and crushing. Thus, seeking trends in the pattern of TL peak temperatures must be made with caution. Tentative trend lines have been sketched on the Fig. 16. Starting with the crystalline and powder values of set (A) the crushed samples show a swathe of
Fig. 15. Comparison of thermoluminescence glow curves from the rare earth doped powders(a) high response group (b) low response group.
points labelled “a” which approximately fit into two subsets with wavelength dependent points. There is one set at approximately the same temperature of the two powder examples that extends across to the cerium end of the diagram. Plus, a second set, shown
Table I Summary of TL peaks for rare earth doped Mg2SiO4. Sample
Atomic number
R3þ (nm)
Volume (nm3) x 103
Glow peak temperatures ( C)
Ce
58
0.102
4.45
Pr
59
0.099
4.06
Nd
60
0.098
3.94
124 118 112 112 124
Sm Eu Tb Dy
62 63 65 66
0.096 0.095 0.092 0.091
3.71 3.59 3.26 3.16
Er Tm
68 69
0.089 0.088
2.95 2.85
Yb
70
0.087
2.76
Powder 12 Mg Single crystal Mg2þ
0.072
1.56
227 221
245 202
116 112 100
94 100 118 112 112 120
Comments 245
257 245 229 239
196 166 190 196
165
341
>300
>400
Blue end Red end ~625 nm >800 nm Blue Red >900 nm
323
184 214
323 347 317 323 353
244 240
263 281 269 ~270
>430
Near 625 nm
>430 >430
Changing near 625 nm
281
>350
>900 nm
>425?
Different samples
304 265
329 341 395
At 3 C/min
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expected if there is RE occupancy at both the Mg lattice sites. (iv) The broadening of the RE emission spectra is consistent with accommodation of very large dopant ions into localized clusters, rather than at isolated sites. A positive understanding of this complexity is that one can adjust the emission spectra to favour particular types of detector by the choice of the RE dopant, and with careful thermal processing optimize the size of the dopant clusters to intensify the signals. Acknowledgement The work has been supported by the Fundamental Research Funds for the Central Universities in China and the National Natural Science Foundation of China (No.11205134). References Fig. 16. A graphical view of the glow peak temperatures for the various rare earth dopants recorded in Mg2SiO4. To reveal the size mismatch their volumes are plotted. The Mg ion size for several powder samples are indicated by circles, but the TL from, a crystal [33] are marked as squares. The trend lines and possible interpretations are discussed in the text.
with a dashed line from Tm to Ce, which clearly increases smoothly in the TL peak temperature with the size of the dopants. The line b runs from the TL peak temperature of powder sites across to one of the Ce examples. The sketched fit is not unique. However, the C set of lines are at higher temperature and thus imply deeper trapping levels, and perhaps greater stability of possible sites. From the powder examples, two lines (c1, c2) decrease as the lattice is more distorted by larger increasing dopants and the number of points is justified on the basis of wavelength dependent signals. The extreme starting point of the C set is for crystalline TL data. And again, as expected, lattice distortion from both large dopants and/or crushing into the powder form follows the calcite pattern of reducing the TL peak temperatures when there is more lattice damage. Overall this cautious interpretation is that sites which existed in crystalline material can be retained in some of the powder samples, but the act of crushing will weaken the lattice and generate equivalent sites with slightly shallower traps. Less speculative is that some trend lines rise with increasing RE size, whilst others are falling. This is consistent with the presence of two different Mg sites that can be occupied by the RE ions. The most important spectral observations are that two Mg sites are involved, and the act of crushing has lowered the energy (and temperature) required to move electrons from traps into the RE emission sites. Earlier literature has noted that, because the 4f electrons of trivalent rare-earth ions are well shielded from the surroundings, the emission transitions can yield sharp line spectra, but many of the examples noted here (Fig. 4) are somewhat broadened which may imply that there are further links caused by association of the dopants into clusters of dopants at each type of TL site. In isolation, if the 4f states only weakly interact with the host lattice, the spectra are very similar in different host lattices [34].
6. Conclusion The current data set of rare earth doped powder samples of Mg2SiO4 confirm that (i) dopant size alters the glow peak temperatures. (ii) Crushing into powder produces lower peak temperatures than reported for single crystals. (iii) The fact that emission at different wavelengths can differ noticeably (e.g. for cerium) is
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thermoluminescence of manganese in carbonate lattices, Radiat. Meas. 26 (1996) 719e731. Y. Wang, N. Can, P.D. Townsend, Influence of Li dopants on thermoluminescence spectra of CaSO4 doped with Dy or Tm, J. Lumin. 131 (2011) 1864e1868. T. Karali, N. Can, P.D. Townsend, A.P. Rowlands, J. Hanchar, Radioluminescence and thermoluminescence of rare earth element and phosphorus-doped zircon, Am. Mineral. 85 (2000) 668e681. P.D. Townsend, B. Yang, Y. Wang, Luminescence detection of phase transitions, local environment and nanoparticle inclusions, Contemp. Phys. 49 (2008) 255e280. Y. Wang, B. Yang, N. Can, P.D. Townsend, Indications of bulk property changes from surface ion implantation, Philos. Mag. 91 (2011) 259e271. B. Yang, P.D. Townsend, R. Fromknecht, Low temperature detection of phase transitions and relaxation processes in strontium titanate by means of cathodoluminescence, J. Phys. Condens. Matter 16 (2004) 8377e8386. Y. Wang, B. Ma, W. Zhang, D. Li, Y. Zhao, A.A. Finch, P.D. Townsend, Substrate lattice relaxations, spectral distortions, and nanoparticle inclusions of ion implanted zinc oxide, J. Appl. Phys. 118 (2015), 095703-1-10. S.A. Holgate, T.H. Sloane, P.D. Townsend, D.R. White, A.V. Chadwick,
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Thermoluminescence of calcium fluoride doped with neodymium, J. Phys. Condens. Matter 6 (1994) 9255e9266. N. Can, P.D. Townsend, D.E. Hole, H.V. Snelling, J.M. Ballesteros, C.N. Afonso, Enhancement of luminescence by pulse laser annealing of ion implanted europium in sapphire and silica, J. Appl. Phys. 78 (1995) 6737e6744. M. Stef, A. Pruna, N. Pecingina-Garjoaba, I. Nicoara, Influence of various impurities on the optical properties of YbF3-doped CaF2 crystals, Acta Phys. Pol. 112 (2007) 1007e1012. P.D. Townsend, D.R. White, Interpretation of rare earth thermoluminescence spectra, Radiat. Protect. Dosim. 65 (1996) 83e88. P.D. Townsend, A.A. Finch, M. Maghrabi, V. Ramachandran, G.V. V azquez, Y. Wang, D.R. White, Spectral changes and wavelength dependent thermoluminescence of rare earth ions after X-ray irradiation, J. Lumin. 192 (2017) 574e581. G. Okada, T. Kojima, J. Ushizawa, N. Kawaguchi, T. Yanagida, Radio-photoluminescence observed in non-doped Mg2SiO4 single crystal, Curr. Appl. Phys. 17 (2017) 422e426. G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994, p. 40.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Thermoluminescence spectra of rare earth doped magnesium orthosilicate Y. Zhao a, Y. Wang a, *, H. Jin a, L. Yin a, X. Wu a, Y. Ma b, P.D. Townsend c a
School of Science, China University of Geosciences, Beijing, 100083, China Beijng Center for Deceases Prevention and Control, Beijing, 100013, China c Physics Building, University of Sussex, Brighton, BN1 9QH, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2019 Received in revised form 9 May 2019 Accepted 13 May 2019 Available online 16 May 2019
Thermoluminescence (TL) spectra of undoped and rare earth doped magnesium orthosilicate, Mg2SiO4: RE3þ (RE ¼ Yb, Tm, Er, Dy, Tb, Eu, Sm, Nd, Pr, Ce) samples, were recorded from room temperature to 450 C. The rare earth dopants mostly suppress signals from the host material. The glow peak temperatures are a function of rare earth size relative to the magnesium ions. The patterns differ between glow peaks, and this is interpreted as evidence for rare earth ions on the two possible magnesium lattice sites. Further, the peak TL temperatures can vary between the transition wavelengths of the same dopant, which is consistent with close association of the rare earth and charge trapping sites. Overall, this large body of spectrally resolved thermoluminescence data indicate systematic changes with the size of the dopants. Powder formation introduces lattice damage which lowers the peak temperatures relative to single crystals, and inherent temperature sensitive spectral complexity of the data emphasis that energy analyses of polychromatic signals are inappropriate. © 2019 Elsevier B.V. All rights reserved.
Keywords: Rare earth Thermoluminescence Mg2SiO4:RE
1. Introduction Silicates are an attractive class of materials among inorganic phosphors with a wide range of applications due to desirable properties such as water resistance, chemical stability and transparency of visible light. Luminescence efficiency in inorganic phosphors doped with trivalent rare earth cations has advantages, both in terms of overall efficiency, and in matching the spectra to specific types of detector. At the more fundamental level, the dopants and their emission spectra, can offer indications on the models for luminescence sites. Among the silicate range of materials, the Mg2SiO4 (forsterite) host, doped with rare earth ions, exhibit interesting applications such as a long-lasting phosphor, X-ray imaging, light emitting display (LED), environmental monitoring, etc. Further, luminescent materials, doped with Eu3þ ions are widely studied because of their high efficiency and their CIE chromaticity coordinates. Eu3þ doped phosphors are effectively excited by near-UV or blue light, and as a result it emits a strong red color which attributes to the 4fe4f transitions [1e5], additionally RE3þ (RE ¼ Eu, Sm, Tb, Dy) doped
* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jallcom.2019.05.173 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Mg2SiO4 exhibit a range of colors [6e9]. Because of its high sensitivity, magnesium orthosilicate doped with terbium (Mg2SiO4: Tb) has been studied since 1971 as a radiation dosimeter [10,11]. The material is mostly formed of relatively light elements and its effective atomic number is approximately 11, so that it is useful in environmental and personnel monitoring [12]. Despite the diversity of applications, the details of how the rare earth ions are incorporated into the host lattice of magnesium orthosilicate are far less clear, and over time there have been a variety of ever more complex models. It is assumed that lattice defects are directly linked and stabilized by the rare-earth impurities. This is an ideal situation as trapped charges can directly couple to the emission site and hence offer a high efficiency TL material, and the spectra may reveal characteristics of the defect sites. The details of trapping and recombination sites are elusive as many factors need to be considered. The wealth of experience gained from defect and TL studies in the last 70 years has revealed patterns which are worth recalling in order to consider if optimal materials processing has been used. This experience is based on a wide range of host lattices that have been activated with rare earth (RE) ions [13e16].
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2. Key features of the prior TL literature A summary of the key factors seen in a diversity of materials can indicate how thermoluminescence might be produced more efficiently, but the requirements are not always compatible with commercial production of the material. In general, dopants and lattice imperfections are involved, often in close association, and these define the temperature, spectra and efficiency of the TL responses. A brief list of factors (a to e), with examples and consequences will now follow. Unfortunately, some are experimentally overlooked, or not always considered, unless there is spectral recording of the TL. (a) Lattice defects and distortions are inevitable from doping, but many imperfections have extremely long-range interactions that can reduce TL efficiency. For example, surfaces, dislocations and grain boundaries will all have extensive strain fields that reduce luminescence efficiency. Comparisons of TL from single crystals and crushed and powdered material demonstrate this quite dramatically. An early comparison of calcite TL from crystals, before and after crushing into powder, showed lower intensity, changes in spectra and the number of glow peaks, and the TL peak temperatures decreased [17]. For example, a major peak at 230 C dropped below 200 C, plus other changes in the number and relative intensities of the peaks. Similarly, luminescence from dislocation free layers of Nd:YAG were 50% more intense than normal crystals, but destruction of the crystallinity dropped the signals by more than 200 times and shifted the emission wavelengths [18]. (b) Alternative growth techniques can alter the defect structures, for example, in successful attempts to grow stoichiometric yttrium orthovanadate, YVO4, but in terms of TL spectra and temperatures, two growth methods (Czochralski and top seeded solution growth methods) generated totally different TL signals, because of subtle differences in their intrinsic defect structures [19]. (c) The physical size of dopants may fairly efficiently replace host lattice sites but they will always differ in size, even if they have the same valency. In rare earth doped LaF3 the RE ions occupy the La site, but their TL peak temperatures scale with the ionic size of the dopants [14], and it appears that to minimize the local strain they associate into groups [20]. The pattern is repeated in many materials as diverse as bismuth germanate [15], calcite [21], calcium sulphate [22] etc. In extreme cases the lattice accommodates the dopants as nanoparticles of dopant inclusion phases [23,24]. Other TL examples indicate that rare earth dopants may even modify the overall lattice structure (i.e. not just in the immediate neighbourhood of the dopant) [25e27]. (d) The pairing and clustering of the dopants can be critically dependent on the thermal treatments, both for separation whilst at high temperature, and retention via thermal quenching. The TL patterns of CaF2 can be totally altered by such thermal treatments [28]. An extreme thermal example is to use laser pulse anneals, as were applied for enhancements to lifetime and efficiency in both Er implanted sapphire, and Eu implanted silica [29], where both properties were enhanced by 5e8 times relative to furnace treatments. In general, all such processing can modify wavelength, line width, lifetime and relative intensity of the rare earth transitions [30] and the TL [31]. (e) Because rare earth dopants and trapping sites may be associated in a single complex the charge transfer can be via tunneling or barrier crossing, and then the temperature of
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the TL peaks will differ between luminescence transitions of the RE ion. These variations have been seen in spectral measurements of many materials [32] (and will be further identified in this current data). Such examples clearly indicate that dissociation of defect clusters can raise luminescence efficiency and should be considered in enhancing the performance of dosimetry materials, and the preceding patterns (a to e) have been observed in a wide range of host materials. The current data were obtained in an exploration of rare earth doped magnesium orthosilicate. A series of rare earth ions were selected because they are the commonly used ones to have different colors so that the result are valuable for the studies. Besides, their ion sizes vary and they could give us a general idea on how the ion size alters peak temperatures of the TL process. A particular interest with this material is that the magnesium is incorporated at two different sites, thus the TL behavior may differ depending on which Mg site the RE ions occupy. With such inherent complexity for the rare earth doped materials, accurate and unequivocal interpretation of the details of the TL sites is unlikely. Nevertheless, for applications, progress becomes a mixture of modelling and empirical data, and thus a broad understanding of the size effects is valuable. 3. Experimental Mg2SiO4: RE used here for this study of TL has been synthesized by a high temperature solid state reaction technique. The raw material of Mg(OH)2$4MgCO3$5H2O and SiO2, with the addition of rare earth oxides were mixed according to the stoichiometric ratio. The mixtures were then heated in the air at 1650 C for an hour. The concentration of rare earth ions is 1 wt% for all the Mg2SiO4: RE samples. The sintered materials were then ground and sieved through a 200-mesh sieve into a fine powder form for the experiment. Three dimensional thermoluminescence spectral plots (intensity versus wavelength and temperature) were recorded through a TOSL-3DS spectrometer made by Guangzhou Ruidi Aisheng Technology Co. Ltd. All the Mg2SiO4: RE samples (RE ¼ Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yb) were exposed to X-rays with the irradiation dose of 120 mGy for the TL measurements. The temperature range is from room temperature to 450 C with a heating rate of 3 C/s. 4. Results and comments Before considering deliberately doped material it was necessary to monitor the TL of the “pure” starting material. This sample was produced with the same origin material as rare earth doped samples, and the same preparation condition but longer sintering time (2 h). In Fig. 1 are presented isometric and contour maps for the magnesium orthosilicate powder. Several glow peaks are apparent which have the dominant emission at different wavelengths. The spectral pattern is broadly similar to the data obtained with single crystal samples [33] where the emission spectra are centered near 630 nm (i.e. as for the powder material). However, the crystalline data did not record any low temperature signal near 460 nm. Matching their crystal TL, with three main glow peaks, to the powder data requires a minor adjustment as, at a heating rate of 1 C/min, the crystal data show peaks near 160, 260 and 390 C. The powder data are all for a higher heating rate of 3 C/s, so the equivalent crystal data should appear nearer 165, 265 and 395 C. In spectral terms there is a match between an emission band for “pure” powder and crystal data near 625 nm. The powder peak temperatures are lower at ~112, 244 and 329 C (plus a possible
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Fig. 2. Thermoluminescence from another nominally pure powder sample of magnesium orthosilicate which has a strong low temperature signal at short wavelengths.
Fig. 1. Thermoluminescence from a nominally pure powder sample of magnesium orthosilicate. (N.B. The long wavelength high temperature signal is primarily from black body radiation, but there is an indication of signal near 625 nm above 425 C).
feature above 425 C). For this example, there are a second set of strong peaks nearer 710 nm which maximize near ~120 and 240 C. Whilst a 625 nm region emission is seen in all the “pure” powder samples (and some of the doped ones) other features have been noted, and a further example is shown in Fig. 2 from another pure powder sample with definite peaks near 112, 244 and 329 C. The sample was produced in the same starting material but a different heating procedure. Once again, the 625 nm band is present, plus a stronger short wavelength low temperature band, but the 710 nm emission is absent. A simplistic integration of the signals of Mg2SiO4 samples doped with different rare earths is shown in Fig. 3. Note however that these signals have been corrected for the wavelength sensitivity of the equipment and so will differ significantly for recordings of polychromatic or filtered light. The emission spectra of all the RE samples are shown in Fig. 4. TL spectra and contour maps of Mg2SiO4 samples doped with different rare earths are shown in Fig. 5 to Fig. 14. Note that some appear to contain the 625 nm emission characteristic of the undoped material, in addition to TL at RE line transitions. Comments on each set of data follow the figures. There are two broad band emission zones with peaks near
467 nm and 615 nm as shown in Fig. 5. The emission at 467 nm and the other bands may be from Ce3þ (5D0-7F2) as the charge state of cerium can vary with the local environments. The glow peak temperatures are roughly at 124, 227, 245 and 323 C for the shorter wavelengths. At 615 nm the values are ~118, 221, and 347 C. Note that cerium is the largest dopant ion used, and thus will provide most lattice distortion. The two TL patterns may be the result of Ce on the two possible Mg sites. As shown in Fig. 6, the TL spectra of Pr doped Mg2SiO4 are in the red. There are at least five main emission lines which include those at 612 nm, 650 nm and 718 nm from the transition of 3P0 to 3H6, 3F2 and 3F4 separately. Peak values are at ~112, 245, 317 C. At longer wavelengths there is a peak near 323 C. As shown in Fig. 7, the TL spectra from Nd peak near 124, 200, 257 and 353 C at shorter wavelengths but slightly differently, at ~202 and 245 C in the red end of the spectra. All the emission bands are characteristic Sm3þ, being at 579 nm, 610 nm，660 nm and 715 nm as shown in Fig. 8, which are assigned to the transitions of 4G5/2 to 6H5/2, 7/2, 9/2, 11/2, plus there are infrared lines at 787 nm, 926 nm and 959 nm. There are two very intense glow peak temperatures which are at 116 C and 229 C, plus a possible unresolved peak above 300 C, but beyond ~900 nm there is evidence for a peak above 400 C. The emission intensity of the Eu doped material is far more intense than for the previous samples, as shown in Fig. 9, this is immediately obvious by noting that the black body signal is
Y. Zhao et al. / Journal of Alloys and Compounds 797 (2019) 1338e1347
Fig. 3. Integrated thermoluminescence signals for the rare earth doped powders.
Fig. 4. Integrated spectra from different RE doped Mg2SiO4 powder samples.
minimal by comparison with the Eu emission bands. This is expected, as Eu as a preferred dopant for this dosimeter. The main peak of the TL is around 610 nm which is assigned as the transition of 5D0-7F2. The others are at 597 nm, 660 nm, 694 nm and 711 nm. There is no sign of Eu2þ since the sample was prepared in the air
Fig. 5. Thermoluminescence spectra of Mg2SiO4: Ce.
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Fig. 6. Thermoluminescence spectra of Mg2SiO4: Pr.
Fig. 7. Thermoluminescence spectra of Mg2SiO4: Nd.
atmosphere. The peak temperature for all the emission lines are at 112 and 239 C. Tb is also an excellent dopant for the TL with many lines extending into the blue end of the spectrum. In this sense it is a more favourable dopant than Eu as most photomultiplier detectors are more efficient at the blue end of the spectrum. Similar to the Eu doped Mg2SiO4, Mg2SiO4: Tb response is also high, as noted by the minimal blackbody signal. All the emission lines are the characteristic lines of Tb3þ. The most intense signal is at 196 C, but there are higher peaks near 323 and 415 C, as shown in Fig. 10. The data shown here are somewhat unusual, as in most examples of TL with Dy dopants there are often just the 4 Dy lines of very similar intensity, whereas here the 582 nm line is considerably stronger than the others, as shown in Fig. 11. The second oddity is that the Dy line signals seem to be accompanied by a weak broad band intrinsic emission TL centered around 625 nm. Dy TL peaks occur near 100, 166 and 263 C, whereas the intrinsic peaks around 625 nm are near 190, 281 and > 430 C. A possible reason is that there is a resonance coupling between an intrinsic and the adjacent Dy emission line. The TL spectra of Er doped Mg2SiO4 powder has only one broad band emission centered near 625 nm as shown in Fig. 12, but progressively moving to shorter wavelengths at higher temperatures. The response of this sample to the irradiation is very low compared to the other samples. The peak temperature of the glow curves are at 196, 269 and above 430 C. There is thus a suspicion that the signal might arise from intrinsic features.
As shown in Fig. 13, the TL spectra of Mg2SiO4:Tm show the normal Tm3þ lines. There is one obvious Tm glow peak at ~94 C, and near 625 nm there is a wavelength dependent signal at 103, 270 and > 430 C which might well be intrinsic. The Yb3þ lines are in the infrared region at 977 nm and 1000 nm as shown in Fig. 14. Whilst the TL extends over a wide temperature range, the only resolvable peaks are near 118 C but unresolved peaks are approximately near 184, 281 and above 350 C. Overall, the patterns of TL from the various dopants lack an obvious visual pattern, and some samples have both host and dopant signals. A integration of the signals of differently doped phosphor is shown in Fig. 15. Since the signals vary with doping, two sets of the samples are presented as Fig. 15a and b. A tabular listing (Table I) of component peaks in terms of the ionic radii of the RE ions suggests that there may be 5 or 6 common peaks that alter in temperature with ionic size. Values for undoped “pure” powder and a single crystal [33] are included. Some smaller changes between glow peak temperature with wavelength have not been listed. A cautionary note is that not all peaks are clearly resolved (as for Yb). Secondly, although literature discussions are in terms of ionic radii the more important parameter is the volume of the dopant ion relative to the available space, and moving from a magnesium ion to cerium implies a volume increase of ~5 times. This impresses considerable lattice distortions, which in turn alters the lattice parameters around the defect zone, and probably extends changes over many neighbouring lattice and imperfection sites. As noted earlier, even for LaF3 doped with other rare earth
Fig. 10. Thermoluminescence spectra of Mg2SiO4: Tb.
Fig. 8. Thermoluminescence spectra of Mg2SiO4: Sm.
Fig. 11. Thermoluminescence spectra of Mg2SiO4: Dy. Fig. 9. Thermoluminescence spectra of Mg2SiO4: Eu.
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Fig. 13. Thermoluminescence spectra of Mg2SiO4: Tm. Fig. 12. Thermoluminescence spectra of Mg2SiO4:Er.
ions, where there are only small distortions, the TL peaks alter with ionic size and to minimize lattice strains there is evidence for dopant clustering (in that case with a group of at least three RE ions). Thus, one may expect even greater site complexity for the very large RE ions entering one or other (or both) of the two Mg sites since all the RE ions are much larger than the Mg sites. The Mg ions occupy two crystallographically distinct positions, which have inversion (i) and mirror (m) symmetry and are designated as Mg(I) and Mg(II) respectively, and Si tetrahedra have mirror symmetry. The salient feature of the structure is that all of the polyhedra are obviously distorted. With such a complex table a visual display, as in Fig. 16, may help to indicate how the data are interlinked.
5. Discussion The initial consideration must be to ask why the TL peaks occur at different temperatures in the single crystal data [33] compared with the various undoped powder samples listed here (e.g. Figs. 1 and 2 and Table 1). In part this might be differences in purity since the current aim is to develop improved dosimeters and so the starting material may be less pure than that used in the crystalline data. However, as already mentioned, earlier data for CaCO3 (in both calcite and aragonite crystalline versions [17]) offered very different TL curves, and spectra, between the original crystals and crushed powder variants of the same pieces of material. In each
case the glow peak temperatures were reduced by up to 40 C, with changes in their relative intensities, and sensitivity to storage time between irradiation and measurement. Etching to remove surface damage of the powder did not alter the effect. Therefore, it appeared that the crushing had generated dislocations, grain boundaries and other localized defects which in turn weaken the lattices, and so reduce the thermal energy required for charge transfer and TL. For the current material, magnesium orthosilicate, the differences between single crystal TL with 3 peaks, and a range of lower peaks in different powder samples, follows the same pattern. To emphasise this on Fig. 16 the crystalline and powder TL peaks are bracketed into three groups to suggest that the lower temperature powder peaks are actually variants of the same crystalline example in which peak values have variously been depressed by as much as 60e100 . On the plot, in Celsius, this appears to be a large percentage change but in the more appropriate temperature scale in Kelvin, then these decreases are around just 10% (i.e. both a modest change and comparable with the effects or RE dopants in LaF3). Two caveats are that crushing may produce a mixture of original and a variety of distorted sites, plus there are two Mg site. Hence there may be more than one signal from the powder samples for nominally equivalent defects. The arrows A, B and C indicate how the temperature changes may group the crystalline and powder TL sets of signals. Adding rare earth dopants will be a further complication to the overall data presented in Fig. 16. In the case of cerium doped
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Fig. 14. Thermoluminescence spectra of Mg2SiO4: Yb.
powder, it is very clear (Fig. 5) that there are two sets of TL peak temperatures emitting at different wavelengths, which could support the view that the Ce is occupying two different Mg sites. As seen from Table 1 wavelength peak sensitivity is noted for many other RE dopants, and expected from the suggestion of differences between the two Mg sites. A far greater problem is that since the undoped powder peaks differ between samples, these effects may equally imply variations between differently prepared sets of samples and crushing. Thus, seeking trends in the pattern of TL peak temperatures must be made with caution. Tentative trend lines have been sketched on the Fig. 16. Starting with the crystalline and powder values of set (A) the crushed samples show a swathe of
Fig. 15. Comparison of thermoluminescence glow curves from the rare earth doped powders(a) high response group (b) low response group.
points labelled “a” which approximately fit into two subsets with wavelength dependent points. There is one set at approximately the same temperature of the two powder examples that extends across to the cerium end of the diagram. Plus, a second set, shown
Table I Summary of TL peaks for rare earth doped Mg2SiO4. Sample
Atomic number
R3þ (nm)
Volume (nm3) x 103
Glow peak temperatures ( C)
Ce
58
0.102
4.45
Pr
59
0.099
4.06
Nd
60
0.098
3.94
124 118 112 112 124
Sm Eu Tb Dy
62 63 65 66
0.096 0.095 0.092 0.091
3.71 3.59 3.26 3.16
Er Tm
68 69
0.089 0.088
2.95 2.85
Yb
70
0.087
2.76
Powder 12 Mg Single crystal Mg2þ
0.072
1.56
227 221
245 202
116 112 100
94 100 118 112 112 120
Comments 245
257 245 229 239
196 166 190 196
165
341
>300
>400
Blue end Red end ~625 nm >800 nm Blue Red >900 nm
323
184 214
323 347 317 323 353
244 240
263 281 269 ~270
>430
Near 625 nm
>430 >430
Changing near 625 nm
281
>350
>900 nm
>425?
Different samples
304 265
329 341 395
At 3 C/min
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expected if there is RE occupancy at both the Mg lattice sites. (iv) The broadening of the RE emission spectra is consistent with accommodation of very large dopant ions into localized clusters, rather than at isolated sites. A positive understanding of this complexity is that one can adjust the emission spectra to favour particular types of detector by the choice of the RE dopant, and with careful thermal processing optimize the size of the dopant clusters to intensify the signals. Acknowledgement The work has been supported by the Fundamental Research Funds for the Central Universities in China and the National Natural Science Foundation of China (No.11205134). References Fig. 16. A graphical view of the glow peak temperatures for the various rare earth dopants recorded in Mg2SiO4. To reveal the size mismatch their volumes are plotted. The Mg ion size for several powder samples are indicated by circles, but the TL from, a crystal [33] are marked as squares. The trend lines and possible interpretations are discussed in the text.
with a dashed line from Tm to Ce, which clearly increases smoothly in the TL peak temperature with the size of the dopants. The line b runs from the TL peak temperature of powder sites across to one of the Ce examples. The sketched fit is not unique. However, the C set of lines are at higher temperature and thus imply deeper trapping levels, and perhaps greater stability of possible sites. From the powder examples, two lines (c1, c2) decrease as the lattice is more distorted by larger increasing dopants and the number of points is justified on the basis of wavelength dependent signals. The extreme starting point of the C set is for crystalline TL data. And again, as expected, lattice distortion from both large dopants and/or crushing into the powder form follows the calcite pattern of reducing the TL peak temperatures when there is more lattice damage. Overall this cautious interpretation is that sites which existed in crystalline material can be retained in some of the powder samples, but the act of crushing will weaken the lattice and generate equivalent sites with slightly shallower traps. Less speculative is that some trend lines rise with increasing RE size, whilst others are falling. This is consistent with the presence of two different Mg sites that can be occupied by the RE ions. The most important spectral observations are that two Mg sites are involved, and the act of crushing has lowered the energy (and temperature) required to move electrons from traps into the RE emission sites. Earlier literature has noted that, because the 4f electrons of trivalent rare-earth ions are well shielded from the surroundings, the emission transitions can yield sharp line spectra, but many of the examples noted here (Fig. 4) are somewhat broadened which may imply that there are further links caused by association of the dopants into clusters of dopants at each type of TL site. In isolation, if the 4f states only weakly interact with the host lattice, the spectra are very similar in different host lattices [34].
6. Conclusion The current data set of rare earth doped powder samples of Mg2SiO4 confirm that (i) dopant size alters the glow peak temperatures. (ii) Crushing into powder produces lower peak temperatures than reported for single crystals. (iii) The fact that emission at different wavelengths can differ noticeably (e.g. for cerium) is
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