- Email: [email protected]
Electron irradiation of single crystal thorium dioxide and electron transfer reactions
www.elsevier.nl/locate/ica Inorganica Chimica Acta 300 – 302 (2000) 305 – 313
Electron irradiation of single crystal thorium dioxide and electron transfer reactions Trevor R. Griffiths *, James Dixon School of Chemistry, Uni6ersity of Leeds, Leeds LS2 9JT, UK Received 9 September 1999; accepted 11 November 1999
Abstract This first study of electron irradiation (3 MeV) of single crystals of thoria has revealed that the crystals turn blue, as seen with neutron irradiation, but the absorption profiles in the 12 000 – 17 260 cm − 1 (1.49 – 2.14 eV) region were similar but not identical. Calculations showed that the electrons transferred energy sufficient to displace lattice oxygen atoms, but not thorium atoms. The crystals were the purest currently available and supported the proposal that the fundamental absorption edge occurs close to 47 600 cm − 1 (5.9 eV). Theoretically, thoria is colourless and cannot exhibit intervalence bands, but aggregates of impurity ions having more than one oxidation state can produce such bands, and some crystals showed pale amber or grey zones. The most likely impurity involved was lead. The effect of electron irradiation on defect generation after various oxidising and reducing anneals, up to 1000°C, was studied, together with the loss of the blue radiation-induced absorption on isochronal anneals at temperatures up to 400°C. The generated profile could be resolved equally well into three or four Gaussian bands and hence band assignments are not possible. It is concluded that electrons transfer energy to thoria to create defects of shallow energy, and electron transfer involving intervalence bands can occur in the presence of impurity ions. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Single crystal; Electron irradiation; Defect formation; Intervalence spectra; Aggregate centres
1. Introduction The fluorite oxides CeO2, UO2, ThO2 and PuO2 have been of considerable interest to chemists for four decades. Ceria and urania can lose and gain, respectively, oxygen atoms, thereby becoming the non-stoichiometric materials CeO2 − x and UO2 + x. The presence of two cations having differing oxidation states, for example, Ce3 + and Ce4 + in hypo-stoichiometric ceria, now allows electron transfer in the form of intervalence spectra. This is readily observed as the initially white ceria powder, or a transparent single crystal, becomes first a pale then a deeper blue as oxygen is lost in a reducing atmosphere. Thoria, on the other hand, only exhibits oxidation state four and thus is not an apparent candidate for electron transfer reactions. However, thoria has one of the highest melting points known, * Corresponding author. Tel.: + 44-113-233 6408; fax: + 44-113233 6565. E-mail address: [email protected] (T.R. Griffiths)
above 3000°C, and thus, in the generation of single crystals, crystals having some impurity ions are generally the final product. Such impurity ions can give colour to thoria single crystals, and the colour changes upon heating in oxidising or reducing atmospheres. Single crystals of thoria can also be turned blue by subjecting them to high energy electron radiation. While the colour is brought about by transferring the energy of the electrons to the crystal, the colour this time is not due to intervalence bands. This paper is therefore concerned with the effects of electron radiation on thoria single crystals and the possibility of intervalence bands involving crystal impurities. Our recent literature survey showed that few papers have appeared in recent years on spectroscopic studies of single crystal thoria, apart from our own analysis of oxidising and reducing anneals and gamma and ultraviolet radiation on both flux-grown and arc-fused single crystals [1], and the identification of the presence of F° colour centres [2], the first time that the F° centre had
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 5 9 7 - 6
306
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
been identified in crystals having the fluorite lattice. This present paper analyses the results of a series of experiments undertaken over the past few years involving the electron irradiation of the above-mentioned arc-fused thoria single crystals, and the spectra of the blue crystals generated. Briefly, our literature survey revealed that Bodine and Theiss [3] have studied the effect of high temperature anneals, in air and under vacuum, on the optical absorption spectrum of single crystal thoria. Vacuum annealing at 1500°C coloured the crystal yellow but when the annealing temperature was lowered to 1000°C, the crystal returned to colourless; high temperature annealing in air turned the crystal red. Weinreich and Danforth [4] had earlier subjected their crystals to a cycle of high temperature (up to 1800°C) anneals in vacuum and air and also reported yellow and red colorations. They also noted that the red colour could be bleached at the much lower temperature of 200°C upon annealing in vacuum, provided the crystals had previously been subjected to ultraviolet radiation. They further reported that rapid quenching, following heat treatments, produced a deep black coloration; mixed valence compounds are often deep blue to black in colour. Both groups of workers stated that their crystals were 99.9% pure. Linares [5] has examined the effect of various flux conditions on crystal growth and of various dopant ions on the colour of thoria. The addition of 0.1% Ca2 + ions to the flux (a NaFB2O3 melt) yielded crystals with a pale permanent orange colour. The addition of Y3 + , Nb5 + and F− ions produced no observable effect, but when a lead-containing flux was employed the resulting crystals contained around 100 ppm lead impurity. Ultraviolet radiation generated a greenish colour that could be bleached by annealing at 300°C or by storing in the dark for several days. Neeley et al. [6] irradiated colourless thoria crystals (grown from lead-based fluxes) with 60Co gamma rays and with 2 MeV electrons. Both treatments turned the crystals dark yellow, but when their spectra were recorded the process initiated bleaching that was completed in a few minutes. Bates [7] annealed his thoria single crystals and observed that after a high temperature anneal in hydrogen the crystal became blue, with two bands at 24 800 and 14 100 cm − 1 (3.076 and 1.746 eV). A high temperature oxidising anneal in air reversed the process. Childs et al. [8] have performed a detailed investigation of annealing and radiation treatments on arc-fused single crystals of thoria. Their colourless crystals had an average 1445 ppm total metal impurity and 120 ppm non-metal impurities and were probably the purest crystals then available. Crystals that had received a reducing anneal at 1400°C, when irradiated with 60Co, subsequently contained four bands at 12 020, 14 250,
16 370 and 18 710 cm − 1 (1.49, 1.80, 2.03 and 2.32 eV). Their intensity reached saturation after approximately 1 h in a flux of 7×105 rad h − 1 and in a few crystals the highest energy band was relatively large but decayed soon after radiation was terminated. Our earlier measurements [1] similarly showed that 60 Co gamma radiation affected flux-grown and arcfused crystals, the effects decaying with time. We now report the effects of electron radiation, from a 3 MeV Van der Graaf generator, on a number of the arc-fused crystals used previously [1], and consider possible electron transfer processes initiated by electron radiation.
2. Experimental
2.1. Crystals and crystal annealing Thorium dioxide single crystals were kindly supplied by the Canadian Atomic Energy Commission, Chalk River, Ont., Canada. They had been grown by the arc-fusion technique, had a face area of approximately 0.3 cm2, and had already been cut and polished. The crystals were annealed in a silica tube, sealed at one end and mounted vertically in a furnace. A crystal was placed at the bottom of the tube, which was then located in the hot zone of the furnace. The open end, outside the furnace, contained a silicone rubber stopper that admitted a thermocouple and a silica capillary tube for delivering dried air, oxygen or hydrogen above the crystal surface. The gases exited through a side-arm near the stopper and through which back-diffusion was prevented. The maximum annealing temperature used was 1000°C, and the optimum annealing time at this temperature was determined as 2 h, with longer times at lower temperatures. At the end of each anneal the furnace was rapidly lowered to quench the crystal without disturbing it.
2.2. Irradiation by electrons Electron radiation was performed using electrons of energy 3 MeV from a Van der Graaf-type electron accelerator (High Voltage Engineering Corporation, Model KF3000) at the Cookridge Radiation Research Centre attached to the University of Leeds. Two types of crystal holders were employed. Type A consisted of a hollow copper cylinder, cooled by the passage of liquid nitrogen, and with the crystal fixed to the cylinder base with Apiezon vacuum grease. Type B was an aluminium cylindrical block that held a circular reservoir of water of diameter 10 cm and 2 mm deep. The crystal was again fixed with grease to the bottom of the reservoir and its upper surface covered by a water layer 1 mm deep. The aluminium block was cooled by water circulating through its base.
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Using the type A crystal holder and electron beam current intensities of 11.14 and 8.82 mamps cm − 2 caused the crystals to crack. This was surprising and implied a large temperature differential between the upper and lower surfaces of the crystal slice, due to the intensity of the radiation and the inability of the liquid nitrogen-cooled crystal holder to dissipate heat effectively. Some irradiated crystals from holder A exhibited significant upwards curvature before fracturing on handling, again demonstrating progressively reduced contact with the cold metal surface and a large thermal gradient from the upper to the lower surface of the crystals, due to the electron bombardment. It was however possible to study the dose dependence of the absorption properties by recording the spectra of the crystal submerged beneath a thin water film, (type B holder), and a reduced electron beam intensity of 2.48 mamps cm − 2 allowed the crystal to remain intact. The temperature of the crystal probably did not now exceed 100°C during electron radiation.
2.3. Spectroscopic measurements and relationships Absorption spectra were recorded in the energy range 50 000–5000 cm − 1, with air used as reference. A Unicam SP 700 spectrophotometer was utilised in order to minimise the optical bleaching of crystals; spectrophotometers that employ a component sequence that irradiates the sample with all the source lamp energy before dispersing the light in the monochromator are not suitable. Other details have been published elsewhere [1]. The following relationships apply to absorption spectra. The recorded absorbance, A, is given by log (Io/I), where Io is the intensity of the incident light, and I the transmitted intensity. For a crystal the absorption coefficient, a =A/d, where d is the crystal thickness. To avoid confusion with commonly presented spectra, A versus wavelength, the spectra displayed here employ a and kK energy units, where 1 kK=1000 cm − 1. In the text their eV equivalents are also given (1 eV = 8066 cm − 1).
307
3. Results
3.1. Crystal quality The arc-fused thoria crystals studied were first examined with a polarising microscope and found to be perfectly isotropic. Subsequent examination showed that they were free from dislocations throughout the various anneals. The crystals had all originated from the same batch, while some were colourless the others contained grey and pale amber zones. A sample from each zone was subjected to spectrographic analysis and no difference was detected in their chemical composition. Additional evidence arose from hydrogen anneals. After annealing at 800°C, but not at lower temperatures, the crystal was colourless [1]. A more detailed analysis by spark source mass spectrometry was undertaken by CAEC, Chalk River, on two crystals. Table 1 is a partial listing containing those elements found in atomic concentration above 20 ppm.
3.2. Basic spectral features The spectra recorded all contained three basic features: a discrete profile (responsible for the blue colour) and generally consisting of three or four partially overlapping bands; the fundamental absorption edge [1] in the ultraviolet region and a well-pronounced shoulder containing several bands sited thereon.
3.3. Dose dependence of electron radiation induced absorption Fig. 1(A) illustrates the dose dependence of the radiation-induced absorption in crystal TA(10), which had previously been annealed in oxygen at 1000°C (spectrum 1). (We have earlier described the effects of oxidising and reducing anneals on this and ten other arc-fused single crystals [1] and the same crystal labelling has been retained here.) Spectrum 2 was obtained from a fragment of TA(10), denoted TA(10,i),
Table 1 Analysis of two arc-fused thoria single crystals by spark source mass spectrometry [10] in atomic concentrations (ppm) Element
Crystal 1
Crystal 2
Element
Crystal 1
Crystal 2
Na Mg Al Si P S Cl Ca Ti Fe
20 120 63 71
20 105 390 314 65 77 43 220 50 51
Y Zr La Ce Pr Nd Sm Gd Dy Er
115 120 37 63
80 130 43 65 20 45 29
57 125 37 47
45 23 34 28
57 21
308
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Fig. 1. (A) Effect of electron irradiation dose on absorption spectrum of an oxidised single crystal of thoria. (1) Crystal after annealing in oxygen at 1000°C; (2) after electron irradiation dose of 8.58 ×1016 electrons cm − 2; (3) after electron irradiation dose of 3.43 × 1017 electron cm − 2. (B) Effect of different cooling procedures during electron irradiation of single crystal thoria. (4) Crystal just submerged in water and holder water-cooled; (5) crystal exposed in metal holder and with liquid nitrogen flowing through its base. Dosages essentially the same, 3.35 and 3.45×10 − 17 electron cm − 2.
Fig. 2. Effect of electron irradiation (3.35× 10 − 17 electron cm − 2) on thoria crystal TA(10,ii) after previous oxidising and reducing anneals at 1000°C. (A) (1) Annealed in oxygen; (2) followed by electron irradiated. (B) (3) Annealed in hydrogen; (4) followed by electron irradiation.
after a radiation dose of 8.58 ×1016 electrons cm − 2. This equates to a radiation intensity of 3.82 mamps cm − 2. As supplied, TA(10) contained grey and pale amber zones and after annealing in oxygen became amber, due largely to the shoulder at 32 500 cm − 1 (4.029 eV). After electron radiation it was a faint grey –blue colour due to the two low intensity bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV). The band at 22 900 cm − 1 (2.840 eV) had disappeared and a band at 24 500 cm − 1 (4.029 eV) had been generated. The broad shoulder at 32 500 cm − 1 (4.029 eV) had decreased in the region 26 000 – 38 000 cm − 1 (3.223– 4.711 eV).
TA(10,i) was further irradiated using an increased flow of liquid nitrogen coolant, but again the crystal fractured. Spectrum 3 was obtained from a portion of this crystal, now designated TA(10,ii), that had received a total radiation dose of 3.43× 1017 electrons cm − 2. This crystal portion was now deep blue due to the increased intensity of the now well-resolved bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV). At energies above 20 000 cm − 1 (2.480 eV) the absorption in spectrum 3 has been uniformly increased relative to spectrum 2. Thus, from 10 000 to 28 000 cm − 1 (1.240– 3.471 eV) the absorption is greater that that for the annealed state of the crystal, and from 27 000 to 35 000 cm − 1 (3.347–4.340 eV) it is slightly decreased, and slightly increased above 35 000 cm − 1 (4.340 eV).
3.4. Effect of crystal cooling on electron radiation-induced absorption Following the electron radiation of crystal TA(10,ii), using the type A crystal holder, the radiation-induced defects were annealed out by re-annealing in oxygen at 1000°C, and regenerating a colourless crystal section. The crystal was then irradiated again with electrons, but under water, using the type B holder, and with a radiation dose of 3.35× 1017 electrons cm − 2 and a radiation intensity of 2.48 mamp cm − 2. Fig. 1(B) compares the spectrum of the original irradiated crystal with that of the re-irradiated crystal. The latter crystal now had a faint blue colour and exhibited bands at 14 630 and 16 460 cm − 1 (1.814 and 2.028 eV). The intensities of these bands were considerably lower than for the original irradiation treatment, and the intensity of the absorption profile was uniformly decreased over the whole energy range. Since the radiation doses were almost the same, changing the holders and cooling characteristics of the crystal decreased the concentration of defects formed because the absorption coefficient of the bands in the region 11 000–20 000 cm − 1 (1.364–2.480 eV) is reduced. This is most likely related to the higher temperature on the upper surface of the crystal attained when using holder A. However, the radiation intensity had also been decreased from 3.82 to 2.48 mamp cm − 2 and this is also likely to be a contributing factor. A minor component may be due to the slow decay of the spectrum with time that would only arise if there was a significant time difference between electron irradiation and spectral measurement.
3.5. Effect of electron radiation after 6arious anneal treatments Irradiations were performed on the thoria crystal segment TA(10,ii) using the type B water-cooled holder and a radiation intensity of 2.48 mamp cm − 2. Fig. 2(A)
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
shows the effect on the crystal of a radiation dose of 3.35× 1017 electrons cm − 2 after previously annealing in oxygen at 1000°C. After irradiation, the band at 22 900 cm − 1 (2.840 eV) had been removed and the intensity of the broad shoulder at 32 500 cm − 1 (4.029 eV) was decreased in the region 26 000 – 38 000 cm − 1 (3.223– 4.711 eV), while bands at 24 500, 16 360 and 14 630 cm − 1 (3.037, 2.028 and 1.814 eV) were generated. The decrease in the broad shoulder may well arise from a decrease in the intensities of the bands at 27 290, 32 070, 36 490 and 38 930 cm − 1 (2.840, 3.383, 3.976 and 4.826 eV, respectively). The crystal was then annealed in hydrogen at 1000°C and given the same radiation dose as when it was oxidised (Fig. 2(B)). The absorption now showed a slight increase from 12 000 to 28 000 cm − 1 (1.488– 3.471 eV), with the bands at 14 630, 16 360 and 24 500 cm − 1 (1.814, 2.028 and 3.037 eV) regenerated. At energies above 28 000 cm − 1 (3.471 eV) the absorption was slightly decreased. An increase in the intensity of the
Fig. 3. Comparison of the spectra generated upon electron irradiation in un-annealed and annealed thoria single crystal. (A) (1) oxidised; (2) reduced and (B) (3) un-annealed.
309
band at 27 290 cm − 1 (2.840 eV) and a decrease in the intensity of the bands at 32 070, 36 490 and 38 930 cm − 1 (3.383, 3.976 and 4.826 eV) may account for or contribute to the observed changes. Comparing the spectra of the irradiated crystal after oxidising and reducing annealing pre-treatments (Fig. 3(A)) reveals that the intensities of the absorption profiles from 14 500 to 16 400 cm − 1 (1.798–2.037 eV) are almost the same. However, the shape of the bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV) are more pronounced in the pre-oxidised crystal because the absorption below 14 000 cm − 1 (1.129 eV) and above 17 000 cm − 1 (2.108 eV) is slightly less than that in the pre-reduced crystal, making the intensities of the two bands appear slightly greater than when this crystal had been pre-oxidised. In the region 17 000–33 000 cm − 1 (2.108–4.091 eV) the absorption of the irradiated pre-reduced crystal is slightly greater, whereas above 33 000–38 000 cm − 1 (2.727–4.711 eV) the difference between the two absorption profiles is much less than the difference between the profiles of the annealed crystals prior to irradiation. Fig. 3(B) shows the effect of electron radiation on an un-annealed crystal, TA11. This crystal was given a total radiation dose of 3.41× 1017 electrons cm − 2 in two stages, initially at a radiation intensity of 11.14 and then of 3.82 mamp cm − 2. After irradiation the crystal was an intense blue, but only in a localised area, the remainder of the crystal being pale grey. Fig. 3(B) shows the spectrum of the blue area. The expected bands at 14 640 and 16 360 cm − 1 (1.814 and 2.028 eV) were present but unresolved, unlike the spectra of the annealed crystals subjected to electron or neutron [2] radiation. The shoulder at 18 000 cm − 1 (2.232 eV) is doubtless due to the expected band at 18 050 cm − 1 (2.238 eV). Also prominent is the band at 24 500 cm − 1 (3.037 eV), which also occurs in irradiated annealed crystals. The distinct band at 12 000 cm − 1 (1.488 eV) would appear to be the same as that found in the neutron-irradiated crystal, TA5 [2]. Unfortunately the un-annealed crystal later shattered into too small pieces for spectral measurements and so no further studies could be undertaken.
3.6. Isochronal annealing of radiation-induced absorption
Fig. 4. Effect of isochronal anneals on the spectrum of the defects produced by electron irradiation. (A) 1 h oxygen anneals of electron irradiated thoria crystal TA(10,ii) previously annealed in oxygen at 1000°C; (1) after irradiation and before anneal; (2) anneal at 100°C; (3) anneal at 200°C. (B) 1 h hydrogen anneals of electron irradiated thoria crystal TA(10,ii) previously annealed in hydrogen at 1000°C; (4) after irradiation and before anneal; (5) anneal at 200°C; (6) anneal at 400°C; (7) anneal at 600°C.
Isochronal annealing of the defects that give rise to the bands in the 12 000–22 000 cm − 1 region (1.488– 2.727 eV) was performed on crystal TA(10,ii). Each anneal duration was 1 h, and the annealing atmosphere the same as that of the pre-irradiation anneal. Fig. 4(A) shows the effect of annealing in oxygen. Previously, the crystal had received a dose of 3.43× 1017 electrons cm − 2 in the type A crystal holder. The anneal at 100°C produced a uniform decrease in the
310
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Fig. 5. Analysis of profile generated by electron irradiation of thoria single crystal into Gaussian bands. Open circles, data points. Line through circles comprises sum of resolved Gaussian bands and indicates quality of fit. (A) Resolution into three bands. (B) Resolution into four bands.
profile intensity, and the absorption was completely bleached after the anneal at 200°C. The effect of hydrogen anneals on the same crystal, but which had previously received an electron radiation dose of 3.35× 1017 electrons cm − 2 in the type B holder, is shown in Fig. 4(B). Now the 200°C anneal only decreases the intensity of the bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV) and they are only completely bleached by the anneal at 400°C. These differences cannot be attributed to the slight differences in electron radiation dose.
3.7. Gaussian analysis of radiation-induced absorption A suitable residual profile was obtained by subtracting the spectrum of the anneal at 200°C in oxygen from that at 100°C. Initially we attempted to describe the absorption profiles by the same three bands that fitted the neutron-irradiated crystal [1] (Fig. 5(A) 1). Several inconsistencies were noted when these bands were compared with the three bands resolved for the neutron-irradiated crystal [1]. Specifically, the width of the central band was doubled and that of the highest energy band reduced by half, as well as its position being shifted from 18 050 to 19 530 cm − 1 (2.238 – 2.421 eV); the resolved peak heights were also markedly different. Only the width and position of the lowest energy band remained reasonably consistent. These differences be-
tween the parameters of the three bands for the electron and neutron-irradiated crystals suggest that in the former the central band has increased in width, or additional hidden bands are present. Therefore, a second analysis was performed to ascertain if a good fit could still be obtained with an additional, fourth, band (Fig. 5(B)). A good fit was still obtained between the recombined bands and the original profile, but a comparison of band widths and positions with those of the three band analysis (Fig. 5(A)) shows that the intense central band had shifted from 16 630 to 16 900 cm − 1 (2.062–2.095 eV), and its width decreased by approximately a quarter. The additional fourth band, at 16 060 cm − 1 (1.991 eV), was relatively narrow and there was no real consistency between the parameters of this band and that at 16 360 cm − 1 (2.028 eV), characteristic of the neutron-irradiated crystal. It is therefore clear that the bands generated by electron-irradiation cannot be adequately described by the three bands that were resolved for the neutron-irradiated crystal. Additional bands may have been generated but this cannot be confirmed from the experimental data. The best evidence for additional bands may come from the additional band apparent in the spectrum of the electron-irradiated un-annealed crystal (Fig. 3(B)) and from further work on fresh samples.
4. Discussion
4.1. Inter6alence bands and impurity ions We have briefly reviewed above earlier reports on coloured thoria crystals. Pure thoria cannot give rise to intervalence bands but impurity ions capable of existing in two or more oxidation states, and having their relative concentrations of these states altered by oxidising or reducing anneals, can in principle bring about intervalence spectra. The most likely impurity candidate is lead. Oxidising anneals at 1000°C may oxidise some Pb2 + ions to the + 3 or + 4 oxidation state. An intervalence absorption band would then arise from an electron transfer from a Pb2 + ion to a lead ion of higher oxidation state, provided the ions are sufficiently close. The red colour characteristic of some oxidised thoria crystals is also characteristic of lead dioxide and the mixed oxide Pb3O4. In the latter, Pb2 + ions occupy pyramidal sites and Pb4 + ions octahedral sites. However, its colour has been suggested [9] as a combination of the yellow PbO and red PbO2, and that the transition is not sufficiently intense to be due to intervalence absorption. Until the concentration and distribution of lead ions in the crystal has been established this sugges-
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
tion cannot be taken as dispelling evidence for intervalence electron transfer. The broadening of the absorption profile as the cut-off shifts to lower energy with temperature increase may arise from an increased distortion in the oxide sub-lattice. This may be produced by the formation of aggregate centres involving the migration of impurity ions. Evidence for this comes from the knowledge that dislocations were formed in the crystal after the anneal at 870°C. The anneal in hydrogen at 540°C would reduce a significant number of the Pb3 + or Pb4 + ions to the + 2 oxidation state. Since the absorption cut-off occurs at 25 660 cm − 1 (3.181 eV) and not 33 160 cm − 1 (4.111 eV) after oxygen anneals, this may be a consequence of the change in the site symmetries of the impurity lead ions. The anneal in hydrogen at the higher temperature of 820°C produces a brownish-black colour that could be attributed to further reduction of Pb2 + to metallic lead. This colour could now arise from intervalence absorption between the different oxidation states of lead ions having almost equivalent site symmetries. In the mixed oxide Pb12O19 the lead ions reside in nearly equivalent cubic sites, and the colour of the crystal is a deep brownish-black [9].
4.2. The absorption cut-off and electron transfer The rising absorption seen in Figs. 1 – 3 around 40 000 cm − 1 (5.0 eV) is the low energy, low intensity side of the absorption cut-off. However, it may be the low energy side of the fundamental absorption edge but more likely composed of the combined bands at 39 600 and 41 860 cm − 1 (4.91 and 5.19 eV), observed by Childs [10] in very pure arc-fused crystals. These bands had been enhanced by doping with CaO and Y2O3. Since charge-compensating anion vacancies are formed with these ions the bands may have been due to excitons localised at the vacancies. Such transitions would involve the transfer of an electron from an oxide ion to the vacancy on an impurity cation adjacent to the vacancy. This type of transition occurs in alkali halides, and is situated on the low energy side of the fundamental edge [11]. The absorption cut-off at 47 600 cm − 1 (5.9 eV) in the relatively purer thoria crystals [10] thus may be the fundamental absorption edge. Supporting evidence that the edge is close to this value comes from the thermoluminescence studies of Rodine and Land [12]. They found that the maximum excitation efficiency of the thermoluminescence occurred at 47 600 cm − 1 (5.9 eV), and concluded that the excitation process was dominated by electron transitions across the band gap. However, since the magnitude of the absorption coefficient values associated with the fundamental edge are of the order of 106, the edge can only conclusively be recorded in the spectra of ultra-thin films of thoria crystal, to which we do not have access.
311
4.3. Defects produced by electron radiation With neutron and electron radiation there is the distinct possibility of producing defects that involve the displacement ions via collision processes. We have discussed the defects produced by 2 MeV neutrons [1], the mean energy of reactor neutrons [13]. With 3 MeV electrons fewer defects are obtained. This is because neutrons, having no charge, transfer energy to the crystal via hard sphere elastic collisions with the lattice ions. Since electrons are charged particles they displace lattice ions by Coulomb interaction with the nuclei, and the maximum energy (Emax) that can be transferred by a relativistic electron is given by Emax = 2Ee(Ee + 2me c 2)/M c 2, where Ee is the energy of the incident electron, me the electron mass, M the mass of the struck atom and c the velocity of light. The mean energy transferred in a single collision is given by Ep = {Ed Emax/(Emax − Ed)} ln (Emax/Ed), where Ed is the threshold displacement energy required to displace an ion into an interstitial position. In many cases Emax is no more than a few times Ed and hence the cascade process that occurs with neutron bombardment does not readily take place with electron bombardment. The total number of displaced ions produced by electron bombardment is given by NT = ud f NA Nd, where NT is the total number of displaced ions, ud is the electron scattering cross-section, f the integrated electron dose and NA the number of lattice ions per unit volume. Minimisation of the errors that arise from the energy degradation and multiple scattering of electrons as they traverse the sample is achieved by using as thin crystals as possible [14]; the problems we experienced with crystal buckling may be associated with these features. With 3 MeV electrons the mean energy transferred to the primary displaced ion was 80.2 eV (Emax = 110.7 eV) for thorium and 205 eV (Emax = 1605 eV) for oxygen. Hence, 3 MeV electrons are not sufficiently energetic to displace many cations. For oxygen ions the value of Nd is approximately 2, and assuming Ee = me c 2 the Mackinley and Feshback formulation [15] gives a scattering cross-section ud of approximately 9 barns. These values give a final concentration of 8.4 cation vacancies and 1016 anion vacancies cm − 3 for a dose of 1017 electrons cm − 2. The actual concentration of defects will probably be significantly lower than these theoretical values since the Kinchin and Pease model considerably over-estimates the radiation damage for alkaline earth oxides [16]. Even so, these calculations rightly predict that the damage will greatest for neutron irradiation. We have observed a significant shift in the position of the absorption edge with neutron irradiation [1], but no observable effect was seen here with a numerically equivalent electron dose.
312
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
The process of defect formation is complex, and we have noted above (Section 3.7) that Gaussian analyses of the electron radiation-induced absorption profiles showed that additional bands were involved, compared with the spectra from neutron irradiated samples. Further, these bands were increased by annealing in hydrogen the electron irradiated crystals. Thus, the nature of these defects is different from those generated by neutrons and may involve ion vacancy clusters. The anomalous lack of absorption increase in the similar post electron-irradiation anneals in oxygen add to the complexity. Whether or not the defects involve trapped holes or electrons all appear to be shallow traps, since the complete absorption profile exhibits thermal decay at room temperature, and it can also be bleached by light of energy B17 180 cm − 1 (2.13 eV). The decay process may involve the release of the trapped holes or electrons from the defects, with the subsequent re-trapping at deeper traps. The removal of the defects at anneal temperatures above 800°C will be via the recombination of the displaced ions and vacancies. At these elevated temperatures either one or both of the entities may be mobile. The spectral profile of the defects may be described as unfortunate since it is equally well fitted by three or four bands. Although it is a general principle that spectra should be resolved into the minimum number of overlapping bands, comparisons with a similar profile obtained by neutron irradiation present problems (Section 3.7). Band assignments are therefore not possible for the present crystals, even though they are by far the purest currently available. It will therefore be necessary to wait until purer and larger ones have been prepared at some future date.
4.4. Low energy band at 12 020 cm − 1 (1.49 eV) Fig. 3(B) shows a band so far not discussed, at 12 020 cm − 1 (1.49 eV), after electron irradiation of an un-annealed crystal. Childs and et al. have reported it [8] in their proton irradiated thoria. It was observed in some of the neutron irradiated crystals [1], and when seen was not as intense or sharp as in Fig. 3. It can also be seen as a weak profile in some of the spectra after electron irradiation in Figs. 1 – 3. Although Childs et al. [8] found that the defects annealed out at temperatures around 1000°C, they did not report any increase in absorption after annealing at lower temperatures, as observed here. We propose that this band arises from a defect involving an impurity ion.
transfer can occur at the fundamental absorption edge, and the results here support the proposal that the edge occurs close to 47 600 cm − 1 (5.9 eV). The high melting point of thoria means that it is very difficult to obtain pure single crystals, and hence they generally contains impurity ions. When these ions can exist in more than one oxidation state then electron transfer can in principle occur, and provided the ions are close enough for this to occur, or have or can be made to aggregate, then the crystal is coloured and intervalence spectra can be recorded. The most likely impurity involved is lead. When thoria crystals are irradiated with 3 MeV electrons the energy of the electrons is transferred into the crystal. This energy is not sufficient to displace more than a few thorium ions into interstitial sites but readily displaces oxygen ions. The electron irradiated crystal turns blue, as it does when irradiated with neutrons and subjected to controlled annealing. The absorption spectrum thereby generated, in the region 12 000–17 260 cm − 1 (1.49– 2.14 eV), is similar but not identical to the profile generated by neutron irradiation. The spectra obtained here can be satisfactorily resolved into three Gaussian bands, but the profiles are equally well fitted by four Gaussian bands, thereby making assignments essentially impossible. The defects generated arise from aggregate centres. These defects are of shallow energy, since they can be annealed out at elevated temperatures. Their precise identity has still to be determined, and they may involve impurity ions. This study is therefore effectively complete. The effects of electron irradiation on single crystal thoria are here reported and analysed for the first time, and the crystals are now too fragmented for further work. Further definitive work awaits the availability of larger and purer thoria crystals.
Acknowledgements We thank Dr B.G. Childs, Canadian AEC, for donating the crystals and staff at the Cookridge Radiation Research Centre for assistance with the electron irradiation experiments. Valuable discussions with Drs H.V.St.A. Hubbard and V.A. Volkovich are acknowledged. J.D. thanks the UKAEA for financial support.
References 5. Conclusions Single crystals of thoria are not intrinsically capable of exhibiting intervalence bands, although electron
[1] T.R. Griffiths, J. Dixon, J. Chem. Soc., Faraday Trans. 88 (1992) 1149. [2] T.R. Griffiths, J. Dixon, J. Chem. Soc., Faraday Trans. 88 (1992) 3475. [3] J.M. Bodine, F.B. Theiss, Phys. Rev. 98 (1955) 1532.
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313 [4] [5] [6] [7]
O.A. Weinreich, W.E. Danforth, Phys. Rev. 88 (1952) 953. R.C. Linares, J. Phys. Chem. Solids 28 (1967) 1285. V.I. Neeley, J.B. Gruber, W.J. Gray, Phys. Rev. 158 (1967) 809. J.L. Bates, Techn. Rep. BNWL-457, Contract At (45-1)-1830 (1967) 16. [8] B.G. Childs, P.J. Harvey, J.B. Hallett, J. Am. Chem. Soc. 53 (1970) 431. [9] M.B. Robin, P. Day, Adv. Inorg. Radiochem. 10 (1967) 247.
.
313
[10] B.G. Childs, private communication. [11] W.B. Fowler, in: W.B. Fowler (Ed.), Physics of Color Centers, Academic Press, New York, 1968, p. 182. [12] E.T. Rodine, P.L. Land, Phys. Rev. 130 (1963) 2701. [13] R.S. Wilks, A.E.R.E. Rep. No. R5543 (1967). [14] J.W. Corbett, Solid State Suppl. 7 (1966). [15] W.A. Mackinley, H. Feshback, Phys. Rev. 74 (1958) 1759. [16] B. Henderson, J.E. Wertz, Adv. Phys. 17 (1968) 749.
Electron irradiation of single crystal thorium dioxide and electron transfer reactions Trevor R. Griffiths *, James Dixon School of Chemistry, Uni6ersity of Leeds, Leeds LS2 9JT, UK Received 9 September 1999; accepted 11 November 1999
Abstract This first study of electron irradiation (3 MeV) of single crystals of thoria has revealed that the crystals turn blue, as seen with neutron irradiation, but the absorption profiles in the 12 000 – 17 260 cm − 1 (1.49 – 2.14 eV) region were similar but not identical. Calculations showed that the electrons transferred energy sufficient to displace lattice oxygen atoms, but not thorium atoms. The crystals were the purest currently available and supported the proposal that the fundamental absorption edge occurs close to 47 600 cm − 1 (5.9 eV). Theoretically, thoria is colourless and cannot exhibit intervalence bands, but aggregates of impurity ions having more than one oxidation state can produce such bands, and some crystals showed pale amber or grey zones. The most likely impurity involved was lead. The effect of electron irradiation on defect generation after various oxidising and reducing anneals, up to 1000°C, was studied, together with the loss of the blue radiation-induced absorption on isochronal anneals at temperatures up to 400°C. The generated profile could be resolved equally well into three or four Gaussian bands and hence band assignments are not possible. It is concluded that electrons transfer energy to thoria to create defects of shallow energy, and electron transfer involving intervalence bands can occur in the presence of impurity ions. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Single crystal; Electron irradiation; Defect formation; Intervalence spectra; Aggregate centres
1. Introduction The fluorite oxides CeO2, UO2, ThO2 and PuO2 have been of considerable interest to chemists for four decades. Ceria and urania can lose and gain, respectively, oxygen atoms, thereby becoming the non-stoichiometric materials CeO2 − x and UO2 + x. The presence of two cations having differing oxidation states, for example, Ce3 + and Ce4 + in hypo-stoichiometric ceria, now allows electron transfer in the form of intervalence spectra. This is readily observed as the initially white ceria powder, or a transparent single crystal, becomes first a pale then a deeper blue as oxygen is lost in a reducing atmosphere. Thoria, on the other hand, only exhibits oxidation state four and thus is not an apparent candidate for electron transfer reactions. However, thoria has one of the highest melting points known, * Corresponding author. Tel.: + 44-113-233 6408; fax: + 44-113233 6565. E-mail address: [email protected] (T.R. Griffiths)
above 3000°C, and thus, in the generation of single crystals, crystals having some impurity ions are generally the final product. Such impurity ions can give colour to thoria single crystals, and the colour changes upon heating in oxidising or reducing atmospheres. Single crystals of thoria can also be turned blue by subjecting them to high energy electron radiation. While the colour is brought about by transferring the energy of the electrons to the crystal, the colour this time is not due to intervalence bands. This paper is therefore concerned with the effects of electron radiation on thoria single crystals and the possibility of intervalence bands involving crystal impurities. Our recent literature survey showed that few papers have appeared in recent years on spectroscopic studies of single crystal thoria, apart from our own analysis of oxidising and reducing anneals and gamma and ultraviolet radiation on both flux-grown and arc-fused single crystals [1], and the identification of the presence of F° colour centres [2], the first time that the F° centre had
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 5 9 7 - 6
306
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
been identified in crystals having the fluorite lattice. This present paper analyses the results of a series of experiments undertaken over the past few years involving the electron irradiation of the above-mentioned arc-fused thoria single crystals, and the spectra of the blue crystals generated. Briefly, our literature survey revealed that Bodine and Theiss [3] have studied the effect of high temperature anneals, in air and under vacuum, on the optical absorption spectrum of single crystal thoria. Vacuum annealing at 1500°C coloured the crystal yellow but when the annealing temperature was lowered to 1000°C, the crystal returned to colourless; high temperature annealing in air turned the crystal red. Weinreich and Danforth [4] had earlier subjected their crystals to a cycle of high temperature (up to 1800°C) anneals in vacuum and air and also reported yellow and red colorations. They also noted that the red colour could be bleached at the much lower temperature of 200°C upon annealing in vacuum, provided the crystals had previously been subjected to ultraviolet radiation. They further reported that rapid quenching, following heat treatments, produced a deep black coloration; mixed valence compounds are often deep blue to black in colour. Both groups of workers stated that their crystals were 99.9% pure. Linares [5] has examined the effect of various flux conditions on crystal growth and of various dopant ions on the colour of thoria. The addition of 0.1% Ca2 + ions to the flux (a NaFB2O3 melt) yielded crystals with a pale permanent orange colour. The addition of Y3 + , Nb5 + and F− ions produced no observable effect, but when a lead-containing flux was employed the resulting crystals contained around 100 ppm lead impurity. Ultraviolet radiation generated a greenish colour that could be bleached by annealing at 300°C or by storing in the dark for several days. Neeley et al. [6] irradiated colourless thoria crystals (grown from lead-based fluxes) with 60Co gamma rays and with 2 MeV electrons. Both treatments turned the crystals dark yellow, but when their spectra were recorded the process initiated bleaching that was completed in a few minutes. Bates [7] annealed his thoria single crystals and observed that after a high temperature anneal in hydrogen the crystal became blue, with two bands at 24 800 and 14 100 cm − 1 (3.076 and 1.746 eV). A high temperature oxidising anneal in air reversed the process. Childs et al. [8] have performed a detailed investigation of annealing and radiation treatments on arc-fused single crystals of thoria. Their colourless crystals had an average 1445 ppm total metal impurity and 120 ppm non-metal impurities and were probably the purest crystals then available. Crystals that had received a reducing anneal at 1400°C, when irradiated with 60Co, subsequently contained four bands at 12 020, 14 250,
16 370 and 18 710 cm − 1 (1.49, 1.80, 2.03 and 2.32 eV). Their intensity reached saturation after approximately 1 h in a flux of 7×105 rad h − 1 and in a few crystals the highest energy band was relatively large but decayed soon after radiation was terminated. Our earlier measurements [1] similarly showed that 60 Co gamma radiation affected flux-grown and arcfused crystals, the effects decaying with time. We now report the effects of electron radiation, from a 3 MeV Van der Graaf generator, on a number of the arc-fused crystals used previously [1], and consider possible electron transfer processes initiated by electron radiation.
2. Experimental
2.1. Crystals and crystal annealing Thorium dioxide single crystals were kindly supplied by the Canadian Atomic Energy Commission, Chalk River, Ont., Canada. They had been grown by the arc-fusion technique, had a face area of approximately 0.3 cm2, and had already been cut and polished. The crystals were annealed in a silica tube, sealed at one end and mounted vertically in a furnace. A crystal was placed at the bottom of the tube, which was then located in the hot zone of the furnace. The open end, outside the furnace, contained a silicone rubber stopper that admitted a thermocouple and a silica capillary tube for delivering dried air, oxygen or hydrogen above the crystal surface. The gases exited through a side-arm near the stopper and through which back-diffusion was prevented. The maximum annealing temperature used was 1000°C, and the optimum annealing time at this temperature was determined as 2 h, with longer times at lower temperatures. At the end of each anneal the furnace was rapidly lowered to quench the crystal without disturbing it.
2.2. Irradiation by electrons Electron radiation was performed using electrons of energy 3 MeV from a Van der Graaf-type electron accelerator (High Voltage Engineering Corporation, Model KF3000) at the Cookridge Radiation Research Centre attached to the University of Leeds. Two types of crystal holders were employed. Type A consisted of a hollow copper cylinder, cooled by the passage of liquid nitrogen, and with the crystal fixed to the cylinder base with Apiezon vacuum grease. Type B was an aluminium cylindrical block that held a circular reservoir of water of diameter 10 cm and 2 mm deep. The crystal was again fixed with grease to the bottom of the reservoir and its upper surface covered by a water layer 1 mm deep. The aluminium block was cooled by water circulating through its base.
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Using the type A crystal holder and electron beam current intensities of 11.14 and 8.82 mamps cm − 2 caused the crystals to crack. This was surprising and implied a large temperature differential between the upper and lower surfaces of the crystal slice, due to the intensity of the radiation and the inability of the liquid nitrogen-cooled crystal holder to dissipate heat effectively. Some irradiated crystals from holder A exhibited significant upwards curvature before fracturing on handling, again demonstrating progressively reduced contact with the cold metal surface and a large thermal gradient from the upper to the lower surface of the crystals, due to the electron bombardment. It was however possible to study the dose dependence of the absorption properties by recording the spectra of the crystal submerged beneath a thin water film, (type B holder), and a reduced electron beam intensity of 2.48 mamps cm − 2 allowed the crystal to remain intact. The temperature of the crystal probably did not now exceed 100°C during electron radiation.
2.3. Spectroscopic measurements and relationships Absorption spectra were recorded in the energy range 50 000–5000 cm − 1, with air used as reference. A Unicam SP 700 spectrophotometer was utilised in order to minimise the optical bleaching of crystals; spectrophotometers that employ a component sequence that irradiates the sample with all the source lamp energy before dispersing the light in the monochromator are not suitable. Other details have been published elsewhere [1]. The following relationships apply to absorption spectra. The recorded absorbance, A, is given by log (Io/I), where Io is the intensity of the incident light, and I the transmitted intensity. For a crystal the absorption coefficient, a =A/d, where d is the crystal thickness. To avoid confusion with commonly presented spectra, A versus wavelength, the spectra displayed here employ a and kK energy units, where 1 kK=1000 cm − 1. In the text their eV equivalents are also given (1 eV = 8066 cm − 1).
307
3. Results
3.1. Crystal quality The arc-fused thoria crystals studied were first examined with a polarising microscope and found to be perfectly isotropic. Subsequent examination showed that they were free from dislocations throughout the various anneals. The crystals had all originated from the same batch, while some were colourless the others contained grey and pale amber zones. A sample from each zone was subjected to spectrographic analysis and no difference was detected in their chemical composition. Additional evidence arose from hydrogen anneals. After annealing at 800°C, but not at lower temperatures, the crystal was colourless [1]. A more detailed analysis by spark source mass spectrometry was undertaken by CAEC, Chalk River, on two crystals. Table 1 is a partial listing containing those elements found in atomic concentration above 20 ppm.
3.2. Basic spectral features The spectra recorded all contained three basic features: a discrete profile (responsible for the blue colour) and generally consisting of three or four partially overlapping bands; the fundamental absorption edge [1] in the ultraviolet region and a well-pronounced shoulder containing several bands sited thereon.
3.3. Dose dependence of electron radiation induced absorption Fig. 1(A) illustrates the dose dependence of the radiation-induced absorption in crystal TA(10), which had previously been annealed in oxygen at 1000°C (spectrum 1). (We have earlier described the effects of oxidising and reducing anneals on this and ten other arc-fused single crystals [1] and the same crystal labelling has been retained here.) Spectrum 2 was obtained from a fragment of TA(10), denoted TA(10,i),
Table 1 Analysis of two arc-fused thoria single crystals by spark source mass spectrometry [10] in atomic concentrations (ppm) Element
Crystal 1
Crystal 2
Element
Crystal 1
Crystal 2
Na Mg Al Si P S Cl Ca Ti Fe
20 120 63 71
20 105 390 314 65 77 43 220 50 51
Y Zr La Ce Pr Nd Sm Gd Dy Er
115 120 37 63
80 130 43 65 20 45 29
57 125 37 47
45 23 34 28
57 21
308
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Fig. 1. (A) Effect of electron irradiation dose on absorption spectrum of an oxidised single crystal of thoria. (1) Crystal after annealing in oxygen at 1000°C; (2) after electron irradiation dose of 8.58 ×1016 electrons cm − 2; (3) after electron irradiation dose of 3.43 × 1017 electron cm − 2. (B) Effect of different cooling procedures during electron irradiation of single crystal thoria. (4) Crystal just submerged in water and holder water-cooled; (5) crystal exposed in metal holder and with liquid nitrogen flowing through its base. Dosages essentially the same, 3.35 and 3.45×10 − 17 electron cm − 2.
Fig. 2. Effect of electron irradiation (3.35× 10 − 17 electron cm − 2) on thoria crystal TA(10,ii) after previous oxidising and reducing anneals at 1000°C. (A) (1) Annealed in oxygen; (2) followed by electron irradiated. (B) (3) Annealed in hydrogen; (4) followed by electron irradiation.
after a radiation dose of 8.58 ×1016 electrons cm − 2. This equates to a radiation intensity of 3.82 mamps cm − 2. As supplied, TA(10) contained grey and pale amber zones and after annealing in oxygen became amber, due largely to the shoulder at 32 500 cm − 1 (4.029 eV). After electron radiation it was a faint grey –blue colour due to the two low intensity bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV). The band at 22 900 cm − 1 (2.840 eV) had disappeared and a band at 24 500 cm − 1 (4.029 eV) had been generated. The broad shoulder at 32 500 cm − 1 (4.029 eV) had decreased in the region 26 000 – 38 000 cm − 1 (3.223– 4.711 eV).
TA(10,i) was further irradiated using an increased flow of liquid nitrogen coolant, but again the crystal fractured. Spectrum 3 was obtained from a portion of this crystal, now designated TA(10,ii), that had received a total radiation dose of 3.43× 1017 electrons cm − 2. This crystal portion was now deep blue due to the increased intensity of the now well-resolved bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV). At energies above 20 000 cm − 1 (2.480 eV) the absorption in spectrum 3 has been uniformly increased relative to spectrum 2. Thus, from 10 000 to 28 000 cm − 1 (1.240– 3.471 eV) the absorption is greater that that for the annealed state of the crystal, and from 27 000 to 35 000 cm − 1 (3.347–4.340 eV) it is slightly decreased, and slightly increased above 35 000 cm − 1 (4.340 eV).
3.4. Effect of crystal cooling on electron radiation-induced absorption Following the electron radiation of crystal TA(10,ii), using the type A crystal holder, the radiation-induced defects were annealed out by re-annealing in oxygen at 1000°C, and regenerating a colourless crystal section. The crystal was then irradiated again with electrons, but under water, using the type B holder, and with a radiation dose of 3.35× 1017 electrons cm − 2 and a radiation intensity of 2.48 mamp cm − 2. Fig. 1(B) compares the spectrum of the original irradiated crystal with that of the re-irradiated crystal. The latter crystal now had a faint blue colour and exhibited bands at 14 630 and 16 460 cm − 1 (1.814 and 2.028 eV). The intensities of these bands were considerably lower than for the original irradiation treatment, and the intensity of the absorption profile was uniformly decreased over the whole energy range. Since the radiation doses were almost the same, changing the holders and cooling characteristics of the crystal decreased the concentration of defects formed because the absorption coefficient of the bands in the region 11 000–20 000 cm − 1 (1.364–2.480 eV) is reduced. This is most likely related to the higher temperature on the upper surface of the crystal attained when using holder A. However, the radiation intensity had also been decreased from 3.82 to 2.48 mamp cm − 2 and this is also likely to be a contributing factor. A minor component may be due to the slow decay of the spectrum with time that would only arise if there was a significant time difference between electron irradiation and spectral measurement.
3.5. Effect of electron radiation after 6arious anneal treatments Irradiations were performed on the thoria crystal segment TA(10,ii) using the type B water-cooled holder and a radiation intensity of 2.48 mamp cm − 2. Fig. 2(A)
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
shows the effect on the crystal of a radiation dose of 3.35× 1017 electrons cm − 2 after previously annealing in oxygen at 1000°C. After irradiation, the band at 22 900 cm − 1 (2.840 eV) had been removed and the intensity of the broad shoulder at 32 500 cm − 1 (4.029 eV) was decreased in the region 26 000 – 38 000 cm − 1 (3.223– 4.711 eV), while bands at 24 500, 16 360 and 14 630 cm − 1 (3.037, 2.028 and 1.814 eV) were generated. The decrease in the broad shoulder may well arise from a decrease in the intensities of the bands at 27 290, 32 070, 36 490 and 38 930 cm − 1 (2.840, 3.383, 3.976 and 4.826 eV, respectively). The crystal was then annealed in hydrogen at 1000°C and given the same radiation dose as when it was oxidised (Fig. 2(B)). The absorption now showed a slight increase from 12 000 to 28 000 cm − 1 (1.488– 3.471 eV), with the bands at 14 630, 16 360 and 24 500 cm − 1 (1.814, 2.028 and 3.037 eV) regenerated. At energies above 28 000 cm − 1 (3.471 eV) the absorption was slightly decreased. An increase in the intensity of the
Fig. 3. Comparison of the spectra generated upon electron irradiation in un-annealed and annealed thoria single crystal. (A) (1) oxidised; (2) reduced and (B) (3) un-annealed.
309
band at 27 290 cm − 1 (2.840 eV) and a decrease in the intensity of the bands at 32 070, 36 490 and 38 930 cm − 1 (3.383, 3.976 and 4.826 eV) may account for or contribute to the observed changes. Comparing the spectra of the irradiated crystal after oxidising and reducing annealing pre-treatments (Fig. 3(A)) reveals that the intensities of the absorption profiles from 14 500 to 16 400 cm − 1 (1.798–2.037 eV) are almost the same. However, the shape of the bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV) are more pronounced in the pre-oxidised crystal because the absorption below 14 000 cm − 1 (1.129 eV) and above 17 000 cm − 1 (2.108 eV) is slightly less than that in the pre-reduced crystal, making the intensities of the two bands appear slightly greater than when this crystal had been pre-oxidised. In the region 17 000–33 000 cm − 1 (2.108–4.091 eV) the absorption of the irradiated pre-reduced crystal is slightly greater, whereas above 33 000–38 000 cm − 1 (2.727–4.711 eV) the difference between the two absorption profiles is much less than the difference between the profiles of the annealed crystals prior to irradiation. Fig. 3(B) shows the effect of electron radiation on an un-annealed crystal, TA11. This crystal was given a total radiation dose of 3.41× 1017 electrons cm − 2 in two stages, initially at a radiation intensity of 11.14 and then of 3.82 mamp cm − 2. After irradiation the crystal was an intense blue, but only in a localised area, the remainder of the crystal being pale grey. Fig. 3(B) shows the spectrum of the blue area. The expected bands at 14 640 and 16 360 cm − 1 (1.814 and 2.028 eV) were present but unresolved, unlike the spectra of the annealed crystals subjected to electron or neutron [2] radiation. The shoulder at 18 000 cm − 1 (2.232 eV) is doubtless due to the expected band at 18 050 cm − 1 (2.238 eV). Also prominent is the band at 24 500 cm − 1 (3.037 eV), which also occurs in irradiated annealed crystals. The distinct band at 12 000 cm − 1 (1.488 eV) would appear to be the same as that found in the neutron-irradiated crystal, TA5 [2]. Unfortunately the un-annealed crystal later shattered into too small pieces for spectral measurements and so no further studies could be undertaken.
3.6. Isochronal annealing of radiation-induced absorption
Fig. 4. Effect of isochronal anneals on the spectrum of the defects produced by electron irradiation. (A) 1 h oxygen anneals of electron irradiated thoria crystal TA(10,ii) previously annealed in oxygen at 1000°C; (1) after irradiation and before anneal; (2) anneal at 100°C; (3) anneal at 200°C. (B) 1 h hydrogen anneals of electron irradiated thoria crystal TA(10,ii) previously annealed in hydrogen at 1000°C; (4) after irradiation and before anneal; (5) anneal at 200°C; (6) anneal at 400°C; (7) anneal at 600°C.
Isochronal annealing of the defects that give rise to the bands in the 12 000–22 000 cm − 1 region (1.488– 2.727 eV) was performed on crystal TA(10,ii). Each anneal duration was 1 h, and the annealing atmosphere the same as that of the pre-irradiation anneal. Fig. 4(A) shows the effect of annealing in oxygen. Previously, the crystal had received a dose of 3.43× 1017 electrons cm − 2 in the type A crystal holder. The anneal at 100°C produced a uniform decrease in the
310
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
Fig. 5. Analysis of profile generated by electron irradiation of thoria single crystal into Gaussian bands. Open circles, data points. Line through circles comprises sum of resolved Gaussian bands and indicates quality of fit. (A) Resolution into three bands. (B) Resolution into four bands.
profile intensity, and the absorption was completely bleached after the anneal at 200°C. The effect of hydrogen anneals on the same crystal, but which had previously received an electron radiation dose of 3.35× 1017 electrons cm − 2 in the type B holder, is shown in Fig. 4(B). Now the 200°C anneal only decreases the intensity of the bands at 14 630 and 16 360 cm − 1 (1.814 and 2.028 eV) and they are only completely bleached by the anneal at 400°C. These differences cannot be attributed to the slight differences in electron radiation dose.
3.7. Gaussian analysis of radiation-induced absorption A suitable residual profile was obtained by subtracting the spectrum of the anneal at 200°C in oxygen from that at 100°C. Initially we attempted to describe the absorption profiles by the same three bands that fitted the neutron-irradiated crystal [1] (Fig. 5(A) 1). Several inconsistencies were noted when these bands were compared with the three bands resolved for the neutron-irradiated crystal [1]. Specifically, the width of the central band was doubled and that of the highest energy band reduced by half, as well as its position being shifted from 18 050 to 19 530 cm − 1 (2.238 – 2.421 eV); the resolved peak heights were also markedly different. Only the width and position of the lowest energy band remained reasonably consistent. These differences be-
tween the parameters of the three bands for the electron and neutron-irradiated crystals suggest that in the former the central band has increased in width, or additional hidden bands are present. Therefore, a second analysis was performed to ascertain if a good fit could still be obtained with an additional, fourth, band (Fig. 5(B)). A good fit was still obtained between the recombined bands and the original profile, but a comparison of band widths and positions with those of the three band analysis (Fig. 5(A)) shows that the intense central band had shifted from 16 630 to 16 900 cm − 1 (2.062–2.095 eV), and its width decreased by approximately a quarter. The additional fourth band, at 16 060 cm − 1 (1.991 eV), was relatively narrow and there was no real consistency between the parameters of this band and that at 16 360 cm − 1 (2.028 eV), characteristic of the neutron-irradiated crystal. It is therefore clear that the bands generated by electron-irradiation cannot be adequately described by the three bands that were resolved for the neutron-irradiated crystal. Additional bands may have been generated but this cannot be confirmed from the experimental data. The best evidence for additional bands may come from the additional band apparent in the spectrum of the electron-irradiated un-annealed crystal (Fig. 3(B)) and from further work on fresh samples.
4. Discussion
4.1. Inter6alence bands and impurity ions We have briefly reviewed above earlier reports on coloured thoria crystals. Pure thoria cannot give rise to intervalence bands but impurity ions capable of existing in two or more oxidation states, and having their relative concentrations of these states altered by oxidising or reducing anneals, can in principle bring about intervalence spectra. The most likely impurity candidate is lead. Oxidising anneals at 1000°C may oxidise some Pb2 + ions to the + 3 or + 4 oxidation state. An intervalence absorption band would then arise from an electron transfer from a Pb2 + ion to a lead ion of higher oxidation state, provided the ions are sufficiently close. The red colour characteristic of some oxidised thoria crystals is also characteristic of lead dioxide and the mixed oxide Pb3O4. In the latter, Pb2 + ions occupy pyramidal sites and Pb4 + ions octahedral sites. However, its colour has been suggested [9] as a combination of the yellow PbO and red PbO2, and that the transition is not sufficiently intense to be due to intervalence absorption. Until the concentration and distribution of lead ions in the crystal has been established this sugges-
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
tion cannot be taken as dispelling evidence for intervalence electron transfer. The broadening of the absorption profile as the cut-off shifts to lower energy with temperature increase may arise from an increased distortion in the oxide sub-lattice. This may be produced by the formation of aggregate centres involving the migration of impurity ions. Evidence for this comes from the knowledge that dislocations were formed in the crystal after the anneal at 870°C. The anneal in hydrogen at 540°C would reduce a significant number of the Pb3 + or Pb4 + ions to the + 2 oxidation state. Since the absorption cut-off occurs at 25 660 cm − 1 (3.181 eV) and not 33 160 cm − 1 (4.111 eV) after oxygen anneals, this may be a consequence of the change in the site symmetries of the impurity lead ions. The anneal in hydrogen at the higher temperature of 820°C produces a brownish-black colour that could be attributed to further reduction of Pb2 + to metallic lead. This colour could now arise from intervalence absorption between the different oxidation states of lead ions having almost equivalent site symmetries. In the mixed oxide Pb12O19 the lead ions reside in nearly equivalent cubic sites, and the colour of the crystal is a deep brownish-black [9].
4.2. The absorption cut-off and electron transfer The rising absorption seen in Figs. 1 – 3 around 40 000 cm − 1 (5.0 eV) is the low energy, low intensity side of the absorption cut-off. However, it may be the low energy side of the fundamental absorption edge but more likely composed of the combined bands at 39 600 and 41 860 cm − 1 (4.91 and 5.19 eV), observed by Childs [10] in very pure arc-fused crystals. These bands had been enhanced by doping with CaO and Y2O3. Since charge-compensating anion vacancies are formed with these ions the bands may have been due to excitons localised at the vacancies. Such transitions would involve the transfer of an electron from an oxide ion to the vacancy on an impurity cation adjacent to the vacancy. This type of transition occurs in alkali halides, and is situated on the low energy side of the fundamental edge [11]. The absorption cut-off at 47 600 cm − 1 (5.9 eV) in the relatively purer thoria crystals [10] thus may be the fundamental absorption edge. Supporting evidence that the edge is close to this value comes from the thermoluminescence studies of Rodine and Land [12]. They found that the maximum excitation efficiency of the thermoluminescence occurred at 47 600 cm − 1 (5.9 eV), and concluded that the excitation process was dominated by electron transitions across the band gap. However, since the magnitude of the absorption coefficient values associated with the fundamental edge are of the order of 106, the edge can only conclusively be recorded in the spectra of ultra-thin films of thoria crystal, to which we do not have access.
311
4.3. Defects produced by electron radiation With neutron and electron radiation there is the distinct possibility of producing defects that involve the displacement ions via collision processes. We have discussed the defects produced by 2 MeV neutrons [1], the mean energy of reactor neutrons [13]. With 3 MeV electrons fewer defects are obtained. This is because neutrons, having no charge, transfer energy to the crystal via hard sphere elastic collisions with the lattice ions. Since electrons are charged particles they displace lattice ions by Coulomb interaction with the nuclei, and the maximum energy (Emax) that can be transferred by a relativistic electron is given by Emax = 2Ee(Ee + 2me c 2)/M c 2, where Ee is the energy of the incident electron, me the electron mass, M the mass of the struck atom and c the velocity of light. The mean energy transferred in a single collision is given by Ep = {Ed Emax/(Emax − Ed)} ln (Emax/Ed), where Ed is the threshold displacement energy required to displace an ion into an interstitial position. In many cases Emax is no more than a few times Ed and hence the cascade process that occurs with neutron bombardment does not readily take place with electron bombardment. The total number of displaced ions produced by electron bombardment is given by NT = ud f NA Nd, where NT is the total number of displaced ions, ud is the electron scattering cross-section, f the integrated electron dose and NA the number of lattice ions per unit volume. Minimisation of the errors that arise from the energy degradation and multiple scattering of electrons as they traverse the sample is achieved by using as thin crystals as possible [14]; the problems we experienced with crystal buckling may be associated with these features. With 3 MeV electrons the mean energy transferred to the primary displaced ion was 80.2 eV (Emax = 110.7 eV) for thorium and 205 eV (Emax = 1605 eV) for oxygen. Hence, 3 MeV electrons are not sufficiently energetic to displace many cations. For oxygen ions the value of Nd is approximately 2, and assuming Ee = me c 2 the Mackinley and Feshback formulation [15] gives a scattering cross-section ud of approximately 9 barns. These values give a final concentration of 8.4 cation vacancies and 1016 anion vacancies cm − 3 for a dose of 1017 electrons cm − 2. The actual concentration of defects will probably be significantly lower than these theoretical values since the Kinchin and Pease model considerably over-estimates the radiation damage for alkaline earth oxides [16]. Even so, these calculations rightly predict that the damage will greatest for neutron irradiation. We have observed a significant shift in the position of the absorption edge with neutron irradiation [1], but no observable effect was seen here with a numerically equivalent electron dose.
312
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313
The process of defect formation is complex, and we have noted above (Section 3.7) that Gaussian analyses of the electron radiation-induced absorption profiles showed that additional bands were involved, compared with the spectra from neutron irradiated samples. Further, these bands were increased by annealing in hydrogen the electron irradiated crystals. Thus, the nature of these defects is different from those generated by neutrons and may involve ion vacancy clusters. The anomalous lack of absorption increase in the similar post electron-irradiation anneals in oxygen add to the complexity. Whether or not the defects involve trapped holes or electrons all appear to be shallow traps, since the complete absorption profile exhibits thermal decay at room temperature, and it can also be bleached by light of energy B17 180 cm − 1 (2.13 eV). The decay process may involve the release of the trapped holes or electrons from the defects, with the subsequent re-trapping at deeper traps. The removal of the defects at anneal temperatures above 800°C will be via the recombination of the displaced ions and vacancies. At these elevated temperatures either one or both of the entities may be mobile. The spectral profile of the defects may be described as unfortunate since it is equally well fitted by three or four bands. Although it is a general principle that spectra should be resolved into the minimum number of overlapping bands, comparisons with a similar profile obtained by neutron irradiation present problems (Section 3.7). Band assignments are therefore not possible for the present crystals, even though they are by far the purest currently available. It will therefore be necessary to wait until purer and larger ones have been prepared at some future date.
4.4. Low energy band at 12 020 cm − 1 (1.49 eV) Fig. 3(B) shows a band so far not discussed, at 12 020 cm − 1 (1.49 eV), after electron irradiation of an un-annealed crystal. Childs and et al. have reported it [8] in their proton irradiated thoria. It was observed in some of the neutron irradiated crystals [1], and when seen was not as intense or sharp as in Fig. 3. It can also be seen as a weak profile in some of the spectra after electron irradiation in Figs. 1 – 3. Although Childs et al. [8] found that the defects annealed out at temperatures around 1000°C, they did not report any increase in absorption after annealing at lower temperatures, as observed here. We propose that this band arises from a defect involving an impurity ion.
transfer can occur at the fundamental absorption edge, and the results here support the proposal that the edge occurs close to 47 600 cm − 1 (5.9 eV). The high melting point of thoria means that it is very difficult to obtain pure single crystals, and hence they generally contains impurity ions. When these ions can exist in more than one oxidation state then electron transfer can in principle occur, and provided the ions are close enough for this to occur, or have or can be made to aggregate, then the crystal is coloured and intervalence spectra can be recorded. The most likely impurity involved is lead. When thoria crystals are irradiated with 3 MeV electrons the energy of the electrons is transferred into the crystal. This energy is not sufficient to displace more than a few thorium ions into interstitial sites but readily displaces oxygen ions. The electron irradiated crystal turns blue, as it does when irradiated with neutrons and subjected to controlled annealing. The absorption spectrum thereby generated, in the region 12 000–17 260 cm − 1 (1.49– 2.14 eV), is similar but not identical to the profile generated by neutron irradiation. The spectra obtained here can be satisfactorily resolved into three Gaussian bands, but the profiles are equally well fitted by four Gaussian bands, thereby making assignments essentially impossible. The defects generated arise from aggregate centres. These defects are of shallow energy, since they can be annealed out at elevated temperatures. Their precise identity has still to be determined, and they may involve impurity ions. This study is therefore effectively complete. The effects of electron irradiation on single crystal thoria are here reported and analysed for the first time, and the crystals are now too fragmented for further work. Further definitive work awaits the availability of larger and purer thoria crystals.
Acknowledgements We thank Dr B.G. Childs, Canadian AEC, for donating the crystals and staff at the Cookridge Radiation Research Centre for assistance with the electron irradiation experiments. Valuable discussions with Drs H.V.St.A. Hubbard and V.A. Volkovich are acknowledged. J.D. thanks the UKAEA for financial support.
References 5. Conclusions Single crystals of thoria are not intrinsically capable of exhibiting intervalence bands, although electron
[1] T.R. Griffiths, J. Dixon, J. Chem. Soc., Faraday Trans. 88 (1992) 1149. [2] T.R. Griffiths, J. Dixon, J. Chem. Soc., Faraday Trans. 88 (1992) 3475. [3] J.M. Bodine, F.B. Theiss, Phys. Rev. 98 (1955) 1532.
T.R. Griffiths, J. Dixon / Inorganica Chimica Acta 300–302 (2000) 305–313 [4] [5] [6] [7]
O.A. Weinreich, W.E. Danforth, Phys. Rev. 88 (1952) 953. R.C. Linares, J. Phys. Chem. Solids 28 (1967) 1285. V.I. Neeley, J.B. Gruber, W.J. Gray, Phys. Rev. 158 (1967) 809. J.L. Bates, Techn. Rep. BNWL-457, Contract At (45-1)-1830 (1967) 16. [8] B.G. Childs, P.J. Harvey, J.B. Hallett, J. Am. Chem. Soc. 53 (1970) 431. [9] M.B. Robin, P. Day, Adv. Inorg. Radiochem. 10 (1967) 247.
.
313
[10] B.G. Childs, private communication. [11] W.B. Fowler, in: W.B. Fowler (Ed.), Physics of Color Centers, Academic Press, New York, 1968, p. 182. [12] E.T. Rodine, P.L. Land, Phys. Rev. 130 (1963) 2701. [13] R.S. Wilks, A.E.R.E. Rep. No. R5543 (1967). [14] J.W. Corbett, Solid State Suppl. 7 (1966). [15] W.A. Mackinley, H. Feshback, Phys. Rev. 74 (1958) 1759. [16] B. Henderson, J.E. Wertz, Adv. Phys. 17 (1968) 749.