Radiation stabilization of U 5 in CaO matrix and its thermal stability: Electron paramagnetic resonance and thermally stimulated luminescence studies

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1 PRAMANA Printed in India Vol. 47, No. 4, journal of October 1996 physics pp Radiation stabilization of U 5 in CaO matrix and its thermal stability: Electron paramagnetic resonance and thermally stimulated luminescence studies V NATARAJAN, T K SESHAGIRI and M D SASTRY Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay , India MS received 24 May 1996; revised 6 September 1996 Abstract. Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO matrix. From the theoretical predictions of g value for U s in axial symmetries, it was concluded that U 5 at Ca 2 + site is associated with a second neighbour charge compensating Ca 2 vacancy. EPR measurements also revealed the presence of Mn 2 +, Mn 4 + and Cu 2 impurities in the samples. The thermal stability of U 5 was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the intensity of EPR signal of U 5 ion around this temperature. This peak is associated with the process U hole - U 6 + * ~ U hr. The activation energy for this process was determined to be 1"4 ev. Keywords. Electron paramagnetic resonance; thermally stimulated luminescence. PACS Nos 33.30; 76-30; Introduction Among the different oxidation states of uranium, U s with 5f 1 electronic configuration is a very attractive ion for magnetic investigations. Because it is a single electron system, there is no electron-electron repulsion among the 5f electrons enabling the description of the energy levels and wave functions with greater certainty. Recently Miyake [1] has reviewed the magnetochemistry of U 5 complexes and compounds. U s + was also reported to be stabilized at Nb sites in LiNbO 3 [2]. An important aspect of U s in octahedral symmetry is its relatively long spin-lattice relaxation time when the site symmetry is perfectly octahedral. Under this symmetry, the lowest crystal field state of U 5 is a doublet, F z, which takes the form of F 7 under spin-orbit interaction. Miyake [1] had discussed the effect of different extents of axial (C4v) and orthorhombic (CEv) distortions on g value of U s. In spite of low symmetry perturbations on octahedral crystalline field, the g value was reported to be nearly isotropic for small distortions. The absolute ~ value for overall octahedral symmetry is expected to be around 1.1 which remains constant up to a distortion (e) of 800 cm- 1 (for a fixed value of z = ) beyond which the g value decreases [3]. The axial distortion (e) and orthorhombic component (z) were described by the Hamiltonian [1] no..,a, r,o, =

2 V Natarajan et al where O and 0 2 are the appropriate operator equivalents [1]. In C4v symmetry, the g value smoothly increases with increase in e from the octahedral value of 1.1. Therefore g = 1.1 can be taken as a finger-print for near-perfect octahedral symmetry at the site of U 5+" U 5+ can be produced by irradiation of U6 /U 4+ doped crystals. CaO lattice is an ideal one for stabilizing U s+ in perfectly octahedral symmetries in view of its NaCl-like structure. However, there can be charge compensating sites which would reduce the site symmetry. Furthermore, as U 5+ is a radiation induced defect, 7- irradiation of calcium oxide doped with uranium can produce other concomitant defects which would also be paramagnetic. The thermal stability of U 5+ in turn depends on the thermal stabilities of these defects and also due to any other paramagnetic impurity that may be present in the starting material. These aspects are investigated in the present work using electron paramagnetic resonance and thermally stimulated luminescence. These two techniques are complimentary. Whereas EPR gives information on the static condition of the trapped charges, TSL gives information on the thermally stimulated electron transfer reactions. In this paper, clear evidence is presented for the stabilization of U 5+ ion with a weak C4v perturbation and the thermally stimulated electron transfer reactions involving U 5+ are discussed. 2. Experimental Analar grade Ca(NO3) 2 was dissolved in distilled water and slightly acidified with 2M HNO 3. Using a slight excess of(nh4)2 CO 3, calcium was preeipitated as carbonate. In uranium doped samples, uranyl nitrate solution (at 3 different uranium concentrations - 0.1% (A), 0"5% (B) and 1.0% (C) by weight of CaO) was added to calcium nitrate solution followed by precipitation of CaCO 3. The precipitate was filtered, dried and thoroughly ground and then heated in sintered alumina crucibles at 1273 K for 24 h. The samples were immediately transferred to a dessiccator containing P205 to avoid pick up of moisture/co 2 from air. The x-ray diffraction patterns of the final products agreed well with those reported for CaO in the literature. The TSL studies were conducted on a home built unit [4] in the K range. The samples were wrapped in para films during irradiation and prior to TSL recording to avoid moisture pick-up by samples. Spectral studies were conducted using narrow band interference filters. EPR spectra of the samples were recorded at room temperature and at 77 K on a Bruker ESP-300 X-band EPR spectrometer. For EPR studies, about 60 mg of the samples were sealed in quartz tubes immediately after preparation. Temperature dependence of the EPR spectra was studied by annealing the irradiated samples at different temperatures for 2-3 min and recording the EPR spectra at 77 K. A CTI cryogenic closed cycle helium refrigerator was used for studying the EPR spectra of the uranium doped CaO sample in pellet form in the K region. A gamma chamber with 6 Co source (dose rate 2 kgy/h) was used for irradiation. Spectroscopic analysis of the samples using an inductively coupled plasma-atomic emission direct reading spectrometer showed the presence of Mn, Cu and Fe at about ppm level in all the samples. The uranium content in all the uranium doped samples was estimated by fission track method and spectroscopic analysis [5]. 326 Pramana - J. Phys., Vol. 47, No. 4, October 1996

3 Radiation stabilization of U 5 + in CaO matrix 3. Results 3.1 EPR studies On gamma irradiation to a dose of 4 kgy, a new EPR signal at g = was observed at 77 K in all the uranium doped samples (figure 1). Thiswas absent in undoped CaO sample. The temperature dependence of this signal showed that it is observable between 10 K and 95 K and its g value is temperature independent. This is assigned to U s [6] at near octahedral symmetry in CaO matrix. On annealing the irradiated CaO:U samples at 540 K, drastic reduction in the intensity of this signal was observed. EPR signals from Mn 2+ (g = and A = 86-3 G), Cu 2 (g = 2.222, A = 21 G) and Fe 3+ (g = 2.005) were observed in unirradiated samples. These ions were the residual impurities in the CaO matrix. In one of the samples (C) with relatively higher concentration of the dopant (U 3 O a), a weak signal from Mn 4 was also observed. (The stabilization of this higher oxidation state of Mn is probably due to the presence of excess oxygen ions.) In view of the possible role of these impurity ions in the TSL processes, their EPR was also followed in ~/-irradiated samples. In sample C, strong signals from Mn 4+ (g = 1"9948 and A = 78 G were observed on irradiation (figure 2). In gamma irradiated CaO sample, no new signal other than from Mn 2 +, Cu 2 and Fe 3 + was observed. Temperature dependence studies of the EPR spectra of the irradiated samples did not show any significant change in the intensities of Mn 2 +, Mn 4 and Fe 3 + in the K range. In ~-irradiated CaO:U pellet sample, a signal at gx = was observed up to 55 K. This was attributed to O- ion on the basis of earlier reports I-7, 8]. 3.2 TSL studies The TSL glow curves of CaO samples gamma irradiated at room temperature to a dose of 4 kgy showed weak peaks around 403, 443 and 485 K (heating rate = 5 K/s). In HDPPH G MAGNETIC FIELD, (GAUSS) Figure 1. EPR spectra of U 5 + in gamma irradiated CaO:U sample (dose = 4 kgy) recorded at 77 K. Pramana - J. Phys., Vol. 47, No. 4, October

4 V Natarajan et al uranium doped samples, in addition to these peaks, an intense peak around K was present. The intensity of this peak was found to be maximum in sample C. Figure 3 shows the TSL glow curves of the different uranium doped CaO samples. Table 1 shows the TSL peaks observed in these samples. The trap parameters for the peaks were evaluated from the different heating rates method [9] and are included in table 1. Thermal bleaching of the lower temperature peaks was done to get the correct T m for the higher temperature peaks. Spectral studies of the TSL glow peaks at 403, 443 and 485K showed emission around 600nm. For the peak at 540K present in uranium ~I"" ~ "D~'H = 3400 c. IOOG I o. - M~ b - Mn ~-+ C. - C~ ~'+ H ( GAUSS ) Figure 2. EPR spectrum of CaO:U sample (C) after gamma irradiation to a dose of 4 kgy (recorded at 77 K)..< Z I-- Z.,.i I,- /% * ~. f Figure 3. i i,;, ~;3 s..3 TEMP. ('K) TSL glow curves of gamma irradiated CaO and CaO: U samples. 328 Pramana - J. Phys., Vol. 47, No. 4, October 1996

5 Radiation stabilization of U 5 + in CaO matrix Table 1. TSL peaks observed in the CaO samples. Peak temperatures are expressed in K (heating rate = 5 K/s- i) Peak number Sample CaO CaO:U-A CaO:U-B (vw) (E = 0"64 ev, (E = 1-4 ev, s = 3"7 x 108) s = 1"6 x 1013) CaO:U-C (E = 0.79 ev, (vw) (E = 1.38 ev, s = 3 x 1012) s = 2.3 x 10 t2) E = trap depth and s = frequency factor, vw = very weak. A - 0" 1% by weight (20 ppm), B - 0"5% by weight (500 ppm) C - 1.0% (1500ppm). The % values indicate the amount of uranium added as uranyl nitrate at the precipitation stage whereas figures in the bracket indicate amount of uranium estimated in the final samples. doped samples, emission around 550, 580 and 598 nm was observed. On the basis of the earlier reports [ 10,11], Mn 2 was identified as the luminescent centre for the first three weak peaks and U 6 for the peak at 540 K. 4. Discussion CaO matrix has NaC1 type of structure and uranium entering in 4 + or 6 + oxidation state would go substitutionally in association with lattice defects for charge compensation. The charge compensation can be achieved either by cation vacancies or interstitial anions in the lattice. As mentioned earlier, the g value for U s in perfectly octahedral symmetry (U s+ at Ca site with no nearby charge compensating defect) would be 1.1. The present value of shows that the local symmetry has small perturbation of axial distortion. The value ofe is estimated to be about 1000 cm- 1 [1]. The 9 value greater than 1.1 for U 5+ suggests that it is more likely due to cation vacancy at the second nearest neighbour site which produces local C4~ symmetry. Interstitial 0 2- ion in close neighbourhood would have increased the coordination number and resulted in a much lower g-value. When the sample is heated to 540 K, U 5 intensity is reduced. This suggests that e- or hole released from a trap elsewhere recombines at U 5 site. The spectral character of emission for the peak (viz. emission around 550, 580 and 598 nm) at 540 K shows that the emission centre is U 6 4. Therefore the formation ofu 4 through capture of electron is considered less probable. In view of the nature of the emission centre, the reduction in U 5 + signal is ascribed to recombination with a hole. The probable mechanism is as follows: Pramana - J. Phys., Vol. 47, No. 4, October

6 V Natarajan et al On gamma irradiation, On heating to 540 K, ~ O- + e-, (1) U 6+ +e- ~ U 5+ (2) O- ~ hole, (3) U 5+ + hole ~ U 6+*, (4) U 6 + * ---~ U 6 + -t- hv, (5) i.e, U s + + O- --* U 6 + * ; U 6 + * ~ U 6 + -~- hr. The EPR of O- was observed in the CaO:U pellet sample up to 55 K. This observation of O- only at low temperatures is not surprising since it is reported to have been observed only at 20 K in high symmetry crystals like KCI [12] as it is a Jahn-Teller ion. The annealing behaviour of O- was examined by recording the EPR spectra at 20 K of gamma irradiated CaO: U pellet heated to 540 K. The percentage change in the intensity of O- signal was not large but a definite decrease was noticed. However it may be recalled that sensitivity of TSL is significantly more when compared to EPR [13]. In summary, EPR evidence is presented for radiation stabilization ofu 5 in CaO. It appears to be associated with a second neighbour cation vacancy, producing low symmetry distortion of about 1000 cm- 1. U 5 + acts as the recombination centre for the TSL glow peak at 540 K. References [1] C Miyake, in Handbook on the physics and chemistry of actinides edited by A J Freeman and C Keller (Elsevier Science Publishers, Amsterdam, 1991) ch. 7, p. 337 [2] W Burton Lewis, H G Hecht and M P Eastman, lnorg. Chem. 12, 1634 (1973) [3] A F Leung and Y Poon, Can. J. Phys. 55, 937 (1977) [4] A G I Dalvi, M D Sastry and B D Joshi, J. Phys. El3, 1106 (1980) [5] V Natarajan, T K Seshagiri, S K Thulasidas, A K Pandey, P C Kalsi, M D Sastry and R H Iyer, Proc. IX National Symposium on Solid State Track Detectors, Bombay (1995) p. 150 [6] P M Llewellyn, Ph.D. thesis (Oxford University, Oxford, 1956) [7] W C O'Mara and J E Wertz, Solid State Commun. 8, 807 (1970) [8] A J Tench and M J Duck, Solid State Commun. 15, 333 (1974) [9] W Hoogenstraaten, Philips Res. Rep. 13, 515 (1958) [10] W Lehmann, J. Lumin. 6, 455 (1973) [11] T K Seshagiri, V Natarajan, A G I Dalvi and M D Sastry, Pramana - J. Phys. 33, 685 (1989) [12] W Sander, Naturwissenschaften 51,404 (1964) [13] M D Sastry, A G I Dalvi, A G Page and B D Joshi, J. Phys. C8, 3232 (1975) 330 Pramana - J. Phys., Vol. 47, No. 4, October 1996