Americium alloys with gold and copper

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Americium alloys with gold and copper
V M Radchenko, M A Ryabinin, Chernakova and S V Tomilin
Joint Stock Company “State Scientific Center – Research Institute of Atomic Reactors” (JSC «SSC RIAR»), Dimitrovgrad-10, Ulyanovsk region
E-mail: [email protected]
Abstract. Presented are results of the production and X-ray examination of micro-samples of americium-241 compounds with gold and copper produced by high-temperature condensation of metal americium vapor onto corresponding substrates. No mutual solubility of the investigated system components was revealed at room temperature. The following three intermetallic compounds were revealed in the Am-Au system: Au6Am with tetragonal lattice of the Au6Sm structural type, AuAm with orthorhombic lattice of the CuCe structural type and AuAm with cubic lattice. The Am-Cu system showed the intermetallic compound Cu5Am (Cu7Am) with a hexagonal lattice of the Cu5Ca(Cu7Tb) structure type. An effect of the 241Am nuclide alpha-activity on the crystal structure of the produced intermetallide was studied.
1. Introduction Over many years JSC “SSC RIAR” has been active in the production and investigation of metals of transplutonium elements (TPE), their alloys and compounds. This paper presents a summary of results of the production and X-ray examination of micro-samples of americium-241 compounds with gold and copper, i.e. an identification of crystal structures of the compounds produced and determination of crystal lattice parameters as well as study of the effect of alpha-decay on the intermetallide crystal structures. Formation of curium-244 compounds with copper was presented in paper [1]. This system was demonstrated to form intermetallide Cu5Cm with a hexagonal lattice of the Cu5Ca type. The m- Cu system was expected to form an intermetallide of similar stoichiometric composition.
2. Experimental Americium metal used in the experiment was produced by thermal reduction of americium-241 oxide with lanthanum including simultaneous condensation of americium vapor onto a flat tantalum substrate. Samples of Au - 241Am alloys were produced by subsequent distillation of the americium metal including condensation of its vapor onto a gold substrate.
An Am-Cu sample was produced by thermal reduction of americium oxide with lanthanum including simultaneous condensation of metal vapor onto a copper substrate that was preliminary annealed in vacuum.The process was carried out in high vacuum. Table 1 shows production conditions of the samples. Equipment and methods of the americium metal production as well as a device used for its evaporation and condensation are described in papers [2, 3]. The 241Am content in the samples was determined by α- and γ- spectrometry and by their comparison with standard samples.
The samples were examined by the X-ray diffraction (diffractometer DRON-3M) at room temperature. Non-monochromatic copper α-radiation with Ni filter was applied for this purpose. The initial processing of the XRD patterns was carried out using special software. The angular position of
Actinides 2009 IOP Publishing IOP Conf. Series: Materials Science and Engineering 9 (2010) 012093 doi:10.1088/1757-899X/9/1/012093
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reflections was corrected on the basis of reflections of diamond cubic lattice which was applied as a thin layer onto the sample surface at each X-ray diffraction experiment. An analytical extrapolation technique of definition of precise crystal lattice parameters (CLP) based on the least squares method and a mathematical multiple regression model were used to calculate CLP of phases identified in the X-ray pattern.
ASTM standards [4] and computer databank for crystal structures of non-organic materials were used to identify crystal lattices and their corresponding phases (compounds).
Table 1. Production conditions of Am-Au and Am-Cu samples.
Production conditions System Sample , º τ, min Americium mass in sample, mg
1 1400 2 0.140 2 1100 3 0.053 Am-Au 3 1250 3 0.079
Am-Cu 1 1600 3 1.450 3. Results and discussion 3.1 Americium intermetallics with gold Decoding of all XRD patterns started with the identification of reflections of the gold FCC lattice. The calculated parameter of this structure corresponded to literature value a = 4.079(1) Å within the error limits [4]. This result proves the non-solubility of americium in the gold FCC lattice.
For sample 1 XRD patterns of the initial gold substrate and those of samples after sputtering of americium were recorded in 1.7 and 75 days. The substrate XRD pattern contained an ordinary set of reflections of the gold FCC lattice ( = 4.08 Å), however, the intensity of a number of reflections was very distorted, obviously, due to the sample texture.
The XRD pattern of fresh sample 1 contained more than 40 reflections of different intensity, of which 9 reflections belonged undoubtedly to the gold FCC lattice. It should be noted that after thermal treatment in the process of americium sputtering the gold metal reflections returned to their regular intensity characteristic of a fine-dispersed untexturized sample. Further observation of the dynamics of the change in the angular position and intensity of other reflections allowed a conclusion that most of them (30 reflections) belonged to a phase which contained gold and americium. During 75 days of self-irradiation at room temperature the number of reflections of this phase decreased approximately by a factor of 4. In this case most of the reflections, which initially showed higher intensity, became weak, and their angular position changed towards lower scattering angles 2Θ. This indicates to the swelling processes of the crystal lattice and its gradual amorphization. The latter is possible only if a source of the radiation damage (241Am nuclide) is present in the phase composition.
High-temperature during the samples production followed by a high cooling rate often leads to formation of non-equilibrium phases. On subsequent self-irradiation and/or annealing such phases transform to more stable compounds in terms of thermodynamics. After a long-term storage for 300 days sample 1 was annealed in vacuum at 800°. Its XRD pattern showed 37 reflections (figure 1). The most intensive reflections belonged to the FCC lattice of Au ( = 4.08(1) Å). The angular position of other reflections differed significantly from the initial XRD pattern. New reflections were observed to appear. Thus, the long-term self-irradiation of this sample at room temperature and its subsequent annealing led to a re-crystallization of the sample.
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Figure 1. XRD pattern of sample 1 of the Am-Au system (after annealing).
Identification of the expected intermetallics Am-Au (Am-Cu) was carried out by the method that
became standard for identification of new TPE compounds with other elements of the periodic table, i.e. by comparing value sets of interplanar distances and reflections intensity of the known lanthanide and actinide compounds with the data obtained in the XRD pattern of the sample under study, a difference in the metal radii of lanthanides and americium being considered. The lanthanide-gold system is known to contain 27 structural types of intermetallics. Mainly they represent complex (non- cubic) structures [4]. Such analysis of sample 1 resulted in the detection of a tetragonal structure of the Au6Sm type interpreted as Au6Am and also orthorhombic structure of the CuCe type interpreted as AuAm. Crystal lattice parameters of the phases found in sample 1 are listed in table 2.
Table 2. Calculated parameters of crystal lattices detected in the initial XRD patterns of the produced samples of the Am-Au and Am-Cu systems.
Lattice parameters Phase Syngony (space group) n a Å b, Å c, Å V, Å3
Sample 1 Am-Au Au6Am Tetragonal (P42/ncm) 24 10.3894(7) - 9.7036(7) 1047.4(2) AuAm Orthorhombic (Pnma) 10 7.402(2) 4.564(1) 5.826(1) 196.8(1) Sample 2 Am-Au AuAm Cubic (Fd3m) 5 4.784(2) - - - Sample 3 Am-Au AuAm Cubic (Fd3m) 5 4.786(3) - - - Sample Am-Cu Cu5Am Hexagonal (P6/mmm) 13 4.958(1) - 4.175(2) 88.90(9)
Note: n – number of reflections in the calculated value set. V – elementary cell volume.
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In the XRD patterns there are an addition seen reflections of a monoclinic lattice such as Am2O3 of the B-form, whose volume corresponds to literature value (V = 444.1(2) Å3 [4]).
The reflections recorded in the initial XRD experiment on sample 2 were of very poor intensity. We succeeded in the recording of 25 reflections belonging to: FCC lattice of Au; FCC lattice with parameter 4.784(2) Å and to monoclinic lattice of the C2/m Am2O3 space group. In the XRD experiment repeated in 255 days no reflections were revealed except those of the Au FCC lattice and separate reflections of the Am sesquioxide.
Based on the results obtained an assumption was made that as a result of the 241Am alpha-decay an X-ray re-amorphization of the FCC phase with parameter a = 4.784(2) Å took place. A search of an analogue of the FCC phase having a close parameter value among lanthanides and actinides with gold in the known data bases has not produced any positive result. On extension of the search area of the structural analogue a similar FCC lattice AgTh with parameter = 4.80 Å [4] was found in the system of lanthanides and actinides with silver, which was chosen for identification of AuAm. Gold, silver and copper belong to the 1B group of elements of the periodic system. The analysis of the known lanthanide and actinide compounds with these elements revealed a similarity of the structural types, atomic ratio and crystal lattice parameters. This led to an assumption of the possibility of the existence of an intermetallide of stoichiometric composition AuAm with a FCC lattice.
XRD patterns of sample 3 were recorded 1 day and 197 days after its production. The initial pattern contained 48 reflections: among them the Au FCC lattice was observed; intensive reflections belonged to DHCP lattice of metallic Am; multiple reflections corresponded to the Am2O3 monoclinic lattice of the B-form. Besides, reflections of the FCC lattice have been recorded, which were further interpreted as AuAm.
The revealed metal americium structure (DHCP lattice) has a marked texture attributed to intensification of reflections of the 00l type i.e. 004, 008, 0 0 12. Within the period of 1 -197 days the situation has not changed, the DHCP lattice texture being retained along the axis (00l). Calculated parameters of the americium DHCP lattice, within the error limit, correspond to the literature value [4] and have not changed significantly during 197 days.
Further observation of the dynamics of the angle position change and intensity of residual reflections allowed a conclusion that they belonged to a phase containing gold and americium. An analysis of the known compounds of lanthanides and first actinides with gold made it possible to reveal an FCC lattice interpreted as AuAm (analogue of the AgTh FCC lattice [4]).
3.2 Americium intermetallics with copper Decoding the X-ray pattern started with the identification of the reflections, which belonged to the substrate material i.e. to copper metal. The intensity of reflections of the copper FCC lattice was found to be markedly lower compared to its state before sputtering of americium. The calculated parameter of this structure corresponds to the literature value a = 3.615(1) Å [4] within the error limits. This result proves to non-solubility of americium in the copper FCC- lattice.
The initial XRD pattern of the Am-Cu sample was recorded one day after its production. The subsequent 7 patterns were recorded in 6, 13, 23, 37, 51, 76 and 146 days in the course of the sample self-irradiation at room temperature to observe the dynamics of the diffraction pattern change.
The initial pattern showed 47 reflections (figure 2): among them the most intensive reflections belong to a hexagonal lattice of the Cu5Ca type; the remaining ones belong to a DHCP lattice of the americium metal. Table 2 presents crystal lattice parameters of the intermetallic compound Cu5Am.
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Figure 2. Initial XRD pattern of the Am-u system sample.
Actually, all Ln-Cu systems contain compounds of the CuLn type with the CsCl type cubic lattice
and (or) an orthorhombic one with a space group Pnma, and also compounds of the Cu2Ln type having the Cu2Ce type orthorhombic lattice; many compounds represent the Cu6Ln type with an orthorhombic Cu6Ce type lattice and Cu5Ln compounds have a Be5Au type cubic lattice [4]. No similar compounds were found in the Am-Cu system.
The effect of self-irradiation on the Cu5Am intermetallide crystal structure could be observed in 7 subsequently recorded patterns. In the course of self-irradiation of the sample (i.e. with increasing self-irradiation dose) the intensity of the Cu5Am reflections gradually decreased. Simultaneously they broadened and shifted (predominantly to smaller angles); their number gradually decreased – at the beginning mainly low-intensity reflections disappeared at large angles 2Θ, and then reflections of higher intensity etc. On the whole the increase in the elementary cell volume of intermetallide Cu5Am (Cu7Am) reached 2% during 76 days of their self-irradiation. Full X-ray amorphization of its crystal lattice was actually completed within 146 days of its self-irradiation.
Figure 3 shows a dependence of the elementary cell volume of Cu5Am on the sample self- irradiation time at room temperature.
Figure 3. Change in the elementary cell volume of Cu5Am as a function of self-irradiation time.
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References [1] Radchenko V M, Seleznev A G, Ryabinin M A et al. Actinides-2001 Int. Conf. Hayama (Japan),
November 4-9, 2001. Paper P 7009 [2] Radchenko V M, Seleznev A G, Ryabinin M A et al. 1994 Radiokhimiya 36 299–303 [3] Seleznev A G, Stupin V A, Radchenko V M et al. 1987 Production and Properties of
Transplutonium Metals: Review. .: TSNIIatominform, P.57 [4] X-Ray Diffraction Data Cards. Joint Committee on Powder Diffraction Standards. Amer. Soc.
for Testing Materials (ASTM). Philadelphia 1999
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Americium alloys with gold and copper