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The structure of β-U2 N3
Journal of Nuclear Materials 58 (1975) 241-243 3 North-Holland Publishing Company
THESTRUCTUREOFfl-U,N, N. MASAKI and H. TAGAWA Japan Atomic Energy Research Institute,
Tokai-mura, Naka-gun, Ibaraki-ken, Japan
Received 22 July 1975 Revised form received 18 August 1975
It is reported by Vaughan [l] that the structure of &U2N3 is of the La20g-type based on X-ray powder diffraction data. However, the exact value of the structural parameter for nitrogen atoms has not been determined because of the small X-ray scattering power of nitrogen compared to uranium. The present study has been made with the neutron and X-ray powder diffraction method at room temperature to determine the crystal structure of P-UzN3. The P-U2N3 powder sample was prepared by heat treating alpha uranium sesquinitride (bixbyite-type structure) at 1100°C for 1 hour under 28.7 torr nitrogen. The obtained o-phase specimen (ca. Sg) was contained in a thin-walled aluminum tube of 8 mm diameter and 35 mm height for neutron diffraction. The neutron diffraction data at room temperature were obtained with a diffractometer installed at the JRR-3 reactor of JAERI [2]. Neutrons were monochromated by the (111) plane of a copper single crystal in transmitting position which reflects 1.53 A. A soller slit of 15’ angular divergence was placed between the reactor and the copper, and a soller slit of 30’ in front of the BF3-detector. The conventional 8-28 measurement was made in the range 0.08 < sin 8/h < 0.50 A-‘. The contamination of h/2 was estimated to be about 2% by comparison of the reflexion intensities between 110 and 220 of U02 (CaF2type structure). The X-ray diffraction work upon the specimen was also made to determine the cell constants using Cu Ka, radiation monochromated with the (10.1) plane of curved quartz. Fig. 1 shows the collected neutron diffraction pattern. A minor amount of o-phase was detected in
the neutron as well as in the X-ray diffraction pattern. The amount of o-phase was estimated to be (2 + 1) vol. % by means of the direct comparison method [3] based on the X-ray diffraction data. The contamination, however, did not interfere with the collection of the integrated intensities of 24 observed neutron reflexions. The structure analysis was carried out with the neutrondiffraction data thus obtained. Since the space group for the La203type structure is Pyml [ 1,4], the analysis was started with locating the uranium atoms on the special position 2(d), and the nitrogen atoms on two sets of the special positions 2(d) and l(a). The I(a) position is parameter free, so that the analysis consisted in varying the structural parameters of 2(d). The numerical values of the parameters for the uranium and nitrogen atoms in 2(d) were derived under the consideration of overall agreement in terms of the R-index in the form
where pi is the sum over all peaks which can be separated in the pattern, Zr is the sum over the overlapping reflexions in such peaks and j is the multiplicity factor of a reflexion. jFz is the observed structure factor and jF: the calculated one. The best fit between the jF$ and jFz was found at U(1) : 2(d) z = 0.250, N(1) : 2(d) z = 0.641 and N(2) : l(a). The R-index is 0.08. The lattice constants based on the X-ray diffraction data are hexagonal: II = 3.700 f 0.002, c = 5.825 + 0.03 A. The final results are given in table 1. The determined crystal structure of &U,N, is
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N. Masaki, H. Tagawa / The structure of p-U2N3
shown in fig. 2 as the projections onto the (00.1) and the (11 .O) planes. From fig. 2 it is seen that each uranium atom is surrounded by seven nitrogen atoms. The atomic distances between U and N are found to be: three are U( 1) - N(2) = 2.585 A, one is U( 1) - N( 1) = 2.278 and the remaining three are U( 1) - N( 1) = 2.229. The authors would like to thank Dr. Y. Hamaguchi for his interest and encouragement. They also wish to express their gratitude to the members of their group.
243
References [1] D.A. Vaughan, J. Metals 8 (1956) 78. [2] N. Masaki and H. Tagawa, J. Nucl. Mater. 57 (1975) 187. [ 31 B.D. Cullity, Elements of X-ray diffraction, (AddisonWesley Publishing Co., Inc., Reading, Massachusetts, 1956) p. 391. [4] International Tables for X-ray Crystallography, vol. 1, (Kynoch Press, Birmingham, 1952) p. 270.
THESTRUCTUREOFfl-U,N, N. MASAKI and H. TAGAWA Japan Atomic Energy Research Institute,
Tokai-mura, Naka-gun, Ibaraki-ken, Japan
Received 22 July 1975 Revised form received 18 August 1975
It is reported by Vaughan [l] that the structure of &U2N3 is of the La20g-type based on X-ray powder diffraction data. However, the exact value of the structural parameter for nitrogen atoms has not been determined because of the small X-ray scattering power of nitrogen compared to uranium. The present study has been made with the neutron and X-ray powder diffraction method at room temperature to determine the crystal structure of P-UzN3. The P-U2N3 powder sample was prepared by heat treating alpha uranium sesquinitride (bixbyite-type structure) at 1100°C for 1 hour under 28.7 torr nitrogen. The obtained o-phase specimen (ca. Sg) was contained in a thin-walled aluminum tube of 8 mm diameter and 35 mm height for neutron diffraction. The neutron diffraction data at room temperature were obtained with a diffractometer installed at the JRR-3 reactor of JAERI [2]. Neutrons were monochromated by the (111) plane of a copper single crystal in transmitting position which reflects 1.53 A. A soller slit of 15’ angular divergence was placed between the reactor and the copper, and a soller slit of 30’ in front of the BF3-detector. The conventional 8-28 measurement was made in the range 0.08 < sin 8/h < 0.50 A-‘. The contamination of h/2 was estimated to be about 2% by comparison of the reflexion intensities between 110 and 220 of U02 (CaF2type structure). The X-ray diffraction work upon the specimen was also made to determine the cell constants using Cu Ka, radiation monochromated with the (10.1) plane of curved quartz. Fig. 1 shows the collected neutron diffraction pattern. A minor amount of o-phase was detected in
the neutron as well as in the X-ray diffraction pattern. The amount of o-phase was estimated to be (2 + 1) vol. % by means of the direct comparison method [3] based on the X-ray diffraction data. The contamination, however, did not interfere with the collection of the integrated intensities of 24 observed neutron reflexions. The structure analysis was carried out with the neutrondiffraction data thus obtained. Since the space group for the La203type structure is Pyml [ 1,4], the analysis was started with locating the uranium atoms on the special position 2(d), and the nitrogen atoms on two sets of the special positions 2(d) and l(a). The I(a) position is parameter free, so that the analysis consisted in varying the structural parameters of 2(d). The numerical values of the parameters for the uranium and nitrogen atoms in 2(d) were derived under the consideration of overall agreement in terms of the R-index in the form
where pi is the sum over all peaks which can be separated in the pattern, Zr is the sum over the overlapping reflexions in such peaks and j is the multiplicity factor of a reflexion. jFz is the observed structure factor and jF: the calculated one. The best fit between the jF$ and jFz was found at U(1) : 2(d) z = 0.250, N(1) : 2(d) z = 0.641 and N(2) : l(a). The R-index is 0.08. The lattice constants based on the X-ray diffraction data are hexagonal: II = 3.700 f 0.002, c = 5.825 + 0.03 A. The final results are given in table 1. The determined crystal structure of &U,N, is
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9 3
3
3 _
.e _.: *2 I Z/f ‘01 I
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N. Masaki, H. Tagawa / The structure of p-U2N3
shown in fig. 2 as the projections onto the (00.1) and the (11 .O) planes. From fig. 2 it is seen that each uranium atom is surrounded by seven nitrogen atoms. The atomic distances between U and N are found to be: three are U( 1) - N(2) = 2.585 A, one is U( 1) - N( 1) = 2.278 and the remaining three are U( 1) - N( 1) = 2.229. The authors would like to thank Dr. Y. Hamaguchi for his interest and encouragement. They also wish to express their gratitude to the members of their group.
243
References [1] D.A. Vaughan, J. Metals 8 (1956) 78. [2] N. Masaki and H. Tagawa, J. Nucl. Mater. 57 (1975) 187. [ 31 B.D. Cullity, Elements of X-ray diffraction, (AddisonWesley Publishing Co., Inc., Reading, Massachusetts, 1956) p. 391. [4] International Tables for X-ray Crystallography, vol. 1, (Kynoch Press, Birmingham, 1952) p. 270.