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The crystal structure of uranium pentabromide by powder neutron diffraction
J. inorg, nucl. Chem. Vol. 40, pp. 1055--1057
0022-190217810S01-10551502.0010
© Pergamon Press Ltd., 1978. Printed in Great Britain
THE CRYSTAL S T R U C T U R E OF U R A N I U M P E N T A B R O M I D E BY P O W D E R N E U T R O N DIFFRACTION J. H. LEVY, J. C. TAYLOR and P. W. WILSON Australian Atomic Energy Commission, Research Establishment, Private Mail Bag, Sutherland, NSW, 2232, Australia (First received 8 June 1977; in revised [orm 18 August 1977: received [or publication 27 October 1977)
Abstract--Analysis of the neutron powder pattern of uranium pentabromide shows its structure to be of the triclinic/3-UCI5type, and not the a-PaBr5 type as proposed by earlier workers. Refinement by the profile fitting technique shows it to consist of U2Br~odimers, the anions being in a simple hexagonal close packed arrangement. INTRODUCTION Uranium pentabromide has been known for many years, but its crystal structure has remained unsolved. A method for preparing UBr5 was given by Lux, Wirth and Bagnall[1] who stated that their X-ray powder pattern indicated that it had a-PaBr5 structure[2]. The structure of a-PaBr5 is unknown, but ~-PaBr5 is of the monoclinic a-UCI5 type [3, 4]. We have collected a neutron powder pattern of UBr~ to determine this new structure type. EXPERIMENTAL
Uranium pentabromide (about 20g) was prepared by the method of Lux, Wirth and Bagnall, with acetonitrile catalyst[l]. It was placed in vanadium can (12 mm dia.)which was sealed against atmospheric moisture, and a neutron powder diffraction pattern was collected on the reactor HIFAR using the elastic diffraction technique[5]. The pattern (Fig. 1) was collected from 20 = 5° to 39.6°, in steps of 0.1° 20, with ,~ = 1.077A.
The structure was, however, clearly similar to the WSBr4 type. Since WSBr4 has a simple hexagonal close packed anion arrangement, we thought it possible that UBr~ could have the triclinic /~-UCI5 structure[9], which also has simple hexagonal close packed anion layers. With a B-UCI5 model, the refinement of the UBr5 dataconverged to R = 0.136, only slightly greater than the R-factor expected from the statistics of the pattern, 0.095. This confirmed the UBrs structure as//-UCI5 type; the good agreement between observed and calculated neutron powder diffraction profiles is illustrated in Fig. I. Values of the neutron scattering lengths used were b u = 8 . 5 and bnr=6.8fm, and there were 217 (hkl) reflexions in the range of the pattern. Variables refined were the positional parameters, three half-width parameters, the scale factor, the cell constants, an 'overall isotropic Debye-Waller factor and the two-theta zero. Refined parameters are shown in Table 1, and the interatomic distances and angles in Table 2.
ANALYSIS OF DATA The a-PaBr5 unit cell[3] was large, so the UBr5 structure type was thought to be complex. Before attempting to analyse the X-ray powder pattern of UBr5 for the uranium locations, as in our direct analysis of the UBr4 structure from powders[6], we calculated some theoretical neutron powder patterns for some pentahalide (and oxide and sulphide tetrahalide) structure types. Surprisingly, the UBr, pattern was similar to that of the monoclinic WSBr, type[7], although subsequent refinement with the program of Rietveid[8] reduced the reliability index R = X(yo-yc)/Xyo (where the y's are background-corrected pattern intensities) to only 0.24.
DISCUSSION The//-UCI5 type structure of UBr5 is shown in Fig. 2. The bromine atoms form a nearly regular simple hexagonal close packed array, and the uranium atoms fill one-fifth of the octaghdral holes in such a way as to form U2Brto dimers. The dimers pack in parallel orientation and have a centre of symmetry. Except for PaCI5 and the fluorides, all the pentahalides as well as the oxide and sulphide tetrahalides of the actinides and the 2nd and 3rd row transition metals are based on such dimers. The structure types in this family were summarised recently[10].
2000 ~ UBr 5 Neufron
= c
Powder
Pro'tern
1 5 0 0 r-
I 5 O 0 ,I
5
i0
15
20
. . . . . 28,
• 25
-
-
,°- -
i 30
. . . .
5J5 "
~
'
deg
Fig. 1. Observed and calculated neutron powder pattern profiles for uranium pentabromide. The vertical bars show the positions of the 217 (hkl) reflexions in this range. 1055
1056
J.H. LEVY et
al. i
C
Fig. 2. A stereo pair of the ~-UCIs(UBrs) structure type, calculated with the programORTEP[12I. The hexagonal close-packed anion layers are seen end-on, and inclusion of uraniumatoms in the octahedral holes forms layers of U2Brlodimers. Table 1. Uranium pentabromide structural parameters, derived Table 2. Interatomic distances and angles in uranium from the neutronprofile analysis pentabromide aunds (AI Atom
103 x
U
103 y
224(6)
A~qles (Deq)
103 Z
223(4)
(bridging)
B r (1) - U - B r (1)
1
U - B r (1)
2.94
99(8)
O-Br (I) 1
2.81 (brldqing)
Br (1)-U-Br (2)
79.3
U-Br (2)
2,72 (terminal)
Br (1) -U-ar (3)
85.9
U-Br (3)
2.60 (terminal)
Br (1) -U-BE (4)
172.7
84.8
Br{l)
-197(6)
43(4)
52(7)
Br(2)
391(8)
163(4)
531(8)
Br(3)
2(6)
264(4)
-316(6)
U ~ r ~4}
2,58 (terminal)
Br (1)-U-Br (5)
89.0
Br(4)
595(8)
352(5)
121(7)
U--Br (5)
2,70 (terminal)
BE (1) 1-U-Br (2)
87.9
Br(5)
189(8)
445(6)
280(6)
U .... U 1 4.43
Br (i) I-U-Br (3)
92.6
Br(1) 1-U-Br (4)
93.4
BU
-
4 . 4 5 ~2 overall
Bnr =
B
nr (1)-Br (1) 1
refined
4.95 ~2
1 2 X 2 = Zw(y ° - ~ yc ) /NO-NV = 2.1 Final cell constants
a ~ 7.449(7), ~ 89.25(12),
(A = 1.077 A)
b = i0.127(14), B = 117.56(4),
c = 6.686(4) A, A = 108.87(9)
deg.
Unit cell volume = 417.5 ~3 = volume per dimer unit.
3.67
Br 41) 1-U-BE (5) 166.9
BE (1)-Br (2)
3.82
8r (2) -U-Br (3)
170.5
Br (1)-BE(3)
3.79
Br (2) -U-Br (4)
95.1
Br (1)-Br (5)
3.96
Br (2) -U-Br (5)
85.4
Br (i) l-Br (2)
3.83
Br (3)-U-Br (4)
94.4
Br (I) l-Br (3)
3.91
Br (3)-U-Br (5)
92.2
Br (1) 1-Br (4)
3.92
Br (4) -U-Br (5)
Br (1) 1-Br (5)
5.47
U-Br (1)-U 1
98.3 100.7
8r ~2) -Br (3)
4.17
U1-U-Br (2)
85.2
The dimensions of the U2Br,o dimer are not significantly different from those of the dimer in ~PaBr~. As in ~-PaBrs, the terminal U-Br bonds are about 0.25 ,~ shorter than the bridge bonds, Table 2. The bridge bonds are not significantly different in length. The octahedra in the dimer are distorted from regularity by the U-U repulsion. Thus, the angle Br(I)-U-Br(1)' is 79.3 ° instead of 90°, Br(3)-U-Br(2) is 170°, and Br(4)-UBr(5)o is 98°. There is a short Br(l)-Br(l) contact of
Br (2) -Br (4)
3.91
01-U-Br (3)
88.9
Br (2) -Br (5)
3.68
Br (3) -Br (4)
3.80
3.67 A.
(i) In the calculated X-ray powder patterns for aUCIs, /3-PaBr~, /~-UCI~, UBr5 and a-PaBr~, shown in Fig. 3, a-UCl5 and ~-PaBr5 are of the same type with a larger unit cell for the latter, and similarly fl-UCI~ and UBrs are of the other type with a larger cell for UBrs. Some lines are split with UBrs, whereas they accidentally coincide with fl-UCIs, but the patterns are isostructural. The 20 values for a-PaBr~ are shown (the lines are all given the same intensity in this case as the structure is
The WSBr4 structure[7] is derived from the UBrs structure by substituting a sulphur for a halogen atom; both structures have the same cation arrangement in the hexagonal close packed anion layers. As already mentioned, UBr5 has been incorrectly assigned the monoclinic a-PaBr~ structure[l,3]. The following considerations show that UBrs, prepared according to Ref. [1], cannot have this structure:
Br (3)-Br (5)
3.82
Br(4) -8r (5)
4.00
,
o
The errors in the distances are about 0.07 A, and in the angles 2-3 degrees.
Table 3. Unit cell data and dimer volumes for some MX~ structures. (Cell constants in ~ and degrees, volume in B
a
b
u-UC15
7.99
10.69
8.48
90
~-UCl s
7.07
9,65
6,35
89.1
12.69
12.82
9.92
90
Compound
~-PaBr 5
c
91.5 117.36 108 91.1
~-PaBr 5
8.385
11.205
8.950
90
UBr 5
7.449
10.127
6.686
89.25 117.56
¥
90 108.54
Volume per DimerUnit
362 360
90
384
90
420
108.87
418
The crystal structure of uranium pentabromide by powder neutron diffraction
, iI
i
1057
IIIIh J,,d 3
d(hk/)
a -UCl~
J
J
I
,il,l ,
],J Ill
6
5'1-PoBr5I
lJJ
4l
.,
llJdLl
I dl
/~-UCa~ i,
II
.... ,,,.J,J.
hI
UBr5
l
I
I
I1
Ull
I IIIIIII
II
a - PoBr5 Fig. 3. Calculated X-ray powder patterns for a-UCIs, ~-PaBrs, ~-UC6, UBr5 and a-PaBr~. As the a-PaBr~ structure is unknown, all intensities in this pattern are assumed the same (intensity scale of each pattern arbitrary). unknown). Clearly, UBr5 and a-PaBrs are not isostrucrural. The UBrs X-ray pattern given by Lux, Wirth and Bagnall[l] is of the ~-UCIs type. (ii) A Dclaunay reduction for the UBrs ceil, Table 1, showed that the cell was truly triclinic, and not monoclinic. (iii) The unit cell dimensions for the relevant pentahalide structures are given in Table 3. Since Pav and U v are almost identical in ionic radius[IlL it would be expected that the volume per dimer unit should be about the same for a-PaBrs and UBrs, if they were isostructural. They are, in fact, quite different (384 and 418.1,3). The UBrs volume does, however, agree closely with that of/3-PaBrs ~, as expected. The polymorphs a-UC6 and /3-UC6 show little difference in dimer volumes also. The present neutron profile analysis of UBrs has filled another gap in the list of actinide penthalide structures by unexpectedly forming a //-UCIs-type structure for UBrs.
REFERENCES I. F. Lux, G. Wirth and K. W. Bagnall, Chem. Ber. 103, 2807 (1970). 2. D. Brown, Private communication to F. Lux, G. Wirth and K. W. Bagnall (see Ref. [1]). 3, D, Brown, T, J. Petcher and A. J. Smith, Acta Cryst. B25, 178 (1969). 4. G. S. Smith, Q. Johnson and R. E. Elson, Acta Cryst. 22, 300 (1967). 5. G. Caglioti, In Thermal Neutron Diffraction, (Edited by B. T. M. Willis). Oxford University Press, Oxford (1970). 6. J. C. Taylor and P. W. Wilson, Acta Cryst. B30, 2664 (1974). 7. M. G. B. Drew and R. Mandyczewsky, J. Chem. Soc. (A), 2815 (1970). 8. H. M. Rietveld, J. AppL Cryst. 2, 65 (1969). 9. U. Miiller and W. Kolitsch, Z. Anorg. Allg. Chem. 410, 32 (1974). 10. J. C. Taylor, Coord. Chem. Rev. 20, 197 (1976). IL R. D. Shannon and C. T. Prewitt, Acta Cryst. B~, 925 (1969). 12. C. K. Johnson, Oak Ridge National Laboratory Report ORNL-3794 (1965).
0022-190217810S01-10551502.0010
© Pergamon Press Ltd., 1978. Printed in Great Britain
THE CRYSTAL S T R U C T U R E OF U R A N I U M P E N T A B R O M I D E BY P O W D E R N E U T R O N DIFFRACTION J. H. LEVY, J. C. TAYLOR and P. W. WILSON Australian Atomic Energy Commission, Research Establishment, Private Mail Bag, Sutherland, NSW, 2232, Australia (First received 8 June 1977; in revised [orm 18 August 1977: received [or publication 27 October 1977)
Abstract--Analysis of the neutron powder pattern of uranium pentabromide shows its structure to be of the triclinic/3-UCI5type, and not the a-PaBr5 type as proposed by earlier workers. Refinement by the profile fitting technique shows it to consist of U2Br~odimers, the anions being in a simple hexagonal close packed arrangement. INTRODUCTION Uranium pentabromide has been known for many years, but its crystal structure has remained unsolved. A method for preparing UBr5 was given by Lux, Wirth and Bagnall[1] who stated that their X-ray powder pattern indicated that it had a-PaBr5 structure[2]. The structure of a-PaBr5 is unknown, but ~-PaBr5 is of the monoclinic a-UCI5 type [3, 4]. We have collected a neutron powder pattern of UBr~ to determine this new structure type. EXPERIMENTAL
Uranium pentabromide (about 20g) was prepared by the method of Lux, Wirth and Bagnall, with acetonitrile catalyst[l]. It was placed in vanadium can (12 mm dia.)which was sealed against atmospheric moisture, and a neutron powder diffraction pattern was collected on the reactor HIFAR using the elastic diffraction technique[5]. The pattern (Fig. 1) was collected from 20 = 5° to 39.6°, in steps of 0.1° 20, with ,~ = 1.077A.
The structure was, however, clearly similar to the WSBr4 type. Since WSBr4 has a simple hexagonal close packed anion arrangement, we thought it possible that UBr~ could have the triclinic /~-UCI5 structure[9], which also has simple hexagonal close packed anion layers. With a B-UCI5 model, the refinement of the UBr5 dataconverged to R = 0.136, only slightly greater than the R-factor expected from the statistics of the pattern, 0.095. This confirmed the UBrs structure as//-UCI5 type; the good agreement between observed and calculated neutron powder diffraction profiles is illustrated in Fig. I. Values of the neutron scattering lengths used were b u = 8 . 5 and bnr=6.8fm, and there were 217 (hkl) reflexions in the range of the pattern. Variables refined were the positional parameters, three half-width parameters, the scale factor, the cell constants, an 'overall isotropic Debye-Waller factor and the two-theta zero. Refined parameters are shown in Table 1, and the interatomic distances and angles in Table 2.
ANALYSIS OF DATA The a-PaBr5 unit cell[3] was large, so the UBr5 structure type was thought to be complex. Before attempting to analyse the X-ray powder pattern of UBr5 for the uranium locations, as in our direct analysis of the UBr4 structure from powders[6], we calculated some theoretical neutron powder patterns for some pentahalide (and oxide and sulphide tetrahalide) structure types. Surprisingly, the UBr, pattern was similar to that of the monoclinic WSBr, type[7], although subsequent refinement with the program of Rietveid[8] reduced the reliability index R = X(yo-yc)/Xyo (where the y's are background-corrected pattern intensities) to only 0.24.
DISCUSSION The//-UCI5 type structure of UBr5 is shown in Fig. 2. The bromine atoms form a nearly regular simple hexagonal close packed array, and the uranium atoms fill one-fifth of the octaghdral holes in such a way as to form U2Brto dimers. The dimers pack in parallel orientation and have a centre of symmetry. Except for PaCI5 and the fluorides, all the pentahalides as well as the oxide and sulphide tetrahalides of the actinides and the 2nd and 3rd row transition metals are based on such dimers. The structure types in this family were summarised recently[10].
2000 ~ UBr 5 Neufron
= c
Powder
Pro'tern
1 5 0 0 r-
I 5 O 0 ,I
5
i0
15
20
. . . . . 28,
• 25
-
-
,°- -
i 30
. . . .
5J5 "
~
'
deg
Fig. 1. Observed and calculated neutron powder pattern profiles for uranium pentabromide. The vertical bars show the positions of the 217 (hkl) reflexions in this range. 1055
1056
J.H. LEVY et
al. i
C
Fig. 2. A stereo pair of the ~-UCIs(UBrs) structure type, calculated with the programORTEP[12I. The hexagonal close-packed anion layers are seen end-on, and inclusion of uraniumatoms in the octahedral holes forms layers of U2Brlodimers. Table 1. Uranium pentabromide structural parameters, derived Table 2. Interatomic distances and angles in uranium from the neutronprofile analysis pentabromide aunds (AI Atom
103 x
U
103 y
224(6)
A~qles (Deq)
103 Z
223(4)
(bridging)
B r (1) - U - B r (1)
1
U - B r (1)
2.94
99(8)
O-Br (I) 1
2.81 (brldqing)
Br (1)-U-Br (2)
79.3
U-Br (2)
2,72 (terminal)
Br (1) -U-ar (3)
85.9
U-Br (3)
2.60 (terminal)
Br (1) -U-BE (4)
172.7
84.8
Br{l)
-197(6)
43(4)
52(7)
Br(2)
391(8)
163(4)
531(8)
Br(3)
2(6)
264(4)
-316(6)
U ~ r ~4}
2,58 (terminal)
Br (1)-U-Br (5)
89.0
Br(4)
595(8)
352(5)
121(7)
U--Br (5)
2,70 (terminal)
BE (1) 1-U-Br (2)
87.9
Br(5)
189(8)
445(6)
280(6)
U .... U 1 4.43
Br (i) I-U-Br (3)
92.6
Br(1) 1-U-Br (4)
93.4
BU
-
4 . 4 5 ~2 overall
Bnr =
B
nr (1)-Br (1) 1
refined
4.95 ~2
1 2 X 2 = Zw(y ° - ~ yc ) /NO-NV = 2.1 Final cell constants
a ~ 7.449(7), ~ 89.25(12),
(A = 1.077 A)
b = i0.127(14), B = 117.56(4),
c = 6.686(4) A, A = 108.87(9)
deg.
Unit cell volume = 417.5 ~3 = volume per dimer unit.
3.67
Br 41) 1-U-BE (5) 166.9
BE (1)-Br (2)
3.82
8r (2) -U-Br (3)
170.5
Br (1)-BE(3)
3.79
Br (2) -U-Br (4)
95.1
Br (1)-Br (5)
3.96
Br (2) -U-Br (5)
85.4
Br (i) l-Br (2)
3.83
Br (3)-U-Br (4)
94.4
Br (I) l-Br (3)
3.91
Br (3)-U-Br (5)
92.2
Br (1) 1-Br (4)
3.92
Br (4) -U-Br (5)
Br (1) 1-Br (5)
5.47
U-Br (1)-U 1
98.3 100.7
8r ~2) -Br (3)
4.17
U1-U-Br (2)
85.2
The dimensions of the U2Br,o dimer are not significantly different from those of the dimer in ~PaBr~. As in ~-PaBrs, the terminal U-Br bonds are about 0.25 ,~ shorter than the bridge bonds, Table 2. The bridge bonds are not significantly different in length. The octahedra in the dimer are distorted from regularity by the U-U repulsion. Thus, the angle Br(I)-U-Br(1)' is 79.3 ° instead of 90°, Br(3)-U-Br(2) is 170°, and Br(4)-UBr(5)o is 98°. There is a short Br(l)-Br(l) contact of
Br (2) -Br (4)
3.91
01-U-Br (3)
88.9
Br (2) -Br (5)
3.68
Br (3) -Br (4)
3.80
3.67 A.
(i) In the calculated X-ray powder patterns for aUCIs, /3-PaBr~, /~-UCI~, UBr5 and a-PaBr~, shown in Fig. 3, a-UCl5 and ~-PaBr5 are of the same type with a larger unit cell for the latter, and similarly fl-UCI~ and UBrs are of the other type with a larger cell for UBrs. Some lines are split with UBrs, whereas they accidentally coincide with fl-UCIs, but the patterns are isostructural. The 20 values for a-PaBr~ are shown (the lines are all given the same intensity in this case as the structure is
The WSBr4 structure[7] is derived from the UBrs structure by substituting a sulphur for a halogen atom; both structures have the same cation arrangement in the hexagonal close packed anion layers. As already mentioned, UBr5 has been incorrectly assigned the monoclinic a-PaBr~ structure[l,3]. The following considerations show that UBrs, prepared according to Ref. [1], cannot have this structure:
Br (3)-Br (5)
3.82
Br(4) -8r (5)
4.00
,
o
The errors in the distances are about 0.07 A, and in the angles 2-3 degrees.
Table 3. Unit cell data and dimer volumes for some MX~ structures. (Cell constants in ~ and degrees, volume in B
a
b
u-UC15
7.99
10.69
8.48
90
~-UCl s
7.07
9,65
6,35
89.1
12.69
12.82
9.92
90
Compound
~-PaBr 5
c
91.5 117.36 108 91.1
~-PaBr 5
8.385
11.205
8.950
90
UBr 5
7.449
10.127
6.686
89.25 117.56
¥
90 108.54
Volume per DimerUnit
362 360
90
384
90
420
108.87
418
The crystal structure of uranium pentabromide by powder neutron diffraction
, iI
i
1057
IIIIh J,,d 3
d(hk/)
a -UCl~
J
J
I
,il,l ,
],J Ill
6
5'1-PoBr5I
lJJ
4l
.,
llJdLl
I dl
/~-UCa~ i,
II
.... ,,,.J,J.
hI
UBr5
l
I
I
I1
Ull
I IIIIIII
II
a - PoBr5 Fig. 3. Calculated X-ray powder patterns for a-UCIs, ~-PaBrs, ~-UC6, UBr5 and a-PaBr~. As the a-PaBr~ structure is unknown, all intensities in this pattern are assumed the same (intensity scale of each pattern arbitrary). unknown). Clearly, UBr5 and a-PaBrs are not isostrucrural. The UBrs X-ray pattern given by Lux, Wirth and Bagnall[l] is of the ~-UCIs type. (ii) A Dclaunay reduction for the UBrs ceil, Table 1, showed that the cell was truly triclinic, and not monoclinic. (iii) The unit cell dimensions for the relevant pentahalide structures are given in Table 3. Since Pav and U v are almost identical in ionic radius[IlL it would be expected that the volume per dimer unit should be about the same for a-PaBrs and UBrs, if they were isostructural. They are, in fact, quite different (384 and 418.1,3). The UBrs volume does, however, agree closely with that of/3-PaBrs ~, as expected. The polymorphs a-UC6 and /3-UC6 show little difference in dimer volumes also. The present neutron profile analysis of UBrs has filled another gap in the list of actinide penthalide structures by unexpectedly forming a //-UCIs-type structure for UBrs.
REFERENCES I. F. Lux, G. Wirth and K. W. Bagnall, Chem. Ber. 103, 2807 (1970). 2. D. Brown, Private communication to F. Lux, G. Wirth and K. W. Bagnall (see Ref. [1]). 3, D, Brown, T, J. Petcher and A. J. Smith, Acta Cryst. B25, 178 (1969). 4. G. S. Smith, Q. Johnson and R. E. Elson, Acta Cryst. 22, 300 (1967). 5. G. Caglioti, In Thermal Neutron Diffraction, (Edited by B. T. M. Willis). Oxford University Press, Oxford (1970). 6. J. C. Taylor and P. W. Wilson, Acta Cryst. B30, 2664 (1974). 7. M. G. B. Drew and R. Mandyczewsky, J. Chem. Soc. (A), 2815 (1970). 8. H. M. Rietveld, J. AppL Cryst. 2, 65 (1969). 9. U. Miiller and W. Kolitsch, Z. Anorg. Allg. Chem. 410, 32 (1974). 10. J. C. Taylor, Coord. Chem. Rev. 20, 197 (1976). IL R. D. Shannon and C. T. Prewitt, Acta Cryst. B~, 925 (1969). 12. C. K. Johnson, Oak Ridge National Laboratory Report ORNL-3794 (1965).