- Email: [email protected]
A cubic form of uranium trioxide
3. lneepnic and NuSur Cbe MM7,1955. Vol.1 . pp. 309-312. Parpmou Pear Ltd ., Land=
A CUBIC FORM OF URANIUM TRIOXIDE By E.
WAIT
Atomic Energy Research Establishment, Harwell, Didcot, Berks (Received 7 March 1955) Abstract-The crystal structure of a uranium oxide phase of composition UO,.,, has been determined by X-ray powder diffraction techniques using a Guinier-type focusing camera ., The unit cell is cubic with a = 4138 t 0.005 kX. A uranium atom is located at (000) and oxygens at (}00), (040), and (00}) with some anion vnncies. The compound is isostructural with ReO, . The U--O bond distance of 2073 A agrees with that predicted by ZACHARIASEN for a bond strength S = 1 .
formation of non-stou-i iometric uranium oxide phases as a consequence of the easy variability of valency of uranium has been observed for compounds of ideal composition U0 2 , U3 0 8 , and U03 . In each case, the crystal structure is essentially unchanged over a limited range of composition near the ideal ." ) The compound to be described has the composition U0 2. 82. It is, however, convenient to describe its crystal structure in terms of the ideal U0 3 . A number of different crystalline modifications of uranium trioxide have been reported,(.2, 3, 4) but in only one case has a detailed crystal structure analysis been carried out .lbl Many methods of preparation are reputed to give an amorphous product, and most of the work in this field has been handicapped by the small crystallite size of the preparations, which results in very diffuse X-ray diffraction patterns. A considerable improvement on the ordinary Debye-Scherrer powder photograph may be obtained by means of a Guinier-type focusing powder camera using monochromatic radiation .° The work to be described was possible only with the aid of such a camera . THE
EXPERIMENTAL Preparation and Analysis. The specimen examined was prepared by the thermal decomposition of one of several available crystalline modifications of uranium trioxide monohydrate in air at 415°C . It was deep brick-red in colour . Oxygen in excess of UO,.,, was determined by reducing a weighed sample of the material in carbon monoxide at 800°C and measuring the carbon dioxide produced by a gas volumetric method . The composition as determined by this method corresponded to a formula U0 2.83. Attempts to prepare samples with compositions nearer to the ideal U0, by decomposing the hydrate at lower temperatures were unsuccessful . The hydrate had lost no appreciable amount of water after heating for one hour at 375°C . Morphology and Optics . Under the microscope in unpolarized light, the trioxide appeared to consist of tablets about 20 p in length . This was, however, the habit of the monohydrate precursor . A more detailed examination under a linear magnification of 1000 showed the pitted surface and translucent appearance characteristic of a mass of disordered crystallites . The bulk of the material appeared to be optically isotropic . A few fragments of birefringent material, different in colour from the main phase, were present and may have been formed by the rehydration of the anhydrous oxide . (1) (2) (3) (4) (6) (a)
and E . RABINOWITCH, p. 227, .The Chemistry of Uranium, National Nuclear Energy Series, McGraw Hill, New York (1951) . 1 . SHEET, S . FRIED, and N . R. DAVIDSON, J. Amer . Chem. Soc ., 72, 2172 (1950). J. J. KATz and D . M. GRUEN, J. Amer . Chem . Soc ., 71, 2106 (1949) and 73, 1475 (1951) . P . PERio, Bull. Soc . Chim. Fr., p . 776 (1953) . W. H. ZACHARIASEN, Acta Cryst., 1, 265 (1948) . R. W. M . D•EYE, Harwell Report A .E.R.E. C/R 1524 (1954) . J. J . KATZ
309
3 10
E. WArr
X-ray Photography. A thin layer of the oxide was mounted on a base of "Sellotape" and protected from atmospheric moisture by a further layer of the same material. The resulting "sandwich" was mounted in the specimen holder of the Guinier camera . Before processing, a scale graduated at 0 . 1 mm intervals was printed along one edge of the film so as to correct for film shrinkage errors . 161 The X-ray diffraction photograph obtained after an exposure of twenty-four hours using CuKQ radiation is reproduced in Fig. 1 . THE DETERMINATION OF THE STRUCTURE
The most prominent lines on the photograph are very diffuse and correspond to a crystallite size of less than 0 . 1 It. For comparison purposes, the diffraction photograph of aluminium foil is also presented in Fig . I as an example of the type of line to be expected from a well crystallized specimen . The very faint sharp lines on the trioxide photograph may be attributed to the hydrate phase which was noticed in the optical examination . They are also present in the diffraction pattern of the monohydrate precursor . The lines of the trioxide phase may be indexed on the basis of a simple cubic cell with the cell constant :ao = 4. 138 ± 0 .005 kX . = 4-146± 0.005 A. The calculated and observed values of sins 0 are compared in Table 1 . TABLE 1 .-CALCULATED AND OBSERVED VALUES OF SIN' O AND THE REFLECTION INTENSIT.FS
hkl
Sin' °obs
sin' °cake
100 110 111 200 210 211 220
0. 0346 0 . 0691 0 . 1027 0 . 1385 0 . 1728 0 . 2069 0 .2740 0 . 3113 0 . 3452 0. 3798
0 .0346 0 .0691 0. 1036 0. 1382 0. 1728 0. 2073 0. 2765 0 . 3110 0 . 3456 0. 3802
300 and 221 310 311
obs
ICU)
h(2)
4(U)
S M W_ . W M M V W W VW VW
27 15 4 5 11 7 3 7 3 2
19 15 10 5 8 7 3 5 3 5
23 18 7 4 10 8 3 6 4 3
The volume of the unit cell V,, = 71 .27 As.' This corresponds to a cell content of one formula unit UO a on the reasonable assumption of an oxygen packing volume of 20 As. The calculated density p, = 6.663 g/cc. The uranium atoms must lie in positions of one-fold and the oxygens in positions of threefold multiplicity, respectively, In the cubic system there are one-fold positions at (000) and (}}}) and three-fold positions at (400), (040), . and (00}), and at (041), (40}); and (}40). These latter correspond to the midpoints of the cube edges and faces respectively . The uranium atom may be arbitrarily, assigned to the (000) position by a suitable choice of origin . There are then two alternative sets of three-fold positions for the oxygens. The values of the X-ray intensities to be expected in the, two cases were now calculated .
J t b . V V.
C C C
k
A cubic form of uranium trioxide
311
We have I oc ALp F12
where I is the intensity, A the absorption factor, L the combined Lorentz and polarization factor, p the number of crystal planes co-operating in a given reflection, and F the structure factor . The constant of proportionality is independent of the Bragg angle 0 . The absorption factor A was assumed to be near unity for the thin powder specimen used . The Lorentz-polarization factor used was the approximate form quoted by D'EYE. (6) The simplified structure factors for the two sets of atomic positions considered, are set out in Table 2 for the observed reflections . TABLE 2 .-SIMPLIFIED STRUCTURE FACTORS FOR THE ALTERNATIVE SETS OF ATOMIC POSITIONS
hkl
I
2
100
fu +f. fu - A fu - 3f0 fu +3f fu + o fu - f, fu + 3f„ fu + A fu +A If. - A fu -3fo
fu - //o' f - ./ a fu+ 3f f + 3fo fu - f(t f4 fu+3fo fu - fA A - . f0 f - .f. fu + 3fo
110 111 200 210 211 220 300 221 310 311
A
./
The atomic scattering factors used were those of THOMAS and FERMI for uranium (7) and of McWEENEY for oxygen . (8 ' No correction was made for the effects of thermal vibration . In Table 1, the observed intensities are compared with those calculated for the two alternative sets of oxygen positions and with the values given by the uranium alone with no oxygen contribution . The overall agreement, and in particular the relative intensities of 111 and 200, are best represented by the first set of intensities, from oxygen positions (400), (040), and (004) . Hence we may conclude that the cell has uranium at (000) and oxygens at (400), (040), and (00}) . DISCUSSION
Fig. 2 is a diagram of the proposed structure in isometric projection . Each uranium is octahedrally co-ordinated by oxygen and each oxygen linearly by uranium. The compound is isostructural with rhenium trioxide . 191 A feature of this structure is the "hole" at (444). In the related perowskite structure ABX.a this position is occupied by the larger cation A . The bond length U-O is equal to a/2 = 2 . 073 A. This is not significantly different from the value 2 . 08 A obtained by ZACHARIASEN for the U-0 1 distance in (7' ~B~
" Internationale Tabellen zur Bestimmung von Krystallstrukturen." Berlin (1935). R . McWEngEy, Acta Cryst., 4, 513 (1951) .
Bd. II p . 571 .
Borntraeger,
312
R WAIT
hexagonal U03 (loc. cit.). The close agreement between these bond lengths is consistent with ZACHARIASEN'S observationt10l of a monotonic increase in bond length with decreasing bond strength over a series of oxygen compounds of hexavalent uranium . Each oxygen forms two bonds only to adjacent uranium, so that the bond strength S = I as in the primary U-0 1 bonds in hexagonal UO B.
Fw. 2.
The chemical analyses indicate an oxygen content corresponding to the formula U02.82. The structure is one of considerable rigidity, and it would easily be possible to remove an occasional oxygen at random without causing the collapse of the lattice . The net charge balance would be maintained, by a . diminution of the average uranium valency. ACKNOWLEDGEMENTS
The author is greatly indebted to Drs . R . W. M. D'EYE and many valuable discussions .
J. K. DAWSON
R. W. G. WYCKOFF, "Crystal Structures," Vol . I. tnterscicnce Publishers Inc., New York, 1948. 1101 W. H. ZACHARIASEN, .4cta Cryst., 7, 795 (1954).
for
A CUBIC FORM OF URANIUM TRIOXIDE By E.
WAIT
Atomic Energy Research Establishment, Harwell, Didcot, Berks (Received 7 March 1955) Abstract-The crystal structure of a uranium oxide phase of composition UO,.,, has been determined by X-ray powder diffraction techniques using a Guinier-type focusing camera ., The unit cell is cubic with a = 4138 t 0.005 kX. A uranium atom is located at (000) and oxygens at (}00), (040), and (00}) with some anion vnncies. The compound is isostructural with ReO, . The U--O bond distance of 2073 A agrees with that predicted by ZACHARIASEN for a bond strength S = 1 .
formation of non-stou-i iometric uranium oxide phases as a consequence of the easy variability of valency of uranium has been observed for compounds of ideal composition U0 2 , U3 0 8 , and U03 . In each case, the crystal structure is essentially unchanged over a limited range of composition near the ideal ." ) The compound to be described has the composition U0 2. 82. It is, however, convenient to describe its crystal structure in terms of the ideal U0 3 . A number of different crystalline modifications of uranium trioxide have been reported,(.2, 3, 4) but in only one case has a detailed crystal structure analysis been carried out .lbl Many methods of preparation are reputed to give an amorphous product, and most of the work in this field has been handicapped by the small crystallite size of the preparations, which results in very diffuse X-ray diffraction patterns. A considerable improvement on the ordinary Debye-Scherrer powder photograph may be obtained by means of a Guinier-type focusing powder camera using monochromatic radiation .° The work to be described was possible only with the aid of such a camera . THE
EXPERIMENTAL Preparation and Analysis. The specimen examined was prepared by the thermal decomposition of one of several available crystalline modifications of uranium trioxide monohydrate in air at 415°C . It was deep brick-red in colour . Oxygen in excess of UO,.,, was determined by reducing a weighed sample of the material in carbon monoxide at 800°C and measuring the carbon dioxide produced by a gas volumetric method . The composition as determined by this method corresponded to a formula U0 2.83. Attempts to prepare samples with compositions nearer to the ideal U0, by decomposing the hydrate at lower temperatures were unsuccessful . The hydrate had lost no appreciable amount of water after heating for one hour at 375°C . Morphology and Optics . Under the microscope in unpolarized light, the trioxide appeared to consist of tablets about 20 p in length . This was, however, the habit of the monohydrate precursor . A more detailed examination under a linear magnification of 1000 showed the pitted surface and translucent appearance characteristic of a mass of disordered crystallites . The bulk of the material appeared to be optically isotropic . A few fragments of birefringent material, different in colour from the main phase, were present and may have been formed by the rehydration of the anhydrous oxide . (1) (2) (3) (4) (6) (a)
and E . RABINOWITCH, p. 227, .The Chemistry of Uranium, National Nuclear Energy Series, McGraw Hill, New York (1951) . 1 . SHEET, S . FRIED, and N . R. DAVIDSON, J. Amer . Chem. Soc ., 72, 2172 (1950). J. J. KATz and D . M. GRUEN, J. Amer . Chem . Soc ., 71, 2106 (1949) and 73, 1475 (1951) . P . PERio, Bull. Soc . Chim. Fr., p . 776 (1953) . W. H. ZACHARIASEN, Acta Cryst., 1, 265 (1948) . R. W. M . D•EYE, Harwell Report A .E.R.E. C/R 1524 (1954) . J. J . KATZ
309
3 10
E. WArr
X-ray Photography. A thin layer of the oxide was mounted on a base of "Sellotape" and protected from atmospheric moisture by a further layer of the same material. The resulting "sandwich" was mounted in the specimen holder of the Guinier camera . Before processing, a scale graduated at 0 . 1 mm intervals was printed along one edge of the film so as to correct for film shrinkage errors . 161 The X-ray diffraction photograph obtained after an exposure of twenty-four hours using CuKQ radiation is reproduced in Fig. 1 . THE DETERMINATION OF THE STRUCTURE
The most prominent lines on the photograph are very diffuse and correspond to a crystallite size of less than 0 . 1 It. For comparison purposes, the diffraction photograph of aluminium foil is also presented in Fig . I as an example of the type of line to be expected from a well crystallized specimen . The very faint sharp lines on the trioxide photograph may be attributed to the hydrate phase which was noticed in the optical examination . They are also present in the diffraction pattern of the monohydrate precursor . The lines of the trioxide phase may be indexed on the basis of a simple cubic cell with the cell constant :ao = 4. 138 ± 0 .005 kX . = 4-146± 0.005 A. The calculated and observed values of sins 0 are compared in Table 1 . TABLE 1 .-CALCULATED AND OBSERVED VALUES OF SIN' O AND THE REFLECTION INTENSIT.FS
hkl
Sin' °obs
sin' °cake
100 110 111 200 210 211 220
0. 0346 0 . 0691 0 . 1027 0 . 1385 0 . 1728 0 . 2069 0 .2740 0 . 3113 0 . 3452 0. 3798
0 .0346 0 .0691 0. 1036 0. 1382 0. 1728 0. 2073 0. 2765 0 . 3110 0 . 3456 0. 3802
300 and 221 310 311
obs
ICU)
h(2)
4(U)
S M W_ . W M M V W W VW VW
27 15 4 5 11 7 3 7 3 2
19 15 10 5 8 7 3 5 3 5
23 18 7 4 10 8 3 6 4 3
The volume of the unit cell V,, = 71 .27 As.' This corresponds to a cell content of one formula unit UO a on the reasonable assumption of an oxygen packing volume of 20 As. The calculated density p, = 6.663 g/cc. The uranium atoms must lie in positions of one-fold and the oxygens in positions of threefold multiplicity, respectively, In the cubic system there are one-fold positions at (000) and (}}}) and three-fold positions at (400), (040), . and (00}), and at (041), (40}); and (}40). These latter correspond to the midpoints of the cube edges and faces respectively . The uranium atom may be arbitrarily, assigned to the (000) position by a suitable choice of origin . There are then two alternative sets of three-fold positions for the oxygens. The values of the X-ray intensities to be expected in the, two cases were now calculated .
J t b . V V.
C C C
k
A cubic form of uranium trioxide
311
We have I oc ALp F12
where I is the intensity, A the absorption factor, L the combined Lorentz and polarization factor, p the number of crystal planes co-operating in a given reflection, and F the structure factor . The constant of proportionality is independent of the Bragg angle 0 . The absorption factor A was assumed to be near unity for the thin powder specimen used . The Lorentz-polarization factor used was the approximate form quoted by D'EYE. (6) The simplified structure factors for the two sets of atomic positions considered, are set out in Table 2 for the observed reflections . TABLE 2 .-SIMPLIFIED STRUCTURE FACTORS FOR THE ALTERNATIVE SETS OF ATOMIC POSITIONS
hkl
I
2
100
fu +f. fu - A fu - 3f0 fu +3f fu + o fu - f, fu + 3f„ fu + A fu +A If. - A fu -3fo
fu - //o' f - ./ a fu+ 3f f + 3fo fu - f(t f4 fu+3fo fu - fA A - . f0 f - .f. fu + 3fo
110 111 200 210 211 220 300 221 310 311
A
./
The atomic scattering factors used were those of THOMAS and FERMI for uranium (7) and of McWEENEY for oxygen . (8 ' No correction was made for the effects of thermal vibration . In Table 1, the observed intensities are compared with those calculated for the two alternative sets of oxygen positions and with the values given by the uranium alone with no oxygen contribution . The overall agreement, and in particular the relative intensities of 111 and 200, are best represented by the first set of intensities, from oxygen positions (400), (040), and (004) . Hence we may conclude that the cell has uranium at (000) and oxygens at (400), (040), and (00}) . DISCUSSION
Fig. 2 is a diagram of the proposed structure in isometric projection . Each uranium is octahedrally co-ordinated by oxygen and each oxygen linearly by uranium. The compound is isostructural with rhenium trioxide . 191 A feature of this structure is the "hole" at (444). In the related perowskite structure ABX.a this position is occupied by the larger cation A . The bond length U-O is equal to a/2 = 2 . 073 A. This is not significantly different from the value 2 . 08 A obtained by ZACHARIASEN for the U-0 1 distance in (7' ~B~
" Internationale Tabellen zur Bestimmung von Krystallstrukturen." Berlin (1935). R . McWEngEy, Acta Cryst., 4, 513 (1951) .
Bd. II p . 571 .
Borntraeger,
312
R WAIT
hexagonal U03 (loc. cit.). The close agreement between these bond lengths is consistent with ZACHARIASEN'S observationt10l of a monotonic increase in bond length with decreasing bond strength over a series of oxygen compounds of hexavalent uranium . Each oxygen forms two bonds only to adjacent uranium, so that the bond strength S = I as in the primary U-0 1 bonds in hexagonal UO B.
Fw. 2.
The chemical analyses indicate an oxygen content corresponding to the formula U02.82. The structure is one of considerable rigidity, and it would easily be possible to remove an occasional oxygen at random without causing the collapse of the lattice . The net charge balance would be maintained, by a . diminution of the average uranium valency. ACKNOWLEDGEMENTS
The author is greatly indebted to Drs . R . W. M. D'EYE and many valuable discussions .
J. K. DAWSON
R. W. G. WYCKOFF, "Crystal Structures," Vol . I. tnterscicnce Publishers Inc., New York, 1948. 1101 W. H. ZACHARIASEN, .4cta Cryst., 7, 795 (1954).
for