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Crystallographic study of Rb3UO2F5
J. in,,rg, nut/. (,'hem.. 1972,Vol. 34. pp. 1575-1580. PergamonPress. Printedin Great Britain
CRYSTALLOGRAPHIC STUDY OF Rb3UO2F~ H. BRUSSET. N G U Y E N QUY DAO and A. R U B I N S T E I N - A U B A N Ecole Centrale des Arts et Manufactures, lnstitut de Chimie. 92. ('hatenay-Malabry. France (Received 9 A ttgost 1971 )
Abstract- R~UO~F~ crystallizes in the tetragonal lattice with the space group 14,/a. The dimensions of the cell are a - 9.53~ ,~,; c = 18.573 ,~,. Most of the crystals formed are twinned. One type of the twin observed is described. The compound is isostructural with K:,UO~F:,.
THIS crystallographic study of Rb3UO2F 5 is a part of our work on the physicochemical properties of the compounds formed in the ternary system UO2F2R b F - H 2 0 . The crystal structures of the homologous compounds M~UO2F~ (where M = K, NH4 and Cs) have already been elucidated. K3UO2F~ crystallizes in the tetragonal system[l], (NH4)3UO.,F~ in the monoclinic system[2] and Cs,~UO2F5 in the cubic system[31. Therefore it seemed interesting to know in which system Rb3UO2F5 would crystallize and to make a classification of these salts. EXPERIMENTAl. Rb:~UO2F~ is prepared by evaporating a concentrated solution which contains 20 moles of RbF and one mole of UOzF~. Small well-formed crystals of pseudocubic habit and yellow fluorescent colour were obtained. They have the same form as the ones described by Davidovich, Sergienko and Kalacheva[4]. All the crystals examined under the polarizing microscope showed that they were twinned in a complicated manner. The crystallographic studies were made using both single crystal and powder methods. For the single crystal methods, v,e used a Weissenberg camera with a Cu anticathode. After several attempts we obtained one crystal which was simply twinned and gave simple X-Ray diagrams for interpretation. ]'he powder diagrams were obtained with a Philips PW 1330 diffractometer with a goniometer mounted in the reflection position. The precision obtained in the determination of 0 is 0.01 ° for the Ka, Cu component. These diagrams were used for the refinement of tile parameters of the cell and the estimation of the observed intensities. T H E S T R U C T U R E O F Rb.~UO2F~
The symmetry observed in the Weissenberg diagrams showed that the compound crystallizes in the tetragonal system. The dimensions of the cell were measured from the diagrams of the rotation method and the Weissenberg method and then refined with the values of dh~t observed in the powder diagram using a least-square refinement method [5]. The values of the cell parameters are: a = 9.538 ± 0.002 c - - 18.573 ±0.004 A. W . H. Zachariasen,Acta crystallogr. 7,783 (1954). Nguyen Quy Dao, Bull. Soc. C h i n . Fr. 9, 3542 (1968). H. Brusset and Nguyen Quy Dao, J. inorg, nucl. (}hem. 33, 1365 ( 1971 ). R. L. Davidovich, V. 1. Sergienko and T. A. Kalacheva. Izv. Akad. N a u k , S S S R . Ser. K h i m . 8. 1678 (1968). 5. F. Madaule-Aubry. Unpublished work.
1. 2. 3. 4.
1575
1576
H. B R U S S E T , N. Q. D A O and A. R U B I N S T E I N - A U B A N
An examination of the diagrams of the single crystal diffraction method showed that the crystal is twinned. Before studying the extinction laws we had to explain the type of twin. At first, one can observe that lel and 12al have very similar values. The relative difference ( 2 a - c ) / 2 a is only 0.027. This small difference makes it easy for a twin to be formed in which two components i and ii have a common (012) plane, called the twin-plane, playing the role of a pseudosymmetry plane. The common axis a becomes a pseudo-tetrad axis. Figure 1 shows a section of such a crystal. The twin-angle ~, defined as the difference of the
~s
S~
/ I
Co12) Fig. 1. Section perpendicular to the pseudo-tetrad axis o f a RbaUO~F~ twinned crystal. (012) = twin plane; a = pseudo-tetrad axis; ~ = twin angle; et~ = rotation axis.
angle of a twinned crystal and of the well-formed single crystal, has the following value: 2b-c t g ¢ = 2b = 0 " 0 2 7 a n d ¢ = 1 0 3 3 ' . Figure 2 represents the (b*, ¢*) plane of the twinned crystal. As the rotation axis is the c, axis, which is not quite superposed on theb~ axis, the following phenomena will be seen in the rotation diagram: (a) The diagram can be well aligned for only one of the two elements of the twin. In our case, the aligned axis was the e, axis. (b) the angles formed by the same layers of the two elements increase from ~ (zero layer) to ~pn (higher layers). In theory, one can deduce ~ from the rotation diagram. Unfortunately, because of the (a) effect, it is impossible to estimate ~ experimentally. However, these two points were qualitatively observed in the diagrams. Figure 3 represents the Weissenberg plan of the twinned crystal, i.e. (a*, b*), and (a*, e*)~. in theory, the following effects should be seen: (a) The e~ direction is practically superposed on the b, direction with this difference: c = 2b. Then c~* is half b,*. For all the okl reflections where I = 2n + i, we have a single lattice which is indicated by the dotted lines in the figure.
Crystallographic study of
,~-~.v2 ~ /
~.
//
Rb3UO2Fs
1577
\
.
\\
ayer2 layer1 layerO Fig.
2.
Okl reciprocal lattice plane of the twinned crystal.
/
x3 ,4 I ' - -
x~ ~
/
.4 J 2/
I"
xll
K
| r
I,--
J
P
i
Fig. 3. Reciprocal lattice plane perpendicular to Ihc rotation axis.
in the Weissenberg diagrams, we did in fact observe single diffraction spots forming a single lattice (a, c)~ whose lines were twice as close as the (a, b), lattice. (b) a is common to the two components i and ii. Therefore the recorded diffraction spots hoo~ and hoo~ must be superposed (Fig. 4(a)). The reflections oko, and ool~ will be separate in the direction of b~* (or e?) a quantity which is proportional to xt = l ( a - (c/2)) cosec (Fig. 4(b)). The reflections hko, and ho6 will be differ-
1578
H. BRUSSET, N. Q. D A O and A. R U B I N S T E I N - A U B A N
ent in the two directions: in the ca direction, the distance is proportional to a while in the b~ direction, it is proportional to xt (Fig. 4(c)). All these points were observed on the Weissenberg diagrams. The interpretatiofi of the nature of the twin by X-Ray diffraction allowed us to observe the following conditions limiting possible reflections: hkl h + k + l = 2n hko h, ( k ) = 2n ool I = 4n.
The space group is 14,/a.
aii
(o)
bii t ci
I
(bJ
hi
-
hOI.
I
(c)
Fig. 4. Observed diffraction spots on Weissenberg O level diagrams.
The existence of the twin makes the determination of the structure of RthUO2Fs impossible using single crystal methods. However, the identity of the space group of Rb3UO2F5 and KaUO2F5 and also the very close values of the parameters of the cell seem to indicate that the two compounds are at least isostructural. To consolidate this hypothesis, we have calculated the theoretical intensities of the powder diagram. The powder diagram was obtained by the reflection method to avoid the very considerable absorption of the compound. The experimental intensities were estimated at their maximum intensity. The Lorentz-polarisation and the Kc~l-Kot2 corrections were worked out. The theoretical intensities were calculated with uranium and rubidium atoms in place of uranium and potassium atoms with their thermal values as in the KaUOzF~ structure[I]. The extremely close agreement between the observed and calculated intensities (Table l) suggests that the two compounds are isostructural and possibly isomorphous. CONCLUSION
In the structure of RbzUO2Fs, as in the other similar structures already studied, we can observe the pentagonal bipyramid UO2Fs 3- and alkali ions which crystallize in a pseudo-cubic cell. The comparison of these structures has already been made [3]. This study adds to our knowledge of the structures of the series. if we plot the values of the dimension of the cell, whose direction coincides with
I579
Crystallographic study of Rb,UO,F, Table I. Powder diagrams of Rb,,UO,F, Obs. dhkl
Cal &ICI
I/lo ohs.
5.45 5.19 4.15 4.bb 4.14 3..50 3.47 3-33 3.17
5.45 5.19
53 4
58 7
4.76
33 X.6
40 I?
‘.Xb 2.82 ?.XO 3.60
2.57 1.37 1.33 2.7-n 2.15 2.12 2.03 I .96 I.93 I .x7
cell A
4.b4 4.15
b
l/lo Cal. ~--
3,Sl 3.46 3.37 3.16 1.Kb 2.82 2.80 2.bl 2.M 2.38 2.33 2.29 2.15
I8.b I7 IO0 2.b 21 4 2.b 8.b
S.8 ‘5 20 IO0 X 24 5 3 b.2
IO 20 13 I? IX
12 I7 3 I? I7
2.12 2.01 I.9b
20 4 3.3
22 4 7
I .93 I.Xb
20 I5
25 20
/r/xl -II’ I03 x0 004 211 213 IO 220. 204 ,?T ___ 312. I lb 303. I I4 215 321 I07 J(X) 008 411.402 316.413.31’ 404.410. ?OX IW. 307 41 4~4.2’8 _ _ 219
dimension
ionic
radii :n
Fig. 5. C‘ell dimension (a for <‘\. Rb and K compounds. b for ammonium compound) vs. ionic radii of Cs’ . Rh-. K‘ and NH,
1580
H. BRUSSET, N. Q. DAO and A. R U B I N S T E I N - A U B A N
the UO2 2÷ axis, against the ionic radii of the alkali ions[6, 7], we find that the points are on a straight line (Fig. 5). As has been seen for the Cs3UO2F5 compound, all these structures are close-packing structures formed by UO2F~ 3and alkali ions. 6. L. Pauling, The Chemical Bond. p. 15 I, Cornell Univ. Press, New York (I 967). 7. R.C. Evans, Crystal Chemistry, p. 138, Cambridge Univ. Press (1964).
CRYSTALLOGRAPHIC STUDY OF Rb3UO2F~ H. BRUSSET. N G U Y E N QUY DAO and A. R U B I N S T E I N - A U B A N Ecole Centrale des Arts et Manufactures, lnstitut de Chimie. 92. ('hatenay-Malabry. France (Received 9 A ttgost 1971 )
Abstract- R~UO~F~ crystallizes in the tetragonal lattice with the space group 14,/a. The dimensions of the cell are a - 9.53~ ,~,; c = 18.573 ,~,. Most of the crystals formed are twinned. One type of the twin observed is described. The compound is isostructural with K:,UO~F:,.
THIS crystallographic study of Rb3UO2F 5 is a part of our work on the physicochemical properties of the compounds formed in the ternary system UO2F2R b F - H 2 0 . The crystal structures of the homologous compounds M~UO2F~ (where M = K, NH4 and Cs) have already been elucidated. K3UO2F~ crystallizes in the tetragonal system[l], (NH4)3UO.,F~ in the monoclinic system[2] and Cs,~UO2F5 in the cubic system[31. Therefore it seemed interesting to know in which system Rb3UO2F5 would crystallize and to make a classification of these salts. EXPERIMENTAl. Rb:~UO2F~ is prepared by evaporating a concentrated solution which contains 20 moles of RbF and one mole of UOzF~. Small well-formed crystals of pseudocubic habit and yellow fluorescent colour were obtained. They have the same form as the ones described by Davidovich, Sergienko and Kalacheva[4]. All the crystals examined under the polarizing microscope showed that they were twinned in a complicated manner. The crystallographic studies were made using both single crystal and powder methods. For the single crystal methods, v,e used a Weissenberg camera with a Cu anticathode. After several attempts we obtained one crystal which was simply twinned and gave simple X-Ray diagrams for interpretation. ]'he powder diagrams were obtained with a Philips PW 1330 diffractometer with a goniometer mounted in the reflection position. The precision obtained in the determination of 0 is 0.01 ° for the Ka, Cu component. These diagrams were used for the refinement of tile parameters of the cell and the estimation of the observed intensities. T H E S T R U C T U R E O F Rb.~UO2F~
The symmetry observed in the Weissenberg diagrams showed that the compound crystallizes in the tetragonal system. The dimensions of the cell were measured from the diagrams of the rotation method and the Weissenberg method and then refined with the values of dh~t observed in the powder diagram using a least-square refinement method [5]. The values of the cell parameters are: a = 9.538 ± 0.002 c - - 18.573 ±0.004 A. W . H. Zachariasen,Acta crystallogr. 7,783 (1954). Nguyen Quy Dao, Bull. Soc. C h i n . Fr. 9, 3542 (1968). H. Brusset and Nguyen Quy Dao, J. inorg, nucl. (}hem. 33, 1365 ( 1971 ). R. L. Davidovich, V. 1. Sergienko and T. A. Kalacheva. Izv. Akad. N a u k , S S S R . Ser. K h i m . 8. 1678 (1968). 5. F. Madaule-Aubry. Unpublished work.
1. 2. 3. 4.
1575
1576
H. B R U S S E T , N. Q. D A O and A. R U B I N S T E I N - A U B A N
An examination of the diagrams of the single crystal diffraction method showed that the crystal is twinned. Before studying the extinction laws we had to explain the type of twin. At first, one can observe that lel and 12al have very similar values. The relative difference ( 2 a - c ) / 2 a is only 0.027. This small difference makes it easy for a twin to be formed in which two components i and ii have a common (012) plane, called the twin-plane, playing the role of a pseudosymmetry plane. The common axis a becomes a pseudo-tetrad axis. Figure 1 shows a section of such a crystal. The twin-angle ~, defined as the difference of the
~s
S~
/ I
Co12) Fig. 1. Section perpendicular to the pseudo-tetrad axis o f a RbaUO~F~ twinned crystal. (012) = twin plane; a = pseudo-tetrad axis; ~ = twin angle; et~ = rotation axis.
angle of a twinned crystal and of the well-formed single crystal, has the following value: 2b-c t g ¢ = 2b = 0 " 0 2 7 a n d ¢ = 1 0 3 3 ' . Figure 2 represents the (b*, ¢*) plane of the twinned crystal. As the rotation axis is the c, axis, which is not quite superposed on theb~ axis, the following phenomena will be seen in the rotation diagram: (a) The diagram can be well aligned for only one of the two elements of the twin. In our case, the aligned axis was the e, axis. (b) the angles formed by the same layers of the two elements increase from ~ (zero layer) to ~pn (higher layers). In theory, one can deduce ~ from the rotation diagram. Unfortunately, because of the (a) effect, it is impossible to estimate ~ experimentally. However, these two points were qualitatively observed in the diagrams. Figure 3 represents the Weissenberg plan of the twinned crystal, i.e. (a*, b*), and (a*, e*)~. in theory, the following effects should be seen: (a) The e~ direction is practically superposed on the b, direction with this difference: c = 2b. Then c~* is half b,*. For all the okl reflections where I = 2n + i, we have a single lattice which is indicated by the dotted lines in the figure.
Crystallographic study of
,~-~.v2 ~ /
~.
//
Rb3UO2Fs
1577
\
.
\\
ayer2 layer1 layerO Fig.
2.
Okl reciprocal lattice plane of the twinned crystal.
/
x3 ,4 I ' - -
x~ ~
/
.4 J 2/
I"
xll
K
| r
I,--
J
P
i
Fig. 3. Reciprocal lattice plane perpendicular to Ihc rotation axis.
in the Weissenberg diagrams, we did in fact observe single diffraction spots forming a single lattice (a, c)~ whose lines were twice as close as the (a, b), lattice. (b) a is common to the two components i and ii. Therefore the recorded diffraction spots hoo~ and hoo~ must be superposed (Fig. 4(a)). The reflections oko, and ool~ will be separate in the direction of b~* (or e?) a quantity which is proportional to xt = l ( a - (c/2)) cosec (Fig. 4(b)). The reflections hko, and ho6 will be differ-
1578
H. BRUSSET, N. Q. D A O and A. R U B I N S T E I N - A U B A N
ent in the two directions: in the ca direction, the distance is proportional to a while in the b~ direction, it is proportional to xt (Fig. 4(c)). All these points were observed on the Weissenberg diagrams. The interpretatiofi of the nature of the twin by X-Ray diffraction allowed us to observe the following conditions limiting possible reflections: hkl h + k + l = 2n hko h, ( k ) = 2n ool I = 4n.
The space group is 14,/a.
aii
(o)
bii t ci
I
(bJ
hi
-
hOI.
I
(c)
Fig. 4. Observed diffraction spots on Weissenberg O level diagrams.
The existence of the twin makes the determination of the structure of RthUO2Fs impossible using single crystal methods. However, the identity of the space group of Rb3UO2F5 and KaUO2F5 and also the very close values of the parameters of the cell seem to indicate that the two compounds are at least isostructural. To consolidate this hypothesis, we have calculated the theoretical intensities of the powder diagram. The powder diagram was obtained by the reflection method to avoid the very considerable absorption of the compound. The experimental intensities were estimated at their maximum intensity. The Lorentz-polarisation and the Kc~l-Kot2 corrections were worked out. The theoretical intensities were calculated with uranium and rubidium atoms in place of uranium and potassium atoms with their thermal values as in the KaUOzF~ structure[I]. The extremely close agreement between the observed and calculated intensities (Table l) suggests that the two compounds are isostructural and possibly isomorphous. CONCLUSION
In the structure of RbzUO2Fs, as in the other similar structures already studied, we can observe the pentagonal bipyramid UO2Fs 3- and alkali ions which crystallize in a pseudo-cubic cell. The comparison of these structures has already been made [3]. This study adds to our knowledge of the structures of the series. if we plot the values of the dimension of the cell, whose direction coincides with
I579
Crystallographic study of Rb,UO,F, Table I. Powder diagrams of Rb,,UO,F, Obs. dhkl
Cal &ICI
I/lo ohs.
5.45 5.19 4.15 4.bb 4.14 3..50 3.47 3-33 3.17
5.45 5.19
53 4
58 7
4.76
33 X.6
40 I?
‘.Xb 2.82 ?.XO 3.60
2.57 1.37 1.33 2.7-n 2.15 2.12 2.03 I .96 I.93 I .x7
cell A
4.b4 4.15
b
l/lo Cal. ~--
3,Sl 3.46 3.37 3.16 1.Kb 2.82 2.80 2.bl 2.M 2.38 2.33 2.29 2.15
I8.b I7 IO0 2.b 21 4 2.b 8.b
S.8 ‘5 20 IO0 X 24 5 3 b.2
IO 20 13 I? IX
12 I7 3 I? I7
2.12 2.01 I.9b
20 4 3.3
22 4 7
I .93 I.Xb
20 I5
25 20
/r/xl -II’ I03 x0 004 211 213 IO 220. 204 ,?T ___ 312. I lb 303. I I4 215 321 I07 J(X) 008 411.402 316.413.31’ 404.410. ?OX IW. 307 41 4~4.2’8 _ _ 219
dimension
ionic
radii :n
Fig. 5. C‘ell dimension (a for <‘\. Rb and K compounds. b for ammonium compound) vs. ionic radii of Cs’ . Rh-. K‘ and NH,
1580
H. BRUSSET, N. Q. DAO and A. R U B I N S T E I N - A U B A N
the UO2 2÷ axis, against the ionic radii of the alkali ions[6, 7], we find that the points are on a straight line (Fig. 5). As has been seen for the Cs3UO2F5 compound, all these structures are close-packing structures formed by UO2F~ 3and alkali ions. 6. L. Pauling, The Chemical Bond. p. 15 I, Cornell Univ. Press, New York (I 967). 7. R.C. Evans, Crystal Chemistry, p. 138, Cambridge Univ. Press (1964).