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Synthesis and crystal growth of some thorium and uranium tetrahalides: ThCl4, ThBr4, UCl4 and UBr4
Journal of Crystal Growth 51(1981)11—16 © North-Holland Publishing Company
SYNTHESIS AND CRYSTAL GROWTH OF SOME THORIUM AND URANIUM TETRAHALIDES: ThCI4, ThBr4, UC14 and UBr4 M. HUSSONNOIS, J.C. KRUPA, M. GENET and L. BRILLARD Université de Paris-Sud, Institut de Physique Nucléaire, Laboratoire de Radiochimie, BP no. 1, F-91406 Orsay Cedex, France
and R. CARLIER Institut du Radium, Laboratoire Curie, F- 75005 Paris, France Received 15 February 1980; manuscript received in final form 29 May 1980
3 in size using the and Bridgman methodinis polycrystalline also described. form Somehas physical propA kilogram scale production of thorium and uranium tetrachioride tetrabromide been develerties of oped. Crystal these growth tetrahalides of single published crystals elsewhere, of several such cmas crystalline structure, refractive index, luminescence, vibrations modes, and phase transition are briefly mentioned. The optical properties ofThBr 4 and ThCL4 and the use of these materials as a host matrix for spectroscopic studies of tetravalent actinide ions is also reported.
1. Introduction
The synthesis and crystal growth procedures for thorium tetrahalides have been successfully used for uranium tetrahalides by a slight change of the different parameters.
The systematic study of the fluorescent [1] and radioluminescent properties [2,3] of ThBr4 and ThC14 and also the use of these materials as a host matrix for the spectroscopic studies of the actinide tetravalent ions require large quantities of these tetrahalides. The poor chemical purity, the undesired finely divided powder form and the high prices of commercially available products led us to produce them on kilogram scale. Among the numerous methods of preparation reviewed by Brown [4], we chose those involving direct combination of the elements. We have developed an original device to recover these very hygroscopic halides in a polycrystalline form which is more suitable for crystal growing. For this purpose, we chose the Bridgman method as the most convenient way to grow single crystals of these materials which are very hygroscopic and have a vapor pressure of about a hundred Torr at the melt. ing point. It was necessary to study the effects of temperature gradients and the speed of the crystal displacement through the furnace to obtain crystals of the best optical quality.
2. Synthesis of thorium and uranium tetrahalides Thorium and uranium tetrabromides and tetrachlorides have been known for many years and several preparations of these compounds have been already published and reviewed [4]. However, most of them lead to a final product which is more or less pure and a further purification (for instance by distillation) must be carried out again. The direct combination of metal and halogen is, from this point of view, more satisfactory and can be used for a rather large scale preparation. We also choose this way because of the easy availability of nuclear grade thorium and uranium metals. The direct reaction takes place in an evacuated silica tube positioned vertically in order to obtain removal of the material from the production place by gravity instead of using the usual gas flow. This is the 11
12
M. Hussonnois et al.
/ Synthesis and c,ystal growth
new feature of our device which can be used to produce large quantities of various halides. 2.1. Apparatus The main part of the reactor (fig. 1) is a silica tube (130 cm in length and 8.5 cm in inner diameter) which passes through a furnace. The upper end of the tube can be connected either to a vacuum system or to a chlorine or a bromine tank. The lower allows successively the introduction of: A thorium or an uranium rod placed in a vitreous carbon crucible which is hung in the hottest zone of the furnace. An inner silica tube lining the main tube. The top of the tube is located at 3 cm down the crucible. This tube is made rough by grounding to obtain a good condensation for the polycrystalline halide. This lining tube also allows us an easy recovery of the compound. In addition, the lower part is connected to a spherical pyrex funnel to collect the powder form which falls down. —
~
8
6
fl
7 2 3 1
Fig. 1. Schematic diagram of the synthesis reactor: 1 main silica tube ended by a pyrex cone, 2 lining silica tube and polycrystalline condensation position (X), 3 Viton 0 ring, 4 recovery of the powder form 5 to the vacuum line, 6 furnace, 7 vitreous carbon crucible and thorium or uranium rod 8 bromine or chlorine tank.
of some thorium and uranium tetrahalides
2.2. Initialproducts Thorium metal rods are of nuclear grade, 25 years old and 10 cm in length, 3 cm in diameter, each weighting about 600 g. Uranium metal rods are 3 cm in diameter, 5 cm in length and weigh about 500 g. Bromine is a RP grade product. Before use, it is frozen at liquid nitrogen temperature and pumped over. This has been repeated twice. Chlorine is delivered from a special gas container through a stainless steel manometer calibrated for absolute pressures. The purity is 99.7%. The handling of a quantity of about a kilogram of thorium or uranium in metallic or compound forms requires some special precautions namely from a point of view of toxicity and radiation hazards. For instance, the use of glove boxes, respirators and labclothes is necessary. 2.3. Operating method After the introduction of the metallic charge in the vitreous carbon crucible the whole system is pumped down to i0~4Torr for about 12 h. While still pumping, the temperature of the furnace is gradually increased to the reaction temperature. At this time, the reactor is isolated and the halogen gas admitted. For bromine, we get a self regulation of the gas pressure due to the vapor pressure which is about 200 Torr at room temperature. For chlorine, the absolute pressure is adjusted at 200 Torr. The reaction corresponding to M(Th or U) + 2 X 2(Cl2 or Br2) MX4 starts immediately with a very fine powder production which falls down at the bottom of the reactor vessel. At the same time, another part condenses at the top of the lining tube. The temperature of this deposition site roughly corresponds to the melting point of the produced halide. After several hours, complete obturation of the tube occurs and therefore powder production stops though the reaction is still going on. After several days, the gas consumption stops which means that the reaction is completed. The vapour condensation on the lining tube produces 3
an aggregate of crystals, several mm in size, that we named the polycrystalline form. The last step is to
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium and uranium tetrahalides
13
cool the reactor and furnace down and to evacuate the excess gas. Then the ampoule containing the powder is vacuum sealed. The reactor is filled up
induced fluorescence of uranyl ions produced by disproportionation of pentavalent uranium according to 2 U” U1” + U’~, when UC14 in polycrystalline
with dry argon gas and the silica jacket is removed as quickly as possible and transferred to a dry glove box. Some characteristics about synthesis of the various halides production are given in the table 1. In the thorium case, after the completion of the reaction, a grey and light residual material shaped like the initial thorium rod remains in the crucible. This residue is certainly due to the fact that thorium is not a pure but a sintered metal. On the other hand, a y-ray analysis spectrometry 232Th of the the radioactive crucible shows thatfamily) 228~ (daughter in natural is fixed inofthe crucible probably as a carbide. This chemical separation
form is dissolved in water. The well known yellowgreen fluorescence color of uranyl ions is greatly enhanced, and in fact can only be seen by cooling down the aqueous solutions to liquid nitrogen temperature. In the case of UBr4, the obtained polycrystalline product is not well crystallized. An interesting point is to note that there is an UV induced fluorescence of ThBr4 and ThC14 which is violet-blue. not But high when enough, the vacuum inside the tube is accidentally a red fluorescence appears for the parts which are in contact with the silica tube. It might be due to a ThOBr 2 or a ThSiO4 formation but the chemical nature of this product is not yet resolved.
during the synthesis, which upsets the equilibrium of 228Th the radioactive also responsible for activity variationchain, in theis newly prepared product. It can be measured by nuclear detection techniques and used for the crystal datation [5]. In the uranium chloride synthesis we observed that the powder is reddish brown which is the pentachloride color, while the condensed polycrystalline form is dark green corresponding to that of tetrachloride. X-rays diffraction patterns confirm the different chemical nature of each of these fractions. This might be explained in the following way: UC1 4 is first produced and a further chlorination occurs during the passage of the powder through the chlorine atmosphere. The deposition site temperature which is in good agreement with the melting point of UC14 is a further evidence of UC14 formation in the primary reaction rather than a UC15 production. Nevertheless the UC14 fraction contains a small amount of UC15. This can be readily shown by uy
3. Crystal growth Taking into account several conditions imposed by the physical properties of our compounds (high vapour pressure at the melting point, hygroscopicity, radioactive materials), we have chosen to grow the crystals in a sealed silica ampoule using the Bridgman method. An advantage of this method is that the apparatus used is quite simple and easy to operate with hygroscopic halides and there is no size limitation for the crystals. Furthermore, this method seems the most convenient and safest for growing thorium halide crystal doped with actinides. Nevertheless two disadvantages arise from contact between the melt and the silica tube: undesired nucleations can give rise to several crystals, and differential thermal contrac-
Table 1 Main synthesis characteristics Products
ThBr4 ThC14 UBr4 UC14
Temperature reaction (°C)
Timing (days)
Color
850 900 750 800
3 3 8 5
White White Brown Dark green
Yield
UV fluorescence
(%) 90 90 90 90
Blue Blue pale
14
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium
tions during cooling can produce thermal stresses that lead to partially cracked crystals. We determine that the best crystal quality was obtained with a temperature gradient of 100°C/cm and a growth rate of less than 1 cm per 24 h.
and uranium tetrahalides
______________ _______
_______________________________________________
3.1. Apparatus To get the right temperature gradient, we used two types of furnace: a classical resistance wire furnace, and a heat-pipe furnace loaded with sodium. We also developed a rather sophisticated device to have both a lowering (0.5 cm/day) and a rotation (30 rev/mm) motion, in vacuum, of the ampoule through the furnace. In this case, the tube supporting the ampoule at the tip, is cooled by water. We mostly used silica ampoules for all tetrahalides growing. For ThCl4 the ampoule is attacked by the melt and that might generate spurious crystals. Several attemps with other materials such as platinum, alumina, carbon nitride, vitreous carbon and graphite do not improve the quality of ThC14 crystals. Usually, the silica ampoule is 0.4 to 4 cm in inner diameter and 10 cm on length. As ThBr4 and ThCl4 present constitutional supercooling phenomena, ampoules ended by a tip (3 cm X 0.4 cm) were helpful, and when we started the growth of the halide, the lower part of the tip contained the solidified product. The ampoules, prior to growth, were filled with the polycrystalline materials in a dry glove box and vacuum sealed. During cooling, the formed crystal is affected by a negative temperature expansion coefficient, the crystal is shrinking and does not stick to the silica wall, which makes the crystal removal easy. The photograph on fig. 2 shows a typical piece of ThBr4 crystal. In all cases, we observed an amorphous black product floating at the surface of the melt whose chemical composition is unknown but in no case it is carbon. It might come from a chemical reaction with silica or with some residual water content. Concerning the chemical purity of our crystals, just ThBr4 has been analyzed. The analytical method used is spark mass spectrometry which needs a nonhygroscopic and electrical conducting material. So we were obliged to use a special procedure. First, ThBr4 in single crystal form was dissolved in very pure distilled water, then this aqueous solution was
Fig. 2: A typical piece of ThBr
4 crystal. Scale in cm.
evaporated to dryness, and finally the resulting pro. duct, which is Th02, was heated for 2 h at 800°C. The thorium dioxide was sintered with pure carbon powder to get electrical conducting electrodes. Data are given in table 2, values are in ppm related to the Th02 weight. We notice that the most important
Table 2 Spark mass spectrometry analysis of ThBr4 single crystal (values are in ppm related to Th02 weight, see text) Al 12 B 0.6
Ca Ce Cl Co Cr Cu F Fe K La Mn Na Nd Ni P Pb Pr pt S Si Ti V Zn
5 0.5 3 0.05 1 3 4 13 0.7 0.1 2 0.3 0.4 1.5 0.1 0.5 0.05 2 3 ~500) 0.15 0.15 2
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium and uranium tetrahalides
impurity is silicon, unfortunately bromine and uranium could not be detected by this method. Elements which are not mentioned in the table correspond to an amount of less than 0.05 ppm. Furthermore, the crystalline quality related to the defects
70 I
15 I
Th Br
4
and rate of dislocations has never been checked. 60 400
4. Some properties of ThBr4 and ThCl4 Among the thorium tetrahalides that we prepared, only ThBr4 and ThC14 have been extensively studied during the last years. We will first briefly review what we have already studied and then mention what is in progress now. ThBr4 and mCi4 are isomorphous and we have checked and found that the crystal structure of the compound that we have prepared is a body centered tetragonal form with an 14 i’~d D~space group symmetry having unit cell dimensions of a0 = 8.93 1 and 8.491 A and c0 = 7.965 and 7.483 A for ThBr4 and ThCl4 respectively. It appears that the structure would be theof~ these high temperature form [6] but the dimorphism halides is controversial [7,8]. —
According to their structure, these compounds are uniaxial and birefrigerent. They easily cleave with a face perpendicular to their optical axis. The refractive indices have been measured with an Abbe refractometer on a single crystal with two polished perpendicular faces, one being perpendicular to the optical axis. The incident wavelengths used are the characteristic emission lines of mercury, thallium, sodium and cadmium lamps, The variation of both indices, the ordinary n1 and extraordinary n2, with wavelengths is shown on fig. 3. Both crystals are optically positive with n2 > n One of the most important particularities of thorium bromide and chloride is their luminescent properties. Both are fluorescent, when they are excited by UV light and radioluminescent when excitation is performed with charged particles or heavy ions. These scintillation properties have been widely studied [2,3,9] and some possible applications using luminescence properties of ThBr4 were patented [10]. In addition, we may point out the permanent self-induced luminescence of these halides because of the radioactivity of the thorium [11]. More recently, we investigated infrared absorption —
~.
—
.
I
500 .
ThCL4
‘I
600
A(nm)
.
Fig. 3. Variation of refractive mdex values with wavelength: (.) ThBr~(.) ThC14.
and Raman spectroscopy experiments on these halides. The vibration energy levels and their symmetry labels have been determined for ThBr4 and ThC14 [12]. By this way, we also found a phase transition occurring for both crystals when they were cooled to liquid helium temperature [12]. Finally, our experiment research program with ThBr4 and ThCl4 is mainly concerned with spectroscopical studies on actinide ions. These crystals are used4~,U4~,Np~). as a host matrix Sf elements Now,forthetetravalent optical properties of (Pa tetravalent uranium in ThBr 4 or ThC14 doped crystal have been intensively developed [13 15]. We also plan to use UC14 and UBr4 in the same manner in order to get more information on the electronic structure of tetravalent uranium ion.
5. Conclusion The kilogram scale synthesis of ThBr4, ThCl4, UBr4 and UC14, and the production of large, pure or doped, single crystals, optically oriented, allow us to measure numerous physical properties of these materials with a good accuracy. The intrinsic qualities of these tetrahalides have opened the wide field of tetravalent actinide ion spectroscopy. Presently, we are growing ThBr4 and ThCl4 crystals doped with U~, Np~and Pa~ions.
References ~ij R. Carlier and M. Genet, Compt. Rend, (Paris) C281 (1975) 671. [2] J.C. Krupa, M. Genet, M. Hussonnois and R. Guillau-
16
M. Hussonnois et al.
/ Synthesis and crystalgrowth of some thorium and uranium tetrahalides
mont, Nucl. Instr. Methods 151 (1978) 405. [3] J.C. Krupa, M. Genet, S. Hubert, M. Hussonnois, F.H. Mentz and R. Guillaumont, IEEE Trans. Nucl. Sci. 26 (1979) 378. [4] D. Brown, Halides of Lanthanides and Actinides (Wiley, New York, 1968). [5] FJ-l. Mentz, Thesis, Université Paris-Sud (1979). [6] J.T. Masson, M.C. Jha, D.M. Bailey and P. Chiotti, J. Less-Common Metals 35 (1974) 331. [7] D. Brown, T. Hail, P.T. Moseley, J. Chem. Soc. Dalton Trans. 6 (1973) 686. [8] R. de Kouchkovsky, M.F. Le Cloarec, P. Delamoye, S. Hubert, to be published. [9] M. Hussonnois, J.C. Krupa, M. Genet and R. Guillaumont, Mater. Res. Bull. 12 (1977) 643.
[10] R. Carlier, M. Genet, H. Hussonnois and J.C. Krupa, French Patent No. 7519961 (1975); US patent No. 4,039,839 (1977). [11] R. Carlier, J.C. Krupa, M. Hussonnuis, M. Genet and R. Guillaumont, Nucl. Instr. Methods 143 (1977) 613. [12] S. Hubert, P. Delamoye, S. Lefrant, M. Lepostollec and M. Hussonnois, J. Solid State Chem., submitted. [13] M. Genet, P. Delamoye, N. Edelstein and J. Conway, J. Chem. Phys. 67 (1977) 1620. [14] P. Delamoye, S. Hubert, M. Hussonnois, J.C. Krupa, M. Genet, R. Guillaumont, C. Naud and R. Parrot, J. Luminescence 18/19 (1979) 76. [15] P. Delarnoye, S. Hubert, M. Hussonnois, J.C. Krupa, M. Genet, R. Guillaumont, N. Edelstein and J. Conway, J. Physique Colloque C4, 4,40 (1979) C4-173.
SYNTHESIS AND CRYSTAL GROWTH OF SOME THORIUM AND URANIUM TETRAHALIDES: ThCI4, ThBr4, UC14 and UBr4 M. HUSSONNOIS, J.C. KRUPA, M. GENET and L. BRILLARD Université de Paris-Sud, Institut de Physique Nucléaire, Laboratoire de Radiochimie, BP no. 1, F-91406 Orsay Cedex, France
and R. CARLIER Institut du Radium, Laboratoire Curie, F- 75005 Paris, France Received 15 February 1980; manuscript received in final form 29 May 1980
3 in size using the and Bridgman methodinis polycrystalline also described. form Somehas physical propA kilogram scale production of thorium and uranium tetrachioride tetrabromide been develerties of oped. Crystal these growth tetrahalides of single published crystals elsewhere, of several such cmas crystalline structure, refractive index, luminescence, vibrations modes, and phase transition are briefly mentioned. The optical properties ofThBr 4 and ThCL4 and the use of these materials as a host matrix for spectroscopic studies of tetravalent actinide ions is also reported.
1. Introduction
The synthesis and crystal growth procedures for thorium tetrahalides have been successfully used for uranium tetrahalides by a slight change of the different parameters.
The systematic study of the fluorescent [1] and radioluminescent properties [2,3] of ThBr4 and ThC14 and also the use of these materials as a host matrix for the spectroscopic studies of the actinide tetravalent ions require large quantities of these tetrahalides. The poor chemical purity, the undesired finely divided powder form and the high prices of commercially available products led us to produce them on kilogram scale. Among the numerous methods of preparation reviewed by Brown [4], we chose those involving direct combination of the elements. We have developed an original device to recover these very hygroscopic halides in a polycrystalline form which is more suitable for crystal growing. For this purpose, we chose the Bridgman method as the most convenient way to grow single crystals of these materials which are very hygroscopic and have a vapor pressure of about a hundred Torr at the melt. ing point. It was necessary to study the effects of temperature gradients and the speed of the crystal displacement through the furnace to obtain crystals of the best optical quality.
2. Synthesis of thorium and uranium tetrahalides Thorium and uranium tetrabromides and tetrachlorides have been known for many years and several preparations of these compounds have been already published and reviewed [4]. However, most of them lead to a final product which is more or less pure and a further purification (for instance by distillation) must be carried out again. The direct combination of metal and halogen is, from this point of view, more satisfactory and can be used for a rather large scale preparation. We also choose this way because of the easy availability of nuclear grade thorium and uranium metals. The direct reaction takes place in an evacuated silica tube positioned vertically in order to obtain removal of the material from the production place by gravity instead of using the usual gas flow. This is the 11
12
M. Hussonnois et al.
/ Synthesis and c,ystal growth
new feature of our device which can be used to produce large quantities of various halides. 2.1. Apparatus The main part of the reactor (fig. 1) is a silica tube (130 cm in length and 8.5 cm in inner diameter) which passes through a furnace. The upper end of the tube can be connected either to a vacuum system or to a chlorine or a bromine tank. The lower allows successively the introduction of: A thorium or an uranium rod placed in a vitreous carbon crucible which is hung in the hottest zone of the furnace. An inner silica tube lining the main tube. The top of the tube is located at 3 cm down the crucible. This tube is made rough by grounding to obtain a good condensation for the polycrystalline halide. This lining tube also allows us an easy recovery of the compound. In addition, the lower part is connected to a spherical pyrex funnel to collect the powder form which falls down. —
~
8
6
fl
7 2 3 1
Fig. 1. Schematic diagram of the synthesis reactor: 1 main silica tube ended by a pyrex cone, 2 lining silica tube and polycrystalline condensation position (X), 3 Viton 0 ring, 4 recovery of the powder form 5 to the vacuum line, 6 furnace, 7 vitreous carbon crucible and thorium or uranium rod 8 bromine or chlorine tank.
of some thorium and uranium tetrahalides
2.2. Initialproducts Thorium metal rods are of nuclear grade, 25 years old and 10 cm in length, 3 cm in diameter, each weighting about 600 g. Uranium metal rods are 3 cm in diameter, 5 cm in length and weigh about 500 g. Bromine is a RP grade product. Before use, it is frozen at liquid nitrogen temperature and pumped over. This has been repeated twice. Chlorine is delivered from a special gas container through a stainless steel manometer calibrated for absolute pressures. The purity is 99.7%. The handling of a quantity of about a kilogram of thorium or uranium in metallic or compound forms requires some special precautions namely from a point of view of toxicity and radiation hazards. For instance, the use of glove boxes, respirators and labclothes is necessary. 2.3. Operating method After the introduction of the metallic charge in the vitreous carbon crucible the whole system is pumped down to i0~4Torr for about 12 h. While still pumping, the temperature of the furnace is gradually increased to the reaction temperature. At this time, the reactor is isolated and the halogen gas admitted. For bromine, we get a self regulation of the gas pressure due to the vapor pressure which is about 200 Torr at room temperature. For chlorine, the absolute pressure is adjusted at 200 Torr. The reaction corresponding to M(Th or U) + 2 X 2(Cl2 or Br2) MX4 starts immediately with a very fine powder production which falls down at the bottom of the reactor vessel. At the same time, another part condenses at the top of the lining tube. The temperature of this deposition site roughly corresponds to the melting point of the produced halide. After several hours, complete obturation of the tube occurs and therefore powder production stops though the reaction is still going on. After several days, the gas consumption stops which means that the reaction is completed. The vapour condensation on the lining tube produces 3
an aggregate of crystals, several mm in size, that we named the polycrystalline form. The last step is to
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium and uranium tetrahalides
13
cool the reactor and furnace down and to evacuate the excess gas. Then the ampoule containing the powder is vacuum sealed. The reactor is filled up
induced fluorescence of uranyl ions produced by disproportionation of pentavalent uranium according to 2 U” U1” + U’~, when UC14 in polycrystalline
with dry argon gas and the silica jacket is removed as quickly as possible and transferred to a dry glove box. Some characteristics about synthesis of the various halides production are given in the table 1. In the thorium case, after the completion of the reaction, a grey and light residual material shaped like the initial thorium rod remains in the crucible. This residue is certainly due to the fact that thorium is not a pure but a sintered metal. On the other hand, a y-ray analysis spectrometry 232Th of the the radioactive crucible shows thatfamily) 228~ (daughter in natural is fixed inofthe crucible probably as a carbide. This chemical separation
form is dissolved in water. The well known yellowgreen fluorescence color of uranyl ions is greatly enhanced, and in fact can only be seen by cooling down the aqueous solutions to liquid nitrogen temperature. In the case of UBr4, the obtained polycrystalline product is not well crystallized. An interesting point is to note that there is an UV induced fluorescence of ThBr4 and ThC14 which is violet-blue. not But high when enough, the vacuum inside the tube is accidentally a red fluorescence appears for the parts which are in contact with the silica tube. It might be due to a ThOBr 2 or a ThSiO4 formation but the chemical nature of this product is not yet resolved.
during the synthesis, which upsets the equilibrium of 228Th the radioactive also responsible for activity variationchain, in theis newly prepared product. It can be measured by nuclear detection techniques and used for the crystal datation [5]. In the uranium chloride synthesis we observed that the powder is reddish brown which is the pentachloride color, while the condensed polycrystalline form is dark green corresponding to that of tetrachloride. X-rays diffraction patterns confirm the different chemical nature of each of these fractions. This might be explained in the following way: UC1 4 is first produced and a further chlorination occurs during the passage of the powder through the chlorine atmosphere. The deposition site temperature which is in good agreement with the melting point of UC14 is a further evidence of UC14 formation in the primary reaction rather than a UC15 production. Nevertheless the UC14 fraction contains a small amount of UC15. This can be readily shown by uy
3. Crystal growth Taking into account several conditions imposed by the physical properties of our compounds (high vapour pressure at the melting point, hygroscopicity, radioactive materials), we have chosen to grow the crystals in a sealed silica ampoule using the Bridgman method. An advantage of this method is that the apparatus used is quite simple and easy to operate with hygroscopic halides and there is no size limitation for the crystals. Furthermore, this method seems the most convenient and safest for growing thorium halide crystal doped with actinides. Nevertheless two disadvantages arise from contact between the melt and the silica tube: undesired nucleations can give rise to several crystals, and differential thermal contrac-
Table 1 Main synthesis characteristics Products
ThBr4 ThC14 UBr4 UC14
Temperature reaction (°C)
Timing (days)
Color
850 900 750 800
3 3 8 5
White White Brown Dark green
Yield
UV fluorescence
(%) 90 90 90 90
Blue Blue pale
14
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium
tions during cooling can produce thermal stresses that lead to partially cracked crystals. We determine that the best crystal quality was obtained with a temperature gradient of 100°C/cm and a growth rate of less than 1 cm per 24 h.
and uranium tetrahalides
______________ _______
_______________________________________________
3.1. Apparatus To get the right temperature gradient, we used two types of furnace: a classical resistance wire furnace, and a heat-pipe furnace loaded with sodium. We also developed a rather sophisticated device to have both a lowering (0.5 cm/day) and a rotation (30 rev/mm) motion, in vacuum, of the ampoule through the furnace. In this case, the tube supporting the ampoule at the tip, is cooled by water. We mostly used silica ampoules for all tetrahalides growing. For ThCl4 the ampoule is attacked by the melt and that might generate spurious crystals. Several attemps with other materials such as platinum, alumina, carbon nitride, vitreous carbon and graphite do not improve the quality of ThC14 crystals. Usually, the silica ampoule is 0.4 to 4 cm in inner diameter and 10 cm on length. As ThBr4 and ThCl4 present constitutional supercooling phenomena, ampoules ended by a tip (3 cm X 0.4 cm) were helpful, and when we started the growth of the halide, the lower part of the tip contained the solidified product. The ampoules, prior to growth, were filled with the polycrystalline materials in a dry glove box and vacuum sealed. During cooling, the formed crystal is affected by a negative temperature expansion coefficient, the crystal is shrinking and does not stick to the silica wall, which makes the crystal removal easy. The photograph on fig. 2 shows a typical piece of ThBr4 crystal. In all cases, we observed an amorphous black product floating at the surface of the melt whose chemical composition is unknown but in no case it is carbon. It might come from a chemical reaction with silica or with some residual water content. Concerning the chemical purity of our crystals, just ThBr4 has been analyzed. The analytical method used is spark mass spectrometry which needs a nonhygroscopic and electrical conducting material. So we were obliged to use a special procedure. First, ThBr4 in single crystal form was dissolved in very pure distilled water, then this aqueous solution was
Fig. 2: A typical piece of ThBr
4 crystal. Scale in cm.
evaporated to dryness, and finally the resulting pro. duct, which is Th02, was heated for 2 h at 800°C. The thorium dioxide was sintered with pure carbon powder to get electrical conducting electrodes. Data are given in table 2, values are in ppm related to the Th02 weight. We notice that the most important
Table 2 Spark mass spectrometry analysis of ThBr4 single crystal (values are in ppm related to Th02 weight, see text) Al 12 B 0.6
Ca Ce Cl Co Cr Cu F Fe K La Mn Na Nd Ni P Pb Pr pt S Si Ti V Zn
5 0.5 3 0.05 1 3 4 13 0.7 0.1 2 0.3 0.4 1.5 0.1 0.5 0.05 2 3 ~500) 0.15 0.15 2
M. Hussonnois et al.
/ Synthesis and crystal growth of some thorium and uranium tetrahalides
impurity is silicon, unfortunately bromine and uranium could not be detected by this method. Elements which are not mentioned in the table correspond to an amount of less than 0.05 ppm. Furthermore, the crystalline quality related to the defects
70 I
15 I
Th Br
4
and rate of dislocations has never been checked. 60 400
4. Some properties of ThBr4 and ThCl4 Among the thorium tetrahalides that we prepared, only ThBr4 and ThC14 have been extensively studied during the last years. We will first briefly review what we have already studied and then mention what is in progress now. ThBr4 and mCi4 are isomorphous and we have checked and found that the crystal structure of the compound that we have prepared is a body centered tetragonal form with an 14 i’~d D~space group symmetry having unit cell dimensions of a0 = 8.93 1 and 8.491 A and c0 = 7.965 and 7.483 A for ThBr4 and ThCl4 respectively. It appears that the structure would be theof~ these high temperature form [6] but the dimorphism halides is controversial [7,8]. —
According to their structure, these compounds are uniaxial and birefrigerent. They easily cleave with a face perpendicular to their optical axis. The refractive indices have been measured with an Abbe refractometer on a single crystal with two polished perpendicular faces, one being perpendicular to the optical axis. The incident wavelengths used are the characteristic emission lines of mercury, thallium, sodium and cadmium lamps, The variation of both indices, the ordinary n1 and extraordinary n2, with wavelengths is shown on fig. 3. Both crystals are optically positive with n2 > n One of the most important particularities of thorium bromide and chloride is their luminescent properties. Both are fluorescent, when they are excited by UV light and radioluminescent when excitation is performed with charged particles or heavy ions. These scintillation properties have been widely studied [2,3,9] and some possible applications using luminescence properties of ThBr4 were patented [10]. In addition, we may point out the permanent self-induced luminescence of these halides because of the radioactivity of the thorium [11]. More recently, we investigated infrared absorption —
~.
—
.
I
500 .
ThCL4
‘I
600
A(nm)
.
Fig. 3. Variation of refractive mdex values with wavelength: (.) ThBr~(.) ThC14.
and Raman spectroscopy experiments on these halides. The vibration energy levels and their symmetry labels have been determined for ThBr4 and ThC14 [12]. By this way, we also found a phase transition occurring for both crystals when they were cooled to liquid helium temperature [12]. Finally, our experiment research program with ThBr4 and ThCl4 is mainly concerned with spectroscopical studies on actinide ions. These crystals are used4~,U4~,Np~). as a host matrix Sf elements Now,forthetetravalent optical properties of (Pa tetravalent uranium in ThBr 4 or ThC14 doped crystal have been intensively developed [13 15]. We also plan to use UC14 and UBr4 in the same manner in order to get more information on the electronic structure of tetravalent uranium ion.
5. Conclusion The kilogram scale synthesis of ThBr4, ThCl4, UBr4 and UC14, and the production of large, pure or doped, single crystals, optically oriented, allow us to measure numerous physical properties of these materials with a good accuracy. The intrinsic qualities of these tetrahalides have opened the wide field of tetravalent actinide ion spectroscopy. Presently, we are growing ThBr4 and ThCl4 crystals doped with U~, Np~and Pa~ions.
References ~ij R. Carlier and M. Genet, Compt. Rend, (Paris) C281 (1975) 671. [2] J.C. Krupa, M. Genet, M. Hussonnois and R. Guillau-
16
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/ Synthesis and crystalgrowth of some thorium and uranium tetrahalides
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[10] R. Carlier, M. Genet, H. Hussonnois and J.C. Krupa, French Patent No. 7519961 (1975); US patent No. 4,039,839 (1977). [11] R. Carlier, J.C. Krupa, M. Hussonnuis, M. Genet and R. Guillaumont, Nucl. Instr. Methods 143 (1977) 613. [12] S. Hubert, P. Delamoye, S. Lefrant, M. Lepostollec and M. Hussonnois, J. Solid State Chem., submitted. [13] M. Genet, P. Delamoye, N. Edelstein and J. Conway, J. Chem. Phys. 67 (1977) 1620. [14] P. Delamoye, S. Hubert, M. Hussonnois, J.C. Krupa, M. Genet, R. Guillaumont, C. Naud and R. Parrot, J. Luminescence 18/19 (1979) 76. [15] P. Delarnoye, S. Hubert, M. Hussonnois, J.C. Krupa, M. Genet, R. Guillaumont, N. Edelstein and J. Conway, J. Physique Colloque C4, 4,40 (1979) C4-173.