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Preparation of single crystals of EuCl3 and related polyvalent halides
Journal of Crystal Growth 6 (1970) 147—150
North-Ho//and Publishing Co., Amsterdam
PREPARATION OF SINGLE CRYSTALS OF EuC13 AND RELATED POLYVALENT HALIDES
STANLEY MROCZKOWSKI Yale University, Hammond Labora for,, Neu Haven, Connecticut 06520, U.S.A.
Received 10 July 1969
2~have beenofgrown underEuCI3 a protective atmosphere of at~, Cl Single crystals anhydrous containing only 0.05 Eu 2~content has been deter2 using the Bridgman technique. The Eu
mined by measuring the magnetic susceptibility of the grown crystals. Eu2~evenIt inis the found polycrystalline that chlorination raw material reduces of the EuCIamount of 3.
1. Introduction
In addition, we shall describe a method of determining the valence of the Eu in EuCl~using magnetic susceptibility measurements.
The growth of good single crystals containing ions which may occur in more than one valency state is generally quite difficult. If, as is often the case, the preparation involves the use of high temperatures, a state of mixed generally andproperties this will frequently lead valency to undesirable andresults, complex for the final product. In some cases, the mixed valency may even prevent the growth of single crystals of any reasonable size, A good example of this problem is provided by the anhydrous rare earth halides of Sm, Eu and Yb. Since Eu~3has a nonmagnetic ground state (7F 0) EuCI3 is of particular importance as a diluting host lattice for a variety of low temperature experiments in place of LaCl3 with which it is isostructural. This provides an opportunity for investigating the effects of small changes in the lattice parameters on such measurements as the paramagnetic resonance of single ions and pairs’), or for an estimate of the lattice specific heat of the magnetic trichlorides. In this paper, we shall describe a method of growing 3 volume which single crystals of EuCl3 of about 1 cm are essentially free of any divalent europium. The method involves three important stages: 1) the preparation of pure polycrystalline anhydrous EuCl free of oxychlorides and other impurities, 2) chlorina-3 tion to remove the Eu~2formed, and 3) growth of single crystals by the Bridgeman method, using a sealed tube containing Cl 2 gas under pressure.
2. Experimental method Several methods for the preparation of anhydrous 2). The chlorideswhich have isbeen described method generally usedinis the oneliterature which has been very successful in many preparations of other rare earth chlorides, i.e., that of dehydrating the hexahydrated rare earth trichlorides under vacuum in the presence of amnionium chloride3). Unfortunately this method cannot be used in our case because in a vacuum solid EuCI3 completely dissociates into 4) EuCI2. (Cd Other methods using halides of carbon 4, 5) (SOd CHCI3) or sulphur 2, S2C12) were likewise found to be unsatisfactory, as they introduce unwanted impurities such as EuS~or small carbon particles which prevented the growth of single crystals. To overcome these various difficulties we have tried a rather different method, using a stream of HC1 gas to dehydrate the hydrated trichloride. With suitable control the of temperature this yielded materialoffree any impurties, and it the is, inanhydrous fact, the method we have used to prepare many of the other rare earth trichlorides. In the case of europium it is still 2which is formed to necessary to was convert the Eu~ Eu~3 and this achieved by exposing the dehydrated material to a stream of dry chlorine gas at a carefully controlled temperature of 5 10—530 °C. When the chlorination was judged to be complete the material
148
STANLEY MROCZKOWSKI
was sealed off together with Cl2 gas at 2.5 atm pressure (corresponding to 6 atm at the melting point), The apparatus used for this procedure was similar to that described by Fong6). andInYocom for we the have purificaour case the tion of SrCI2 and BaCI2to a large H exhaust side connected 2S04 trap which gives us a slight over-pressure condition and prevents diffusion of air into the system. The whole apparatus is very flexible and can be easily modified for each step of the whole preparation process with only minor changes. One of the most important parts in the system is the _____
“quartz frit tube” (see fig. I) which allows one to manipulate the material from step to step without exposure to the air. The quartz frit tube has two side arms, one for equipped introducingwith a stream of dry ampule, Cl2 gas, and the second, a transfer for tapping material into a thick walled quartz crystal growing tube. The crystal growing tube has a 2 inch long capillary tube at the bottom and a reinforced wall on the seal-off neck to eliminate the danger of rapid expansion and bursting during seal-off. The crystal growing and annealing process is achieved by passing the material through the temperature gradient of a Bridgernan furnace 24 inches long. Care is taken to keep the temperature in the top part of the furnace just aboveofthe pO~flt(T~,eit + 14(see °C)while bottom part themelting furnace is unheated fig. 2). the In passing
Cl 2 OUT
:
3
Cl2 lN—~
4
~
,y
200
~.
I1~
400
600
T(°C)
Fig. 2.
Temperature gradient of a Bridgeman furnace 24 inches long.
through the temperature gradient from the top to the bottom of the furnace the crystal undergoes a natural annealing process. We have also discovered that the actual temperature is not critical, but rather the purity of the material and the high pressure of Cl2 are of prime importance (as is true for all R. E. trichlorides). It was found that a clear yellow crystal (Tmeit 623 °C+ 2°)could be obtained in the above manner, but if a significantly lower chlorine pressure was used a dark green polycrystaline material was formed corresponding to a mixture 2byofthe EuCl2 presence and EuCI3. of Cl The suppression of the Eu~ 2 gas has been based on the work done by Novikov and =
—
________________
Fig. I. Quartz frit tube showing: (I) side arm for dry Cl2 gas; (2) side arm for tapping material into crystal growing boat; 3) transfer ampule; (4) quartz frit; (5) crystal growing boat with capillary tip; (6) 35/40 changeable ball and socket joint; (7) furnace.
7
Polyachenok
.
.
.
) who studied the dissociation of Eud13
PREPARATION
OF SINGLE CRYSTALS OF
EuCI3
and other polyvalent rare earth chlorides. It was also possible to use essentially the same method to grow single crystals of SmCI3. 3. Crystal analysis The purity and composition of the grown crystals have been checked by several methods, 3. I. CHEMICAL ANALYSIS We have used the gravimetric and volumetric methods recommended by McCoy8). The results are tabulated in table I. Using these two conventional methods the
AND RELATED
POLYVALENT
149
HALIDES
centration. Moreover, as l/T—~0, , (l—x---y)y0. The result for the sample with the lowest impurity concentration shows very little deviation from the Van Vleck temperature independent susceptibility. Extrapolating to l/T = 0 we can compare the actual and predicted3.Theoretically magnitudes ofone the finds9) Van Vleck susceptibility 0.00625 emu/mole for Eu~ for this term while our measurements give a value of 0.002 emu/mole. We have checked this method by measuring the impurities in a sample with a known amount of dopant (~ I ~ Gd~3in EuCl 3). From the susceptibility measurements (see fig. 3) we find an —÷
TABLE I
.irn~.
Result of volumetric and gravimetric analysis of EuCI3 Run
Cl calculated
2 3
(°)
Cl found by gravimetric method (°/)
41.17 41.17 41.17
41.2 41.3 41.3
mote
Cl found by titration
.120
-
.110
-
(°,,)
.100
-
41.2 41.2
.090
-
MAGNETIC SUSCEPTIBiLiTY
where Xo
=
(I —x—y)
-
.040
-
.030 .025 0(5 ~020
-
~
0.0
If the total susceptibility ~is the sum of contributions from the various substances which comprise the sample, and if x and y denote the concentration of the most prominent impurities 42, x = the concentration of Eu 3, y =write the concentration of Gd~ then one can =
.070
xc
Xo +
+
T
yc’ -
T’
susceptibility of pure Eu~3.Since Eu43
is diamagnetic Xo is the sum of a diamagnetic term, which is negligible, and a much larger Van Vleck term, which3isis temperature since isoelectronicindependent. with Eu~2Furthermore, then c = c’ (c’ = Gd~ 7.9 emu/mole of Gd43). A measurement of~ versus l/Tshould yield a straight line whose slope is proportional to the impurity con-
3 .---
% Gd
.060.050 -
chlorine content can only be estimated to a rather limited accuracy of ±0.5~’, and this does not solve the problem of Eu42 in EuCl 3. Therefore magnetic susceptibility criterion for purity. measurements have been used as a 3.2.
.080
-
-~
.26% Eo~ 2
0.2
oi~r.;~~
-
~_-~‘
0.4
0.6
0.8
09% E~
1.0
12
Fig. 3. Result of magnetic susceptibility measurement between 1.7 and 4.2 K on four samples: (—--————-—) polycrystaline material before chlorination; ( ) polycrystalline material after chlorination;(———)singlecrystalEuCl 3(———--)singlecrystal EuCI 3 ---I ‘Gd.
impurity concentration of 0.986°/. We thus conclude that we can determine any concentration of impurities which is greater than the uncertainty in the measurements due to our apparatus; this uncertainty corresponds to 0.001 of Gd43 in EuCl 3. ‘~
3. Laue-reflection that the X-ray sampleanalysis is a single crystal and has sixindicates fold symmetry. by the powder method shows that the crystal has an hexagonal structure with space group C6 3/m and a0 = 7,380 A, c0 = 4.128 A. .
X-RAY
150
STANLEY
MROCZKOWSKI
4. Conclusion It would appear that the general idea of growing crystals under pressure using the Bridgeman method with a sealed quartz tube could be used for many other systems where dissociation is a problem. Magnetic susceptibility techniques can then provide a simple and highly accurate criterion for sample purity in cases where mixed valencies may occur. Acknowledgements The author would like to thank Dr. W. P. Wolf who suggested the use of the magnetic susceptibility method as a very sensitive and accurate tool in the analysis used here and he would also like to thank Dr. R. ~ Birgeneau, C. A. Catanese, and J. C. Doran for their
very helpful discussions. This work was supported by the U.S. Atomic Energy Commission. References I) R. J. Birgeneau, M. T. Hutchings and W. P. Wolf, Phys. Rev. 179 (1969) 275. 2) N. I-I. Kiess, J. Res. NatI. Bur. Std. A 67 (1963) 343. 3) G. Garton, M. T. Hutchings, R. Shore and W. P. Wolf, J. Chem. Phys. 41 (1964) 1970. 4) J. F. Miller, S. E. Miller and R. C. Himes, J. Am. Chem. Soc. 81(1959) 4449. 5) C. Matignon and F. Bourion, Compt. Rend.(Paris)138(I904). 6) F. K. Fong and P. N. Yocom, J. Chem. Phys. 41 (l964) 383. 7) G. I. Novikov and 0. G. Polyachenok, Russ. Chcrn. Rev. 33 (l964) 343. 8) N. van McCoy, J. Am. Chem. Soc. 57(1935). 9) H. J, H. Vleck, Electric and Magnetic Susceptibilities (Oxford Univ. Press, 1932).
North-Ho//and Publishing Co., Amsterdam
PREPARATION OF SINGLE CRYSTALS OF EuC13 AND RELATED POLYVALENT HALIDES
STANLEY MROCZKOWSKI Yale University, Hammond Labora for,, Neu Haven, Connecticut 06520, U.S.A.
Received 10 July 1969
2~have beenofgrown underEuCI3 a protective atmosphere of at~, Cl Single crystals anhydrous containing only 0.05 Eu 2~content has been deter2 using the Bridgman technique. The Eu
mined by measuring the magnetic susceptibility of the grown crystals. Eu2~evenIt inis the found polycrystalline that chlorination raw material reduces of the EuCIamount of 3.
1. Introduction
In addition, we shall describe a method of determining the valence of the Eu in EuCl~using magnetic susceptibility measurements.
The growth of good single crystals containing ions which may occur in more than one valency state is generally quite difficult. If, as is often the case, the preparation involves the use of high temperatures, a state of mixed generally andproperties this will frequently lead valency to undesirable andresults, complex for the final product. In some cases, the mixed valency may even prevent the growth of single crystals of any reasonable size, A good example of this problem is provided by the anhydrous rare earth halides of Sm, Eu and Yb. Since Eu~3has a nonmagnetic ground state (7F 0) EuCI3 is of particular importance as a diluting host lattice for a variety of low temperature experiments in place of LaCl3 with which it is isostructural. This provides an opportunity for investigating the effects of small changes in the lattice parameters on such measurements as the paramagnetic resonance of single ions and pairs’), or for an estimate of the lattice specific heat of the magnetic trichlorides. In this paper, we shall describe a method of growing 3 volume which single crystals of EuCl3 of about 1 cm are essentially free of any divalent europium. The method involves three important stages: 1) the preparation of pure polycrystalline anhydrous EuCl free of oxychlorides and other impurities, 2) chlorina-3 tion to remove the Eu~2formed, and 3) growth of single crystals by the Bridgeman method, using a sealed tube containing Cl 2 gas under pressure.
2. Experimental method Several methods for the preparation of anhydrous 2). The chlorideswhich have isbeen described method generally usedinis the oneliterature which has been very successful in many preparations of other rare earth chlorides, i.e., that of dehydrating the hexahydrated rare earth trichlorides under vacuum in the presence of amnionium chloride3). Unfortunately this method cannot be used in our case because in a vacuum solid EuCI3 completely dissociates into 4) EuCI2. (Cd Other methods using halides of carbon 4, 5) (SOd CHCI3) or sulphur 2, S2C12) were likewise found to be unsatisfactory, as they introduce unwanted impurities such as EuS~or small carbon particles which prevented the growth of single crystals. To overcome these various difficulties we have tried a rather different method, using a stream of HC1 gas to dehydrate the hydrated trichloride. With suitable control the of temperature this yielded materialoffree any impurties, and it the is, inanhydrous fact, the method we have used to prepare many of the other rare earth trichlorides. In the case of europium it is still 2which is formed to necessary to was convert the Eu~ Eu~3 and this achieved by exposing the dehydrated material to a stream of dry chlorine gas at a carefully controlled temperature of 5 10—530 °C. When the chlorination was judged to be complete the material
148
STANLEY MROCZKOWSKI
was sealed off together with Cl2 gas at 2.5 atm pressure (corresponding to 6 atm at the melting point), The apparatus used for this procedure was similar to that described by Fong6). andInYocom for we the have purificaour case the tion of SrCI2 and BaCI2to a large H exhaust side connected 2S04 trap which gives us a slight over-pressure condition and prevents diffusion of air into the system. The whole apparatus is very flexible and can be easily modified for each step of the whole preparation process with only minor changes. One of the most important parts in the system is the _____
“quartz frit tube” (see fig. I) which allows one to manipulate the material from step to step without exposure to the air. The quartz frit tube has two side arms, one for equipped introducingwith a stream of dry ampule, Cl2 gas, and the second, a transfer for tapping material into a thick walled quartz crystal growing tube. The crystal growing tube has a 2 inch long capillary tube at the bottom and a reinforced wall on the seal-off neck to eliminate the danger of rapid expansion and bursting during seal-off. The crystal growing and annealing process is achieved by passing the material through the temperature gradient of a Bridgernan furnace 24 inches long. Care is taken to keep the temperature in the top part of the furnace just aboveofthe pO~flt(T~,eit + 14(see °C)while bottom part themelting furnace is unheated fig. 2). the In passing
Cl 2 OUT
:
3
Cl2 lN—~
4
~
,y
200
~.
I1~
400
600
T(°C)
Fig. 2.
Temperature gradient of a Bridgeman furnace 24 inches long.
through the temperature gradient from the top to the bottom of the furnace the crystal undergoes a natural annealing process. We have also discovered that the actual temperature is not critical, but rather the purity of the material and the high pressure of Cl2 are of prime importance (as is true for all R. E. trichlorides). It was found that a clear yellow crystal (Tmeit 623 °C+ 2°)could be obtained in the above manner, but if a significantly lower chlorine pressure was used a dark green polycrystaline material was formed corresponding to a mixture 2byofthe EuCl2 presence and EuCI3. of Cl The suppression of the Eu~ 2 gas has been based on the work done by Novikov and =
—
________________
Fig. I. Quartz frit tube showing: (I) side arm for dry Cl2 gas; (2) side arm for tapping material into crystal growing boat; 3) transfer ampule; (4) quartz frit; (5) crystal growing boat with capillary tip; (6) 35/40 changeable ball and socket joint; (7) furnace.
7
Polyachenok
.
.
.
) who studied the dissociation of Eud13
PREPARATION
OF SINGLE CRYSTALS OF
EuCI3
and other polyvalent rare earth chlorides. It was also possible to use essentially the same method to grow single crystals of SmCI3. 3. Crystal analysis The purity and composition of the grown crystals have been checked by several methods, 3. I. CHEMICAL ANALYSIS We have used the gravimetric and volumetric methods recommended by McCoy8). The results are tabulated in table I. Using these two conventional methods the
AND RELATED
POLYVALENT
149
HALIDES
centration. Moreover, as l/T—~0, , (l—x---y)y0. The result for the sample with the lowest impurity concentration shows very little deviation from the Van Vleck temperature independent susceptibility. Extrapolating to l/T = 0 we can compare the actual and predicted3.Theoretically magnitudes ofone the finds9) Van Vleck susceptibility 0.00625 emu/mole for Eu~ for this term while our measurements give a value of 0.002 emu/mole. We have checked this method by measuring the impurities in a sample with a known amount of dopant (~ I ~ Gd~3in EuCl 3). From the susceptibility measurements (see fig. 3) we find an —÷
TABLE I
.irn~.
Result of volumetric and gravimetric analysis of EuCI3 Run
Cl calculated
2 3
(°)
Cl found by gravimetric method (°/)
41.17 41.17 41.17
41.2 41.3 41.3
mote
Cl found by titration
.120
-
.110
-
(°,,)
.100
-
41.2 41.2
.090
-
MAGNETIC SUSCEPTIBiLiTY
where Xo
=
(I —x—y)
-
.040
-
.030 .025 0(5 ~020
-
~
0.0
If the total susceptibility ~is the sum of contributions from the various substances which comprise the sample, and if x and y denote the concentration of the most prominent impurities 42, x = the concentration of Eu 3, y =write the concentration of Gd~ then one can =
.070
xc
Xo +
+
T
yc’ -
T’
susceptibility of pure Eu~3.Since Eu43
is diamagnetic Xo is the sum of a diamagnetic term, which is negligible, and a much larger Van Vleck term, which3isis temperature since isoelectronicindependent. with Eu~2Furthermore, then c = c’ (c’ = Gd~ 7.9 emu/mole of Gd43). A measurement of~ versus l/Tshould yield a straight line whose slope is proportional to the impurity con-
3 .---
% Gd
.060.050 -
chlorine content can only be estimated to a rather limited accuracy of ±0.5~’, and this does not solve the problem of Eu42 in EuCl 3. Therefore magnetic susceptibility criterion for purity. measurements have been used as a 3.2.
.080
-
-~
.26% Eo~ 2
0.2
oi~r.;~~
-
~_-~‘
0.4
0.6
0.8
09% E~
1.0
12
Fig. 3. Result of magnetic susceptibility measurement between 1.7 and 4.2 K on four samples: (—--————-—) polycrystaline material before chlorination; ( ) polycrystalline material after chlorination;(———)singlecrystalEuCl 3(———--)singlecrystal EuCI 3 ---I ‘Gd.
impurity concentration of 0.986°/. We thus conclude that we can determine any concentration of impurities which is greater than the uncertainty in the measurements due to our apparatus; this uncertainty corresponds to 0.001 of Gd43 in EuCl 3. ‘~
3. Laue-reflection that the X-ray sampleanalysis is a single crystal and has sixindicates fold symmetry. by the powder method shows that the crystal has an hexagonal structure with space group C6 3/m and a0 = 7,380 A, c0 = 4.128 A. .
X-RAY
150
STANLEY
MROCZKOWSKI
4. Conclusion It would appear that the general idea of growing crystals under pressure using the Bridgeman method with a sealed quartz tube could be used for many other systems where dissociation is a problem. Magnetic susceptibility techniques can then provide a simple and highly accurate criterion for sample purity in cases where mixed valencies may occur. Acknowledgements The author would like to thank Dr. W. P. Wolf who suggested the use of the magnetic susceptibility method as a very sensitive and accurate tool in the analysis used here and he would also like to thank Dr. R. ~ Birgeneau, C. A. Catanese, and J. C. Doran for their
very helpful discussions. This work was supported by the U.S. Atomic Energy Commission. References I) R. J. Birgeneau, M. T. Hutchings and W. P. Wolf, Phys. Rev. 179 (1969) 275. 2) N. I-I. Kiess, J. Res. NatI. Bur. Std. A 67 (1963) 343. 3) G. Garton, M. T. Hutchings, R. Shore and W. P. Wolf, J. Chem. Phys. 41 (1964) 1970. 4) J. F. Miller, S. E. Miller and R. C. Himes, J. Am. Chem. Soc. 81(1959) 4449. 5) C. Matignon and F. Bourion, Compt. Rend.(Paris)138(I904). 6) F. K. Fong and P. N. Yocom, J. Chem. Phys. 41 (l964) 383. 7) G. I. Novikov and 0. G. Polyachenok, Russ. Chcrn. Rev. 33 (l964) 343. 8) N. van McCoy, J. Am. Chem. Soc. 57(1935). 9) H. J, H. Vleck, Electric and Magnetic Susceptibilities (Oxford Univ. Press, 1932).