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Improved growth method of (SN)x single crystals
Journal of Crystal Growth 55 (1981) 447—452 North-Holland Publishing Company
447
IMPROVED GROWTH METHOD OF (SN)~SINGLE CRYSTALS Ichiroh NAKADA The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Received 25 December 1980; manuscript received in final form 25 June 1981
The crystal growth of pure and sizable single crystals of polysulfur nitride (SN)~was improved by adopting a monitor system with a quadrapole mass spectrometer and a Pirani gauge. The mass spectrometer helped to find a temperature appropriate for trapping (SN) 2 selectively on a cold finger and removing other unnecessary or harmful materials produced by the thermal decomposition of (SN)4 as well as out-gassing water vapour from the glass wall. Leakage ofgasses in the vessel was monitored with the Pirani gauge. With a heat pipe the crystal tube is cooled locally so that only a small number of nuclei start to grow. (SN)~single crystals with dimensions of 1 to 6 mm in edge size have been obtained. The relation between the crystal habit and the crystallographic axes has also been determined.
1. Introduction Since the discovery of the electronic superconducting properties [11, the study of the polysulfur nitride (SN)~has become a subject of absorbing interest. The structure of (SN)~is based on a linear long chain stacked in three dimensions. At present the electrical properties are considered to be characteristic to an anisotropic three-dimensional metal rather than a quasi-one-dimensional metal as at first expected. However, certain problems cannot be understood with such a simple model. For example, in order to explain the Meissner effect, Dee et al. 12—41 had to propose a bundle model, in which the magnetization was related to the crystalline textile structure. Oda et al. [5,61measured DC and AC magnetic susceptibility and proposed a model of a weakly coupled filamentary superconductor in order to explain the susceptibility at the superconducting transition region. Yet, it has not been made clear whether such a behaviour is due to either the intrinsic molecular chain structure or the subboundaries of the bundle structure of the imperfectly crystallized (SN)~.In order to solve the above problems it is necessary to prepare single cyrstals with good molecular arrangement. This has not yet been achieved with present crystal growth technology. Single crystals of(SN)~are generally grown by the 0022-0248/81/0000—0000/$02.50 © 1981 North-Holland
spontaneous solid state polymerization of single crystals of (SN)2 prepared beforehand by the vapour transport method. (SN)2 is produced by the then-na! decomposition of (SN)4 passing through a heated silver wool catalyst. The general method has been described in detail by Mikulski et al. [7]. In order to find the conditions to grow good (SN)~crystals, some modifications were made to the crystal growth apparatus by attaching a quadrapole mass spectrometer and a Pirani gauge. Thus, the species produced by the thermal decomposition either useful and the vapour pressure in the glass vessel could be detected and measured and a preferable condition for the crystal growth process could be determined. For the purpose of producing a sizable crystal a heat pipe was used for local cooling, so that only a few nuclei started to grow at the contact region, and sizable (SN)~single crystals could be obtained.
2. Experimental The crystal growth system, which consists of a sublimation tube, cold finger, growth tube, mass spectrometer and Pirani gauge, is shown schematically in fig. 1. A NAG-531 quadrapole mass filter of Anelva Corporation is used as the mass spectrometer.
I. Nakada / Improved growth method of(SN}~single crystals
448
.e—cold
N2 gas
growth tube was cooled with ice to 0°C.The vessel
was then separated from the evacuation system with mass spe.
cold finger —~vac.PLrnP
(~1~l )..—growth tube
-,
ally at the wall of the cooled growth tube and tiny
I ft~~] I~
single crystals grew. The crystals of (SN)2 were transparent and colourless at first. They changed to opaque crystals with black luster within about one hour. The crystals increased their dimension overnight and then stopped growing. The growth tube was held at room tem-
1-i——ice
Ag-wool
war
heater(l)~iiuJ!?~’ heater(2)-’~Ik~IL,I—
(S
the stop cock. The sublimation of (SN)2 on the cold finger occurred as the temperature rose towards room temperature. The vaporized (SN)2 condensed gradu-
~
N)4’t~.I
perature for more than a week. By this time all the Fig. 1. Crystal growing system.
The typical procedure for crystal preparation is as follows. 4Torn, When the the silver pressure within the vessel wool was heated up toreached about 1 X i0~ 250°C.Then (SN)4 placed at the bottom of the vessel was warmed up to 100°Cto keep a suitable sublimation rate of (SN)4. The vessel was evacuated during the sublimation of (SN)4, which continued for about 5 h. For each experiment about 1 g of (SN)4 was used. The silver wool catalyst is a product of Merck Co. The wool inserted at a time was about 1 g. As remarked [7], silver sulfide and not the silver metal is the catalyst for the decomposition of (SN)4 into (SN)2. First, by passing (SN)4 the silver wool is
changed into Ag2S catalyst by chemical reaction in about I h. During the reaction a considerable amount of nitrogen gas is produced. As the reaction comes to the end the partial pressure of nitrogen in the vessel decreases markedly and (SN)2 is formed efficiently. From the weight increase of the silver wool it was decided that the wool changed into Ag2S all over the fibre. Then, the cold finger was cooled down to o
.
-
crystals changed into shiny golden materials. Then crystals were taken out of the growth tube and transferred to another glass tube in air. The tube was sealed with 0.5 atm of argon after evacuation. The sealed for ampoule then10putdays. in a furnace and kept 60°C morewas than After this, (SN)at 2 changed into (SN)~completely. (SN)~crystal degradntes in air, though quite gradually, and the shiny golden appearance changes to dirty blue-gray colour. Therefore, it is necessary to store the crystals in sealed ampoules. By local cooling of the growth tube with a heat pipe produced by Furukawa Electric Company, we could grow an agglomerate of larger crystals. The contact region is shown schematically in fig. 2. We have not yet been successful to confine the growth to one single piece of single crystal. The precise adjustment of the vapour pressure of (SN)2 in the vessel is neces-
rowth tube g ~~crystal
—
(
.
—40 C by flowing cold nitrogen gas which was produced by heating liquid nitrogen in a large Dewar with an introduced electric heater. The temperature was controlled by adjusting the flow rate of the cold nitrogen gas with the heater. A temperature of —40°C of the cold finger which trapped only (SN)2 selectively was determined by the mass spectrometer.
When all the starting charged source material of (SN)4 sublimed, the heating of the sublimation tube as 4ell as of the catalyst was stopped. Then, the
‘~.
~
epoxy resin
copper piece j1gre~e
th!nT)a~~ ~ V
ice ~—Dewar
Fig. 2. Cooling system with a heat pipe.
449
I. Nakada /Improved growth method of (SN)~single crystals
sary to grow only one crystal at the bottom of the
growth tube. A large single crystal is not always successfully grown with either the heat pipe cooling or simple cooling by iced water. However, it is possible to obtam, out of each batch, several large and good crystals in quality with edge size of 1—3 mm suitable for such a study as the Meissner effect.
3. Results and discussion 3.1. Crystal habit The crystals of both (SN)2 and (SN)~belongs to the monoclinic crystal system and their cyrstal structures have been analyzed by Baughman et al. [8] and Cohen et al. [9]. According to their results we have determined the relation between the crystal habit and the crystallographic orientation with X-ray oscillation method, and the result is shown in fig. 3. In the figure a2, b2 and c2 represent the crystallographic axes for (SN)2, while a~,b~,c~and (bc)~do for (SN)~. The (SN)2 crystal transforms into (SN)~without changing its external form. The fibre texture running along the b-axis of (SN)~can easily be seen with naked eye. As is shown in fig. 3 the typical crystal habit is a hexagonal prism. The prismatic axis runs along the a-axis of (SN)2. The angle between (a2b2)and (b2c2)-plane is about 106°,which is equal to the monoclinic angle 13 of (SN)2.
3.2. Mass-spectroscopic observation The mass spectrometer has detected various molecular species produced by the thermal decomposition during the sublimation of (SN)4. The quantitative analysis of the gases with the quadnapole mass spectrometer is not easy, as the ionization efficiency for each gas molecule has not been determined. In the following the output signals were shown without correction and, therefore, the result is qualitative. Fig. 4 shows the dependence of the formation of thermally decomposed materials in the vessel on the temperature of the silver wool catalyst. The efficiency of the catalyst for decomposing (SN)4 is nearly constant in the temprature range from 140 to 260°C at the usual sublimation rate of (SN)4 of 0.2—0.4 g/h. As the cold finger is cooled to about 10°C,the pressure of (SN)2 begins to decrease. The dependence of the vapour pressure of (SN)2 in the vessel on the temperature of the cold finger is shown in fig. 5. At —40°Cmore than 90% of (SN)2 in the ambient is trapped on the cold finger. From fig. 5 the activation energy for sublimation of (SN)2 is deduced to be 9.2 kcal/mol. The value is characteristic to those of
2~O 290
~
1QO(°c) 50
N° 2
~ioo
~
\b.
~c)—plarie x
2
~
SNH~~ ‘S
10
‘~
c~
c~ \
‘.
SN’
—I ~
\
52Ni
~ Ar
(bc)~—pIar,e Fig. 3. Relation between the crystal habit and the crystallographic axes both for (SN)~ and (SN)2.
th
Z0 22 24 1/TIx io~)
2.6
ZB
ao
Fig. 4. Temperature dependence of the amount of decomposed and out-gassed materials in the sublimation vessel.
-
I. Nakada / Improved growth method of(SN)~single crystals
450 -10-20 -.30 -40-50
If the cold finger is kept at —40°C, out-gassing
-601°C)
water as well as decomposed materials other than
.~100~
(SN)2 is removed from the vessel during sublimation.
—
~ 50’—~
It would provide a preferable condition for the
---—
growth of pure (SN)2 single crystals after the vessel .~.
20
has been separated from the pumping system. Though it is impossible to exclude the trace of water vapour from the vessel, as will be reported elsewhere [141, we have examined with NMR measure-
—\~
—\ ~
ments the degree of incorporation of water into the crystal. However, no signal for hydrogen could be found. the crystal Thismay shows be much that the lessamount than theofseveral hydrogen atomic in percents mentioned previously [13].
2
3)4.6 4.8 1/T(x10 Fig. 5. Dependence of the vapour pressure of (SN) 3.8404.24.4
2 in the
vessel on the temperature of the cold finger.
3.3. Measurement onidinary molecular crystals under the Van der Waals coupling, As far as the temperature of the cold finger is higher than —80°Cthe partial pressure of water in the vessel does not change. When the cold finger is cooled to liquid nitrogen temperature the water vapour in the vessel is tightly trapped there. The source of the water vapour in the vessel would be due to the out-gassing from the glass wall. As it is difficult to out-gas the vessel completely before the start of the experiment with such a simple detachable glass system, water is coming out continuously during the experiment. If the cold finger is cooled to the liquid nitrogen temperature as is usually done [7], much amount of water would be accumulated there. As the cold finger is warmed to room temperature after finishing the sublimation of (SN)4, the water vapour thus trapped comes out simultaneously with (SN)2. At present the harmful effects of the coexisting water vapour and the growth of(SN)~single crys-
tals has not been elucidated. However, the inclusion of hydrogen in (SN)~crystals has been remarked by Seel et a!. [10]. The degradation of (SN)~films with water was examined by Ishida et al. [II] and Rickert et al. [12]. Also the inclusion of hydrogen in the sublimate of(SN)~in vacuum is noted by Smith et a!. [13] by analysis with a mass spectrometer. Their reports suggest that the amount of hydrogen included in (SN)~could be several percents. Anyway, for the crystal growth it is beyond dispute that the growth in the purest ambient is most preferable.
with
Piranigauge
Fig. 6 shows the indication of the Pirani gauge for the change of the total pressure in the vessel after it is separated from the pumping system. The initial main component among the gases in the vessel is (SN)2. Since the pressure unit “Torn” on the ordinate is calibrated with respect to air, the pressure indication in fig. 6 is not quantitative to (SN)2. However, the pressure indication is proportional to the amount of vapour pressure of (SN)2 in the vessel at such a low pressure level. The temperature was about 25°C.The
dotted line in the fig. 6 is the extrapolated pressure component in the vessel due to leakage. From the figure it can be seen that the transport of (SN)2 finishes after about 10 h.
Q25 ~
~
—
~ ~
-~ °~‘~“°~‘
_________
0.022
~
o.oio.__-.~)~-~ii..... -
0.1
1
10
100
TIME (hours) ~.
-
-
-
Fig. 6. Change m the mternal pressure of the sublimation
vessel after the sublimation of (SN)4 was finished and the
vessel was closed to
the evacuating system.
I. Nakada / Improved growth method of (SN)~ single crystals
451
3.4. Grown crystals Fig. 7 shows the crystals grown by the ordinary method, where the growth tube was only immersed in a Dewar with ice [7] as is the case of fig. 1. As the cooled area is rather large, many crystals grew dispersedly on the wall at the place where the nucleation started. Each crystal is tiny, as the transported (SN)2 must be shared with many crystals. However, as a result of the fluctuation of the growing condition around them, some crystals happened to grow to a 3. We can select dimension about sizable 3 X 3 Xcrystals 3 mm suitable for meafrom severalofbatches surement of various solid properties. An agglomerate of crystals grown by the heat pipe method is shown in fig. 8. As the cooled region is small, the number of the nuclei is suppressed. But, at present we are not successful to reduce the number of nuclei to only one single piece. At any rate each piece of the grown crystals is larger than that produced by the former method. Some crystals are seen to grow into dimensions of more than 5 mm.
I
Fig. 8. Agglomerate of crystals of(SN)~grown with the heat pipe method. Marker represents 1 cm.
4. Characterization The crystals were not yet fully characterized. However, as it was described elsewhere [5], the reproducibility for the Meissner effect is satisfactory ______ __________
~
-~
among crystals from different batches as confirmed by the behaviour of the magnetic susceptibility in the superconducting transition region. Acknowledgement
~
_________
_____________
______
The author wishes to thank Professor S. Tamura and Mrs. F. Sakai for advice for the preparation of (SN) 4 which is the starting material of (SN)~.This work was supported by a Grant-in-Aid for Special
_____
Project Research from the Ministry of Education, -
3.
Science and Culture for “Chemical Research in DevelS
~es”~
Utilization
of
Nitrogen-Organic
References Fig. 7, Single crystals of (SN)~grown with the ordinary
method. Marker represents 1 cm.
(11 R.L. Greene, G.B. Street and L.J. Suter, Phys. Rev. Letters 34(1975)577.
452
I. Nakada / Improved growth method of (SN)~single crystals
[2J R.H. Dee, AJ. Berlinsky, J.F. Carolan, E. Klein, NJ. Stone, B.G. Turrell and G.B. Street, Solid State Cornmun. 22 (1977) 303. [31 R.H. Dee, D.H. Dollard, B.G. Turrell and J.F. Carolan, Solid State Comrnun. 24 (1977) 469. [41 R.H. Dee, J.F. Carolan, B.G. Turrell and R.L. Greene, Phys. Rev. B22 (1980) 174. [51 Y. Oda, H. Takenaka, H. Nagano and I. Nakada, Solid State Commun. 32 (1979) 659. . [61 Y. Oda, H. Takenaka, H. Nagano and I. Nakada, Solid State Commun. 35 (1980) 887. [71C.M. Mikulski, P.J. Russo, M.S. Saran, A.G. MacDiarmid, A.F. Garito and A.J. Heeger, J. Am. Chem. Soc. 97 (1975) 6358. [81 R.H. Baughman and R.R. Chance, J. Chem. Phys. 64 (1976) 1869.
[9] M.J. Cohen, A.F. Garito, A.J. Heeger, A.G. MacDiarmid, C.M. Mikuiski, M.S. Saran and J. Kleppinger, J. Am. Chem. Soc. 98 (1976) 3844. [101 M. Seel, T.C. Collins, F. Martino, D.K. Rai and J. Ladik, Phys. Rev. B18 (1978) 6460. [ill H. Ishida, S.E. Richert, A.J. Hopfinger, J.B. Lando, E. Baer and J.L. Koening, J. Appl. Phys. 51(1980) 5188. [12~ S.E. Richert, H. Ishida, J.B. Lando, J.L. Koening and E. Baer, J. Appl. Phys. 51(1980) 5194. [131 R.D. Smith, J.R. Wyatt, D.C. Weber, JJ. DeCorpo and F.E. Saalfeld, Inorg. Chem. 17 (1978) 1639. [14] H. Nishihara, I. Nakada and K. Satoh, J. Phys. Chem. Solids, submitted.
447
IMPROVED GROWTH METHOD OF (SN)~SINGLE CRYSTALS Ichiroh NAKADA The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Received 25 December 1980; manuscript received in final form 25 June 1981
The crystal growth of pure and sizable single crystals of polysulfur nitride (SN)~was improved by adopting a monitor system with a quadrapole mass spectrometer and a Pirani gauge. The mass spectrometer helped to find a temperature appropriate for trapping (SN) 2 selectively on a cold finger and removing other unnecessary or harmful materials produced by the thermal decomposition of (SN)4 as well as out-gassing water vapour from the glass wall. Leakage ofgasses in the vessel was monitored with the Pirani gauge. With a heat pipe the crystal tube is cooled locally so that only a small number of nuclei start to grow. (SN)~single crystals with dimensions of 1 to 6 mm in edge size have been obtained. The relation between the crystal habit and the crystallographic axes has also been determined.
1. Introduction Since the discovery of the electronic superconducting properties [11, the study of the polysulfur nitride (SN)~has become a subject of absorbing interest. The structure of (SN)~is based on a linear long chain stacked in three dimensions. At present the electrical properties are considered to be characteristic to an anisotropic three-dimensional metal rather than a quasi-one-dimensional metal as at first expected. However, certain problems cannot be understood with such a simple model. For example, in order to explain the Meissner effect, Dee et al. 12—41 had to propose a bundle model, in which the magnetization was related to the crystalline textile structure. Oda et al. [5,61measured DC and AC magnetic susceptibility and proposed a model of a weakly coupled filamentary superconductor in order to explain the susceptibility at the superconducting transition region. Yet, it has not been made clear whether such a behaviour is due to either the intrinsic molecular chain structure or the subboundaries of the bundle structure of the imperfectly crystallized (SN)~.In order to solve the above problems it is necessary to prepare single cyrstals with good molecular arrangement. This has not yet been achieved with present crystal growth technology. Single crystals of(SN)~are generally grown by the 0022-0248/81/0000—0000/$02.50 © 1981 North-Holland
spontaneous solid state polymerization of single crystals of (SN)2 prepared beforehand by the vapour transport method. (SN)2 is produced by the then-na! decomposition of (SN)4 passing through a heated silver wool catalyst. The general method has been described in detail by Mikulski et al. [7]. In order to find the conditions to grow good (SN)~crystals, some modifications were made to the crystal growth apparatus by attaching a quadrapole mass spectrometer and a Pirani gauge. Thus, the species produced by the thermal decomposition either useful and the vapour pressure in the glass vessel could be detected and measured and a preferable condition for the crystal growth process could be determined. For the purpose of producing a sizable crystal a heat pipe was used for local cooling, so that only a few nuclei started to grow at the contact region, and sizable (SN)~single crystals could be obtained.
2. Experimental The crystal growth system, which consists of a sublimation tube, cold finger, growth tube, mass spectrometer and Pirani gauge, is shown schematically in fig. 1. A NAG-531 quadrapole mass filter of Anelva Corporation is used as the mass spectrometer.
I. Nakada / Improved growth method of(SN}~single crystals
448
.e—cold
N2 gas
growth tube was cooled with ice to 0°C.The vessel
was then separated from the evacuation system with mass spe.
cold finger —~vac.PLrnP
(~1~l )..—growth tube
-,
ally at the wall of the cooled growth tube and tiny
I ft~~] I~
single crystals grew. The crystals of (SN)2 were transparent and colourless at first. They changed to opaque crystals with black luster within about one hour. The crystals increased their dimension overnight and then stopped growing. The growth tube was held at room tem-
1-i——ice
Ag-wool
war
heater(l)~iiuJ!?~’ heater(2)-’~Ik~IL,I—
(S
the stop cock. The sublimation of (SN)2 on the cold finger occurred as the temperature rose towards room temperature. The vaporized (SN)2 condensed gradu-
~
N)4’t~.I
perature for more than a week. By this time all the Fig. 1. Crystal growing system.
The typical procedure for crystal preparation is as follows. 4Torn, When the the silver pressure within the vessel wool was heated up toreached about 1 X i0~ 250°C.Then (SN)4 placed at the bottom of the vessel was warmed up to 100°Cto keep a suitable sublimation rate of (SN)4. The vessel was evacuated during the sublimation of (SN)4, which continued for about 5 h. For each experiment about 1 g of (SN)4 was used. The silver wool catalyst is a product of Merck Co. The wool inserted at a time was about 1 g. As remarked [7], silver sulfide and not the silver metal is the catalyst for the decomposition of (SN)4 into (SN)2. First, by passing (SN)4 the silver wool is
changed into Ag2S catalyst by chemical reaction in about I h. During the reaction a considerable amount of nitrogen gas is produced. As the reaction comes to the end the partial pressure of nitrogen in the vessel decreases markedly and (SN)2 is formed efficiently. From the weight increase of the silver wool it was decided that the wool changed into Ag2S all over the fibre. Then, the cold finger was cooled down to o
.
-
crystals changed into shiny golden materials. Then crystals were taken out of the growth tube and transferred to another glass tube in air. The tube was sealed with 0.5 atm of argon after evacuation. The sealed for ampoule then10putdays. in a furnace and kept 60°C morewas than After this, (SN)at 2 changed into (SN)~completely. (SN)~crystal degradntes in air, though quite gradually, and the shiny golden appearance changes to dirty blue-gray colour. Therefore, it is necessary to store the crystals in sealed ampoules. By local cooling of the growth tube with a heat pipe produced by Furukawa Electric Company, we could grow an agglomerate of larger crystals. The contact region is shown schematically in fig. 2. We have not yet been successful to confine the growth to one single piece of single crystal. The precise adjustment of the vapour pressure of (SN)2 in the vessel is neces-
rowth tube g ~~crystal
—
(
.
—40 C by flowing cold nitrogen gas which was produced by heating liquid nitrogen in a large Dewar with an introduced electric heater. The temperature was controlled by adjusting the flow rate of the cold nitrogen gas with the heater. A temperature of —40°C of the cold finger which trapped only (SN)2 selectively was determined by the mass spectrometer.
When all the starting charged source material of (SN)4 sublimed, the heating of the sublimation tube as 4ell as of the catalyst was stopped. Then, the
‘~.
~
epoxy resin
copper piece j1gre~e
th!nT)a~~ ~ V
ice ~—Dewar
Fig. 2. Cooling system with a heat pipe.
449
I. Nakada /Improved growth method of (SN)~single crystals
sary to grow only one crystal at the bottom of the
growth tube. A large single crystal is not always successfully grown with either the heat pipe cooling or simple cooling by iced water. However, it is possible to obtam, out of each batch, several large and good crystals in quality with edge size of 1—3 mm suitable for such a study as the Meissner effect.
3. Results and discussion 3.1. Crystal habit The crystals of both (SN)2 and (SN)~belongs to the monoclinic crystal system and their cyrstal structures have been analyzed by Baughman et al. [8] and Cohen et al. [9]. According to their results we have determined the relation between the crystal habit and the crystallographic orientation with X-ray oscillation method, and the result is shown in fig. 3. In the figure a2, b2 and c2 represent the crystallographic axes for (SN)2, while a~,b~,c~and (bc)~do for (SN)~. The (SN)2 crystal transforms into (SN)~without changing its external form. The fibre texture running along the b-axis of (SN)~can easily be seen with naked eye. As is shown in fig. 3 the typical crystal habit is a hexagonal prism. The prismatic axis runs along the a-axis of (SN)2. The angle between (a2b2)and (b2c2)-plane is about 106°,which is equal to the monoclinic angle 13 of (SN)2.
3.2. Mass-spectroscopic observation The mass spectrometer has detected various molecular species produced by the thermal decomposition during the sublimation of (SN)4. The quantitative analysis of the gases with the quadnapole mass spectrometer is not easy, as the ionization efficiency for each gas molecule has not been determined. In the following the output signals were shown without correction and, therefore, the result is qualitative. Fig. 4 shows the dependence of the formation of thermally decomposed materials in the vessel on the temperature of the silver wool catalyst. The efficiency of the catalyst for decomposing (SN)4 is nearly constant in the temprature range from 140 to 260°C at the usual sublimation rate of (SN)4 of 0.2—0.4 g/h. As the cold finger is cooled to about 10°C,the pressure of (SN)2 begins to decrease. The dependence of the vapour pressure of (SN)2 in the vessel on the temperature of the cold finger is shown in fig. 5. At —40°Cmore than 90% of (SN)2 in the ambient is trapped on the cold finger. From fig. 5 the activation energy for sublimation of (SN)2 is deduced to be 9.2 kcal/mol. The value is characteristic to those of
2~O 290
~
1QO(°c) 50
N° 2
~ioo
~
\b.
~c)—plarie x
2
~
SNH~~ ‘S
10
‘~
c~
c~ \
‘.
SN’
—I ~
\
52Ni
~ Ar
(bc)~—pIar,e Fig. 3. Relation between the crystal habit and the crystallographic axes both for (SN)~ and (SN)2.
th
Z0 22 24 1/TIx io~)
2.6
ZB
ao
Fig. 4. Temperature dependence of the amount of decomposed and out-gassed materials in the sublimation vessel.
-
I. Nakada / Improved growth method of(SN)~single crystals
450 -10-20 -.30 -40-50
If the cold finger is kept at —40°C, out-gassing
-601°C)
water as well as decomposed materials other than
.~100~
(SN)2 is removed from the vessel during sublimation.
—
~ 50’—~
It would provide a preferable condition for the
---—
growth of pure (SN)2 single crystals after the vessel .~.
20
has been separated from the pumping system. Though it is impossible to exclude the trace of water vapour from the vessel, as will be reported elsewhere [141, we have examined with NMR measure-
—\~
—\ ~
ments the degree of incorporation of water into the crystal. However, no signal for hydrogen could be found. the crystal Thismay shows be much that the lessamount than theofseveral hydrogen atomic in percents mentioned previously [13].
2
3)4.6 4.8 1/T(x10 Fig. 5. Dependence of the vapour pressure of (SN) 3.8404.24.4
2 in the
vessel on the temperature of the cold finger.
3.3. Measurement onidinary molecular crystals under the Van der Waals coupling, As far as the temperature of the cold finger is higher than —80°Cthe partial pressure of water in the vessel does not change. When the cold finger is cooled to liquid nitrogen temperature the water vapour in the vessel is tightly trapped there. The source of the water vapour in the vessel would be due to the out-gassing from the glass wall. As it is difficult to out-gas the vessel completely before the start of the experiment with such a simple detachable glass system, water is coming out continuously during the experiment. If the cold finger is cooled to the liquid nitrogen temperature as is usually done [7], much amount of water would be accumulated there. As the cold finger is warmed to room temperature after finishing the sublimation of (SN)4, the water vapour thus trapped comes out simultaneously with (SN)2. At present the harmful effects of the coexisting water vapour and the growth of(SN)~single crys-
tals has not been elucidated. However, the inclusion of hydrogen in (SN)~crystals has been remarked by Seel et a!. [10]. The degradation of (SN)~films with water was examined by Ishida et al. [II] and Rickert et al. [12]. Also the inclusion of hydrogen in the sublimate of(SN)~in vacuum is noted by Smith et a!. [13] by analysis with a mass spectrometer. Their reports suggest that the amount of hydrogen included in (SN)~could be several percents. Anyway, for the crystal growth it is beyond dispute that the growth in the purest ambient is most preferable.
with
Piranigauge
Fig. 6 shows the indication of the Pirani gauge for the change of the total pressure in the vessel after it is separated from the pumping system. The initial main component among the gases in the vessel is (SN)2. Since the pressure unit “Torn” on the ordinate is calibrated with respect to air, the pressure indication in fig. 6 is not quantitative to (SN)2. However, the pressure indication is proportional to the amount of vapour pressure of (SN)2 in the vessel at such a low pressure level. The temperature was about 25°C.The
dotted line in the fig. 6 is the extrapolated pressure component in the vessel due to leakage. From the figure it can be seen that the transport of (SN)2 finishes after about 10 h.
Q25 ~
~
—
~ ~
-~ °~‘~“°~‘
_________
0.022
~
o.oio.__-.~)~-~ii..... -
0.1
1
10
100
TIME (hours) ~.
-
-
-
Fig. 6. Change m the mternal pressure of the sublimation
vessel after the sublimation of (SN)4 was finished and the
vessel was closed to
the evacuating system.
I. Nakada / Improved growth method of (SN)~ single crystals
451
3.4. Grown crystals Fig. 7 shows the crystals grown by the ordinary method, where the growth tube was only immersed in a Dewar with ice [7] as is the case of fig. 1. As the cooled area is rather large, many crystals grew dispersedly on the wall at the place where the nucleation started. Each crystal is tiny, as the transported (SN)2 must be shared with many crystals. However, as a result of the fluctuation of the growing condition around them, some crystals happened to grow to a 3. We can select dimension about sizable 3 X 3 Xcrystals 3 mm suitable for meafrom severalofbatches surement of various solid properties. An agglomerate of crystals grown by the heat pipe method is shown in fig. 8. As the cooled region is small, the number of the nuclei is suppressed. But, at present we are not successful to reduce the number of nuclei to only one single piece. At any rate each piece of the grown crystals is larger than that produced by the former method. Some crystals are seen to grow into dimensions of more than 5 mm.
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Fig. 8. Agglomerate of crystals of(SN)~grown with the heat pipe method. Marker represents 1 cm.
4. Characterization The crystals were not yet fully characterized. However, as it was described elsewhere [5], the reproducibility for the Meissner effect is satisfactory ______ __________
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among crystals from different batches as confirmed by the behaviour of the magnetic susceptibility in the superconducting transition region. Acknowledgement
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The author wishes to thank Professor S. Tamura and Mrs. F. Sakai for advice for the preparation of (SN) 4 which is the starting material of (SN)~.This work was supported by a Grant-in-Aid for Special
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Project Research from the Ministry of Education, -
3.
Science and Culture for “Chemical Research in DevelS
~es”~
Utilization
of
Nitrogen-Organic
References Fig. 7, Single crystals of (SN)~grown with the ordinary
method. Marker represents 1 cm.
(11 R.L. Greene, G.B. Street and L.J. Suter, Phys. Rev. Letters 34(1975)577.
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[9] M.J. Cohen, A.F. Garito, A.J. Heeger, A.G. MacDiarmid, C.M. Mikuiski, M.S. Saran and J. Kleppinger, J. Am. Chem. Soc. 98 (1976) 3844. [101 M. Seel, T.C. Collins, F. Martino, D.K. Rai and J. Ladik, Phys. Rev. B18 (1978) 6460. [ill H. Ishida, S.E. Richert, A.J. Hopfinger, J.B. Lando, E. Baer and J.L. Koening, J. Appl. Phys. 51(1980) 5188. [12~ S.E. Richert, H. Ishida, J.B. Lando, J.L. Koening and E. Baer, J. Appl. Phys. 51(1980) 5194. [131 R.D. Smith, J.R. Wyatt, D.C. Weber, JJ. DeCorpo and F.E. Saalfeld, Inorg. Chem. 17 (1978) 1639. [14] H. Nishihara, I. Nakada and K. Satoh, J. Phys. Chem. Solids, submitted.