- Email: [email protected]
Czochralski growth of Li3N crystals
Journal of Crystal Growth 43(1978)469—472 © North-Holland Publishing Company
CZOCHRALSKI GROWTH OF Li3N CRYSTALS E. SCHONHERR, G. MULLER and E. WINCKLER Max-Planck-Institut für Festkorperforschung, 7 Stuttgart 80, Federal Republic of Germany Received 14 October 1977; manuscript received in final form 28 November 1977
Large Li3N single crystals were grown by the Czochralski method. Starting material for crystal growth was synthesized from the elements. Decomposition of the melt was prevented by use of a stagnant N2 atmosphere (p = 700 Torr). Tungsten was found to be the most suitable crucible material which was not severely attacked by the Li3N melt. The growth of single crystals was complicated by the easy formation of numerous misorientated grains during the seeding process.
1. Introduction
by a BTS catalyst ~tand a molecular sieve with 4 A pores ~ respectively. The synthesis was carried out by two methods: In the first case [8,10] lithium was loaded into a Ni beaker which was placed into a stainless steel vessel of about 2 liters in volume. The vessel was first evacuated and then filled with nitrogen to a pressure of 10 atm. The vessel was then sealed and the lithium was heated. The reaction between Li and N2 took place when the lithium reached the temperature of approximately 150°C. Because the reaction exothermic = —39.4 ± 1 [11]) was further increase in(H~ temperature 0.3 kcal mol of starting material to about 500°Ctook place. The from nitrogen
Lithium nitride is an interesting material because of its high ionic conductivity [l—4]. Li3N is hexagonal [5], space group P6/mm [6], with lattice parametars a = 3.648 A, c = 3.875 A [61.In the structure of L13N there are Li2N layers perpendicular to the hexagonal c-axis. Li atoms which occupy the sites between respective N atoms of two layers can presumably easily move in the direction parallel to the layers [5]. Growth of small Li3N crystals up to 0.5 mm in diameter reported Zintl and from BrauerLi—Li3N [Sj andmelt laterwas byfirst other authorsby [6,7]. Since Li 3N melts congruently at 8 14°C [8,9] crystal growth of Li3N can be carried out by the Crochralski method. The present paper describes the growth of large Li3N single crystals by the Czochralski method and some phenomena observed during the growth of these crystals.
obtained product had glossy brown-red color. The fractured surface displayed small hexagonal platelets
/
2. Synthesis of Li3N Li3N was synthesized from 3N * lithium metal and purified nitrogen. Oxygen and water were removed *
*
.
.
k
.
Cerac/Pure, Inc. Milwaukee, Wisconsin, USA. Badisehe Anilin- and Soda-Fabrik AG, Ludwigshafen/ Rhein, Germany. Merck AG, Darmstadt, Germany.
-
Fig. 1. Fracture surface of Li3N ingot synthesized at 12 atm N2 and 150°Cinitial temperature. 469
470
F. Schönherr et al.
/ Czochralski growth of Li3N crystals
as shown by the SEM photograph in fig. 1. In a later stage the synthesis of Li3N was accomplished in the crystal growth chamber. The growth vessel containing Li metal was evacuated and when a vacuum of l0—~Torr was reached the chamber was filled with N2 (PN2 = 500 Torr) and30C/h. then heated up The reacwith rate of heating about 2 X I 0total pressure of tion started at aboutof 500°C.The nitrogen was then raised to approximately 700 Torr. This pressure was held constant up to the melting
Fig. 3. A 2 mm thick Li
point of Li3N.
growth axis (3 mm grid).
3N slice perpendicularly cut to the
‘if
3. Crystal growth
77
,
10
9
6
12 2
5
_____
_________
I
___________
Li3N crystals were grown in an equipment similar to that used previously for the growth of MnF2 crystals [12]. The apparatus is drawn schematically in fig. 2. It consists of a stainless stell vessel (2) and a stainless steel resistance heater (4) which is mounted on water cooled electrodes of nickel (3). The heat loss is reduced by one or two molybdenum shields (12). The outside wallcopper of the tubes vessel which is cooled by water from three contain smallcoming holes. The furnace temperature is controlled by a Pt/PtRh thermocouple (8) which is protected by an Al203 tube. The seed holder (9) is made of nickel and is water-cooled for removing the heat of solidification. Tungsten crucibles were used as containers for Li3N. New crucibles were first deoxidized by Li3N melt. The growth of crystals was carried out in a stagnant atmosphere of purified N2 at a pressure of 700 Torr. The Li3N crystals were pulled at a rate of about 5 mm/h and rotation speed of approximately 30 rpm. Boules up to 3 cm in diameter and 5 cm in length were grown. A slice of 2 mm in thickness perpendicularly cut to the growth axis is displayed in fig. 3.
I
e
3~
~
4. Results and discussion 5 cm
Fig. 2. Growth equipment: (1) basic plate (Ni); (2) vessel (SS); (3) electrodes water cooled; (4) furnace (SS), (5) pedestal; (6) crucible; (7) heat shields (Mo); (8) thermocouple (Pt/Pt Rh, shielded by A12O3); (9) seed holder water cooled (Ni); (10) water cooling tubes; (11) gas inlet; (12) heat shields (Mo).
The stagnant N2 atmosphere (PN2 = 700 Torr) prevented decomposition of Li3N into liquid Li and N2 gas. The decomposition of the melt was observed when the N2 pressure was lowered to about 400 Torr. This pressure is in good agreement with the extrapolated value of T 814°C[81.
~N2
=
275
Torr
at the melting point
F. Schönherr et al.
/ Czochralski growth of Li3N crystals
Deposition of Li3N was observed in the cool parts of the growth chamber. The mechanism of the transport in the vapor phase dissociative sublimation
isornotvaporization known but it may be enhanced by the presence of small amount ofmay water.be Major difficulties in growing single crystal boules arose during the seeding process. When the melt was touched with the seed the liquid crept along the seed as shown schematically in fig. 4a. Such a behaviour is probably due to the high surface tension and low density of the melt. The wetting 2was aboutsection. 0.8 cm In in in cross height fortoa other seed of 0.7 X 0.3(Ge, cm MnF contrast materials 2, Mg2Sn etc.) which were grown in the same equipment, it was impossible to melt the seed and achieve stationary condition as shown in fig. 4b [13,14]. Numerous misorientated grains originated their growth within the melt on the side walls of the seed as shown in fig. 4c. Moreover, it was impossible to neck the seed or to reduce the crystal radius to its initial size in a reasonable period of time. This indicated that the minimum joining angle [14] for Li3N was possibly slightly negative or even positive for small diameters of the seed. The reason for such a behavior could he incomplete wetting, high surface tension and low density of the melt, Most of the parasitic grains formed on the side walls of the seed ceased to grow when coming out of the melt during pulling. Polygonizatiori or twinning was not observed if pure melts and pulling rates less than 6 mm/h were used. The Li3N boules consisted of a few grains elongated iii the growth direction. Their main growth direction was found to be perpendicular to the c-axis. When seeds were orientated perpendicularly to the c-axis, some of the boules grew as single crystals at the lower end. Additional difficulties came from the strong chem ical reactivity of Li3N. Initially used graphite heaters reacted with residual H20 to form CO2 gas which caused a high loss of the melt and decomposition of the Li3N crystals. Crucibles of iron were badly attacked and the crystals decomposed during cooling while the crucible content foamed over. Crucibles of vitreous carbon broke in several small pieces before *
Preliminary analysis: The compound contained 20.4 wt% Li.
471
1LJLJL. mett
b
a
c
Fig. 4. Seeding: (a) initial stage, (b) stationary stage, (c) real stage.
the melting point of Li3N was reached. crucibles were strongly attacked by theMolybdenum melt which led to constitutional supercooling. Much less severe disturbance in crystal growth was observed in the case of W crucibles. Tungsten crucibles were found to be the most suitable for the growth of crystals, although the loss of W was 0.37 ±0.04 g/h for 16 runs when crucibles 50 mm in diameter were used and when the melt level was 30 mm high. The crucible loss decreased by a factor of 3 when previously grown crystals of Li3N were used as starting material. The loss was enormously increased when the nitrogen was contaminated by H20 or 02. The solidified melt which was left in the crucible after the crystal had been withdrawn was investigated by a scanning electron microscope (SEM) for the purpose of revealing the influence of the crucible material on crystal growth. Distribution of impurities was investigated using X-rays excited by electrons of the SEM.
.—i~-
---
__________________________________________ -______
_______
r10~ Fig. 5. Part of remaining melt which was in contact with W crucible. Bright layer: W—Li—N compound; upper dark layer: Li3N.
472
F. SchOnherr et al.
/ Gzochralski growth of
I,i
3N crystals
in pulled crystals. Investigation by optical emission analysis revealed an Al content of the order of 5 X l0—~wt%, and a Mg and Cu content of the order of 5 X l0~wt%.
Acknowledgement
-
-
11g. 6. The H3N layer is removed from the W- Il N compound shown in fig. 5. Crystals of a W—Li—N compound are displayed.
The solidified melt was always covered by a layer of an unknown fine crystalline W—Li---N compound * at the places where the melt was in contact with the crucible. Fig. 5 shows this layer as a bright zone. This layer could be easily separated from the solidified melt. Small cyrstals still of the same W—Li—N cornpound were found between the two layers as shown in fig. 6. In no case could W be detected in the Li3N part of the solidified melt by EDAX but Ca and Al and traces of Cu were present. Calcium was found to be precipitated along the grain boundaries of the Li3N crystals in the solidified melt. Similar results were obtained when Mo crucibles were used with the exception that Mo was found along grain boundaries. We assume that the solubiity of W in the Li3N melt is small compared to Mo since no W was found in crystals nor on grain boundaries. The better quality of crystals grown from W crucibles might be due to a reduceá constitutional supercooling. Small amounts of Al and Cu were found by EDAX
We thank Professor A. Rabenau for helpful discussions and Mrs. G. Fröhlich for preparing the seed crystals. We thank Mr. L. Viczian for the analysis of the W--Li--N compound.
References [11 E. Masdupuy, Ann. Chim. (Paris) 13 (1948) 1527. Boukamp and R.A. Huggins, Phys. Letters 58A (1976) 231. [3f R.A. Huggins, Electrochim. Acta 22 (1977) 773. [4] U. v. Alpen, A. Rabenau and G.H. Talat, App!. Phys. Letters 30 (1977) 621. [51F. Zinti and G. Brauer, Z. Elektrochcm. 41(1935)102. [61A. Rabenau and H. Schulz, J. Less Common Metals 50
[21 B.A.
(1976) 155.
[71M.D. (1970)Lyataya 1753.
and T.S. Bartnitskaya, Neorgan. Mater. 6
[81R.M. Yonco, F. Veleckis and V.A. Maroni, 1. Nucl. Mater. 57 (1975) 317. [91P.F. Adams, P. Hubberstey and RJ. Puiham, J. Less Common Metals 42 (1975) 1. [10] B. Neumann, C. Kroger and H. Haebler, Z. Anorg. Allgem. Chem. 204 (1932) 81. [11] P.A.G. O’Hare and Gerald K. Johnson, J. Chem. Thermodynamics 7 (1975) 13. [121E. Schonherr, DFG-Kristal!zuchtung, Konsmission für Festkorpterforschung, Bonn- Bad Godesbcrg, Germany, (1971) p. 10. [13] D. Geist and P. Grosse, Z. Angew. Physik 14 (1962) 105. [141K. Mika and W. Uelhoff, J. Crystal Growth 30 (1975) 9.
CZOCHRALSKI GROWTH OF Li3N CRYSTALS E. SCHONHERR, G. MULLER and E. WINCKLER Max-Planck-Institut für Festkorperforschung, 7 Stuttgart 80, Federal Republic of Germany Received 14 October 1977; manuscript received in final form 28 November 1977
Large Li3N single crystals were grown by the Czochralski method. Starting material for crystal growth was synthesized from the elements. Decomposition of the melt was prevented by use of a stagnant N2 atmosphere (p = 700 Torr). Tungsten was found to be the most suitable crucible material which was not severely attacked by the Li3N melt. The growth of single crystals was complicated by the easy formation of numerous misorientated grains during the seeding process.
1. Introduction
by a BTS catalyst ~tand a molecular sieve with 4 A pores ~ respectively. The synthesis was carried out by two methods: In the first case [8,10] lithium was loaded into a Ni beaker which was placed into a stainless steel vessel of about 2 liters in volume. The vessel was first evacuated and then filled with nitrogen to a pressure of 10 atm. The vessel was then sealed and the lithium was heated. The reaction between Li and N2 took place when the lithium reached the temperature of approximately 150°C. Because the reaction exothermic = —39.4 ± 1 [11]) was further increase in(H~ temperature 0.3 kcal mol of starting material to about 500°Ctook place. The from nitrogen
Lithium nitride is an interesting material because of its high ionic conductivity [l—4]. Li3N is hexagonal [5], space group P6/mm [6], with lattice parametars a = 3.648 A, c = 3.875 A [61.In the structure of L13N there are Li2N layers perpendicular to the hexagonal c-axis. Li atoms which occupy the sites between respective N atoms of two layers can presumably easily move in the direction parallel to the layers [5]. Growth of small Li3N crystals up to 0.5 mm in diameter reported Zintl and from BrauerLi—Li3N [Sj andmelt laterwas byfirst other authorsby [6,7]. Since Li 3N melts congruently at 8 14°C [8,9] crystal growth of Li3N can be carried out by the Crochralski method. The present paper describes the growth of large Li3N single crystals by the Czochralski method and some phenomena observed during the growth of these crystals.
obtained product had glossy brown-red color. The fractured surface displayed small hexagonal platelets
/
2. Synthesis of Li3N Li3N was synthesized from 3N * lithium metal and purified nitrogen. Oxygen and water were removed *
*
.
.
k
.
Cerac/Pure, Inc. Milwaukee, Wisconsin, USA. Badisehe Anilin- and Soda-Fabrik AG, Ludwigshafen/ Rhein, Germany. Merck AG, Darmstadt, Germany.
-
Fig. 1. Fracture surface of Li3N ingot synthesized at 12 atm N2 and 150°Cinitial temperature. 469
470
F. Schönherr et al.
/ Czochralski growth of Li3N crystals
as shown by the SEM photograph in fig. 1. In a later stage the synthesis of Li3N was accomplished in the crystal growth chamber. The growth vessel containing Li metal was evacuated and when a vacuum of l0—~Torr was reached the chamber was filled with N2 (PN2 = 500 Torr) and30C/h. then heated up The reacwith rate of heating about 2 X I 0total pressure of tion started at aboutof 500°C.The nitrogen was then raised to approximately 700 Torr. This pressure was held constant up to the melting
Fig. 3. A 2 mm thick Li
point of Li3N.
growth axis (3 mm grid).
3N slice perpendicularly cut to the
‘if
3. Crystal growth
77
,
10
9
6
12 2
5
_____
_________
I
___________
Li3N crystals were grown in an equipment similar to that used previously for the growth of MnF2 crystals [12]. The apparatus is drawn schematically in fig. 2. It consists of a stainless stell vessel (2) and a stainless steel resistance heater (4) which is mounted on water cooled electrodes of nickel (3). The heat loss is reduced by one or two molybdenum shields (12). The outside wallcopper of the tubes vessel which is cooled by water from three contain smallcoming holes. The furnace temperature is controlled by a Pt/PtRh thermocouple (8) which is protected by an Al203 tube. The seed holder (9) is made of nickel and is water-cooled for removing the heat of solidification. Tungsten crucibles were used as containers for Li3N. New crucibles were first deoxidized by Li3N melt. The growth of crystals was carried out in a stagnant atmosphere of purified N2 at a pressure of 700 Torr. The Li3N crystals were pulled at a rate of about 5 mm/h and rotation speed of approximately 30 rpm. Boules up to 3 cm in diameter and 5 cm in length were grown. A slice of 2 mm in thickness perpendicularly cut to the growth axis is displayed in fig. 3.
I
e
3~
~
4. Results and discussion 5 cm
Fig. 2. Growth equipment: (1) basic plate (Ni); (2) vessel (SS); (3) electrodes water cooled; (4) furnace (SS), (5) pedestal; (6) crucible; (7) heat shields (Mo); (8) thermocouple (Pt/Pt Rh, shielded by A12O3); (9) seed holder water cooled (Ni); (10) water cooling tubes; (11) gas inlet; (12) heat shields (Mo).
The stagnant N2 atmosphere (PN2 = 700 Torr) prevented decomposition of Li3N into liquid Li and N2 gas. The decomposition of the melt was observed when the N2 pressure was lowered to about 400 Torr. This pressure is in good agreement with the extrapolated value of T 814°C[81.
~N2
=
275
Torr
at the melting point
F. Schönherr et al.
/ Czochralski growth of Li3N crystals
Deposition of Li3N was observed in the cool parts of the growth chamber. The mechanism of the transport in the vapor phase dissociative sublimation
isornotvaporization known but it may be enhanced by the presence of small amount ofmay water.be Major difficulties in growing single crystal boules arose during the seeding process. When the melt was touched with the seed the liquid crept along the seed as shown schematically in fig. 4a. Such a behaviour is probably due to the high surface tension and low density of the melt. The wetting 2was aboutsection. 0.8 cm In in in cross height fortoa other seed of 0.7 X 0.3(Ge, cm MnF contrast materials 2, Mg2Sn etc.) which were grown in the same equipment, it was impossible to melt the seed and achieve stationary condition as shown in fig. 4b [13,14]. Numerous misorientated grains originated their growth within the melt on the side walls of the seed as shown in fig. 4c. Moreover, it was impossible to neck the seed or to reduce the crystal radius to its initial size in a reasonable period of time. This indicated that the minimum joining angle [14] for Li3N was possibly slightly negative or even positive for small diameters of the seed. The reason for such a behavior could he incomplete wetting, high surface tension and low density of the melt, Most of the parasitic grains formed on the side walls of the seed ceased to grow when coming out of the melt during pulling. Polygonizatiori or twinning was not observed if pure melts and pulling rates less than 6 mm/h were used. The Li3N boules consisted of a few grains elongated iii the growth direction. Their main growth direction was found to be perpendicular to the c-axis. When seeds were orientated perpendicularly to the c-axis, some of the boules grew as single crystals at the lower end. Additional difficulties came from the strong chem ical reactivity of Li3N. Initially used graphite heaters reacted with residual H20 to form CO2 gas which caused a high loss of the melt and decomposition of the Li3N crystals. Crucibles of iron were badly attacked and the crystals decomposed during cooling while the crucible content foamed over. Crucibles of vitreous carbon broke in several small pieces before *
Preliminary analysis: The compound contained 20.4 wt% Li.
471
1LJLJL. mett
b
a
c
Fig. 4. Seeding: (a) initial stage, (b) stationary stage, (c) real stage.
the melting point of Li3N was reached. crucibles were strongly attacked by theMolybdenum melt which led to constitutional supercooling. Much less severe disturbance in crystal growth was observed in the case of W crucibles. Tungsten crucibles were found to be the most suitable for the growth of crystals, although the loss of W was 0.37 ±0.04 g/h for 16 runs when crucibles 50 mm in diameter were used and when the melt level was 30 mm high. The crucible loss decreased by a factor of 3 when previously grown crystals of Li3N were used as starting material. The loss was enormously increased when the nitrogen was contaminated by H20 or 02. The solidified melt which was left in the crucible after the crystal had been withdrawn was investigated by a scanning electron microscope (SEM) for the purpose of revealing the influence of the crucible material on crystal growth. Distribution of impurities was investigated using X-rays excited by electrons of the SEM.
.—i~-
---
__________________________________________ -______
_______
r10~ Fig. 5. Part of remaining melt which was in contact with W crucible. Bright layer: W—Li—N compound; upper dark layer: Li3N.
472
F. SchOnherr et al.
/ Gzochralski growth of
I,i
3N crystals
in pulled crystals. Investigation by optical emission analysis revealed an Al content of the order of 5 X l0—~wt%, and a Mg and Cu content of the order of 5 X l0~wt%.
Acknowledgement
-
-
11g. 6. The H3N layer is removed from the W- Il N compound shown in fig. 5. Crystals of a W—Li—N compound are displayed.
The solidified melt was always covered by a layer of an unknown fine crystalline W—Li---N compound * at the places where the melt was in contact with the crucible. Fig. 5 shows this layer as a bright zone. This layer could be easily separated from the solidified melt. Small cyrstals still of the same W—Li—N cornpound were found between the two layers as shown in fig. 6. In no case could W be detected in the Li3N part of the solidified melt by EDAX but Ca and Al and traces of Cu were present. Calcium was found to be precipitated along the grain boundaries of the Li3N crystals in the solidified melt. Similar results were obtained when Mo crucibles were used with the exception that Mo was found along grain boundaries. We assume that the solubiity of W in the Li3N melt is small compared to Mo since no W was found in crystals nor on grain boundaries. The better quality of crystals grown from W crucibles might be due to a reduceá constitutional supercooling. Small amounts of Al and Cu were found by EDAX
We thank Professor A. Rabenau for helpful discussions and Mrs. G. Fröhlich for preparing the seed crystals. We thank Mr. L. Viczian for the analysis of the W--Li--N compound.
References [11 E. Masdupuy, Ann. Chim. (Paris) 13 (1948) 1527. Boukamp and R.A. Huggins, Phys. Letters 58A (1976) 231. [3f R.A. Huggins, Electrochim. Acta 22 (1977) 773. [4] U. v. Alpen, A. Rabenau and G.H. Talat, App!. Phys. Letters 30 (1977) 621. [51F. Zinti and G. Brauer, Z. Elektrochcm. 41(1935)102. [61A. Rabenau and H. Schulz, J. Less Common Metals 50
[21 B.A.
(1976) 155.
[71M.D. (1970)Lyataya 1753.
and T.S. Bartnitskaya, Neorgan. Mater. 6
[81R.M. Yonco, F. Veleckis and V.A. Maroni, 1. Nucl. Mater. 57 (1975) 317. [91P.F. Adams, P. Hubberstey and RJ. Puiham, J. Less Common Metals 42 (1975) 1. [10] B. Neumann, C. Kroger and H. Haebler, Z. Anorg. Allgem. Chem. 204 (1932) 81. [11] P.A.G. O’Hare and Gerald K. Johnson, J. Chem. Thermodynamics 7 (1975) 13. [121E. Schonherr, DFG-Kristal!zuchtung, Konsmission für Festkorpterforschung, Bonn- Bad Godesbcrg, Germany, (1971) p. 10. [13] D. Geist and P. Grosse, Z. Angew. Physik 14 (1962) 105. [141K. Mika and W. Uelhoff, J. Crystal Growth 30 (1975) 9.