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Solid-solution range of LiNbO3
Journal of Crystal Growth 22 (1974) 230—232 © North-Holland Publishing Co.
SOLID-SOLUTION RANGE OF L1NbO3 L. 0. SVAASAND, M. ERIKSRUD, G. NAKKEN and A. P. GRANDE Electronics Research Laboratories, University of Trondheim, Trondheim, Norway Received 14 January 1974 The solid solution range of LiNbO3 has been examined for temperatures below 1000 °C.The experiments indicate that LiNbO3 grown at the congruently melting composition (48.6 mol% L120) decomposes into LiNbO3 and LiNb3O8 at temperatures below 910 ~C.
1. Introduction Many applications of LiNbO3 such as optical parametric interaction, holographic storage and ultrasonic signal processing, require singe!ofcrystals of very 13). large Crystals the necessary uniform composition uniformity have been grown by the Czochralski method from melts at the congruently melting composition (48.6 mol % Li 20) 4)~The stability of congruent LINbO3 crystals is therefore of a particular practical importance. We have examined the properties of LiNbO3 single crystals which have been heated in air or oxygen at atmospheric pressure for long periods of time (100— 1000 h). Crystals with congruent composition exhibited a strong increase in optical scattering when they were heated at temperatures below 910 °C. Guinier X-ray diffraction patterns from these crystals corresponded to a superposition of the patterns from LiNbO 3 and LiNb3O8 5.6). If these crystals were subsequently heated above 910 °Cfor sufficiently long periods of time and then cooled rapidly to room temperature, then the optical scattering was removed and the Guinier X-ray diffraction patterns corresponded to LiNbO3 only. Thus we concluded that the room temperature solidsolution range does not cover crystals of a composition corresponding to the congruently melting composition. 2. Crystal preparation The crystals were grown in air by the Czochralski technique from melts with molecular compositions which were varied from 44 to 56 mol % Li20. The melts
were prepared from ‘Optran’ quality zone-refined LiNbO3 from BDH Chemicals Ltd. and ‘Specpure’ quality Nb205 and Li2CO3 from Johnson Matthey Chemicals Ltd. The melts were contained in Ptwere crucibles were heated by rf power. The crystals pulled which with either c-axis or a-axis along the pulling direction. Growth rates of 3—6 mm/h and crystal rotation rates of 10—30 rpm were used. The molecular compositions of the crystal samples were measured by measurements of the phase-match temperatures for second-harmonic generation of a 11520 A laser beam. Thewere published data Bergman et 7) and Beyer et al.1) used to findbythe relations al. between phase-match temperatures and melt compositions, and the data by Lerner et al.8) and Carruthers et al.4) were used to find the relations between melt and solid compositions. All crystal samples were inspected optically, and only colorless and inclusion-free crystals were used in the experiments. Samples from both poled and unpoled crystal boules were prepared. 3. Measurement techniques The samples were placed upon Pt foils and heated in a resistance heated furnace. The samples were heated at a rate of 100 °C/h,then the temperature was kept constant in a long period of time (100—1000 h), and finally the temperature was lowered to room temperature at 100 °C/h. The atmosphere in the furnace was either air or oxygen at atomospheric pressure. The Guinier X-ray diffraction was recorded from powdered samples with CoKcL radiation.
230
SOLID-SOLUTION RANGE OF
LiNbO3
231
4. Experimental results After heat-annealing and subsequently cooling to room temperature some of the samples exhibited a reduced optical transmission. This reduction in transmission was caused by a rather broad-banded optical scattering; in a visual inspection this scattering gave the crystals a ‘milk-white’ opalescent character. The c-axis pulled crystals exhibited a three-fold symmetric core with higher optical scattering than the outer regions. The scattering was qualitatively independent of the growth axes and domain structures of the samples. The scattering was also independent of the atmosphere in the furnace (air or oxygen at atmospheric pressure). In fig. 1 we show a photograph of c-axis pulled samples of molecular composition 48.7 mol 0/~Li20. The samples in fig. Ia, b and c are heated in air for 170 h at 600 °C,700 °Cand 800 °C,respectively, In fig. 2 we show the change in mean optical trans-
Fig. I.
LiNbO3 crystals with molecular composition 48.7 mo!
Li20 after 170 h heat-annealing. (a) 600 C, (b) 700 C, (c) 800 C.
mission as function of annealing temperature and crystal composition. We proposed that the increase in optical scattering was caused by a segregation of the crystals into LiNbO3 and LiNb3O8. In order to verify this assumption we examined Guinier X-ray diffraction from powdered samples. In samples with a molecular composition less than 48 mol °/~ Li20, we could detect traces of LiNb3O5
db
—30-
a 0)
C
—20-
0
E C
-10-
o
Iv Ill
C
II C
a
-C 0
000
600
700
800
900
1000
~C
Temperature
Fig. 2. Change in optical transmission (specular) at 5900 A in 5 mm thick crystals as function of heat annealing temperature. Annealingtime 170 h in air at atmospheric pressure. (1) 49.0 mol°/0Li20; (11) 48.9 mol% Li20; (III) 48.7 mol% Li20; (IV) 48.6 mol% Li20.
232
L. 0. SVAASAND, M. ERIKSRUD,
after an annealing period of typically 10 h at 800 °C. The congruent samples exhibited traces of LiNb3O8 first after a substantially longer period of time, typically 500 h at 800 °C.This was presumably due to the limited resolution of the Guinier diffraction patterns. If the samples were heated above certain temperatures dependent upon the crystal composition, and subsequently cooled rapidly to room temperature then the scattering vanished and the Guinier diffraction patterns exhibited LiNbO3 lines only. We have used the optical scattering data to find the solid-solution range for LiNbO3 in the temperature region 700—1000 °C. Our data are shown as vertical bars in fig. 3. The lower limits correspond to the highest temperatures where we obtained opalescence in the samples, and the upper limits correspond to the lowest temperatures where this opalescence was removed from the samples again. The high-temperature data in fig. 3 (above 1000 °C) correspond the resultsbars published by Carruthers et 4), and thetohorizontal at 500, 1000 and 1180°C al. 8). correspond to the X-ray results of Lerner et al. Our results agree very closely with published data at 1000 °C,but differs markedly for lower temperatures. We presume that the disagreements may arise from differencies in the time scales used by different experimentalists. We have found that equilibria were reached within 100 h for temperatures above 1000 °C,but that essential changes occured after 100 h in the temperature region below this temperature. 5. Conclusion Our experimental results indicated that crystals grown from melts at the congruently composition (48.6 mol% Li20) were unstable below 910 °C.If these crystals were heated below 910 °Cthe optical scattering increased accumulatively with time. The time rate of
G. NAKKEN AND A. P. GRANDE
1400
Liqo dos
1200
Sod,, LiNbO3
LiNbO3
.
Li6b308
UNbO,
• U,NbO,
600+ -
~‘.~-
-~
46
‘.4
Fig. 3.
Hole p~rc~nlLi
20 Phase-equilibrium diagram for LiNbO
3.
the increase in scattering was maximum at about 800 °C and the rate was reduced to the limit of detection at about 400 °C. References 1) R. L. Dyer, (1970) 2320.J. F. Young and R. S. Feigelson, J. AppI. Phys. 41 2) F. S. Chen, J. T. LaMacchia and D. B. Fraser, AppI. Phys. Letters 13 (1968) 223. 3) L. 0. Svaasand, AppI. Phys. Letters 15 (1969) 300. 4) J. R. Carruthers, 0. E. Peterson, M. Grasso and P. M. Bridenbaugh, J. App!. Phys. 42 (1971) 1846. 5) M. Lundberg, Acta Chern. Scand. 25 (1971) 3337. 6) L. 0. Svaasand, M. Eriksrud, A. P. Grande and F. Mo, J. Crystal Growth 18 (1973) 179. 7) J. G. Bergman, A. Ashkin, A. A. Baliman, J. M. Dziedzic, H. J. Levinstein and R. G. Smith, AppI. Phys. Letters 12 92, C. Legras et J. P. Dumas, J. Crystal Growth 3, 4 8) (1968) P. Lerner, (1968) 231.
SOLID-SOLUTION RANGE OF L1NbO3 L. 0. SVAASAND, M. ERIKSRUD, G. NAKKEN and A. P. GRANDE Electronics Research Laboratories, University of Trondheim, Trondheim, Norway Received 14 January 1974 The solid solution range of LiNbO3 has been examined for temperatures below 1000 °C.The experiments indicate that LiNbO3 grown at the congruently melting composition (48.6 mol% L120) decomposes into LiNbO3 and LiNb3O8 at temperatures below 910 ~C.
1. Introduction Many applications of LiNbO3 such as optical parametric interaction, holographic storage and ultrasonic signal processing, require singe!ofcrystals of very 13). large Crystals the necessary uniform composition uniformity have been grown by the Czochralski method from melts at the congruently melting composition (48.6 mol % Li 20) 4)~The stability of congruent LINbO3 crystals is therefore of a particular practical importance. We have examined the properties of LiNbO3 single crystals which have been heated in air or oxygen at atmospheric pressure for long periods of time (100— 1000 h). Crystals with congruent composition exhibited a strong increase in optical scattering when they were heated at temperatures below 910 °C. Guinier X-ray diffraction patterns from these crystals corresponded to a superposition of the patterns from LiNbO 3 and LiNb3O8 5.6). If these crystals were subsequently heated above 910 °Cfor sufficiently long periods of time and then cooled rapidly to room temperature, then the optical scattering was removed and the Guinier X-ray diffraction patterns corresponded to LiNbO3 only. Thus we concluded that the room temperature solidsolution range does not cover crystals of a composition corresponding to the congruently melting composition. 2. Crystal preparation The crystals were grown in air by the Czochralski technique from melts with molecular compositions which were varied from 44 to 56 mol % Li20. The melts
were prepared from ‘Optran’ quality zone-refined LiNbO3 from BDH Chemicals Ltd. and ‘Specpure’ quality Nb205 and Li2CO3 from Johnson Matthey Chemicals Ltd. The melts were contained in Ptwere crucibles were heated by rf power. The crystals pulled which with either c-axis or a-axis along the pulling direction. Growth rates of 3—6 mm/h and crystal rotation rates of 10—30 rpm were used. The molecular compositions of the crystal samples were measured by measurements of the phase-match temperatures for second-harmonic generation of a 11520 A laser beam. Thewere published data Bergman et 7) and Beyer et al.1) used to findbythe relations al. between phase-match temperatures and melt compositions, and the data by Lerner et al.8) and Carruthers et al.4) were used to find the relations between melt and solid compositions. All crystal samples were inspected optically, and only colorless and inclusion-free crystals were used in the experiments. Samples from both poled and unpoled crystal boules were prepared. 3. Measurement techniques The samples were placed upon Pt foils and heated in a resistance heated furnace. The samples were heated at a rate of 100 °C/h,then the temperature was kept constant in a long period of time (100—1000 h), and finally the temperature was lowered to room temperature at 100 °C/h. The atmosphere in the furnace was either air or oxygen at atomospheric pressure. The Guinier X-ray diffraction was recorded from powdered samples with CoKcL radiation.
230
SOLID-SOLUTION RANGE OF
LiNbO3
231
4. Experimental results After heat-annealing and subsequently cooling to room temperature some of the samples exhibited a reduced optical transmission. This reduction in transmission was caused by a rather broad-banded optical scattering; in a visual inspection this scattering gave the crystals a ‘milk-white’ opalescent character. The c-axis pulled crystals exhibited a three-fold symmetric core with higher optical scattering than the outer regions. The scattering was qualitatively independent of the growth axes and domain structures of the samples. The scattering was also independent of the atmosphere in the furnace (air or oxygen at atmospheric pressure). In fig. 1 we show a photograph of c-axis pulled samples of molecular composition 48.7 mol 0/~Li20. The samples in fig. Ia, b and c are heated in air for 170 h at 600 °C,700 °Cand 800 °C,respectively, In fig. 2 we show the change in mean optical trans-
Fig. I.
LiNbO3 crystals with molecular composition 48.7 mo!
Li20 after 170 h heat-annealing. (a) 600 C, (b) 700 C, (c) 800 C.
mission as function of annealing temperature and crystal composition. We proposed that the increase in optical scattering was caused by a segregation of the crystals into LiNbO3 and LiNb3O8. In order to verify this assumption we examined Guinier X-ray diffraction from powdered samples. In samples with a molecular composition less than 48 mol °/~ Li20, we could detect traces of LiNb3O5
db
—30-
a 0)
C
—20-
0
E C
-10-
o
Iv Ill
C
II C
a
-C 0
000
600
700
800
900
1000
~C
Temperature
Fig. 2. Change in optical transmission (specular) at 5900 A in 5 mm thick crystals as function of heat annealing temperature. Annealingtime 170 h in air at atmospheric pressure. (1) 49.0 mol°/0Li20; (11) 48.9 mol% Li20; (III) 48.7 mol% Li20; (IV) 48.6 mol% Li20.
232
L. 0. SVAASAND, M. ERIKSRUD,
after an annealing period of typically 10 h at 800 °C. The congruent samples exhibited traces of LiNb3O8 first after a substantially longer period of time, typically 500 h at 800 °C.This was presumably due to the limited resolution of the Guinier diffraction patterns. If the samples were heated above certain temperatures dependent upon the crystal composition, and subsequently cooled rapidly to room temperature then the scattering vanished and the Guinier diffraction patterns exhibited LiNbO3 lines only. We have used the optical scattering data to find the solid-solution range for LiNbO3 in the temperature region 700—1000 °C. Our data are shown as vertical bars in fig. 3. The lower limits correspond to the highest temperatures where we obtained opalescence in the samples, and the upper limits correspond to the lowest temperatures where this opalescence was removed from the samples again. The high-temperature data in fig. 3 (above 1000 °C) correspond the resultsbars published by Carruthers et 4), and thetohorizontal at 500, 1000 and 1180°C al. 8). correspond to the X-ray results of Lerner et al. Our results agree very closely with published data at 1000 °C,but differs markedly for lower temperatures. We presume that the disagreements may arise from differencies in the time scales used by different experimentalists. We have found that equilibria were reached within 100 h for temperatures above 1000 °C,but that essential changes occured after 100 h in the temperature region below this temperature. 5. Conclusion Our experimental results indicated that crystals grown from melts at the congruently composition (48.6 mol% Li20) were unstable below 910 °C.If these crystals were heated below 910 °Cthe optical scattering increased accumulatively with time. The time rate of
G. NAKKEN AND A. P. GRANDE
1400
Liqo dos
1200
Sod,, LiNbO3
LiNbO3
.
Li6b308
UNbO,
• U,NbO,
600+ -
~‘.~-
-~
46
‘.4
Fig. 3.
Hole p~rc~nlLi
20 Phase-equilibrium diagram for LiNbO
3.
the increase in scattering was maximum at about 800 °C and the rate was reduced to the limit of detection at about 400 °C. References 1) R. L. Dyer, (1970) 2320.J. F. Young and R. S. Feigelson, J. AppI. Phys. 41 2) F. S. Chen, J. T. LaMacchia and D. B. Fraser, AppI. Phys. Letters 13 (1968) 223. 3) L. 0. Svaasand, AppI. Phys. Letters 15 (1969) 300. 4) J. R. Carruthers, 0. E. Peterson, M. Grasso and P. M. Bridenbaugh, J. App!. Phys. 42 (1971) 1846. 5) M. Lundberg, Acta Chern. Scand. 25 (1971) 3337. 6) L. 0. Svaasand, M. Eriksrud, A. P. Grande and F. Mo, J. Crystal Growth 18 (1973) 179. 7) J. G. Bergman, A. Ashkin, A. A. Baliman, J. M. Dziedzic, H. J. Levinstein and R. G. Smith, AppI. Phys. Letters 12 92, C. Legras et J. P. Dumas, J. Crystal Growth 3, 4 8) (1968) P. Lerner, (1968) 231.