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Crystal growth and characterization of LiPbPO4
,. . . . . . . . C R Y S T A L ONOWTH
ELSEVIER
Journal of Crystal Growth 166 (1996) 361-363
Crystal growth and characterization of LiPbPO 4 M. Burianek *, J. Liebertz, M. Miihlberg Institut fiir Kristallographie, Universitiit zu Ki~ln, Ziilpicher Strasse 49b, D-50674 Ki~ln, Germany
Abstract
Crystals of LiPbPO4 were grown by the Czochralski method and with a top-seeded solution growth (TSSG) technique. The main difficulty in the crystal growth of LiPbPO4 arises from the incongruent melting behaviour of the compound. The incongruent melting temperature is Tp = 900°C and the liquid temperature of a corresponding stoichiometric composition LiPbPO4 is T/= 920°C. Precipitation of a second crystalline phase Li3PO4 could be observed at about 860°C. A small amount of Bi203 was added to the melt in order to prevent the precipitation of Li3PO4.
1. Introduction
LiPbPO 4 crystallizes in the space group Pna2~. Although the crystal can show polar properties, no detailed information about the physical properties could be found in the literature but an interesting pyroelectric effect was expected. For this reason attempts at the growth of single crystals were started in order to investigate physical properties. LiPbPO 4 was first mentioned in 1975 by Brixner and Foils [1]. The authors reported that single crystals were grown from a stoichiometric melt, although a heat anomaly in a DTA measurement was detected at 908°C, 12 K below the liquidus temperature (920°C). In the course of DTA examinations, Ellamari et al. [2] detected two heat anomalies at 898 and 560°C. Phase transformations are supposed to be the reason for the heat anomalies.
* Corresponding author.
2. Crystal g r o w t h of L i P b P O 4
2.1. Czochralski m e t h o d
In order to prove the results of Ref. [1], some growth experiments were carried out using stoichiometric melts. The pulling rate was reduced from 5 m m / h , as described in Ref. [1], to 0.5 m m / h . This should be important because of the high viscosity of the melt. The first obtained specimens were polycrystalline and cloudy. A significant signal in DTA investigations of LiPbPO 4 has shown incongruent melting behaviour at 900°C. The corresponding liquidus temperature of the stoichiometric composition is 920°C. Therefore, it was necessary to carry out crystal growth from non-stoichiometric melts of the system L i 2 0 - P b O - P 2 0 5 , and all the growth experiments confirm that single crystals can be grown from non-stoichiometric melts. The growth was initiated on a small seed crystal with an orientation along [001]. After pulling of 3 - 4 mm, the crystals began to show cloudy parts caused by precipitation of a sec-
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00047-4
362
M. Burianek et a l . / Journal of Crystal Growth 166 (1996) 361-363 2.2. T S S G t e c h n i q u e
Fig. 1. Transparent LiPbPO4 single crystals grown by the Czochralski method. (Melt composition: LiPbPO4 +4.5 mol% BiP309; length 47 and 22 mm; diameter 7 and 4 mm.) Pulling rate: 0.25 mm/h; rotation: 60 min I.
ond crystalline phase. A pulling rate of 0.25 m m / h was used and the rotation of the crystal was 2 5 - 6 0 min -1. To solve the problem of precipitation, another melt composition was used. Several melting experiments showed that the addition of 4.5 mol% BiP30 9 to a melt of stoichiometric composition would prevent precipitation of the second phase (see Fig. 1). Additional problems in crystal growth from nonstoichiometric melts arise from the influence of constitutional supercooling (see Fig. 2). Due to these effects, the crystal volume had to be limited to about 10% ( ~ 5 cm 3) of the melt volume, and the crystal diameter to 6 - 8 mm. The maximum allowed pulling rate was 0.25 m m / h .
In order to grow larger LiPbPO 4 crystals the TSSG technique was applied. It was found that the crystal quality was greatly reduced by the addition of Bi20 3 so another melt composition had to be found. A melt composition consisting of LiPbPO 4 + 5 mol% Pb3P20 8 was cooled down from 890 to 870°C in a resistance furnace. These conditions allowed the growth of crystals with extended transparent regions. After the growth runs, these crystals had to be cooled down rapidly over the range of 5 - 8 K / m i n to prevent the precipitation of a further crystalline phase. By this procedure crystals of up to 220 g could be grown in 300 ml platinum crucibles. Growth was carried out with an uncooled seed holder, without rotation, a pulling rate of 1 m m / d , and a cooling rate of 1 K / d .
3. T h e p r e c i p i t a t i o n
of Li3PO 4 in LiPbPO 4
To understand the reason for the cloudiness of LiPbPO 4, which was observed during the cooling process, some annealing experiments were carried out. If a quenched transparent crystal sample was heated, the precipitation started at about 560°C. At about 850°C the cloudiness disappeared. The cooling down of this transparent crystal sample from 890°C to 840-860°C resulted in renewed cloudiness. The microscopic examination of thin crystal plates, annealed for 2 weeks at 750°C, showed that well-developed precipitates had been grown during this process
f °
-
-
-~ : : A : !
O¸
O J
o
~
•
o
O v
i
~-,
-'..
• 6
- io • | 0 %llm
" Fig. 2. LiPBPOa slice (diameter 7 mm) grown from non-stoichiometric melt showing defects by constitutional supercooling.
,
°
o ~
• alp
•
~ t p;
u
' ~_
J
0
"11"
Fig. 3. Li3PO ~ precipitates in an annealed LiPbPO 4 sample.
M. Burianek et al. / Journal of Crystal Growth 166 (1996) 361-363
363
Table 1 Optical and physical properties of LiPbPO 4
5. Conclusions
Refractive indices
(see Fig. 3). Electron microprobe analyses were used for the determination of the chemical composition of the precipitates. Lead could not be detected in this phase. The stoichiometric composition was calculated to be Li 3P O 4.
In the present paper, we have reported the crystal growth of LiPbPO 4, a compound with an incongruent melting point at 900°C. Both the Czochralski method and the top-seeded solution growth (TSSG) technique are capable of growing single crystals. Suitable non-stoichiometric melt compositions for successful growth were found. In the case of the Czochralski method, the growth of larger crystals ( > 5 c m 3) i s l i m i t e d by constitutional supercooling effects. Large crystals (20-30 cm 3) can be grown by the TSSG technique. As may be seen from the cloudy parts of the crystals, the post-growth process is characterized by a precipitation process. The precipitates were identified as Li3PO 4 by electron microprobe analysis. On the bases of its polar class, mm2, LiPbPO 4 was assumed to show interesting physical properties. Several of these physical properties were measured for the first time and are summarized in Table 1. Technical application should be possible because of the combination of non-ferroelectric behaviour and moderate pyroelectric and dielectric constants. Further research activities are focused on the improvement of the crystal growth conditions. The variation of the physical properties will be investigated by growing mixed crystals of LiPbP l_xVxO4 and by doping with different additives.
4. Physical properties
References
The crystals of LiPbPO4 grown offered the opportunity for the measurement of a number of physical properties for the first time. These are summarized in Table 1.
[1] L.H. Brixner and C.M. Foris, Mater. Res. Bull. 10 (1975) 31. [2] L. Ellamari, B. Elouadi and G. Miiller-Vogt, Phase Transition 13 (1988) 29.
M a x i m u m double refraction at 589 nm Optical character Optic axial angle Longitudinal pyroelectric effect (between T = 180-325 K) [la,C m 2 K I] Relative dielectric constants eii
Longitudinal piezoelectric effect [pC/N] Tensor of elasticity cij [10 j° N m 2]
n o = 1.83103 n~ = 1.85683 n~ = 1.87022 0.0392 2O 2V~ = 109.32 ° 11 ( ~ three times that of Tourmaline) eJl = 17.63 e22 = 19.33 e33 = 21.04 8.3 ( ~ three times that of quartz) clt = 6.001 + 0 . 0 6 7 c22 = 9.043 + 0 . 0 6 5 c33 = 9.883 + 0 . 0 6 8 c44 = 4.313 + 0 . 0 4 2 c55 = 3.055 + 0 . 0 4 2 c66 : 2.508 + 0.043 cl2 = 2 . 4 1 0 + 0 . 0 9 0 cl3 = 0 . 1 2 0 + 0 . 1 2 0 c23 = 4 . 1 2 6 + 0 . 1 1 2
ELSEVIER
Journal of Crystal Growth 166 (1996) 361-363
Crystal growth and characterization of LiPbPO 4 M. Burianek *, J. Liebertz, M. Miihlberg Institut fiir Kristallographie, Universitiit zu Ki~ln, Ziilpicher Strasse 49b, D-50674 Ki~ln, Germany
Abstract
Crystals of LiPbPO4 were grown by the Czochralski method and with a top-seeded solution growth (TSSG) technique. The main difficulty in the crystal growth of LiPbPO4 arises from the incongruent melting behaviour of the compound. The incongruent melting temperature is Tp = 900°C and the liquid temperature of a corresponding stoichiometric composition LiPbPO4 is T/= 920°C. Precipitation of a second crystalline phase Li3PO4 could be observed at about 860°C. A small amount of Bi203 was added to the melt in order to prevent the precipitation of Li3PO4.
1. Introduction
LiPbPO 4 crystallizes in the space group Pna2~. Although the crystal can show polar properties, no detailed information about the physical properties could be found in the literature but an interesting pyroelectric effect was expected. For this reason attempts at the growth of single crystals were started in order to investigate physical properties. LiPbPO 4 was first mentioned in 1975 by Brixner and Foils [1]. The authors reported that single crystals were grown from a stoichiometric melt, although a heat anomaly in a DTA measurement was detected at 908°C, 12 K below the liquidus temperature (920°C). In the course of DTA examinations, Ellamari et al. [2] detected two heat anomalies at 898 and 560°C. Phase transformations are supposed to be the reason for the heat anomalies.
* Corresponding author.
2. Crystal g r o w t h of L i P b P O 4
2.1. Czochralski m e t h o d
In order to prove the results of Ref. [1], some growth experiments were carried out using stoichiometric melts. The pulling rate was reduced from 5 m m / h , as described in Ref. [1], to 0.5 m m / h . This should be important because of the high viscosity of the melt. The first obtained specimens were polycrystalline and cloudy. A significant signal in DTA investigations of LiPbPO 4 has shown incongruent melting behaviour at 900°C. The corresponding liquidus temperature of the stoichiometric composition is 920°C. Therefore, it was necessary to carry out crystal growth from non-stoichiometric melts of the system L i 2 0 - P b O - P 2 0 5 , and all the growth experiments confirm that single crystals can be grown from non-stoichiometric melts. The growth was initiated on a small seed crystal with an orientation along [001]. After pulling of 3 - 4 mm, the crystals began to show cloudy parts caused by precipitation of a sec-
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00047-4
362
M. Burianek et a l . / Journal of Crystal Growth 166 (1996) 361-363 2.2. T S S G t e c h n i q u e
Fig. 1. Transparent LiPbPO4 single crystals grown by the Czochralski method. (Melt composition: LiPbPO4 +4.5 mol% BiP309; length 47 and 22 mm; diameter 7 and 4 mm.) Pulling rate: 0.25 mm/h; rotation: 60 min I.
ond crystalline phase. A pulling rate of 0.25 m m / h was used and the rotation of the crystal was 2 5 - 6 0 min -1. To solve the problem of precipitation, another melt composition was used. Several melting experiments showed that the addition of 4.5 mol% BiP30 9 to a melt of stoichiometric composition would prevent precipitation of the second phase (see Fig. 1). Additional problems in crystal growth from nonstoichiometric melts arise from the influence of constitutional supercooling (see Fig. 2). Due to these effects, the crystal volume had to be limited to about 10% ( ~ 5 cm 3) of the melt volume, and the crystal diameter to 6 - 8 mm. The maximum allowed pulling rate was 0.25 m m / h .
In order to grow larger LiPbPO 4 crystals the TSSG technique was applied. It was found that the crystal quality was greatly reduced by the addition of Bi20 3 so another melt composition had to be found. A melt composition consisting of LiPbPO 4 + 5 mol% Pb3P20 8 was cooled down from 890 to 870°C in a resistance furnace. These conditions allowed the growth of crystals with extended transparent regions. After the growth runs, these crystals had to be cooled down rapidly over the range of 5 - 8 K / m i n to prevent the precipitation of a further crystalline phase. By this procedure crystals of up to 220 g could be grown in 300 ml platinum crucibles. Growth was carried out with an uncooled seed holder, without rotation, a pulling rate of 1 m m / d , and a cooling rate of 1 K / d .
3. T h e p r e c i p i t a t i o n
of Li3PO 4 in LiPbPO 4
To understand the reason for the cloudiness of LiPbPO 4, which was observed during the cooling process, some annealing experiments were carried out. If a quenched transparent crystal sample was heated, the precipitation started at about 560°C. At about 850°C the cloudiness disappeared. The cooling down of this transparent crystal sample from 890°C to 840-860°C resulted in renewed cloudiness. The microscopic examination of thin crystal plates, annealed for 2 weeks at 750°C, showed that well-developed precipitates had been grown during this process
f °
-
-
-~ : : A : !
O¸
O J
o
~
•
o
O v
i
~-,
-'..
• 6
- io • | 0 %llm
" Fig. 2. LiPBPOa slice (diameter 7 mm) grown from non-stoichiometric melt showing defects by constitutional supercooling.
,
°
o ~
• alp
•
~ t p;
u
' ~_
J
0
"11"
Fig. 3. Li3PO ~ precipitates in an annealed LiPbPO 4 sample.
M. Burianek et al. / Journal of Crystal Growth 166 (1996) 361-363
363
Table 1 Optical and physical properties of LiPbPO 4
5. Conclusions
Refractive indices
(see Fig. 3). Electron microprobe analyses were used for the determination of the chemical composition of the precipitates. Lead could not be detected in this phase. The stoichiometric composition was calculated to be Li 3P O 4.
In the present paper, we have reported the crystal growth of LiPbPO 4, a compound with an incongruent melting point at 900°C. Both the Czochralski method and the top-seeded solution growth (TSSG) technique are capable of growing single crystals. Suitable non-stoichiometric melt compositions for successful growth were found. In the case of the Czochralski method, the growth of larger crystals ( > 5 c m 3) i s l i m i t e d by constitutional supercooling effects. Large crystals (20-30 cm 3) can be grown by the TSSG technique. As may be seen from the cloudy parts of the crystals, the post-growth process is characterized by a precipitation process. The precipitates were identified as Li3PO 4 by electron microprobe analysis. On the bases of its polar class, mm2, LiPbPO 4 was assumed to show interesting physical properties. Several of these physical properties were measured for the first time and are summarized in Table 1. Technical application should be possible because of the combination of non-ferroelectric behaviour and moderate pyroelectric and dielectric constants. Further research activities are focused on the improvement of the crystal growth conditions. The variation of the physical properties will be investigated by growing mixed crystals of LiPbP l_xVxO4 and by doping with different additives.
4. Physical properties
References
The crystals of LiPbPO4 grown offered the opportunity for the measurement of a number of physical properties for the first time. These are summarized in Table 1.
[1] L.H. Brixner and C.M. Foris, Mater. Res. Bull. 10 (1975) 31. [2] L. Ellamari, B. Elouadi and G. Miiller-Vogt, Phase Transition 13 (1988) 29.
M a x i m u m double refraction at 589 nm Optical character Optic axial angle Longitudinal pyroelectric effect (between T = 180-325 K) [la,C m 2 K I] Relative dielectric constants eii
Longitudinal piezoelectric effect [pC/N] Tensor of elasticity cij [10 j° N m 2]
n o = 1.83103 n~ = 1.85683 n~ = 1.87022 0.0392 2O 2V~ = 109.32 ° 11 ( ~ three times that of Tourmaline) eJl = 17.63 e22 = 19.33 e33 = 21.04 8.3 ( ~ three times that of quartz) clt = 6.001 + 0 . 0 6 7 c22 = 9.043 + 0 . 0 6 5 c33 = 9.883 + 0 . 0 6 8 c44 = 4.313 + 0 . 0 4 2 c55 = 3.055 + 0 . 0 4 2 c66 : 2.508 + 0.043 cl2 = 2 . 4 1 0 + 0 . 0 9 0 cl3 = 0 . 1 2 0 + 0 . 1 2 0 c23 = 4 . 1 2 6 + 0 . 1 1 2