- Email: [email protected]
Crystal growth and preparation of colourless MgNb2O6 single crystals
j. . . . . . . .
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 151 (1995) 365-368
Crystal growth and preparation of colourless MgNb206 single crystals K, P o l g f i r
a,
A. P&er
a,
j . P a i t z b, C. Z a l d o c,.
a Research Laboratory for Crystal Physics of the Hungarian Academy of Sciences, P.O.B. 132, H-1502 Budapest, Hungary b CentralResearch Institute for Physics of the Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest 114, Hungary c Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Universitario de Cantoblanco, C-W, E-28049Madrid, Spain
Received 14 March 1994
Abstract
MgNb206 single crystals of several cubic centimetres have been grown by the Czochralski technique. The as-grown black crystals were annealed in oxygen at high temperature and became transparent. The process was reversible by high temperature vacuum annealing. The optical absorption observed in as-grown samples has been attributed to oxygen deficiency of the crystal. The grown crystals were resistant against strong mineral acids. The surface tension of the molten material was 0.35 N/m.
1. Introduction
Alkali and alkaline-earth niobate single crystals have been grown for several years in large quantities due to their application in electrooptics and non-linear optics. One of the most widely used crystal of this group is lithium niobate that has found widespread application in non-linear optics and surface acoustic wave filters. Since in lithium niobate the magnesium doping enhances the optical damage resistance, and since it may be grown readily with magnesium concentrations up to several percents, we wanted to examine the optical properties of magnesium niobate in order to estimate its applicability as optical material. The crystalline structure of
* Corresponding author.
MgNb20 6 was first determined by Brandt [1]. The crystal belongs to the Pbcn symmetry space group which involves a centre of symmetry, thus MgNb206 would be appropriate for use as a passive element or substrate. For optical investigations fairly big samples are required. There has been several papers that reported the growth of MgNb20 6 single crystals by different methods: Emmenegger [2,3] and co-workers reported the growth by chemical transport. 5 × 2 × 1 mm 3 platelets were obtained. Greenblatt et al. [4] grew prisms up to 5 x 4 × 3 mm 3 by the flux method. Rakhmankulov and Udalov [5] used Verneuil and Czochralski methods to produce single crystals up to 3-5 mm in diameter and 6 - 8 mm long. All the above papers reported a coloration of the crystals (ranging from clear to black) which prevents the use of the material as optical wave-
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00059-3
366
K. Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
guide substrate. However, no research has b e e n d o n e so far to reveal the origin of this optical absorption. In this p a p e r we describe the growth of large single crystals of M g N b 2 0 6 and the thermal t r e a t m e n t s after the growth which m a d e it possible to modify the colour of the samples. Table 1 summarizes the physical properties o f M g N b 2 0 6 r e p o r t e d up to now.
2. Crystal growth procedure M g N b 2 0 6 p o w d e r was p r e p a r e d f r o m high purity (99.999%) M g O and N b 2 0 5 starting oxides. T h e oxides were fired at 1000°C and mixed in stoichiometric composition in a ball mill. T h e mixture was pressed into tablets by a pressing tool m a d e of plexi-glass. Table 1 Physical properties of MgNb206 single crystals and ceramics Physical property Crystalline structure Orthorhombic space group Lattice parameters (at 300 K)
T h e crystals were grown in a Metals R e s e a r c h M S R - 5 type balance controlled, Czochralski puller, with R F heating. M g N b 2 0 6 was melted in an iridium crucible, 38 m m in height and 38 m m in diameter, s u r r o u n d e d by an alumina insulator and covered by an iridium after-heater ring. To r e d u c e the axial thermal gradient an additional iridium after-heater s u r r o u n d e d the growing crystal. T h e protecting a t m o s p h e r e in the growth c h a m b e r was a N 2 + 4% 0 2 gas mixture. For the first growing runs an Ir stripe was used as seed. A strong wetting of the melt was experienced during the growth, which makes the whole process difficult and deteriorates the crystal quality. D u e to this strong wetting the melt creeps out of the crucible and a high meniscus occurs. A c c o r d ing to our m e a s u r e m e n t the surface tension of the melt at the growing t e m p e r a t u r e is 0.35 N / m . T h e growth conditions were a pulling rate of 2 - 3 m m / h and a rotation speed of 4 - 2 0 rpm.
Ref. [1]
Range of transparency
Pbcn (D 14) a = 5.017 ,~ b = 14.18 ,~ c = 5.665 .A Along (010) planes 1620-1640° C 5.087 g/cm 3 (theoretical) 4.993 g/cm 3 (experimental) HV10o = 727 kg/mm 2 % = 16.4 eb = 20.9 e c = 32.4 300-5700 nm
[This work] [5] [5] [5] [This work] [7] [7] [7] [This work]
Optical absorption edge (at 300 K) Ceramic polycrystalline powder Single crystals oxidized in air Single crystals oxidized in 02
270-290 nm 290 nm 310 nm
[8,9] [This work] [This work]
Cleavage Melting temperature Density Vickers hardness Relative dielectric permittivity (at 300 K, 5-500 kHz)
Intrinsic photoluminescence of single crystals and ceramics (300 K, hexc = 275 nm) Quenching temperature Emission maximum Photoluminescence from defects in ceramics (ceramic, 4-10 K, Aexc= 300 nm) Emission maximum
[This work], [8-10] 350 K 450 nm [91 530 nm
I~ Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
Fig. 1. MgNb206 single crystal obtained by the Czochralski technique. Pulling rate 2 m m / h . Rotation speed 20 rpm.
Under these conditions crystals of several cubic centimetres were obtained. Fig. 1 shows a picture of a typical boule. The as-grown crystals were black. A perfect cleavage was observed along (010) crystallographic planes. The material was resistant to very strong chemical attack, and was not soluble in hot concentrated mineral acids (H2SO4, HNO3, HCI, HF, and H3PO 4) or their mixtures. Slight etching was observed on the surface when molten KHSO4 was used.
tometer (model Cary 17). Fig. 2 shows the optical absorption of as-grown samples. A very broad band centred at 2.0 eV (620 nm) appears in the visible. This absorption may be removed by annealing the sample at high temperature (1200-1300 ° C) in air or in pure oxygen ( 0 2) atmosphere (see also Fig. 2). When the treatment is performed in pure 02, a lower residual optical absorption is observed but a shifting to lower energy of the optical absorption edge of the material is also induced. The optical absorption in the visible may be recovered by thermal reduction in vacuum. Fig. 2 shows the absorption induced by 90 h of heating at 950°C in a residual pressure of 10 -3 mbar. In comparison with as-grown samples, the maximum of the absorption band (at about 1.5 eV) appears shifted to low energy. This fact seems to indicate that the optical absorption is complex and several centres contribute to the spectrum of as-grown and reduced samples. The origin of these optical absorptions has to be related to the loss of molecular oxygen from the crystal lattice. The electrons left in the lattice may be trapped in oxygen vacancies or in niobium ions, giving place to the optical absorption observed. These results look rather similar to those described for LiNbO 3 by Schirmer et al. [6].
7
Optical absorption spectra have been recorded at room temperature with a Varian spectropho-
60
40 E-n. o
20 .< u o
3. Optical characterization
367
0 0.5
1.5
2.5
3.5
4.5
PHOTON ENERGY , eV
Fig. 2. R o o m temperature optical absorption spectra of MgNb206 single crystals. As-grown sample, solid line. After 72 h of annealing in air at 1300 ° C, dashed line. After 90 h of reduction in vacuum (10 -3 mbar) at 950 ° C, solid line.
368
K~ Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
4. Conclusions
References
In s u m m a r y we have shown that large size (several cubic centimetres) M g N b 2 0 6 single crystals can be o b t a i n e d by the Czochralski growth method. T h e transparency of the crystals is improved by high t e m p e r a t u r e annealing in oxygen atmosphere. This indicates that the optical absorption observed in the visible is related to oxygen deftciency of the lattice. F u r t h e r investigation is n e e d e d to reveal the structure of the b r o a d absorption b a n d and thus the precise description of the m e c h a n i s m of the o x i d a t i o n - r e d u c t i o n processes.
[1] K. Brandt, Arkiv Kemi Min. Geol. 17A (1943) No. 15. [2] F. Emmenegger and A. Petermann, J. Crystal Growth 2 (1968) 33. [3] F. Emmenegger, J. Crystal Growth 3/4 (1968) 135. [4] M. Greenblatt, B.M. Wanklyn and B.J. Garrard, J. Crystal Growth 58 (1982) 463. [5] R.M. Rakhmankulov and Yu. P. Udalov, Zh. Neorg. Khim. 21 (1976) 2842. [6] F. Schirmer, O. Thiemann and M. W6hlecke, J. Phys. Chem. Solids 52 (1991) 185. [7] F.P. Emmenegger and H. Roetsehi, J. Phys. Chem. Solids 32 (1971) 787. [8] G. Blasse and A. Bril, Z. Phys. Chem. NF 57 (1968) 187. [9] G. Blasse, G.J. Dirksen and L.H. Brixner, Mater. Chem Phys. 14 (1986) 485. [10] A. Wachtel, J. Electrochem. Soc. 111 (1964) 534.
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 151 (1995) 365-368
Crystal growth and preparation of colourless MgNb206 single crystals K, P o l g f i r
a,
A. P&er
a,
j . P a i t z b, C. Z a l d o c,.
a Research Laboratory for Crystal Physics of the Hungarian Academy of Sciences, P.O.B. 132, H-1502 Budapest, Hungary b CentralResearch Institute for Physics of the Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest 114, Hungary c Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Universitario de Cantoblanco, C-W, E-28049Madrid, Spain
Received 14 March 1994
Abstract
MgNb206 single crystals of several cubic centimetres have been grown by the Czochralski technique. The as-grown black crystals were annealed in oxygen at high temperature and became transparent. The process was reversible by high temperature vacuum annealing. The optical absorption observed in as-grown samples has been attributed to oxygen deficiency of the crystal. The grown crystals were resistant against strong mineral acids. The surface tension of the molten material was 0.35 N/m.
1. Introduction
Alkali and alkaline-earth niobate single crystals have been grown for several years in large quantities due to their application in electrooptics and non-linear optics. One of the most widely used crystal of this group is lithium niobate that has found widespread application in non-linear optics and surface acoustic wave filters. Since in lithium niobate the magnesium doping enhances the optical damage resistance, and since it may be grown readily with magnesium concentrations up to several percents, we wanted to examine the optical properties of magnesium niobate in order to estimate its applicability as optical material. The crystalline structure of
* Corresponding author.
MgNb20 6 was first determined by Brandt [1]. The crystal belongs to the Pbcn symmetry space group which involves a centre of symmetry, thus MgNb206 would be appropriate for use as a passive element or substrate. For optical investigations fairly big samples are required. There has been several papers that reported the growth of MgNb20 6 single crystals by different methods: Emmenegger [2,3] and co-workers reported the growth by chemical transport. 5 × 2 × 1 mm 3 platelets were obtained. Greenblatt et al. [4] grew prisms up to 5 x 4 × 3 mm 3 by the flux method. Rakhmankulov and Udalov [5] used Verneuil and Czochralski methods to produce single crystals up to 3-5 mm in diameter and 6 - 8 mm long. All the above papers reported a coloration of the crystals (ranging from clear to black) which prevents the use of the material as optical wave-
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00059-3
366
K. Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
guide substrate. However, no research has b e e n d o n e so far to reveal the origin of this optical absorption. In this p a p e r we describe the growth of large single crystals of M g N b 2 0 6 and the thermal t r e a t m e n t s after the growth which m a d e it possible to modify the colour of the samples. Table 1 summarizes the physical properties o f M g N b 2 0 6 r e p o r t e d up to now.
2. Crystal growth procedure M g N b 2 0 6 p o w d e r was p r e p a r e d f r o m high purity (99.999%) M g O and N b 2 0 5 starting oxides. T h e oxides were fired at 1000°C and mixed in stoichiometric composition in a ball mill. T h e mixture was pressed into tablets by a pressing tool m a d e of plexi-glass. Table 1 Physical properties of MgNb206 single crystals and ceramics Physical property Crystalline structure Orthorhombic space group Lattice parameters (at 300 K)
T h e crystals were grown in a Metals R e s e a r c h M S R - 5 type balance controlled, Czochralski puller, with R F heating. M g N b 2 0 6 was melted in an iridium crucible, 38 m m in height and 38 m m in diameter, s u r r o u n d e d by an alumina insulator and covered by an iridium after-heater ring. To r e d u c e the axial thermal gradient an additional iridium after-heater s u r r o u n d e d the growing crystal. T h e protecting a t m o s p h e r e in the growth c h a m b e r was a N 2 + 4% 0 2 gas mixture. For the first growing runs an Ir stripe was used as seed. A strong wetting of the melt was experienced during the growth, which makes the whole process difficult and deteriorates the crystal quality. D u e to this strong wetting the melt creeps out of the crucible and a high meniscus occurs. A c c o r d ing to our m e a s u r e m e n t the surface tension of the melt at the growing t e m p e r a t u r e is 0.35 N / m . T h e growth conditions were a pulling rate of 2 - 3 m m / h and a rotation speed of 4 - 2 0 rpm.
Ref. [1]
Range of transparency
Pbcn (D 14) a = 5.017 ,~ b = 14.18 ,~ c = 5.665 .A Along (010) planes 1620-1640° C 5.087 g/cm 3 (theoretical) 4.993 g/cm 3 (experimental) HV10o = 727 kg/mm 2 % = 16.4 eb = 20.9 e c = 32.4 300-5700 nm
[This work] [5] [5] [5] [This work] [7] [7] [7] [This work]
Optical absorption edge (at 300 K) Ceramic polycrystalline powder Single crystals oxidized in air Single crystals oxidized in 02
270-290 nm 290 nm 310 nm
[8,9] [This work] [This work]
Cleavage Melting temperature Density Vickers hardness Relative dielectric permittivity (at 300 K, 5-500 kHz)
Intrinsic photoluminescence of single crystals and ceramics (300 K, hexc = 275 nm) Quenching temperature Emission maximum Photoluminescence from defects in ceramics (ceramic, 4-10 K, Aexc= 300 nm) Emission maximum
[This work], [8-10] 350 K 450 nm [91 530 nm
I~ Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
Fig. 1. MgNb206 single crystal obtained by the Czochralski technique. Pulling rate 2 m m / h . Rotation speed 20 rpm.
Under these conditions crystals of several cubic centimetres were obtained. Fig. 1 shows a picture of a typical boule. The as-grown crystals were black. A perfect cleavage was observed along (010) crystallographic planes. The material was resistant to very strong chemical attack, and was not soluble in hot concentrated mineral acids (H2SO4, HNO3, HCI, HF, and H3PO 4) or their mixtures. Slight etching was observed on the surface when molten KHSO4 was used.
tometer (model Cary 17). Fig. 2 shows the optical absorption of as-grown samples. A very broad band centred at 2.0 eV (620 nm) appears in the visible. This absorption may be removed by annealing the sample at high temperature (1200-1300 ° C) in air or in pure oxygen ( 0 2) atmosphere (see also Fig. 2). When the treatment is performed in pure 02, a lower residual optical absorption is observed but a shifting to lower energy of the optical absorption edge of the material is also induced. The optical absorption in the visible may be recovered by thermal reduction in vacuum. Fig. 2 shows the absorption induced by 90 h of heating at 950°C in a residual pressure of 10 -3 mbar. In comparison with as-grown samples, the maximum of the absorption band (at about 1.5 eV) appears shifted to low energy. This fact seems to indicate that the optical absorption is complex and several centres contribute to the spectrum of as-grown and reduced samples. The origin of these optical absorptions has to be related to the loss of molecular oxygen from the crystal lattice. The electrons left in the lattice may be trapped in oxygen vacancies or in niobium ions, giving place to the optical absorption observed. These results look rather similar to those described for LiNbO 3 by Schirmer et al. [6].
7
Optical absorption spectra have been recorded at room temperature with a Varian spectropho-
60
40 E-n. o
20 .< u o
3. Optical characterization
367
0 0.5
1.5
2.5
3.5
4.5
PHOTON ENERGY , eV
Fig. 2. R o o m temperature optical absorption spectra of MgNb206 single crystals. As-grown sample, solid line. After 72 h of annealing in air at 1300 ° C, dashed line. After 90 h of reduction in vacuum (10 -3 mbar) at 950 ° C, solid line.
368
K~ Polgdr et al. /Journal of Crystal Growth 151 (1995) 365-368
4. Conclusions
References
In s u m m a r y we have shown that large size (several cubic centimetres) M g N b 2 0 6 single crystals can be o b t a i n e d by the Czochralski growth method. T h e transparency of the crystals is improved by high t e m p e r a t u r e annealing in oxygen atmosphere. This indicates that the optical absorption observed in the visible is related to oxygen deftciency of the lattice. F u r t h e r investigation is n e e d e d to reveal the structure of the b r o a d absorption b a n d and thus the precise description of the m e c h a n i s m of the o x i d a t i o n - r e d u c t i o n processes.
[1] K. Brandt, Arkiv Kemi Min. Geol. 17A (1943) No. 15. [2] F. Emmenegger and A. Petermann, J. Crystal Growth 2 (1968) 33. [3] F. Emmenegger, J. Crystal Growth 3/4 (1968) 135. [4] M. Greenblatt, B.M. Wanklyn and B.J. Garrard, J. Crystal Growth 58 (1982) 463. [5] R.M. Rakhmankulov and Yu. P. Udalov, Zh. Neorg. Khim. 21 (1976) 2842. [6] F. Schirmer, O. Thiemann and M. W6hlecke, J. Phys. Chem. Solids 52 (1991) 185. [7] F.P. Emmenegger and H. Roetsehi, J. Phys. Chem. Solids 32 (1971) 787. [8] G. Blasse and A. Bril, Z. Phys. Chem. NF 57 (1968) 187. [9] G. Blasse, G.J. Dirksen and L.H. Brixner, Mater. Chem Phys. 14 (1986) 485. [10] A. Wachtel, J. Electrochem. Soc. 111 (1964) 534.