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Lattice site of trivalent impurities in Mg-doped lithium niobate crystals
Materials Science and Engineering, B9 ( 1991 ) 505-508
505
Lattice site of trivalent impurities in Mg-doped lithium niobate crystals L. Kovfics* lnstituto de Ciencia de Materiales, C.S.I.C. and Departamento de Fisica Aplicada C-IV, Universidad Aut6noma de Madrid, 28049 Madrid (Spain)
L. Rebouta and J. C. Soares Centro de Fisica Nuclear da Universidade de Lisboa, 1699 Lisboa Codex (Portugal)
M. F. da Silva Departamento de Fisica, ICEN/LNETI, 2685 Sacavem (Portugal)
M. Hage-Aii, J. P. Stoquert and P. Siffert Laboratoire PHASE du CNRS, Centre de Recherches Nucleaires, 67037 Strasbourg (France)
C. Zaido Instituto de Ciencia de Materiales, C.S.I.C. and Departamento de Fisica Aplicada C-I V, Universidad A ut6noma de Madrid, 28049 Madrid (Spain)
Zs. Szailer and K. Polgfir Research Laboratory for Crystal Physics"of the Hungarian Academy of Sciences, 1502 Budapest, P.O. Box 132 (Hungary)
Abstract The lattice site of trivalent impurities (lutetium, neodymium and indium) was determined in magnesiumdoped LiNbO 3 crystals by Rutherford backscattering and proton-induced X-ray emission channelling experiments. It was shown that lutetium and indium substitute mainly for lithium ions, while most of the neodymium ions occupy structural vacancy positions. It cannot be excluded, however, that a fraction of the impurities may replace niobium sites. In all of the three crystals an IR absorption band was detected which is attributed to OH stretching vibrations in M~+(niobium site)-OH--MgZ+(lithium site) complex defects.
1. Introduction Ferroelectric LiNbO 3 is widely used in integrated optics because of its excellent acoustooptic, electro-optic and non-linear optical properties [1]. Dopants play an important role in most of the applications: magnesium doping above a threshold level increases the resistance of the crystal to laser damage, transition metal dopants are used in photorefractive applications, while LiNbO3 doped with rare earth ions is a promising laser material. An LiNbO 3 crystal doubly doped with magnesium and neodymium has recently been used to fabricate a miniature *Present address: Research Laboratory for Crystal Physics of the Hungarian Academy of Sciences, 1502 Budapest, P.O. Box 132, Hungary. 0921-5107/91/$3.50
continuous wave laser with high resistance to laser damage [2]. A knowledge of the lattice location of the incorporated impurities may help us to understand the microscopic processes responsible for the physical phenomena mentioned above. In principle the trivalent impurities can be found either in lithium or niobium sites or even in the structurally vacant oxygen octahedral positions. The site occupancy may be changed by varying the Li/Nb ratio or doping the crystal with a high concentation of magnesium. Since the magnesium ions are generally assumed to replace lithium ions it is interesting to know how they influence the site occupation of the trivalent impurities. In this work we studied a series of magnesiumdoped LiNbO 3 crystals codoped with trivalent © Elsevier Sequoia/Printed in The Netherlands
506
impurities (indium, neodymium and lutetium). Rutherford backscatteing (RBS) and protoninduced X-ray emission (PIXE) channelling experiments were used to determine the favoured lattice site positions. The IR absorption spectra of OH ions associated with the impurities provided some additional information about the incorporation of the dopants. A detailed description of the present experiments completed with new data on other doubly doped LiNbO 3 crystals will be presented in a forthcoming paper.
pies the lithium site (see also the plane projections in Fig. 3 below). On the other hand, from a comparison of the maximum yields of the flux peaks for lutetium and erbium, 1.32 and 1.45 respectively [4], it can be concluded that a small amount of lutetium may substitute for niobium ions. The angular scan along the (04L41> axis
(oool
,
0.5
2. Experimental
Congruent LiNbO 3 crystals doped with 6 tool.% Mg and 1-2 mol.% of trivalent impurities were grown by the Czochralski method. The impurity concentrations in the crystals were determined by using the inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) methods. The results are shown in Table 1. The RBS and PIXIE channelling techniques were used to determine the lattice location of the impurities. To localize the lithium ions in the lattice the 7Li(p, a / H e reaction was used [3]. The emitted X-rays and particles were detected by an Si(Li) detector and a surface barrier detector respectively. The O H - ions were monitored by conventional IR absorption spectroscopy.
~
,
,
0.0
..............
.
0.5 0.0 ~ r , , ~, , , - 0 . 5 ' 0.0 ' 0.5 -1.5 0.0 1.5--1.5 0.0 1.5
.
o.o+
......
-~.o
o.o
o.o
~.o
,
-~.oo.o ~.o-~.oo.o
Angle
~.o
(°)
Fig. 1. T h e angular d e p e n d e n c e of the emitted L X-rays for indium- (top) and n e o d y m i u m - d o p e d (middle) samples and of the backscattered a particles from lutetium-doped samples (bottom) of L i N b O 3 : M g : M 3÷ crystals; % niobium: • , indium (top), n e o d y m i u m (middle) or lutetium (bottom).
(ooo~) ; z.o~
•~
o. , . ~
0.5
3. Results and discussion
The RBS results for lutetium-doped LiNbO3:Mg are shown in the bottom section of Fig. 1. The angular distribution of the backscattered a particles along the (0001) plane presents a flux peak indicating that lutetium mainly occu-
•
0.5
~, o.o-~-n-~,~
Z
1.o
-0.5
0.0 0.5 Angle(*)
:~ o.o
-2.0
0.0 ~.0 Angle(*)
Fig. 2. T h e angular distribution of the backscattered protons from n i o b i u m ions ([]) and of the emitted a particles using the 7Li(p,a)4He nuclear reaction (•).
TABLE 1 IR absorption and R B S / P I X E channelling results Cme. mol.%
Ccrv~,~I mo].%
v cm ~
a cm ~
Percentage of M s + in M 3 + - O H - - M g "~+
RBS/PIXE channelling
Er [4] Lu Nd In
I 1 2 1
0.79 0.76 0.14 0.59
3500 3522 3507
< 0.2 0.55 0.63 1.62
< 2% 6% 35% 22%
Li site mainly Li site mainly vacancy site mainly Li site
Cr [7]
0.035
0.043
3506
0.6
100%
C,,c, t and Ccry~,,, are the concentrations of the d o p a n t s in the melt and in the crystal respectively, v, w a v e n u m b e r of the O H tions; a, absorption coefficient at v.
vibra-
507
excludes the occupation of free octahedral positions, while the (0221) axis scan shows the blocking effect of lithium on the lutetium ions occupying the lithium site. A comparison of the lutetium and lithium profiles (Figs. 1 and 2) supports the conclusion that lutetium ions mainly replace lithium ions. The PIXE channelling experiments in LiNbO3 :Mg, In are shown in the top section of Fig. 1. The angular scans for the (0001) planar and the (0441) and (0221) axial directions are essentially similar to those found for the lutetiumdoped crystal. Although the angular distribution along the (0441) direction rules out free octahedral vacancy occupation, a fraction of the indium may be present at niobium sites. For LiNbO 3: Mg,Nd the angular profiles of Nd L X-ray emissions are different from those obtained for indium- and lutetium-doped LiNbO3:Mg. Flux peaks are found along the (0441 ) and (0221) axial directions (see the middle section of Fig. 1). The projections in Fig. 3 show that the intrinsic vacancy sites are at the centre and near to the centre of the (0441) and (0221) channels respectively. Neodymium ions occupying the free octahedral sites are responsible for the flux peaks. To our knowledge we report here the first experimental evidence about the stuctural vacancy site occupation by an impurity in LiNbO 3. The IR absorption spectra of the three doubly doped LiNbO 3 crystals are shown in Fig. 4. The broken curves represent the O H - bands usually found in pure LiNbO 3 (about 3485 cm-1) and in magnesium-doped LiNbO3 (about 3528 and 3538 cm -1) [5]. The absorption bands shown by dotted curves are only present in doubly doped crystals. The wavenumbers and absorption maxima of the bands are collected in Table 1. In an earlier paper [6] we assumed that the new O H - bands in the magnesium- and trivalentimpurity-doped LiNbO3 crystal can be associated
(0001)
(Ozi,-41)
with the vibration of an O H - ion in an M 3 + - O H - - M g 2÷ defect lying along the trigonal c axis where the magnesium ion occupies a lithium site and the trivalent impurity replaces niobium. In an LiNbO3 :Mg, Cr crystal the intensity of the O H - band related to the defect complex has been found to be proportional to the C r 3 ÷ content of the crystal [7]. The estimated absorption strength for the O H - band is equal to 6.7 x 10 -20 cm: [6]. Using this value we calculated the fraction of trivalent impurities (lutetium, neodymium and indium) which forms a defect complex with O H - and magnesium ions (see Table 1). This fraction of the impurity, which is only a minor part of the total impurity concentration, is thought to occupy the niobium site in the crystal. It has to be mentioned that the proportion (35%) of neodymium ions on niobium sites is probably overestimated since the neodymium concentration measured by the low accuracy AAS method is lower than that which can be expected from the usual distribution coefficient of neodymium in LiNbO3 (0.1-0.3)[8]. Taking into account the large uncertainty in the estimation of the amount of trivalent impurities occupying a niobium site (about 50%), the IR absorption results are in good agreement with the channelling experiments. It has to be emphasized that quantitative determination of the relative
1.0 0.5 7
0.0
~0.0
/
(02;~ 1) Wavenumber
Fig. 3. Projections on planes normal to the indicated directions: *, niobium; A, oxygen; D, lithium;~, intrinsic vacancy.
(era -1)
Fig. 4. O H - absorption bands in lutetium- (top), neodymium- (middle) and indium-doped (bottom) LiNbO3:Mg crystals: .... O H - bands attributed to O H - ions in M3+(niobium site)-OH -Mg2+(lithium site) defect complexes; -- -- -O H - usually found in pure LiNbO3 (about 3485 cm 1) and in magnesium-doped LiNbO 3 (about 3528 and 3538 cm- ~).
5(t8
amounts of lithium site, niobium site and vacancy occupancy is not possible from our RBS and PIXE data. It seems to be probable, however, that a small fraction of the impurities does not occupy a lithium or vacancy site, but they do substitute for niobium ions. That fraction of the impurities forms a neutral defect complex with O H - and magnesium ions, giving rise to the appearance of the new OH absorption band. In an LiNbO3:Mg,Er crystal, where the erbium ions were found to substitute only for lithium ions, the OH absorption band was absent [4].
Acknowledgments Financial support from the National Science Fund (OTKA) of Hungary, the Junta Nacional de Investigacao Cientifica e Tecnol6gica (Luis Rebouta) (JNICT(LR)) of Portugal and the Comisi6n Interministerial de Ciencia y Tecnologfa (CICyT) (MAT88-0431-C02) of Spain is acknowl-
edged. The authors are grateful to Mr. I. Cravero and Mr. P. Lassfinyi for the impurity analysis.
References 1 Properties of lithium niobate, EM1S Data Reviews Series No. 5, INSPEC, Institute of Electrical Engineers, London, 1989. 2 A. Cordova-Plaza, M. J. F. Digonnet and K. J. Shaw, IEEE J. Quantum Electron., 23 (1987) 262. 3 L. Rebouta, M. F. da Silva, J. C. Soares, M. Hage-Ali, J. P. Stoquert, E Siffert, J. A. Sanz-Garc[a, E. Di6guez and E Agullo-L6pez, in preparation. 4 L. Kowics, L. Rebouta, J. C. Soarcs and M. F. da Silva, Radiat. El]I, submitted for publication. 5 L. Kovfics, K. Polg~ir and R. Capelletti, CO,st. Lattice l)eJ~,cts, 15 (1987) 115. 6 L. Kov~ics. Zs. Szaller, 1. Cravero, 1. F61dvfiri and C. Zaldo, J. Phys. Chem. Solid3, 5/Ill 990) 417. 7 L. Kovfics. 1. F61dv~iri, I. Cravero, K. Polgs.r and R. Capelletti, Phys. Lett. A, 133 (1988) 433. 8 K. G. Belabaev, A. A. Kaminskii and S. E, Sarkisov, Phys. Status Solidi A, 28 ( 1975 ) K 17.
505
Lattice site of trivalent impurities in Mg-doped lithium niobate crystals L. Kovfics* lnstituto de Ciencia de Materiales, C.S.I.C. and Departamento de Fisica Aplicada C-IV, Universidad Aut6noma de Madrid, 28049 Madrid (Spain)
L. Rebouta and J. C. Soares Centro de Fisica Nuclear da Universidade de Lisboa, 1699 Lisboa Codex (Portugal)
M. F. da Silva Departamento de Fisica, ICEN/LNETI, 2685 Sacavem (Portugal)
M. Hage-Aii, J. P. Stoquert and P. Siffert Laboratoire PHASE du CNRS, Centre de Recherches Nucleaires, 67037 Strasbourg (France)
C. Zaido Instituto de Ciencia de Materiales, C.S.I.C. and Departamento de Fisica Aplicada C-I V, Universidad A ut6noma de Madrid, 28049 Madrid (Spain)
Zs. Szailer and K. Polgfir Research Laboratory for Crystal Physics"of the Hungarian Academy of Sciences, 1502 Budapest, P.O. Box 132 (Hungary)
Abstract The lattice site of trivalent impurities (lutetium, neodymium and indium) was determined in magnesiumdoped LiNbO 3 crystals by Rutherford backscattering and proton-induced X-ray emission channelling experiments. It was shown that lutetium and indium substitute mainly for lithium ions, while most of the neodymium ions occupy structural vacancy positions. It cannot be excluded, however, that a fraction of the impurities may replace niobium sites. In all of the three crystals an IR absorption band was detected which is attributed to OH stretching vibrations in M~+(niobium site)-OH--MgZ+(lithium site) complex defects.
1. Introduction Ferroelectric LiNbO 3 is widely used in integrated optics because of its excellent acoustooptic, electro-optic and non-linear optical properties [1]. Dopants play an important role in most of the applications: magnesium doping above a threshold level increases the resistance of the crystal to laser damage, transition metal dopants are used in photorefractive applications, while LiNbO3 doped with rare earth ions is a promising laser material. An LiNbO 3 crystal doubly doped with magnesium and neodymium has recently been used to fabricate a miniature *Present address: Research Laboratory for Crystal Physics of the Hungarian Academy of Sciences, 1502 Budapest, P.O. Box 132, Hungary. 0921-5107/91/$3.50
continuous wave laser with high resistance to laser damage [2]. A knowledge of the lattice location of the incorporated impurities may help us to understand the microscopic processes responsible for the physical phenomena mentioned above. In principle the trivalent impurities can be found either in lithium or niobium sites or even in the structurally vacant oxygen octahedral positions. The site occupancy may be changed by varying the Li/Nb ratio or doping the crystal with a high concentation of magnesium. Since the magnesium ions are generally assumed to replace lithium ions it is interesting to know how they influence the site occupation of the trivalent impurities. In this work we studied a series of magnesiumdoped LiNbO 3 crystals codoped with trivalent © Elsevier Sequoia/Printed in The Netherlands
506
impurities (indium, neodymium and lutetium). Rutherford backscatteing (RBS) and protoninduced X-ray emission (PIXE) channelling experiments were used to determine the favoured lattice site positions. The IR absorption spectra of OH ions associated with the impurities provided some additional information about the incorporation of the dopants. A detailed description of the present experiments completed with new data on other doubly doped LiNbO 3 crystals will be presented in a forthcoming paper.
pies the lithium site (see also the plane projections in Fig. 3 below). On the other hand, from a comparison of the maximum yields of the flux peaks for lutetium and erbium, 1.32 and 1.45 respectively [4], it can be concluded that a small amount of lutetium may substitute for niobium ions. The angular scan along the (04L41> axis
(oool
,
0.5
2. Experimental
Congruent LiNbO 3 crystals doped with 6 tool.% Mg and 1-2 mol.% of trivalent impurities were grown by the Czochralski method. The impurity concentrations in the crystals were determined by using the inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) methods. The results are shown in Table 1. The RBS and PIXIE channelling techniques were used to determine the lattice location of the impurities. To localize the lithium ions in the lattice the 7Li(p, a / H e reaction was used [3]. The emitted X-rays and particles were detected by an Si(Li) detector and a surface barrier detector respectively. The O H - ions were monitored by conventional IR absorption spectroscopy.
~
,
,
0.0
..............
.
0.5 0.0 ~ r , , ~, , , - 0 . 5 ' 0.0 ' 0.5 -1.5 0.0 1.5--1.5 0.0 1.5
.
o.o+
......
-~.o
o.o
o.o
~.o
,
-~.oo.o ~.o-~.oo.o
Angle
~.o
(°)
Fig. 1. T h e angular d e p e n d e n c e of the emitted L X-rays for indium- (top) and n e o d y m i u m - d o p e d (middle) samples and of the backscattered a particles from lutetium-doped samples (bottom) of L i N b O 3 : M g : M 3÷ crystals; % niobium: • , indium (top), n e o d y m i u m (middle) or lutetium (bottom).
(ooo~) ; z.o~
•~
o. , . ~
0.5
3. Results and discussion
The RBS results for lutetium-doped LiNbO3:Mg are shown in the bottom section of Fig. 1. The angular distribution of the backscattered a particles along the (0001) plane presents a flux peak indicating that lutetium mainly occu-
•
0.5
~, o.o-~-n-~,~
Z
1.o
-0.5
0.0 0.5 Angle(*)
:~ o.o
-2.0
0.0 ~.0 Angle(*)
Fig. 2. T h e angular distribution of the backscattered protons from n i o b i u m ions ([]) and of the emitted a particles using the 7Li(p,a)4He nuclear reaction (•).
TABLE 1 IR absorption and R B S / P I X E channelling results Cme. mol.%
Ccrv~,~I mo].%
v cm ~
a cm ~
Percentage of M s + in M 3 + - O H - - M g "~+
RBS/PIXE channelling
Er [4] Lu Nd In
I 1 2 1
0.79 0.76 0.14 0.59
3500 3522 3507
< 0.2 0.55 0.63 1.62
< 2% 6% 35% 22%
Li site mainly Li site mainly vacancy site mainly Li site
Cr [7]
0.035
0.043
3506
0.6
100%
C,,c, t and Ccry~,,, are the concentrations of the d o p a n t s in the melt and in the crystal respectively, v, w a v e n u m b e r of the O H tions; a, absorption coefficient at v.
vibra-
507
excludes the occupation of free octahedral positions, while the (0221) axis scan shows the blocking effect of lithium on the lutetium ions occupying the lithium site. A comparison of the lutetium and lithium profiles (Figs. 1 and 2) supports the conclusion that lutetium ions mainly replace lithium ions. The PIXE channelling experiments in LiNbO3 :Mg, In are shown in the top section of Fig. 1. The angular scans for the (0001) planar and the (0441) and (0221) axial directions are essentially similar to those found for the lutetiumdoped crystal. Although the angular distribution along the (0441) direction rules out free octahedral vacancy occupation, a fraction of the indium may be present at niobium sites. For LiNbO 3: Mg,Nd the angular profiles of Nd L X-ray emissions are different from those obtained for indium- and lutetium-doped LiNbO3:Mg. Flux peaks are found along the (0441 ) and (0221) axial directions (see the middle section of Fig. 1). The projections in Fig. 3 show that the intrinsic vacancy sites are at the centre and near to the centre of the (0441) and (0221) channels respectively. Neodymium ions occupying the free octahedral sites are responsible for the flux peaks. To our knowledge we report here the first experimental evidence about the stuctural vacancy site occupation by an impurity in LiNbO 3. The IR absorption spectra of the three doubly doped LiNbO 3 crystals are shown in Fig. 4. The broken curves represent the O H - bands usually found in pure LiNbO 3 (about 3485 cm-1) and in magnesium-doped LiNbO3 (about 3528 and 3538 cm -1) [5]. The absorption bands shown by dotted curves are only present in doubly doped crystals. The wavenumbers and absorption maxima of the bands are collected in Table 1. In an earlier paper [6] we assumed that the new O H - bands in the magnesium- and trivalentimpurity-doped LiNbO3 crystal can be associated
(0001)
(Ozi,-41)
with the vibration of an O H - ion in an M 3 + - O H - - M g 2÷ defect lying along the trigonal c axis where the magnesium ion occupies a lithium site and the trivalent impurity replaces niobium. In an LiNbO3 :Mg, Cr crystal the intensity of the O H - band related to the defect complex has been found to be proportional to the C r 3 ÷ content of the crystal [7]. The estimated absorption strength for the O H - band is equal to 6.7 x 10 -20 cm: [6]. Using this value we calculated the fraction of trivalent impurities (lutetium, neodymium and indium) which forms a defect complex with O H - and magnesium ions (see Table 1). This fraction of the impurity, which is only a minor part of the total impurity concentration, is thought to occupy the niobium site in the crystal. It has to be mentioned that the proportion (35%) of neodymium ions on niobium sites is probably overestimated since the neodymium concentration measured by the low accuracy AAS method is lower than that which can be expected from the usual distribution coefficient of neodymium in LiNbO3 (0.1-0.3)[8]. Taking into account the large uncertainty in the estimation of the amount of trivalent impurities occupying a niobium site (about 50%), the IR absorption results are in good agreement with the channelling experiments. It has to be emphasized that quantitative determination of the relative
1.0 0.5 7
0.0
~0.0
/
(02;~ 1) Wavenumber
Fig. 3. Projections on planes normal to the indicated directions: *, niobium; A, oxygen; D, lithium;~, intrinsic vacancy.
(era -1)
Fig. 4. O H - absorption bands in lutetium- (top), neodymium- (middle) and indium-doped (bottom) LiNbO3:Mg crystals: .... O H - bands attributed to O H - ions in M3+(niobium site)-OH -Mg2+(lithium site) defect complexes; -- -- -O H - usually found in pure LiNbO3 (about 3485 cm 1) and in magnesium-doped LiNbO 3 (about 3528 and 3538 cm- ~).
5(t8
amounts of lithium site, niobium site and vacancy occupancy is not possible from our RBS and PIXE data. It seems to be probable, however, that a small fraction of the impurities does not occupy a lithium or vacancy site, but they do substitute for niobium ions. That fraction of the impurities forms a neutral defect complex with O H - and magnesium ions, giving rise to the appearance of the new OH absorption band. In an LiNbO3:Mg,Er crystal, where the erbium ions were found to substitute only for lithium ions, the OH absorption band was absent [4].
Acknowledgments Financial support from the National Science Fund (OTKA) of Hungary, the Junta Nacional de Investigacao Cientifica e Tecnol6gica (Luis Rebouta) (JNICT(LR)) of Portugal and the Comisi6n Interministerial de Ciencia y Tecnologfa (CICyT) (MAT88-0431-C02) of Spain is acknowl-
edged. The authors are grateful to Mr. I. Cravero and Mr. P. Lassfinyi for the impurity analysis.
References 1 Properties of lithium niobate, EM1S Data Reviews Series No. 5, INSPEC, Institute of Electrical Engineers, London, 1989. 2 A. Cordova-Plaza, M. J. F. Digonnet and K. J. Shaw, IEEE J. Quantum Electron., 23 (1987) 262. 3 L. Rebouta, M. F. da Silva, J. C. Soares, M. Hage-Ali, J. P. Stoquert, E Siffert, J. A. Sanz-Garc[a, E. Di6guez and E Agullo-L6pez, in preparation. 4 L. Kowics, L. Rebouta, J. C. Soarcs and M. F. da Silva, Radiat. El]I, submitted for publication. 5 L. Kovfics, K. Polg~ir and R. Capelletti, CO,st. Lattice l)eJ~,cts, 15 (1987) 115. 6 L. Kov~ics. Zs. Szaller, 1. Cravero, 1. F61dvfiri and C. Zaldo, J. Phys. Chem. Solid3, 5/Ill 990) 417. 7 L. Kovfics. 1. F61dv~iri, I. Cravero, K. Polgs.r and R. Capelletti, Phys. Lett. A, 133 (1988) 433. 8 K. G. Belabaev, A. A. Kaminskii and S. E, Sarkisov, Phys. Status Solidi A, 28 ( 1975 ) K 17.