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Electrical and dielectric properties of β-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028 www.elsevier.com/locate/jpcs
Electrical and dielectric properties of b-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals V. Trnovcova´a,, P.P. Fedorovb, A.E. Kokhc, I. Fura´ra a
Department of Physics, Faculty of Materials Science and Technology, Slovak University of Technology, 917 24 Trnava, Slovakia b Institute of General Physics, Russian Academy of Sciences, 119991 Moscow, Russia c Institute of Mineralogy and Petrography, Russian Academy of Sciences, 630090 Novosibirsk, Russia
Abstract In barium borate (BBO) crystals, sodium and potassium ions, inherited due to the preparation technique, are dominant charge carriers. The conductivity between layers is higher; the conductivity activation energy and the conductivity at 350 1C being equal to 1.0170.05 eV and (1.370.2) 108 S/cm, respectively. The conductivity activation energy and the conductivity at 350 1C along the channels are equal to 1.1370.05 eV and to (470.2) 109 S/cm, respectively. Relative static permittivity is almost isotropic, and equal to 7.6570.05. Upon storing of cesium–lithium borate (CLBO) crystals, pre-heating to 600 1C eliminates the influence of surface humidity. At 500 K, the ionic conductivity ranges from 4 1012 to 2 1010 S/cm; the conductivity activation energy ranges from 1.01 to 1.17 eV. Relative static permittivity is equal to 7.470.3. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Optical crystals; B. Crystal growth; D. Electrical conductivity; D. Dielectric properties
1. Introduction The low-temperature modification of barium borate, b-BaB2O4 (BBO), is used in non-linear optics—for a conversion of laser frequencies into the visible or UV range, frequency mixers, and electro-optic Q switch. It has a high effective coefficient of the second harmonic generation, large birefringence (DnE0.12), high optical damage threshold and wide transparency range (190–3500 nm) [1–3]. The cesium–lithium borate, CsLi (B3O5)2 (CLBO), is important because of its high nonlinearity, and high optical damage threshold. It has an outstanding ability for the generation of the second-, fourth-, and fifth-harmonics of an Nd: YAG laser beam [4]. It can generate 193 nm output through sum-frequency generation [5]. Its transparency range extends from 180 to 2750 nm. Applications are limited by its hygroscopic
Corresponding author. Tel.: +421 905 691870; fax: +421 33 5511758.
E-mail addresses: [email protected], [email protected] (V. Trnovcova´), [email protected] (P.P. Fedorov), [email protected] (A.E. Kokh), [email protected] (I. Fura´r). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.03.012
behavior [6]. Water molecules attack the crystal along the a axis much more easily than along the c axis [7]. Barium borate crystals have two polymorphs: a hightemperature trigonal a-phase (above 925 1C, space group ¯ T melt ¼ 110075 1C), and a low-temperature trigonalR3c, like b-phase (space group R3c)—BBO. Both modifications are not hygroscopic. In BBO crystals, a layered structure perpendicular to the c axis is formed. Along the c axis, structural channels are observed. The layered structure contains almost planar, circular [B3O6]3 anions [3,7]. Both the layered structure and structural channels support a possibility of the ionic conductivity in BBO. CLBO, T melt ¼ 852(5) 1C, crystallizes in a tetragonal structure (space group I42d) [6,8]. It melts congruently and has no solid-state phase transformation. The borate network is characterized by a full condensation of (B3O7)5 rings [9]. Eightfold-coordinated Cs+ and fourfold-coordinated Li+ take place in channels formed by borate groups. Ellipsoidal channels are perpendicular to the c axis and contain only Cs+ ions. The channels are wide enough as to enable a mobility of Cs+ ions [8,10]. Channels along the c axis have a squared form. They contain alternately Cs+ and Li+ ions. The Cs+ ions are
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
2. Experimental details BBO was synthesized at 700 1C and single crystals were grown from the BaB2O4–20 m/o Na2O flux, using the TSSG method and [0 0 1]-oriented seeding crystals [10]. The pulling rate was 0.2–0.7 mm/day. Sodium is a natural impurity inherited from the flux [12]. In our crystals, its content ranged from 0.01 to 0.025 wt% Na2O. Moreover, in our BBO crystals, 0.01–0.15 wt% of K2O was detected. All samples were cut perpendicularly to a or c axes. From each type of BBO crystals, five oriented samples were measured. From these measurements, conductivity activation energies and their deviations were calculated (Table 1). CLBO single crystals were prepared in Pt crucibles from the Li2O–Cs2O–B2O3–MoO3 flux, using the TSSG method with a seed [8,13]. Two large, arbitrary oriented samples were prepared two years ago. Three small samples were cut immediately before measurements from a two years’ stored bulk. One of them was cut perpendicularly to the a axis. Samples were stored in tightly closed vessels, in the presence of silica gel. Table 1 Conductivity activation energies (150–400 1C) of BBO along the a axis, Ea, and along the c axis, Ec, and conductivities at 350 1C along both axes, sa, sc, for various samples Sample
Ea (eV)
Ec (eV)
sa (S/cm)
sc (S/cm)
26–13a 26-13 Annealed 29-7b 29-7 Annealed 00-9c 00-9 Annealed
1.0070.05 1.0170.05 1.0070.05 1.0270.05 1.0070.05 0.9970.05
1.0870.05 1.1270.05 1.0970.05 1.1470.05 1.1070.05 —
1 108 1.1 108 1.2 108 1.5 108 1.3 108 2.2 108
9 109 3 109 4 109 5 109 2 109 —
a
0.01 wt % Na2O, 0.011 wt% K2O. 0.025 wt % Na2O, 0.011 wt% K2O. c 0.15 wt % K2O. b
For electrical and dielectric measurements, painted silver contacts were used. For dc measurements, the electrometric voltmeter with a vibrating condenser, VK 2-16, was used. For ac measurements (above 400 1C), the Impedance analyzer SI 1260, Schlumberger-Solartron, working in a frequency range of 100 Hz–10 MHz, was used. For evaluation of the bulk conductivity and static relative permittivity, the impedance or modular spectroscopy was used, respectively. Some samples were pre-annealed at 600 1C, for 30 min, as to dissolve precipitated impurities. At measurements of electrical properties, a heating rate of 2 1C/min was used. Moreover, at 50 1C temperature intervals, isothermal measurements of the conductivity and static permittivity were made. All electrical measurements were done in a dry N2 atmosphere. 3. Results In Fig. 1, Arrhenius plot of the electrical conductivity of BBO crystals of different purity, crystallographic orientation, and thermal history are presented. The temperature dependences exhibit two or three sections with different activation energies. Three sections are observed at measurements along the a axis. The conductivity along the a axis is 3–4 times higher than that along the c axis. Above 500 1C, this anisotropy disappears. In the sample with a high K+ content, a pre-annealing at 600 1C results in a significant increase of the conductivity. Upon multiple heating up to 600 1C, only two sections with different activation energies are observed (Fig. 2). The conductivity
-6
-8 log (σ/S cm-1)
isolated; Li+ ions are strongly bound to the chains formed by B3O7 groups. It is supposed that cationic mobility along this direction will be lower. At high humidity, the CLBO surface hydrates. This hydration changes the physical properties of crystals and causes cracks. The channels along the a axis provide enough space for an infiltration of water molecules [6]. The first stage of dehydroxylation takes place at 85–131 1C [11], the second one at 176–210 1C [7,10]. Due to structural peculiarities of both BBO and CLBO crystals, and their chemical composition (Na+, K+ impurities in BBO, Li+ content in CLBO), we expected a fast ionic conductivity in both materials. Therefore, the aim of this paper is (1) to appreciate the possibility of the fast ionic conductivity and to determine the anisotropy of electrical and dielectric properties of both BBO and CLBO crystals, (2) to determine an influence of alkaline impurities (in BBO) or hydrolysis (in CLBO) on their electrical properties, and (3) to propose conduction mechanisms in BBO and CLBO crystals.
1025
-10
-12
1.2
1.6
2.0
2.4
103/T (K-1) Fig. 1. Temperature dependences of the electrical conductivity of ‘‘asreceived’’ BBO crystals along the a axis (D, x, o), and along the c axis (&, ’). (A sample with a high content of K2O, upon annealing ().)
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
1026
0.75 -7
10 M"
log (σ/S cm-1)
0.5 -9
0.25 -11 0
0.5
1
10
M'
-13
1.0
1.5
2.0
2.5
3.0
Fig. 3. Modular diagrams of BBO crystals along the c axis (r, D), and along the a axis (&, x, o), at temperatures 395 (&,r), 449 (x), 475 (D), and 511 1C (o).
103/T (K-1) Fig. 2. Temperature dependences of the electrical conductivity of BBO crystals along the c axis (non-pre-heated (ooo), pre-heated (xxx)), and along the a axis (non-pre-heated (DDD), pre-heated (mmm)).
-6
log (σ/S cm-1)
-8
along the a axis is not influenced by repeated heatings. The conductivity along the c axis is slightly reduced, upon repeated heatings. The conductivity activation energies of BBO crystals along the a and c axes are equal to 1.0170.05 and 1.1070.02 eV, respectively (Table 1). Below 150–170 1C, adsorbed water molecules result in enhanced surface proton conductivity. Upon pre-heating, this enhanced conductivity disappears (Fig. 2). The static relative permittivity of BBO is almost isotropic being equal to 7.770.1 along the c axis, and 7.670.1 along the a axis. In Fig. 3, corresponding modular diagrams are shown. Up to 600 1C, the temperature dependence of the static permittivity is negligible. In Fig. 4, temperature dependences of the electrical conductivity of two large samples of CLBO are presented. Upon the second heating up to 600 1C, the dependences of both samples were identical and reproducible at repeated measurements; an influence of the adsorbed water molecules on the conductivity has been observed only at temperatures below 100 1C. At higher temperatures, the conductivity activation energy is equal to 1.1770.01 eV; the conductivity at 350 1C is equal to (1.370.1) 109 S/ cm. The static relative permittivity is equal to 7.270.3, in a good agreement with [14]. In Fig. 5, temperature dependences of the electrical conductivity of three small samples of CLBO are presented. The samples were freshly cut; therefore the first measurements were close to the repeated ones. Above 90 1C, we have not seen any influence of the adsorbed water molecules. Usually, the fresh samples had a slightly lower activation energy and higher conductivity than the pre-heated ones. In pre-heated samples, we have found conductivity activation energies: 1.13, 1.02, and 1.01 eV (sample cut perpendicularly to the a axis) and
-10
-12
-14 1.0
1.5
2.0 3/T
10
2.5 (K-1)
Fig. 4. Temperature dependences of the electrical conductivity of large CLBO crystals: ‘‘as received’’ (D), upon the first heating to 600 1C (o), and upon repeated heating to 600 1C (&).
conductivities at 350 1C: 2.1 108 S/cm, 7 109 S/cm, 2.4 109 S/cm, respectively. Some irregularities in the dependences disappeared upon repeated heating. They came probably from high-temperature dehydroxylation processes during first heating. Up to 600 1C, the mean value of the relative static permittivity is equal to 7.470.3. 4. Discussion Considering broad optical gaps and ionic bonds in both BBO and CLBO, it is generally supposed that the ionic conductivity dominates. The most probable charge carriers are alkali metal ions. In the temperature dependences of the conductivity of BBO crystals, measured along the c axis, a low-temperature association region, a medium-temperature migration region,
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
heating to 600 or 250 1C (Fig. 4). Upon pre-heating, the reproducibility of temperature dependences of the conductivity is excellent (Fig. 4). Moreover, no cracking of samples was observed. Measurements of arbitrary oriented small pieces show that the anisotropy of the conductivity of CLBO is more pronounced than that in BBO crystals. At 500 K, the conductivity ranges from 4 1012 to 2 1010 S/cm, and the conductivity activation energy ranges from 1.01 to 1.13 eV. The anisotropy of the relative static permittivity is not very significant, its value being 7.470.3, in a good agreement with [1].
-6
-8 log (σ/S cm-1)
1027
-10
-12
5. Conclusions -14 1.0
1.5
2.0
2.5
103/T (K-1) Fig. 5. Temperature dependences of the electrical conductivity of nonoriented (DDD) and oriented (perpendicular to the a axis (&&&)) small CLBO crystals pre-heated up to 600 1C.
and a high-temperature intrinsic region can be distinguished (Figs. 1 and 2). In these regions, the temperature dependences of the conductivity, s, can be described as s ¼ s0 expðE=kTÞ, where s0 stands for the pre-exponential factor, and E is the conductivity activation energy. In BBO, monovalent naturally inherited Na+ and K+ impurities are more probable charge carriers than large, bivalent Ba2+ ions. Their dominance in the conduction process is confirmed by an enhanced conductivity in pre-heated crystals containing a high concentration of K+ impurities (Fig. 4) or in crystals containing an enhanced concentration of Na+ ions (Table 1 —annealed sample 29–7). Pre-heating brings about dissolution of aliovalent impurities what results in an enhanced conductivity. The influence of pre-heating on the concentration of Na+ charge carriers is usually negligible (Fig. 2). A slight decrease of the conductivity along the c axis indicates a slight dehydration of channels, upon pre-heating. The neutrality condition requires a substitution of Ba2+ ions with splitted interstitials of monovalent alkali ions. An interstitial or interstitialcy conduction mechanism through the channels along the c axis or between the layers perpendicular to the c axis is most probable. Surprisingly, the ionic transport between layers is faster. With regard to the layered structure of BBO the almost isotropic static permittivity is surprising. The values of the permittivity are close to those published earlier [2,15]. The permittivity is important for the evaluation of electro– optical coefficients [2]. We have not found any destructive influence of storing in air on electrical properties of CLBO crystals. The influence of two years’ storing (in silica gel) is eliminated by pre-
In BBO crystals, sodium and potassium ions are dominant charge carriers. These ions are inherited impurities due to the preparation technique. Splitted interstitials of alkali ions are dominant mobile defects. They migrate by an interstitialcy mechanism in channels along the c axis and between the layers in (a,b) planes. The conductivity between the layers is higher, its conductivity activation energy being equal to 1.0170.05 eV, and the conductivity at 350 1C being equal to (1.370.2) 108 S/cm. Upon preheating, the conductivity activation energy along the channels is equal to 1.1370.05 eV, and the conductivity at 350 1C is equal to (470.2) 109 S/cm. Relative static permittivity is almost isotropic, and equal to 7.6570.15. In CLBO crystals, pre-heating to 600 1C eliminates the influence of surface humidity. Upon this treatment, no cracking of samples is observed. At 500 K, the conductivity of arbitrary oriented samples ranges from 4 1012 to 2 1010 S/cm, and the conductivity activation energy ranges from 1.01 to 1.13 eV. Relative static permittivity is almost isotropic, and equal to 7.470.3. Acknowledgment The Scientific Grant Agency VEGA, Slovak Republic, by project no. 1/2100/05 supported the work. The authors thank Mrs. A. Suc˘a´kova´ for her technical assistance. References [1] D.A. Keszler, Curr. Opin. Solid State Mat. Sci. 1 (1996) 204. [2] D. Eimeri, L. Davis, S. Velako, et al., J. Appl. Phys. 62 (1987) 1968. [3] P.P. Fedorov, A.E. Koch, N.G. Kononova, Uspechi chimii 71 (2002) 741. [4] Y. Mori, I. Kuroda, S. Nakajima, et al., Jpn. J. Appl. Phys. 34 (1995) L296. [5] C. Chen, Z. Lin, Z. Wang, Appl. Phys. B 80 (2005) 1. [6] L. Kova´cs, K. Lengyel, A´. Pe´ter, et al., Optical Materials 24 (2003) 457. [7] F. Pan, X. Wang, G. Shen, D. Shen, J. Cryst. Growth 24 (2002) 129. [8] N.G. Kononova, A.E. Koch, P.P. Fedorov, et al., Neorg. Mater. 38 (2002) 1485. [9] J.M. Tu, A. Keszler, Mater. Res. Bull. 30 (1995) 209. [10] P.P. Fedorov, N.G. Kononova, A.E. Kokh, et al., Russ J. Inorg. Chem. 47 (2002) 1042.
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[11] A. Taguchi, A. Miyamoto, Y. Mori, et al., in: C.R. Pollock, W.R. Bosenberg, (Eds.), Advanced Solid State Lasers, vol. 10, 1997, p. 19. [12] A.E. Kokh, N.G. Kononova, P.P. Fedorov, et al., Crystallogr. Rep. 47 (2002) 559.
[13] N.A. Pylneva, N. Kononova, A.M. Yurkin, et al., Proc. SPIE 3610 (1999) 148. [14] K. Bai, S.T. Jung, J. Cryst. Growth 186 (1998) 612. [15] L.I. Ivleva, D.T. Kiseleva, Yu.S. Kuzminov, N.M. Polozkov, Neorg. Mater. 24 (1988) 1153.
Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028 www.elsevier.com/locate/jpcs
Electrical and dielectric properties of b-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals V. Trnovcova´a,, P.P. Fedorovb, A.E. Kokhc, I. Fura´ra a
Department of Physics, Faculty of Materials Science and Technology, Slovak University of Technology, 917 24 Trnava, Slovakia b Institute of General Physics, Russian Academy of Sciences, 119991 Moscow, Russia c Institute of Mineralogy and Petrography, Russian Academy of Sciences, 630090 Novosibirsk, Russia
Abstract In barium borate (BBO) crystals, sodium and potassium ions, inherited due to the preparation technique, are dominant charge carriers. The conductivity between layers is higher; the conductivity activation energy and the conductivity at 350 1C being equal to 1.0170.05 eV and (1.370.2) 108 S/cm, respectively. The conductivity activation energy and the conductivity at 350 1C along the channels are equal to 1.1370.05 eV and to (470.2) 109 S/cm, respectively. Relative static permittivity is almost isotropic, and equal to 7.6570.05. Upon storing of cesium–lithium borate (CLBO) crystals, pre-heating to 600 1C eliminates the influence of surface humidity. At 500 K, the ionic conductivity ranges from 4 1012 to 2 1010 S/cm; the conductivity activation energy ranges from 1.01 to 1.17 eV. Relative static permittivity is equal to 7.470.3. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Optical crystals; B. Crystal growth; D. Electrical conductivity; D. Dielectric properties
1. Introduction The low-temperature modification of barium borate, b-BaB2O4 (BBO), is used in non-linear optics—for a conversion of laser frequencies into the visible or UV range, frequency mixers, and electro-optic Q switch. It has a high effective coefficient of the second harmonic generation, large birefringence (DnE0.12), high optical damage threshold and wide transparency range (190–3500 nm) [1–3]. The cesium–lithium borate, CsLi (B3O5)2 (CLBO), is important because of its high nonlinearity, and high optical damage threshold. It has an outstanding ability for the generation of the second-, fourth-, and fifth-harmonics of an Nd: YAG laser beam [4]. It can generate 193 nm output through sum-frequency generation [5]. Its transparency range extends from 180 to 2750 nm. Applications are limited by its hygroscopic
Corresponding author. Tel.: +421 905 691870; fax: +421 33 5511758.
E-mail addresses: [email protected], [email protected] (V. Trnovcova´), [email protected] (P.P. Fedorov), [email protected] (A.E. Kokh), [email protected] (I. Fura´r). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.03.012
behavior [6]. Water molecules attack the crystal along the a axis much more easily than along the c axis [7]. Barium borate crystals have two polymorphs: a hightemperature trigonal a-phase (above 925 1C, space group ¯ T melt ¼ 110075 1C), and a low-temperature trigonalR3c, like b-phase (space group R3c)—BBO. Both modifications are not hygroscopic. In BBO crystals, a layered structure perpendicular to the c axis is formed. Along the c axis, structural channels are observed. The layered structure contains almost planar, circular [B3O6]3 anions [3,7]. Both the layered structure and structural channels support a possibility of the ionic conductivity in BBO. CLBO, T melt ¼ 852(5) 1C, crystallizes in a tetragonal structure (space group I42d) [6,8]. It melts congruently and has no solid-state phase transformation. The borate network is characterized by a full condensation of (B3O7)5 rings [9]. Eightfold-coordinated Cs+ and fourfold-coordinated Li+ take place in channels formed by borate groups. Ellipsoidal channels are perpendicular to the c axis and contain only Cs+ ions. The channels are wide enough as to enable a mobility of Cs+ ions [8,10]. Channels along the c axis have a squared form. They contain alternately Cs+ and Li+ ions. The Cs+ ions are
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
2. Experimental details BBO was synthesized at 700 1C and single crystals were grown from the BaB2O4–20 m/o Na2O flux, using the TSSG method and [0 0 1]-oriented seeding crystals [10]. The pulling rate was 0.2–0.7 mm/day. Sodium is a natural impurity inherited from the flux [12]. In our crystals, its content ranged from 0.01 to 0.025 wt% Na2O. Moreover, in our BBO crystals, 0.01–0.15 wt% of K2O was detected. All samples were cut perpendicularly to a or c axes. From each type of BBO crystals, five oriented samples were measured. From these measurements, conductivity activation energies and their deviations were calculated (Table 1). CLBO single crystals were prepared in Pt crucibles from the Li2O–Cs2O–B2O3–MoO3 flux, using the TSSG method with a seed [8,13]. Two large, arbitrary oriented samples were prepared two years ago. Three small samples were cut immediately before measurements from a two years’ stored bulk. One of them was cut perpendicularly to the a axis. Samples were stored in tightly closed vessels, in the presence of silica gel. Table 1 Conductivity activation energies (150–400 1C) of BBO along the a axis, Ea, and along the c axis, Ec, and conductivities at 350 1C along both axes, sa, sc, for various samples Sample
Ea (eV)
Ec (eV)
sa (S/cm)
sc (S/cm)
26–13a 26-13 Annealed 29-7b 29-7 Annealed 00-9c 00-9 Annealed
1.0070.05 1.0170.05 1.0070.05 1.0270.05 1.0070.05 0.9970.05
1.0870.05 1.1270.05 1.0970.05 1.1470.05 1.1070.05 —
1 108 1.1 108 1.2 108 1.5 108 1.3 108 2.2 108
9 109 3 109 4 109 5 109 2 109 —
a
0.01 wt % Na2O, 0.011 wt% K2O. 0.025 wt % Na2O, 0.011 wt% K2O. c 0.15 wt % K2O. b
For electrical and dielectric measurements, painted silver contacts were used. For dc measurements, the electrometric voltmeter with a vibrating condenser, VK 2-16, was used. For ac measurements (above 400 1C), the Impedance analyzer SI 1260, Schlumberger-Solartron, working in a frequency range of 100 Hz–10 MHz, was used. For evaluation of the bulk conductivity and static relative permittivity, the impedance or modular spectroscopy was used, respectively. Some samples were pre-annealed at 600 1C, for 30 min, as to dissolve precipitated impurities. At measurements of electrical properties, a heating rate of 2 1C/min was used. Moreover, at 50 1C temperature intervals, isothermal measurements of the conductivity and static permittivity were made. All electrical measurements were done in a dry N2 atmosphere. 3. Results In Fig. 1, Arrhenius plot of the electrical conductivity of BBO crystals of different purity, crystallographic orientation, and thermal history are presented. The temperature dependences exhibit two or three sections with different activation energies. Three sections are observed at measurements along the a axis. The conductivity along the a axis is 3–4 times higher than that along the c axis. Above 500 1C, this anisotropy disappears. In the sample with a high K+ content, a pre-annealing at 600 1C results in a significant increase of the conductivity. Upon multiple heating up to 600 1C, only two sections with different activation energies are observed (Fig. 2). The conductivity
-6
-8 log (σ/S cm-1)
isolated; Li+ ions are strongly bound to the chains formed by B3O7 groups. It is supposed that cationic mobility along this direction will be lower. At high humidity, the CLBO surface hydrates. This hydration changes the physical properties of crystals and causes cracks. The channels along the a axis provide enough space for an infiltration of water molecules [6]. The first stage of dehydroxylation takes place at 85–131 1C [11], the second one at 176–210 1C [7,10]. Due to structural peculiarities of both BBO and CLBO crystals, and their chemical composition (Na+, K+ impurities in BBO, Li+ content in CLBO), we expected a fast ionic conductivity in both materials. Therefore, the aim of this paper is (1) to appreciate the possibility of the fast ionic conductivity and to determine the anisotropy of electrical and dielectric properties of both BBO and CLBO crystals, (2) to determine an influence of alkaline impurities (in BBO) or hydrolysis (in CLBO) on their electrical properties, and (3) to propose conduction mechanisms in BBO and CLBO crystals.
1025
-10
-12
1.2
1.6
2.0
2.4
103/T (K-1) Fig. 1. Temperature dependences of the electrical conductivity of ‘‘asreceived’’ BBO crystals along the a axis (D, x, o), and along the c axis (&, ’). (A sample with a high content of K2O, upon annealing ().)
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
1026
0.75 -7
10 M"
log (σ/S cm-1)
0.5 -9
0.25 -11 0
0.5
1
10
M'
-13
1.0
1.5
2.0
2.5
3.0
Fig. 3. Modular diagrams of BBO crystals along the c axis (r, D), and along the a axis (&, x, o), at temperatures 395 (&,r), 449 (x), 475 (D), and 511 1C (o).
103/T (K-1) Fig. 2. Temperature dependences of the electrical conductivity of BBO crystals along the c axis (non-pre-heated (ooo), pre-heated (xxx)), and along the a axis (non-pre-heated (DDD), pre-heated (mmm)).
-6
log (σ/S cm-1)
-8
along the a axis is not influenced by repeated heatings. The conductivity along the c axis is slightly reduced, upon repeated heatings. The conductivity activation energies of BBO crystals along the a and c axes are equal to 1.0170.05 and 1.1070.02 eV, respectively (Table 1). Below 150–170 1C, adsorbed water molecules result in enhanced surface proton conductivity. Upon pre-heating, this enhanced conductivity disappears (Fig. 2). The static relative permittivity of BBO is almost isotropic being equal to 7.770.1 along the c axis, and 7.670.1 along the a axis. In Fig. 3, corresponding modular diagrams are shown. Up to 600 1C, the temperature dependence of the static permittivity is negligible. In Fig. 4, temperature dependences of the electrical conductivity of two large samples of CLBO are presented. Upon the second heating up to 600 1C, the dependences of both samples were identical and reproducible at repeated measurements; an influence of the adsorbed water molecules on the conductivity has been observed only at temperatures below 100 1C. At higher temperatures, the conductivity activation energy is equal to 1.1770.01 eV; the conductivity at 350 1C is equal to (1.370.1) 109 S/ cm. The static relative permittivity is equal to 7.270.3, in a good agreement with [14]. In Fig. 5, temperature dependences of the electrical conductivity of three small samples of CLBO are presented. The samples were freshly cut; therefore the first measurements were close to the repeated ones. Above 90 1C, we have not seen any influence of the adsorbed water molecules. Usually, the fresh samples had a slightly lower activation energy and higher conductivity than the pre-heated ones. In pre-heated samples, we have found conductivity activation energies: 1.13, 1.02, and 1.01 eV (sample cut perpendicularly to the a axis) and
-10
-12
-14 1.0
1.5
2.0 3/T
10
2.5 (K-1)
Fig. 4. Temperature dependences of the electrical conductivity of large CLBO crystals: ‘‘as received’’ (D), upon the first heating to 600 1C (o), and upon repeated heating to 600 1C (&).
conductivities at 350 1C: 2.1 108 S/cm, 7 109 S/cm, 2.4 109 S/cm, respectively. Some irregularities in the dependences disappeared upon repeated heating. They came probably from high-temperature dehydroxylation processes during first heating. Up to 600 1C, the mean value of the relative static permittivity is equal to 7.470.3. 4. Discussion Considering broad optical gaps and ionic bonds in both BBO and CLBO, it is generally supposed that the ionic conductivity dominates. The most probable charge carriers are alkali metal ions. In the temperature dependences of the conductivity of BBO crystals, measured along the c axis, a low-temperature association region, a medium-temperature migration region,
ARTICLE IN PRESS V. Trnovcova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1024–1028
heating to 600 or 250 1C (Fig. 4). Upon pre-heating, the reproducibility of temperature dependences of the conductivity is excellent (Fig. 4). Moreover, no cracking of samples was observed. Measurements of arbitrary oriented small pieces show that the anisotropy of the conductivity of CLBO is more pronounced than that in BBO crystals. At 500 K, the conductivity ranges from 4 1012 to 2 1010 S/cm, and the conductivity activation energy ranges from 1.01 to 1.13 eV. The anisotropy of the relative static permittivity is not very significant, its value being 7.470.3, in a good agreement with [1].
-6
-8 log (σ/S cm-1)
1027
-10
-12
5. Conclusions -14 1.0
1.5
2.0
2.5
103/T (K-1) Fig. 5. Temperature dependences of the electrical conductivity of nonoriented (DDD) and oriented (perpendicular to the a axis (&&&)) small CLBO crystals pre-heated up to 600 1C.
and a high-temperature intrinsic region can be distinguished (Figs. 1 and 2). In these regions, the temperature dependences of the conductivity, s, can be described as s ¼ s0 expðE=kTÞ, where s0 stands for the pre-exponential factor, and E is the conductivity activation energy. In BBO, monovalent naturally inherited Na+ and K+ impurities are more probable charge carriers than large, bivalent Ba2+ ions. Their dominance in the conduction process is confirmed by an enhanced conductivity in pre-heated crystals containing a high concentration of K+ impurities (Fig. 4) or in crystals containing an enhanced concentration of Na+ ions (Table 1 —annealed sample 29–7). Pre-heating brings about dissolution of aliovalent impurities what results in an enhanced conductivity. The influence of pre-heating on the concentration of Na+ charge carriers is usually negligible (Fig. 2). A slight decrease of the conductivity along the c axis indicates a slight dehydration of channels, upon pre-heating. The neutrality condition requires a substitution of Ba2+ ions with splitted interstitials of monovalent alkali ions. An interstitial or interstitialcy conduction mechanism through the channels along the c axis or between the layers perpendicular to the c axis is most probable. Surprisingly, the ionic transport between layers is faster. With regard to the layered structure of BBO the almost isotropic static permittivity is surprising. The values of the permittivity are close to those published earlier [2,15]. The permittivity is important for the evaluation of electro– optical coefficients [2]. We have not found any destructive influence of storing in air on electrical properties of CLBO crystals. The influence of two years’ storing (in silica gel) is eliminated by pre-
In BBO crystals, sodium and potassium ions are dominant charge carriers. These ions are inherited impurities due to the preparation technique. Splitted interstitials of alkali ions are dominant mobile defects. They migrate by an interstitialcy mechanism in channels along the c axis and between the layers in (a,b) planes. The conductivity between the layers is higher, its conductivity activation energy being equal to 1.0170.05 eV, and the conductivity at 350 1C being equal to (1.370.2) 108 S/cm. Upon preheating, the conductivity activation energy along the channels is equal to 1.1370.05 eV, and the conductivity at 350 1C is equal to (470.2) 109 S/cm. Relative static permittivity is almost isotropic, and equal to 7.6570.15. In CLBO crystals, pre-heating to 600 1C eliminates the influence of surface humidity. Upon this treatment, no cracking of samples is observed. At 500 K, the conductivity of arbitrary oriented samples ranges from 4 1012 to 2 1010 S/cm, and the conductivity activation energy ranges from 1.01 to 1.13 eV. Relative static permittivity is almost isotropic, and equal to 7.470.3. Acknowledgment The Scientific Grant Agency VEGA, Slovak Republic, by project no. 1/2100/05 supported the work. The authors thank Mrs. A. Suc˘a´kova´ for her technical assistance. References [1] D.A. Keszler, Curr. Opin. Solid State Mat. Sci. 1 (1996) 204. [2] D. Eimeri, L. Davis, S. Velako, et al., J. Appl. Phys. 62 (1987) 1968. [3] P.P. Fedorov, A.E. Koch, N.G. Kononova, Uspechi chimii 71 (2002) 741. [4] Y. Mori, I. Kuroda, S. Nakajima, et al., Jpn. J. Appl. Phys. 34 (1995) L296. [5] C. Chen, Z. Lin, Z. Wang, Appl. Phys. B 80 (2005) 1. [6] L. Kova´cs, K. Lengyel, A´. Pe´ter, et al., Optical Materials 24 (2003) 457. [7] F. Pan, X. Wang, G. Shen, D. Shen, J. Cryst. Growth 24 (2002) 129. [8] N.G. Kononova, A.E. Koch, P.P. Fedorov, et al., Neorg. Mater. 38 (2002) 1485. [9] J.M. Tu, A. Keszler, Mater. Res. Bull. 30 (1995) 209. [10] P.P. Fedorov, N.G. Kononova, A.E. Kokh, et al., Russ J. Inorg. Chem. 47 (2002) 1042.
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[11] A. Taguchi, A. Miyamoto, Y. Mori, et al., in: C.R. Pollock, W.R. Bosenberg, (Eds.), Advanced Solid State Lasers, vol. 10, 1997, p. 19. [12] A.E. Kokh, N.G. Kononova, P.P. Fedorov, et al., Crystallogr. Rep. 47 (2002) 559.
[13] N.A. Pylneva, N. Kononova, A.M. Yurkin, et al., Proc. SPIE 3610 (1999) 148. [14] K. Bai, S.T. Jung, J. Cryst. Growth 186 (1998) 612. [15] L.I. Ivleva, D.T. Kiseleva, Yu.S. Kuzminov, N.M. Polozkov, Neorg. Mater. 24 (1988) 1153.