- Email: [email protected]
Structural and nonlinear dielectric properties in fluoride containing SrTiO3 or BaTiO3 ceramics
Journal of Fluorine Chemistry 96 (1999) 25±29
Structural and nonlinear dielectric properties in ¯uoride containing SrTiO3 or BaTiO3 ceramics L. Benziada-TaõÈbi*, H. Kermoun
Institut de Chimie, USTHB, BP 32 El-Alia, 16111, Bab-Ezzouar, Algeria Received 23 July 1997; accepted 24 April 1998
Abstract Cold-pressed pellets were prepared from ATiO3±MF2±LiF mixtures (ASr, Ba; MCa, Sr, Pb) and air-®red at 1223 or 1273 K for 2 h. X-ray diffraction analysis were carried out to control the purity and to identify the different phases. Each sample was a perovskite single phase. Superlattice re¯ections with an orthorhombic symmetry were detected for SrTiO3 compounds whereas no signi®cant change was observed in the structure of BaTiO3 materials. The ceramic microstructure was systematically characterized by scanning electron microscopy. Dielectric measurements were performed as a function of temperature (123 KT473 K) and frequency (20 Hzf109 Hz). At low frequencies, a diffuse ferroelectric phase transition occurred for (Ba, M)(Ti, Li)(O, F)3 ceramics. The room temperature frequency dependence of the complex permittivity has shown a resonance phenomenon for (Sr, M)(Ti, Li)(O, F)3 phases in the range 2108± 5108 Hz and a dielectric relaxation in the range 9106±3107 Hz for BaTiO3 related materials. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Oxy¯uorides; Perovskites; Ferroelectric; SrTiO3; BaTiO3; Ceramics
1. Introduction Advanced ceramics are making news across industry, worldwide. Electrical and electronic ceramics currently dominate applications. Strontium titanate (SrTiO3) and barium titanate (BaTiO3) have a couple of properties that make them attractive in memory devices. For example, the solid solution Ba1ÿxSrxTiO3 is commonly used as a capacitor in DRAMS since these phases have high charge storage densities, low leakage currents and resistance against timedependent dielectric breakdown suf®cient to achieve gigabyte densities and beyond [1±5]. Perovskite SrTiO3 is a paraelectric material having cubic symmetry at room temperature (space group Pm3m, lattice parameter a 0.3904 nm) [6]. A phase transition occurs at about 110 K which involves a symmetry change from cubic to tetragonal [7±9]. On the other hand, BaTiO3 is ferroelectric below 393 K and has two other transitions at about 278 and 203 K [10,11] corresponding to the structural changes: 203 K
278 K
Amm2
2. Experimental The samples used in this study are oxy¯uoride ceramics derived from the perovskites SrTiO3 or BaTiO3. They were prepared by solid state reactions. Stoichiometric strontium titanate and barium titanate were previously synthesized via the reactions: 1373 K
SrCO3 TiO2 ! SrTiO3 CO2
393 K
rhombohedral $ orthorhombic $ tetragonal $ cubic R3m
The relatively simple structures of these two ABO3 perovskites allow the control and modi®cations of the properties over a wide range varying only ionic substitutions on A-site or B-site. The intention of the present work is to sinter SrTiO3 or BaTiO3 using the ¯uoride mixture MF2LiF (MCa, Sr, Pb) for the development of improved materials and to investigate the dielectric properties of the ceramics obtained.
P4mm
Pm3m
*Corresponding author. Tel.: +213-2-247912; fax: +213-2-247311.
1373 K
BaCO3 TiO2 ! BaTiO3 CO2 Various mixtures were then prepared from SrTiO3, BaTiO3, CaF2, SrF2, PbF2 and LiF powders. The initial composition of these samples is summarized in Table 1.
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 3 0 - 3
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
26
The complex permittivity r 0r ÿ i00r was measured at room temperature as a function of frequency (106 Hz f109 Hz) using a Hewlett Packard impedance analyzer HP 4191A.
Table 1 Initial compositions of the samples in moles Sample
Initial compositions in mole
A B C D
0.97 0.97 0.95 0.97
E F G
0.95 BaTiO30.05 CaF20.05 LiF 0.97 BaTiO30.03 PbF20.03 LiF 1 BaTiO30.035 PbF20.031 LiF
SrTiO30.03 SrTiO30.03 SrTiO30.05 SrTiO30.03
CaF20.03 LiF SrF20.03 LiF SrF20.05 LiF PbF20.03 LiF
3. Results and discussion 3.1. X-ray diffraction study The X-ray study has shown each sample to be a perovskite single phase. The lattice parameters (a, b, c) and the unit cell volumes (V) at 300 K are given in Table 2. The diffraction pattern of pure SrTiO3 is greatly modi®ed. Superlattice re¯ections are detected for samples A±D with the formation of a complex perovskite structure isomorphous with NaNbO3. The peaks of the X-ray patterns were indexed by comparison with the NaNbO3 orthorhombic phase at room temperature. The obtained parameters (Table 2) are very close to those of NaNbO3 which are: a00.5566 nm, b01.5520 nm, c00.5506 nm [12]. The orthorhombic unit cell parameters (a0, b0, c0) of the (Sr, M)(Ti, Li)(O, F)3 phases can be deduced from the cubic unit cell parameter of pure SrTiO3 (ac) by the relationships p p a0 ac 2; b0 4ac ; c0 ac 2:
The starting materials SrCO3, BaCO3, CaF2, SrF2, PbF2 and LiF were Merck products of high purity grade. The strontium carbonate, barium carbonate and titanium dioxide were dried at 573 K for several hours. The ¯uorides were dried by heating at 423 K under vacuum for 4 h. The appropriate amounts of the ¯uorides MF2 (MCa, Sr, Pb) and LiF were added to SrTiO3 or BaTiO3. The mixtures were homogenized and dry-ground over half an hour with an agate mortar. The obtained powders were cold-pressed under approximately 108 Pa to pellets of 13 mm diameter and about 1 mm thickness without binder. These disks were sintered in air at 1223 or 1273 K for 2 h. The heating and cooling rates were close to 3 K-minÿ1. The purity and the symmetry of the samples were checked by X-ray diffraction with a powder diffractometer Ê ). The unit cell parausing Cu K 1 radiation (1.54051 A meters were determined and re®ned using a least squares re®nement. The ceramic microstructure was systematically characterized by a scanning electron microscopy observation performed on fractured samples and using a JEOL apparatus, type JSM-840A. The operating voltage was 10 or 15 kV. The in¯uence of the sintering temperature and the holding time on the microstructure and the relative density of the ceramics were followed. For dielectric measurements, the circular faces of the ceramic samples were polished and electroded by gold sputter deposition. The dielectric permittivity (0r ) and the loss factor (tan ) were measured from 123 to 473 K in the frequency range 20 Hzf105 Hz using a HP 4284A LCR meter. The measurements were carried out in gaseous nitrogen (N2) and liquid nitrogen was used as coolant.
This strong structural change is probably due to the ``crumpling'' of the regular array of oxygen octahedra in which the individual octahedra rotates about a fourfold axis [13,14], the rotation being induced by the interactions of neighbouring ions [15]. On the contrary, the structure of pure BaTiO3 is only slightly affected. The tetragonal distortion disappears for all samples (E±G) and the symmetry becomes cubic at 300 K. The lattice parameter ac is around 0.4 nm for (Ba, M)(Ti, Li)(O, F)3 compounds and is very close to those of tetragonal BaTiO3 (at0.3986 nm, bt0.4026 nm) [10]. 3.2. Microstructure The sintering conditions were optimized. The optimal sintering temperature (Tsint) was found to be 1223 K for SrTiO3 compounds (samples A±D) and 1273 K for BaTiO3 derived ceramics (samples E±G), the holding time (sint) being 2 h for all samples. The shrinkage coef®cients ( / )
Table 2 Lattice parameters and unit cell volumes at 300 K Sample
Symmetry
a (nm)
b (nm)
c (nm)
V (nm3)
A B C D
Orthorhombic Orthorhombic Orthorhombic Orthorhombic
0.63750.0002 0.63690.0002 0.63700.0001 0.63670.0001
1.55920.0004 1.56350.0005 1.56240.0003 1.55660.0003
0.65070.0002 0.65130.0002 0.65120.0001 0.65150.0001
0.64680.0005 0.64850.0005 0.64810.0003 0.64680.0003
E F G
Cubic Cubic Cubic
0.39910.0004 0.40080.0002 0.40720.0002
± ± ±
± ± ±
0.06350.0002 0.06430.0001 0.06750.0001
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
27
Table 3 Dielectric characteristics for ceramics (1 kHz) Sample
Tsint (K)
sint (h)
/ (%)
TC (K)
0r (TC)
tan (TC)
0r (293 K)
tan (293 K)
A B C D E F G
1223 1223 1223 1223 1273 1273 1273
2 2 2 2 2 2 2
13.8 13 15.4 10 12.5 16.1 15.8
± ± ± ± 293 283 263
± ± ± ± 3500 7800 7900
± ± ± ± 0.007 0.010 0.014
260 280 274 450 3500 7650 7400
0.06 0.003 0.002 0.007 0.007 0.008 0.009
are in the range 0.10±0.17 (Table 3). The relative density reached 96% for ceramics F and G. Typical micrographs are illustrated in Fig. 1. No interior or surface secondary phases were found. The Sr(Ti, Li)(O, F)3 ceramic (sample C) is more porous and has ®ne (0.3 mm) and coarse (1 mm) grains. Some variation in the size and the shape of grains is observed (Fig. 1(a)). The (Ba, Pb)(Ti, Li)(O, F)3 ceramic (sample F) is composed of larger grains with inter and intragranular porosity. The grain morphology is more regular than in the ®rst one, the grain size being of approximately 5 mm (Fig. 1(b)).
Fig. 1. Scanning electron micrographs of ceramics corresponding to sample C (a) or sample F (b).
3.3. Dielectric properties At low frequencies, no maximum of the dielectric permittivity was observed for (Sr, M)(Ti, Li)(O, F)3 ceramics in the temperature range investigated whereas (Ba, M)(Ti, Li)(O, F)3 samples exhibit a strong maximum of 0r at each frequency. The dielectric characteristics for a measurement frequency of 1 kHz are reported in Table 3. In any case, the value of the ferroelectric Curie temperature (TC) is much lower than that of pure BaTiO3 which is about 393 K. This result is ascribed to the covalency decrease of the B±X bonds (BTi, Li; XO, F) when ¯uorine is substituted for oxygen and is in good agreement with our previous work on ferroelectric oxy¯uorides [16±20]. As examples, Figs. 2 and 3 give the temperature dependence of the permittivity and the dielectric losses for samples B and G. For ceramic B, no dielectric peak is observed in the temperature range investigated, nevertheless 0r and tan curves show an increase from T370 K. Taking into account the lattice symmetry, it is not excluded that the (Sr, M)(Ti, Li)(O, F)3 compounds undergo several phase transitions like NaNbO3 and this increasing of 0r and tan is maybe due to the beginning of a phase transition. Unfortunately, we were limited in our investigations by the system used which operates from 123 to 473 K. So, it would be interesting to investigate the behaviour of SrTiO3 related materials beyond 473 K. The temperature dependence of 0r and the values of tan d are consistent with the class I capacitor norms: at 1 kHz: 0r (298 K)280, tan d (298 K)0.3%, maximum capacitance decreases between 218 and 358 K35%. For sample G, as shown in Fig. 3, the ferroelectric± paraelectric phase transition is diffuse, the maximum of 0r being moderately stable over a wide temperature range whereas the two other transitions corresponding to the sequence change rhombohedral±orthorhombic±tetragonal disappear. Such a ceramic has potential interest as a dielectric for multilayer capacitors of class II, type Z5U. At high frequencies, a resonance phenomenon is observed for (Sr, M)(Ti, Li)(O, F)3 phases whereas a dielectric relaxation is obtained for BaTiO3 derived ceramics. The main results are given in Table 4 where fr is the resonance frequency for samples A±C and the relaxation frequency for samples E±G. The complex relative permittivity 0r ; 00r at room temperature is shown in Figs. 4 and 5 for ceramics B and G. For ceramic B (Fig. 4), derived from
28
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
Fig. 4. Complex permittivity versus frequency for sample B at 300 K.
Fig. 2. Temperature dependence of the dielectric permittivity and the dielectric losses for sample B at 1 kHz.
Fig. 5. Complex permittivity versus frequency for sample G at 300 K.
SrTiO3, the frequency dependence of 0r rises hyperbolically to a maximum, falls roughly, reaches a minimum with a negative value then rises again asymptotically, while the imaginary part 00r exhibits a sharp maximum at 4.4108 Hz. These variations with negative values of 0r are typical of a resonance phenomenon [21]. For all samples the value of fr is lower than that of pure SrTiO3 which is 31012 Hz [22]. For ceramic G (Fig. 5), 0r decreases slightly as the frequency increases. Beyond 15106 Hz 0r decreases quickly whereas 00r traverses a very broad maximum. Such results are characteristic of a dielectric relaxation. As shown in Table 4, the dielectric relaxation in (Ba, M)(Ti, Li)(O, F)3 phases occurs at frequencies lower than in pure BaTiO3: 5108 Hz [23]. The decrease of fr can be mainly related to the coupled substitution Li±Ti, F±O [20,24]. Table 4 Values of fr, 0r and 0r at room temperature
Fig. 3. Temperature dependence of the dielectric permittivity and the dielectric losses for sample G at 1 kHz.
Sample
A
B
D
E
F
G
fr (MHz) 0r (fr) 00r (fr)
288.40 215 500
436.5 11 305
436.5 ÿ57 334
9.1 1500 1500
25 2800 2800
15 2600 2600
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
29
4. Conclusion
References
New oxy¯uoride materialshave been prepared bysolidstate reaction between SrTiO3 or BaTiO3 and the ¯uoride mixture MF2LiF (MCa, Sr, Pb). Ceramics have been sintered at 1223 or 1273 K. The X-ray investigations have shown strong structural perturbations in the cubic SrTiO3 phase which became orthorhombic with the formation of a complex perovskite isomorphous with NaNbO3. Different dielectric behaviors were found for the ceramic samples. For (Ba, M)(Ti, Li)(O, F)3 ceramics, a diffuse phase transition occurred from a ferroelectric tetragonal structure to a paraelectric cubic one, while for (Sr, M)(Ti, Li)(O, F)3 phases, no transition point was discovered in the temperature range investigated. At high frequencies, a resonance phenomenon was observed for SrTiO3-derived ceramics whereas a dielectric relaxation was obtained for BaTiO3-related materials. Up to now, when F was substituted for O in a ferroelectric perovskite such as BaTiO3, NaNbO3, or KNbO3 a decrease of both the distortion lattice and the ferroelectric Curie temperature was observed. From this study it appears that the admixture of ¯uorine in a paraelectric perovskite like SrTiO3 increases the distortion of the octahedra and maybe of TC also. From all the results, the oxy¯uoride ceramics seemed more attractive than the pure SrTiO3 or BaTiO3 for various applications.
[1] J.C. Niepce, J.M. Haussonne, BaTiO3: MateÂriau de base pour les condensateurs ceÂramiques, vol. 1 et 2, Septima 1994. [2] J.F. Scott, Phys. World February (1995) 46. [3] R. Jones, P. Zurcher, P. Chu, Dj. Taylor, Yt. Lii, B. Jiang, Pd. Maniar, S.J. Gillespie, Microelectron. Engrg. 29 (1995) 3. [4] C. Paz De Araujo, J.F. Scott, G.W. Taylor, Ferroelectric Thin Films: Synthesis and Basic Properties, vol. 10, Gordon and Breach, London, 1996. [5] F. MuÈcklich, H. Janocha, Z. Metallkd. 87 (1996) 357. [6] R.S. Roth, J. Research NBS R P 2736 (1957) 58. [7] E. Sawaguchi, A. Kikuchi, Y. Kodera, J. Phys. Soc. Jpn. 17 (1962) 1666. [8] F.W. Lytle, J. Appl. Phys. 35 (1964) 2212. [9] L.E. Cross, D. Chakravorty, Proceedings IMF1, Prague, 1966, p. 394. [10] H.D. Megaw, Nature 154 (1945) 484. [11] B. Matthias, A. Von Hippel, Phys. Rev. 73 (1948) 1378. [12] A.C. Sakowski-Cowely, K. Lukaszewicz, H.D. Megaw, Acta Cryst. B 25 (1969) 581. [13] P.A. Fleury, J.F. Scott, J.M. Worlock, Phys. Rev. Lett. 21 (1968) 16. [14] G. Shirane, K. Yamada, Phys. Rev. 177 (1969) 858. [15] V.A. Isupov, Izr. Akad. Nauk SSSR, Ser. Fiz. 47 (1983) 559. [16] L. Benziada, J. Ravez, P. Hagenmuller, J. Fluor. Chem. 26 (1984) 395. [17] L. Benziada, Z. Ladjeroud, J. Ravez, Ferroelectrics 124 (1991) 73. [18] Z. Ladjeroud, L. Benziada, J. Ravez, Ferroelectrics 154 (1994) 207. [19] L. Benziada, J. Ravez, J. Fluor. Chem. 73 (1995) 69. [20] L. Benziada, J. Claverie, Ferroelectrics 189 (1996) 129. [21] R. Von Hippel, Dielectric Materials and Applications, MIT Press, Cambridge, MA, 1966. [22] A.S. Barker, M. Tinkham, Phys. Rev. 125(5) (1962) 1527. [23] O. Kersten, A. Rost, G. Schmidt, Ferroelectrics 81 (1988) 31. [24] S. Kazaoui, J. Ravez, C. Elissalde, M. Maglione, Ferroelectrics 135 (1992) 85.
Acknowledgements The authors wish to thank Dr. A. Glazounov and Dr. A. Nacer for their kind help in the dielectric measurements.
Structural and nonlinear dielectric properties in ¯uoride containing SrTiO3 or BaTiO3 ceramics L. Benziada-TaõÈbi*, H. Kermoun
Institut de Chimie, USTHB, BP 32 El-Alia, 16111, Bab-Ezzouar, Algeria Received 23 July 1997; accepted 24 April 1998
Abstract Cold-pressed pellets were prepared from ATiO3±MF2±LiF mixtures (ASr, Ba; MCa, Sr, Pb) and air-®red at 1223 or 1273 K for 2 h. X-ray diffraction analysis were carried out to control the purity and to identify the different phases. Each sample was a perovskite single phase. Superlattice re¯ections with an orthorhombic symmetry were detected for SrTiO3 compounds whereas no signi®cant change was observed in the structure of BaTiO3 materials. The ceramic microstructure was systematically characterized by scanning electron microscopy. Dielectric measurements were performed as a function of temperature (123 KT473 K) and frequency (20 Hzf109 Hz). At low frequencies, a diffuse ferroelectric phase transition occurred for (Ba, M)(Ti, Li)(O, F)3 ceramics. The room temperature frequency dependence of the complex permittivity has shown a resonance phenomenon for (Sr, M)(Ti, Li)(O, F)3 phases in the range 2108± 5108 Hz and a dielectric relaxation in the range 9106±3107 Hz for BaTiO3 related materials. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Oxy¯uorides; Perovskites; Ferroelectric; SrTiO3; BaTiO3; Ceramics
1. Introduction Advanced ceramics are making news across industry, worldwide. Electrical and electronic ceramics currently dominate applications. Strontium titanate (SrTiO3) and barium titanate (BaTiO3) have a couple of properties that make them attractive in memory devices. For example, the solid solution Ba1ÿxSrxTiO3 is commonly used as a capacitor in DRAMS since these phases have high charge storage densities, low leakage currents and resistance against timedependent dielectric breakdown suf®cient to achieve gigabyte densities and beyond [1±5]. Perovskite SrTiO3 is a paraelectric material having cubic symmetry at room temperature (space group Pm3m, lattice parameter a 0.3904 nm) [6]. A phase transition occurs at about 110 K which involves a symmetry change from cubic to tetragonal [7±9]. On the other hand, BaTiO3 is ferroelectric below 393 K and has two other transitions at about 278 and 203 K [10,11] corresponding to the structural changes: 203 K
278 K
Amm2
2. Experimental The samples used in this study are oxy¯uoride ceramics derived from the perovskites SrTiO3 or BaTiO3. They were prepared by solid state reactions. Stoichiometric strontium titanate and barium titanate were previously synthesized via the reactions: 1373 K
SrCO3 TiO2 ! SrTiO3 CO2
393 K
rhombohedral $ orthorhombic $ tetragonal $ cubic R3m
The relatively simple structures of these two ABO3 perovskites allow the control and modi®cations of the properties over a wide range varying only ionic substitutions on A-site or B-site. The intention of the present work is to sinter SrTiO3 or BaTiO3 using the ¯uoride mixture MF2LiF (MCa, Sr, Pb) for the development of improved materials and to investigate the dielectric properties of the ceramics obtained.
P4mm
Pm3m
*Corresponding author. Tel.: +213-2-247912; fax: +213-2-247311.
1373 K
BaCO3 TiO2 ! BaTiO3 CO2 Various mixtures were then prepared from SrTiO3, BaTiO3, CaF2, SrF2, PbF2 and LiF powders. The initial composition of these samples is summarized in Table 1.
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 3 0 - 3
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
26
The complex permittivity r 0r ÿ i00r was measured at room temperature as a function of frequency (106 Hz f109 Hz) using a Hewlett Packard impedance analyzer HP 4191A.
Table 1 Initial compositions of the samples in moles Sample
Initial compositions in mole
A B C D
0.97 0.97 0.95 0.97
E F G
0.95 BaTiO30.05 CaF20.05 LiF 0.97 BaTiO30.03 PbF20.03 LiF 1 BaTiO30.035 PbF20.031 LiF
SrTiO30.03 SrTiO30.03 SrTiO30.05 SrTiO30.03
CaF20.03 LiF SrF20.03 LiF SrF20.05 LiF PbF20.03 LiF
3. Results and discussion 3.1. X-ray diffraction study The X-ray study has shown each sample to be a perovskite single phase. The lattice parameters (a, b, c) and the unit cell volumes (V) at 300 K are given in Table 2. The diffraction pattern of pure SrTiO3 is greatly modi®ed. Superlattice re¯ections are detected for samples A±D with the formation of a complex perovskite structure isomorphous with NaNbO3. The peaks of the X-ray patterns were indexed by comparison with the NaNbO3 orthorhombic phase at room temperature. The obtained parameters (Table 2) are very close to those of NaNbO3 which are: a00.5566 nm, b01.5520 nm, c00.5506 nm [12]. The orthorhombic unit cell parameters (a0, b0, c0) of the (Sr, M)(Ti, Li)(O, F)3 phases can be deduced from the cubic unit cell parameter of pure SrTiO3 (ac) by the relationships p p a0 ac 2; b0 4ac ; c0 ac 2:
The starting materials SrCO3, BaCO3, CaF2, SrF2, PbF2 and LiF were Merck products of high purity grade. The strontium carbonate, barium carbonate and titanium dioxide were dried at 573 K for several hours. The ¯uorides were dried by heating at 423 K under vacuum for 4 h. The appropriate amounts of the ¯uorides MF2 (MCa, Sr, Pb) and LiF were added to SrTiO3 or BaTiO3. The mixtures were homogenized and dry-ground over half an hour with an agate mortar. The obtained powders were cold-pressed under approximately 108 Pa to pellets of 13 mm diameter and about 1 mm thickness without binder. These disks were sintered in air at 1223 or 1273 K for 2 h. The heating and cooling rates were close to 3 K-minÿ1. The purity and the symmetry of the samples were checked by X-ray diffraction with a powder diffractometer Ê ). The unit cell parausing Cu K 1 radiation (1.54051 A meters were determined and re®ned using a least squares re®nement. The ceramic microstructure was systematically characterized by a scanning electron microscopy observation performed on fractured samples and using a JEOL apparatus, type JSM-840A. The operating voltage was 10 or 15 kV. The in¯uence of the sintering temperature and the holding time on the microstructure and the relative density of the ceramics were followed. For dielectric measurements, the circular faces of the ceramic samples were polished and electroded by gold sputter deposition. The dielectric permittivity (0r ) and the loss factor (tan ) were measured from 123 to 473 K in the frequency range 20 Hzf105 Hz using a HP 4284A LCR meter. The measurements were carried out in gaseous nitrogen (N2) and liquid nitrogen was used as coolant.
This strong structural change is probably due to the ``crumpling'' of the regular array of oxygen octahedra in which the individual octahedra rotates about a fourfold axis [13,14], the rotation being induced by the interactions of neighbouring ions [15]. On the contrary, the structure of pure BaTiO3 is only slightly affected. The tetragonal distortion disappears for all samples (E±G) and the symmetry becomes cubic at 300 K. The lattice parameter ac is around 0.4 nm for (Ba, M)(Ti, Li)(O, F)3 compounds and is very close to those of tetragonal BaTiO3 (at0.3986 nm, bt0.4026 nm) [10]. 3.2. Microstructure The sintering conditions were optimized. The optimal sintering temperature (Tsint) was found to be 1223 K for SrTiO3 compounds (samples A±D) and 1273 K for BaTiO3 derived ceramics (samples E±G), the holding time (sint) being 2 h for all samples. The shrinkage coef®cients ( / )
Table 2 Lattice parameters and unit cell volumes at 300 K Sample
Symmetry
a (nm)
b (nm)
c (nm)
V (nm3)
A B C D
Orthorhombic Orthorhombic Orthorhombic Orthorhombic
0.63750.0002 0.63690.0002 0.63700.0001 0.63670.0001
1.55920.0004 1.56350.0005 1.56240.0003 1.55660.0003
0.65070.0002 0.65130.0002 0.65120.0001 0.65150.0001
0.64680.0005 0.64850.0005 0.64810.0003 0.64680.0003
E F G
Cubic Cubic Cubic
0.39910.0004 0.40080.0002 0.40720.0002
± ± ±
± ± ±
0.06350.0002 0.06430.0001 0.06750.0001
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
27
Table 3 Dielectric characteristics for ceramics (1 kHz) Sample
Tsint (K)
sint (h)
/ (%)
TC (K)
0r (TC)
tan (TC)
0r (293 K)
tan (293 K)
A B C D E F G
1223 1223 1223 1223 1273 1273 1273
2 2 2 2 2 2 2
13.8 13 15.4 10 12.5 16.1 15.8
± ± ± ± 293 283 263
± ± ± ± 3500 7800 7900
± ± ± ± 0.007 0.010 0.014
260 280 274 450 3500 7650 7400
0.06 0.003 0.002 0.007 0.007 0.008 0.009
are in the range 0.10±0.17 (Table 3). The relative density reached 96% for ceramics F and G. Typical micrographs are illustrated in Fig. 1. No interior or surface secondary phases were found. The Sr(Ti, Li)(O, F)3 ceramic (sample C) is more porous and has ®ne (0.3 mm) and coarse (1 mm) grains. Some variation in the size and the shape of grains is observed (Fig. 1(a)). The (Ba, Pb)(Ti, Li)(O, F)3 ceramic (sample F) is composed of larger grains with inter and intragranular porosity. The grain morphology is more regular than in the ®rst one, the grain size being of approximately 5 mm (Fig. 1(b)).
Fig. 1. Scanning electron micrographs of ceramics corresponding to sample C (a) or sample F (b).
3.3. Dielectric properties At low frequencies, no maximum of the dielectric permittivity was observed for (Sr, M)(Ti, Li)(O, F)3 ceramics in the temperature range investigated whereas (Ba, M)(Ti, Li)(O, F)3 samples exhibit a strong maximum of 0r at each frequency. The dielectric characteristics for a measurement frequency of 1 kHz are reported in Table 3. In any case, the value of the ferroelectric Curie temperature (TC) is much lower than that of pure BaTiO3 which is about 393 K. This result is ascribed to the covalency decrease of the B±X bonds (BTi, Li; XO, F) when ¯uorine is substituted for oxygen and is in good agreement with our previous work on ferroelectric oxy¯uorides [16±20]. As examples, Figs. 2 and 3 give the temperature dependence of the permittivity and the dielectric losses for samples B and G. For ceramic B, no dielectric peak is observed in the temperature range investigated, nevertheless 0r and tan curves show an increase from T370 K. Taking into account the lattice symmetry, it is not excluded that the (Sr, M)(Ti, Li)(O, F)3 compounds undergo several phase transitions like NaNbO3 and this increasing of 0r and tan is maybe due to the beginning of a phase transition. Unfortunately, we were limited in our investigations by the system used which operates from 123 to 473 K. So, it would be interesting to investigate the behaviour of SrTiO3 related materials beyond 473 K. The temperature dependence of 0r and the values of tan d are consistent with the class I capacitor norms: at 1 kHz: 0r (298 K)280, tan d (298 K)0.3%, maximum capacitance decreases between 218 and 358 K35%. For sample G, as shown in Fig. 3, the ferroelectric± paraelectric phase transition is diffuse, the maximum of 0r being moderately stable over a wide temperature range whereas the two other transitions corresponding to the sequence change rhombohedral±orthorhombic±tetragonal disappear. Such a ceramic has potential interest as a dielectric for multilayer capacitors of class II, type Z5U. At high frequencies, a resonance phenomenon is observed for (Sr, M)(Ti, Li)(O, F)3 phases whereas a dielectric relaxation is obtained for BaTiO3 derived ceramics. The main results are given in Table 4 where fr is the resonance frequency for samples A±C and the relaxation frequency for samples E±G. The complex relative permittivity 0r ; 00r at room temperature is shown in Figs. 4 and 5 for ceramics B and G. For ceramic B (Fig. 4), derived from
28
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
Fig. 4. Complex permittivity versus frequency for sample B at 300 K.
Fig. 2. Temperature dependence of the dielectric permittivity and the dielectric losses for sample B at 1 kHz.
Fig. 5. Complex permittivity versus frequency for sample G at 300 K.
SrTiO3, the frequency dependence of 0r rises hyperbolically to a maximum, falls roughly, reaches a minimum with a negative value then rises again asymptotically, while the imaginary part 00r exhibits a sharp maximum at 4.4108 Hz. These variations with negative values of 0r are typical of a resonance phenomenon [21]. For all samples the value of fr is lower than that of pure SrTiO3 which is 31012 Hz [22]. For ceramic G (Fig. 5), 0r decreases slightly as the frequency increases. Beyond 15106 Hz 0r decreases quickly whereas 00r traverses a very broad maximum. Such results are characteristic of a dielectric relaxation. As shown in Table 4, the dielectric relaxation in (Ba, M)(Ti, Li)(O, F)3 phases occurs at frequencies lower than in pure BaTiO3: 5108 Hz [23]. The decrease of fr can be mainly related to the coupled substitution Li±Ti, F±O [20,24]. Table 4 Values of fr, 0r and 0r at room temperature
Fig. 3. Temperature dependence of the dielectric permittivity and the dielectric losses for sample G at 1 kHz.
Sample
A
B
D
E
F
G
fr (MHz) 0r (fr) 00r (fr)
288.40 215 500
436.5 11 305
436.5 ÿ57 334
9.1 1500 1500
25 2800 2800
15 2600 2600
L. Benziada-TaõÈbi, H. Kermoun / Journal of Fluorine Chemistry 96 (1999) 25±29
29
4. Conclusion
References
New oxy¯uoride materialshave been prepared bysolidstate reaction between SrTiO3 or BaTiO3 and the ¯uoride mixture MF2LiF (MCa, Sr, Pb). Ceramics have been sintered at 1223 or 1273 K. The X-ray investigations have shown strong structural perturbations in the cubic SrTiO3 phase which became orthorhombic with the formation of a complex perovskite isomorphous with NaNbO3. Different dielectric behaviors were found for the ceramic samples. For (Ba, M)(Ti, Li)(O, F)3 ceramics, a diffuse phase transition occurred from a ferroelectric tetragonal structure to a paraelectric cubic one, while for (Sr, M)(Ti, Li)(O, F)3 phases, no transition point was discovered in the temperature range investigated. At high frequencies, a resonance phenomenon was observed for SrTiO3-derived ceramics whereas a dielectric relaxation was obtained for BaTiO3-related materials. Up to now, when F was substituted for O in a ferroelectric perovskite such as BaTiO3, NaNbO3, or KNbO3 a decrease of both the distortion lattice and the ferroelectric Curie temperature was observed. From this study it appears that the admixture of ¯uorine in a paraelectric perovskite like SrTiO3 increases the distortion of the octahedra and maybe of TC also. From all the results, the oxy¯uoride ceramics seemed more attractive than the pure SrTiO3 or BaTiO3 for various applications.
[1] J.C. Niepce, J.M. Haussonne, BaTiO3: MateÂriau de base pour les condensateurs ceÂramiques, vol. 1 et 2, Septima 1994. [2] J.F. Scott, Phys. World February (1995) 46. [3] R. Jones, P. Zurcher, P. Chu, Dj. Taylor, Yt. Lii, B. Jiang, Pd. Maniar, S.J. Gillespie, Microelectron. Engrg. 29 (1995) 3. [4] C. Paz De Araujo, J.F. Scott, G.W. Taylor, Ferroelectric Thin Films: Synthesis and Basic Properties, vol. 10, Gordon and Breach, London, 1996. [5] F. MuÈcklich, H. Janocha, Z. Metallkd. 87 (1996) 357. [6] R.S. Roth, J. Research NBS R P 2736 (1957) 58. [7] E. Sawaguchi, A. Kikuchi, Y. Kodera, J. Phys. Soc. Jpn. 17 (1962) 1666. [8] F.W. Lytle, J. Appl. Phys. 35 (1964) 2212. [9] L.E. Cross, D. Chakravorty, Proceedings IMF1, Prague, 1966, p. 394. [10] H.D. Megaw, Nature 154 (1945) 484. [11] B. Matthias, A. Von Hippel, Phys. Rev. 73 (1948) 1378. [12] A.C. Sakowski-Cowely, K. Lukaszewicz, H.D. Megaw, Acta Cryst. B 25 (1969) 581. [13] P.A. Fleury, J.F. Scott, J.M. Worlock, Phys. Rev. Lett. 21 (1968) 16. [14] G. Shirane, K. Yamada, Phys. Rev. 177 (1969) 858. [15] V.A. Isupov, Izr. Akad. Nauk SSSR, Ser. Fiz. 47 (1983) 559. [16] L. Benziada, J. Ravez, P. Hagenmuller, J. Fluor. Chem. 26 (1984) 395. [17] L. Benziada, Z. Ladjeroud, J. Ravez, Ferroelectrics 124 (1991) 73. [18] Z. Ladjeroud, L. Benziada, J. Ravez, Ferroelectrics 154 (1994) 207. [19] L. Benziada, J. Ravez, J. Fluor. Chem. 73 (1995) 69. [20] L. Benziada, J. Claverie, Ferroelectrics 189 (1996) 129. [21] R. Von Hippel, Dielectric Materials and Applications, MIT Press, Cambridge, MA, 1966. [22] A.S. Barker, M. Tinkham, Phys. Rev. 125(5) (1962) 1527. [23] O. Kersten, A. Rost, G. Schmidt, Ferroelectrics 81 (1988) 31. [24] S. Kazaoui, J. Ravez, C. Elissalde, M. Maglione, Ferroelectrics 135 (1992) 85.
Acknowledgements The authors wish to thank Dr. A. Glazounov and Dr. A. Nacer for their kind help in the dielectric measurements.