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Soft mode phase transition of Sr2Bi4Ti5O18
13June1994 PHYSICS
LETTERS
A
Physics Letters A 189 (1994) 257-260
ELSEVIER
Soft mode phase transition of Sr2Bi4Ti5018 Guangtian Zou, Jianjun Liu, Qiliang Cui, Haibin Yang State Key Laboratoryfor Superhardkiaterials,InstituteofAtomic andMolecular Physics,Jilin University,Changchun 130023, China
Received 3 1 January 1994;revised manuscript received 30 March 1994, accepted for publication 6 April 1994 Communicated by J. Plouquet
AbStTWt
High temperature Raman spectra of SrzBi4Ti50,s have been investigated. A first order phase transition induced by a soft mode occurs at its Curie temperature. The soft mode is connected with the unstable Bi3+ ion displacement in the perovskite-like layers.
1. Introduction The layered bismuth compounds first studied by Aurivillius [ 1 ] are known to possess a structure expressed by the general formula (BizO,)2+(A,-1B,03,+ I ) 2-, where A is a combination of ions adequate for twelve-coordinated interstices, B is a combination of ions for octahedrally coordinated sites and m is an integer between 1 to 5. These compounds are built up by the regular intergrowth of (Bi202)‘+ layers and perovskite-type layers (A,_ ,BmOSm+,)2-. Most members of this family are displacive ferroelectrics. Hisano and Toda have found an underdamped soft mode in Bi4Ti30i2 ( m = 3 ) [ 2 1. We have studied the soft mode behavior in SrBi2Nb209 (m = 2) [ 3 ] and Nae.sBi*.STi&,(m=4) [4]. In our study on the behavior of the vibrational mode in layered bismuth compounds it is rather interesting to investigate Sr2Bi4TiS0i8 ( n = 5 ) high temperature Raman spectra. The ferroelectric activity in Sr2Bi4TiS0i8 (SBT) was first observed by Subbarao [ 5 ] in 196 1. At room temperature the crystal structure of SBT is orthorhombicwithQ=5.461 A,b/a=1.OOO,c=48.8A [6]. The dielectric data measured on ceramic disks indi-
cated a ferro- to para-electric phase transition occurring at 285°C [ 71. Above the Curie point the crystal structure is tetragonal and the dielectric constant (e) follows the Curie-Weiss law c = C/ ( T- 0). The Curie constant C and the extrapolated Curie-Weiss temperature 8 are 0.47 x 105°C and 255°C.
2. Experimental A polycrystalline sample was obtained by heating stoichiometric amounts of high purity Bi203, Ti02, St-CO3 in air at 800-l 100°C for 48 h in a covered platinum boat. The grinding and calcination were repeated until the solid state reaction was completed. The result of X-ray diffraction measurements were consistent with that reported by Subbarao [ 61. Raman scattering experiments have been performed with a SPEX-1403 Ramalog system using the 676.4 nm emission line of a Spectra Physics K+ laser as excitation source. The spectra were recorded at high temperature in the right angle scattering geometry. Powder samples were mounted in a SPEX-L? electric oven to raise the sample temperature. A thermocouple was used to monitor the sample temperature.
0375-9601/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDiO375-9601(94)00265-Q
G. Zou et al. /Physics Letters A 189 (I 994) 25 7-260
258
3. Results and discussion
Fig. 1 shows the Raman spectrum of polycrystalline SBT at various temperatures in the frequency range 7-77 cm-‘. Mode A, the lowest frequency Raman band, is highly temperature dependent and becomes overdamped at high temperature. It disappears above the Curie temperature and becomes Raman inactive. The intensity of mode B is very weak. It disappears above room temperature. Mode C is Raman active both below and above T,. It also softens as temperature increases but above T, its frequency hardly changes. Fig. 2 shows the temperature dependence of the peak frequency of modes A and C. In order to extract the temperature dependence of the soft mode frequency and line-width, the line-shape of the soft mode spectrum was fitted to a damped oscillator model [ 8 ] given by
42
7 Ibman
77
shift (cm-=)
Fig. 1. Low frequency part of the Raman spectrum of SBT as a function of temperature.
_I
0
50 loo 150 200 250 300 350 Temperature( “C)
Fig. 2. Temperature dependence of the Raman frequencies for SBT up to 340°C.
where I( w ) is the intensity of the spectrum, n (0) is the Boltzmann factor, n - ’ (0) = exp( fio/kT) - 1, A contains terms that do not depend on w, o, and rare the frequency and half-width of the soft mode, respectively. Figs. 3 and 4 illustrate the temperature dependence of w f and the line-width of the soft mode. of decreases linearly with temperature but it does not have the Curie-Weiss form w* ( T) = p( T,- T) because mode C is also strongly temperature dependent and the phase transition is first order. The reduced soft mode half-width r/r(O) versus reduced temperature t (t = ( T,- T) /T,) satisfies the universal scaling law r(T) =F(O) t-‘I*. The line-widths of many displacive ferroelectrics such as PbTi03, GeTe, SrTi03 follow this law [ 9 1. The room temperature structure of layered bismuth compounds can be described in terms of relatively small amplitude displacive perturbations away from a high symmetry, prototype, parent structure (space group symmetry 14/mmm). This nonpolar, prototype, parent structure consists of perovskite-like A,- ,BmOsm+I slabs regularly interleaved with Bi202 layers. It was previously believed that the overwhelmingly dominant contribution to the ferroelectricity in Bi4Ti30i2(nz=3), BiSTiNbOg(m=2),
259
G. Zou et al. /Physics Letters A 189 (1994) 257-260
150
0
’ ’ 50 loo
’ 150
; Tc ’ / ’
’
’
200 250 300 350
Temperaturec “C) Fig. 3. Temperature dependence of the square of the lowest frequency Raman peak of SBT.
ions in the perovskite A sites with respect to the chains of comer-connected Ti06 octahedra. There is still a contribution to P, arising from a displacement of the perovskite B cation away from the center of its surrounding octahedron of oxygen atoms, but its relative contribution to the spontaneous polarization systematically decreases as n increases. The structure of SBT( n= 5) consists of five perovskite-like layers between the two BizOz oxide layers. One half of the unit cell of SBT is depicted in Fig. 5. It is reasonable to think that the major component of the spontaneous polarization results from the displacement of the A sites in the perovskite-like layers. There are two kinds of cations, the S?+ ion and the Bi3+ ion distributed randomly in the A sites of SBT. According to the refinement of SrBizTaz09 [ 141, Bi4Ti30i2 [ 111, Bi3TiNb09 [ 121, in the A sites of the perovskite-like layers the Sr*+ ion retains a fairly regular 12-coordinate site as reflected in the narrow distribution of Sr-0 bond lengths, while the Bi3+ ion strongly deviates from the center of the dodecahedral coordination sites of the ideal perovskite structure.
eSr,
0.5’ 0
’
0.2
1
0.4
’
0.6
’
0.8
’
1
B=T/Tc
l
Bi
Ti
00
Fig. 4. Reduced soft mode half-widths T/T( 0) for SBT as a function of temperature. (---) T/T(O) = t-‘I*.
Bi2W06 (m = 1) was due to the displacements of the octahedral cations (W, Ti, Nb) away from the center of the surrounding octahedron of oxygen atoms [ 10 1. However, according to the recent refinement of the crystal structure of Bi4Ti3012 [ 111, BisTiNb09 [ 121, Bi*WO, [ 131, the major component of the large spontaneous polarization of these crystals mainly comes from the large u-axis displacements of the Bi3+
Fig. 5. One half of the pseudo-tetragonal unit cell of SBT. (C) ( Bi202)” layers, (B) units of the hypothetical perovskite structure (SrBi)TiOp, (A) peroskite layer (Sr2Bi2Ti5016)2-.
260
G. Zou et al. /Physics Letters A 189 (1994) 25 7-260
The S?+ and Bi3+ ions should have the same behavior in SBT because of the similarity of their structures. Thus, the displacement of the Bi3+ ion contributes the major part to the spontaneous polarization, so we suppose the soft mode is related to the unstable Bi3+ ion displacement, The Bi3+ ion has a pair of 6selectrons beyond the closed shell. The possible hybridization of 6s- and 6p-orbitals causes a tendency to an antisymmetric distortion in a cubically coordinated ion. The distortion is reduced as the temperature increases. When the temperature reaches the Curie point, the phase transition takes place.
4. Conclusion In conclusion, the soft mode has been found for the first time in SBT. The soft mode frequency decreases when the temperature approaches the Curie point, and disappears above the Curie point. The soft mode is related to the displacement of the unstable Bi3+ ion and is associated to the ferro- to para-electric phase transition.
References
[ 1 ] B. Aurivillius, Ark. Kemi 1 ( 1949) 463,499.
[ 21 K. Hisano and K. Toda, Solid State Commun. 18 ( 1976) 585. [ 31 Jianjun Liu, Guangtian Zou, Haibin Yang and Qiliang Cui, Solid State Commun. ( 1994)) in press. [4] Jianjun Liu, Guangtian Zou, Qiliang Cui and Haibin Yang, (1994), to be published. [ 5 ] E.C. Subbarao, IRE Trans. Electron Devices ED-8 ( 1961) 422. [6] E.C. Subbarao, J. Am. Ceram. Sot. 45 (1962) 166. [ 71 E.C. Subbarao, J. Phys. Chem. Solids 23 ( 1962) 665. [ 81 G. Burn and F.H. Dacol, Solid State Commun. 18 ( 1976) 1325. [ 91 J.F. Scott and J.A. Sanjurjo, Solid State Commun. 58 (1986) 687. [ IO] R.E. Newnham, R.W. Wolfe and J.F. Dorrian, Mater. Res. Bull. 6 (1971) 1029. [ 111 A.D. Rae, J.G. Thompson, R.L. Withers and A.C. Willis, Acta Crystallogr. B 46 ( 1990) 474. [ 121 J.G. Thompson, A.D. Rae, R.L. Withers and D.C. Gaig, Acta Crystallogr. B 47 ( 199 1) 174. [ 131 D. Rae, J.G. Thompson and R.L. Withers, Acta Crystallogr. B7 (1991) 870. [ 141 A.D. Rae, J.G. Thompson and R.L. Withers, Acta Crystallogr. B 48 ( 1992) 418.
LETTERS
A
Physics Letters A 189 (1994) 257-260
ELSEVIER
Soft mode phase transition of Sr2Bi4Ti5018 Guangtian Zou, Jianjun Liu, Qiliang Cui, Haibin Yang State Key Laboratoryfor Superhardkiaterials,InstituteofAtomic andMolecular Physics,Jilin University,Changchun 130023, China
Received 3 1 January 1994;revised manuscript received 30 March 1994, accepted for publication 6 April 1994 Communicated by J. Plouquet
AbStTWt
High temperature Raman spectra of SrzBi4Ti50,s have been investigated. A first order phase transition induced by a soft mode occurs at its Curie temperature. The soft mode is connected with the unstable Bi3+ ion displacement in the perovskite-like layers.
1. Introduction The layered bismuth compounds first studied by Aurivillius [ 1 ] are known to possess a structure expressed by the general formula (BizO,)2+(A,-1B,03,+ I ) 2-, where A is a combination of ions adequate for twelve-coordinated interstices, B is a combination of ions for octahedrally coordinated sites and m is an integer between 1 to 5. These compounds are built up by the regular intergrowth of (Bi202)‘+ layers and perovskite-type layers (A,_ ,BmOSm+,)2-. Most members of this family are displacive ferroelectrics. Hisano and Toda have found an underdamped soft mode in Bi4Ti30i2 ( m = 3 ) [ 2 1. We have studied the soft mode behavior in SrBi2Nb209 (m = 2) [ 3 ] and Nae.sBi*.STi&,(m=4) [4]. In our study on the behavior of the vibrational mode in layered bismuth compounds it is rather interesting to investigate Sr2Bi4TiS0i8 ( n = 5 ) high temperature Raman spectra. The ferroelectric activity in Sr2Bi4TiS0i8 (SBT) was first observed by Subbarao [ 5 ] in 196 1. At room temperature the crystal structure of SBT is orthorhombicwithQ=5.461 A,b/a=1.OOO,c=48.8A [6]. The dielectric data measured on ceramic disks indi-
cated a ferro- to para-electric phase transition occurring at 285°C [ 71. Above the Curie point the crystal structure is tetragonal and the dielectric constant (e) follows the Curie-Weiss law c = C/ ( T- 0). The Curie constant C and the extrapolated Curie-Weiss temperature 8 are 0.47 x 105°C and 255°C.
2. Experimental A polycrystalline sample was obtained by heating stoichiometric amounts of high purity Bi203, Ti02, St-CO3 in air at 800-l 100°C for 48 h in a covered platinum boat. The grinding and calcination were repeated until the solid state reaction was completed. The result of X-ray diffraction measurements were consistent with that reported by Subbarao [ 61. Raman scattering experiments have been performed with a SPEX-1403 Ramalog system using the 676.4 nm emission line of a Spectra Physics K+ laser as excitation source. The spectra were recorded at high temperature in the right angle scattering geometry. Powder samples were mounted in a SPEX-L? electric oven to raise the sample temperature. A thermocouple was used to monitor the sample temperature.
0375-9601/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDiO375-9601(94)00265-Q
G. Zou et al. /Physics Letters A 189 (I 994) 25 7-260
258
3. Results and discussion
Fig. 1 shows the Raman spectrum of polycrystalline SBT at various temperatures in the frequency range 7-77 cm-‘. Mode A, the lowest frequency Raman band, is highly temperature dependent and becomes overdamped at high temperature. It disappears above the Curie temperature and becomes Raman inactive. The intensity of mode B is very weak. It disappears above room temperature. Mode C is Raman active both below and above T,. It also softens as temperature increases but above T, its frequency hardly changes. Fig. 2 shows the temperature dependence of the peak frequency of modes A and C. In order to extract the temperature dependence of the soft mode frequency and line-width, the line-shape of the soft mode spectrum was fitted to a damped oscillator model [ 8 ] given by
42
7 Ibman
77
shift (cm-=)
Fig. 1. Low frequency part of the Raman spectrum of SBT as a function of temperature.
_I
0
50 loo 150 200 250 300 350 Temperature( “C)
Fig. 2. Temperature dependence of the Raman frequencies for SBT up to 340°C.
where I( w ) is the intensity of the spectrum, n (0) is the Boltzmann factor, n - ’ (0) = exp( fio/kT) - 1, A contains terms that do not depend on w, o, and rare the frequency and half-width of the soft mode, respectively. Figs. 3 and 4 illustrate the temperature dependence of w f and the line-width of the soft mode. of decreases linearly with temperature but it does not have the Curie-Weiss form w* ( T) = p( T,- T) because mode C is also strongly temperature dependent and the phase transition is first order. The reduced soft mode half-width r/r(O) versus reduced temperature t (t = ( T,- T) /T,) satisfies the universal scaling law r(T) =F(O) t-‘I*. The line-widths of many displacive ferroelectrics such as PbTi03, GeTe, SrTi03 follow this law [ 9 1. The room temperature structure of layered bismuth compounds can be described in terms of relatively small amplitude displacive perturbations away from a high symmetry, prototype, parent structure (space group symmetry 14/mmm). This nonpolar, prototype, parent structure consists of perovskite-like A,- ,BmOsm+I slabs regularly interleaved with Bi202 layers. It was previously believed that the overwhelmingly dominant contribution to the ferroelectricity in Bi4Ti30i2(nz=3), BiSTiNbOg(m=2),
259
G. Zou et al. /Physics Letters A 189 (1994) 257-260
150
0
’ ’ 50 loo
’ 150
; Tc ’ / ’
’
’
200 250 300 350
Temperaturec “C) Fig. 3. Temperature dependence of the square of the lowest frequency Raman peak of SBT.
ions in the perovskite A sites with respect to the chains of comer-connected Ti06 octahedra. There is still a contribution to P, arising from a displacement of the perovskite B cation away from the center of its surrounding octahedron of oxygen atoms, but its relative contribution to the spontaneous polarization systematically decreases as n increases. The structure of SBT( n= 5) consists of five perovskite-like layers between the two BizOz oxide layers. One half of the unit cell of SBT is depicted in Fig. 5. It is reasonable to think that the major component of the spontaneous polarization results from the displacement of the A sites in the perovskite-like layers. There are two kinds of cations, the S?+ ion and the Bi3+ ion distributed randomly in the A sites of SBT. According to the refinement of SrBizTaz09 [ 141, Bi4Ti30i2 [ 111, Bi3TiNb09 [ 121, in the A sites of the perovskite-like layers the Sr*+ ion retains a fairly regular 12-coordinate site as reflected in the narrow distribution of Sr-0 bond lengths, while the Bi3+ ion strongly deviates from the center of the dodecahedral coordination sites of the ideal perovskite structure.
eSr,
0.5’ 0
’
0.2
1
0.4
’
0.6
’
0.8
’
1
B=T/Tc
l
Bi
Ti
00
Fig. 4. Reduced soft mode half-widths T/T( 0) for SBT as a function of temperature. (---) T/T(O) = t-‘I*.
Bi2W06 (m = 1) was due to the displacements of the octahedral cations (W, Ti, Nb) away from the center of the surrounding octahedron of oxygen atoms [ 10 1. However, according to the recent refinement of the crystal structure of Bi4Ti3012 [ 111, BisTiNb09 [ 121, Bi*WO, [ 131, the major component of the large spontaneous polarization of these crystals mainly comes from the large u-axis displacements of the Bi3+
Fig. 5. One half of the pseudo-tetragonal unit cell of SBT. (C) ( Bi202)” layers, (B) units of the hypothetical perovskite structure (SrBi)TiOp, (A) peroskite layer (Sr2Bi2Ti5016)2-.
260
G. Zou et al. /Physics Letters A 189 (1994) 25 7-260
The S?+ and Bi3+ ions should have the same behavior in SBT because of the similarity of their structures. Thus, the displacement of the Bi3+ ion contributes the major part to the spontaneous polarization, so we suppose the soft mode is related to the unstable Bi3+ ion displacement, The Bi3+ ion has a pair of 6selectrons beyond the closed shell. The possible hybridization of 6s- and 6p-orbitals causes a tendency to an antisymmetric distortion in a cubically coordinated ion. The distortion is reduced as the temperature increases. When the temperature reaches the Curie point, the phase transition takes place.
4. Conclusion In conclusion, the soft mode has been found for the first time in SBT. The soft mode frequency decreases when the temperature approaches the Curie point, and disappears above the Curie point. The soft mode is related to the displacement of the unstable Bi3+ ion and is associated to the ferro- to para-electric phase transition.
References
[ 1 ] B. Aurivillius, Ark. Kemi 1 ( 1949) 463,499.
[ 21 K. Hisano and K. Toda, Solid State Commun. 18 ( 1976) 585. [ 31 Jianjun Liu, Guangtian Zou, Haibin Yang and Qiliang Cui, Solid State Commun. ( 1994)) in press. [4] Jianjun Liu, Guangtian Zou, Qiliang Cui and Haibin Yang, (1994), to be published. [ 5 ] E.C. Subbarao, IRE Trans. Electron Devices ED-8 ( 1961) 422. [6] E.C. Subbarao, J. Am. Ceram. Sot. 45 (1962) 166. [ 71 E.C. Subbarao, J. Phys. Chem. Solids 23 ( 1962) 665. [ 81 G. Burn and F.H. Dacol, Solid State Commun. 18 ( 1976) 1325. [ 91 J.F. Scott and J.A. Sanjurjo, Solid State Commun. 58 (1986) 687. [ IO] R.E. Newnham, R.W. Wolfe and J.F. Dorrian, Mater. Res. Bull. 6 (1971) 1029. [ 111 A.D. Rae, J.G. Thompson, R.L. Withers and A.C. Willis, Acta Crystallogr. B 46 ( 1990) 474. [ 121 J.G. Thompson, A.D. Rae, R.L. Withers and D.C. Gaig, Acta Crystallogr. B 47 ( 199 1) 174. [ 131 D. Rae, J.G. Thompson and R.L. Withers, Acta Crystallogr. B7 (1991) 870. [ 141 A.D. Rae, J.G. Thompson and R.L. Withers, Acta Crystallogr. B 48 ( 1992) 418.