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Raman scattering study of Na0.5Bi4.5Ti4O15 and its solid solutions
Vol51, No. 11, pp. 1653-1658, Copyright 0 1996 Elsevicr Science
J. Phys. Chem Solids
PII: 80022~36!q!%)ooO42-x
Pergamon
RAMAN
SCATTERING
Printed
1996 Ltd in Great Britain. AU rights reserved 0022-3697/96 $15.00 + 0.00
STUDY OF Na0.5Bi4,5Ti4015 AND ITS SOLID SOLUTIONS
JIANJUN LIU*, GUANGTIAN ZOU and YANRONG JIN State Key Laboratory for Superhard Materials, Jilin University, Changchun 130023,P. R. China (Received
11 November 1995; accepted 25 January 1996)
Abstract-The Raman spectra of N%,5Bi4.sTi40 is and its solid solutions Nao.sBi4,5_,La,Ti40,5, Na,,sB&,S_,Gd,T&015and (NaBi)o.su_,~Ba,Bi.,Ti.,Oiswere investigated. It was found that a first order phase transition takes place in the systems Na,,sB&.s_,La,Ti401S and Na,,SBi4.5_xGdxTi4015 at room temperature. The microscopic origins of the phase transitions are analysed. On the basis of an internal and external mode analysis, the nature of the various vibrations is discussed. Keywork
C.
Raman spectroscopy, D. lattice dynamics, D. phase transition.
1. INTRODUCTION
2.
The layered bismuth compounds first studied by Aurivillius [l] are known to possess a structure expressed by the general formula (Bir02)2+ (A,_, 2-, where A is Naf, Pb2+, Bi3+, . . .; B is 4n03m+l) Ti4+ NbSf Ta5+ W6+ , . . . ; m is an integer between 1 9 9 , and 5. These compounds are built up by regular intergrowth of (Bi202)2+ layers and perovskite-like layers (A,- 1k@3m+1 I *- along the c-axis. Most members of this family are displacive ferroelectrics. Nao,sBi4,sTi4015 (NBT) is one member of the layered bismuth compounds with m = 4. The structure of NBT consists of four perovskite-like layers between the two (Bi202)*+ oxide layers. One half of the unit cell of NBT is depicted in Fig. 1. The lattice parameters and ferroelectric activity of NBT have been studied by Subbarao [2,3]. At room temperature the crystal structure of NBT is orthorhombic with a = 0.5427 nm, b/a = 1.006, c = 4.065 nm. A ferro- to para-electric phase transition occurs at 655°C. Above Curie temperature the crystal structure is tetragonal with a space group Z4/mmm, and the dielectric constant (E) follows the Curie-Weiss law E = C/ (T - 0). The Curie constant C and the extrapolated Curie-Weiss temperature 0 are 0.79 x 1O’“C and 610°C. The vibrational spectra of NBT have not been investigated up to now. In this paper we report for the first time Raman scattering results on NBT and its solid solution systems.
*Present address: Department of Physics, Nankai University, Tianjin 300071, China
EXPERIMENTAL
In the present experiments polycrystalline samples Nao,sBi4,5_xLa,Ti4015, (x = 0,0.2,. . . ,2.0); NassBi4,5-xGd,Ti40i5, (x = 0,0.2,. . . ,2.6) and (NaBQss(r_,) BaxBi4Ti40r5, (x = O,O.l, . , 1.0) were prepared by the solid state reaction. Stoichiometric amounts of high purity Bi203, La20s, Gd203, Ti02, NarC03 and BaC03 were thoroughly mixed, pressed and fired between 800-1300°C for 48 h. The grinding, pressing and heating of mixed powders were repeated until their X-ray diffraction and Raman spectra remained unchanged. The Raman scattering experiments have been performed with a Jobin-Yvon T64000 system in a backward scattering geometry. The exciting source was the 514.5nm line from an argon ion laser which was filtered by a premonochromator in order to remove unwanted plasma lines. The incident power focused into a spot by an x40 objective lens was less than 1 mW. 3. RESULTS This paper reports the systematic study of composition dependence of lattice dynamical properties on the solid solution systems Nas.sBi4,s_xLa,Ti4015, Nao.sBi4,5_xGd,Ti4015, and (NaBi)o.S(l_,) Ba,Bi4 Ti40r5. The evolutions of the Raman spectra in the frequency range of lo-lOOOcm-’ with dopant concentration are illustrated in Figs 2-7. 3.1. Thesystem Nao,sBi4,5_xLa,Ti4015 (NBLT) Figure 2 shows the composition dependence of low frequency Raman spectra of NBLT. The lowest underdamped mode (27cm-‘) exhibits marked softening
1653
J. LIU et
1654
al.
the La concentration. 212cm-’ band becomes wider as La concentration increases. The high frequency Raman spectra of NBLT is shown in Fig. 3. It was found that the main high frequency bands hardly change. The intensities of the bands at 240 cm-’ and 347 cm-’ decrease. The two bands at Wcm-’ and 562 cm-’ gradually becomes one band. The frequency and linewidth of 856cm-’ mode nearly remain unchanged. The results show that the 27cm-‘, 43 cm-’ and 109 cm-’ bands are drastically influenced by the substitution of La3+ ion for Bi3+ ion. ’ 3.2. The system Na,,sB4,5_,G~Ti4015
eNaBi
l
Ti C 0
Fig. 1. One half of the pseudo-tetragonal unit cell of NBT. (A) perovskite layer (Nae.5Bi2.5T~019)2-. (B) unit of the hypothetical perovskite structure (NaBi)TiOr. (C) (BirO$+ layer. x. It obviously splits into two peaks at x = 1.0. After splitting the new lowest frequency mode continues to soften and suddenly disappears at x = 1.6. The 43cm-’ band rapidly submerges in the shoulder of the 60crn-’ band. 109cm-’ broad band becomes two bands at x = 0.4 and disappears at x = 1.6. 60cm-’ band seems not to be affected by
with increasing
I
IO
60
120
180
240
300
I
Raman frequency (cm-‘) Fig. 2. The low frequency Raman spectra of NBLT at room temperature for different composition x.
(NBGT)
The low frequency and high frequency Raman spectra of the system NBGT are shown in Fig. 4 and Fig. 5, respectively. At low Gd concentration (x < 1.6) the composition dependence of Raman spectra in the system NBGT are similar to that in the system NBLT except the splitting of the lowest frequency mode occurring at lower Gd concentration. However, when x > 1.6 some new features appear in NBGT system. Firstly, an abrupt splitting of 6Ocm-’ mode occurs at x = 1.6 as shown in Fig. 4. Secondly, the behavior of lowest frequency mode is also different from that of NBLT when x > 1.6. It still exists and its frequency begins to increase at x = 1.6. Finally, three new high frequency Raman bands emerge at
I
420
560
700
840
c
98’
Raman frequency (cm-‘) Fig. 3. The high frequency Raman spectra of NBLT at room temperature for different composition x.
Raman scattering study
I
10
1655
I
40
80
160
120
Raman
frequency
200
(cm-‘)
Fig. 4. The low frequency Raman spectra of NBGT at room temperature for different composition x. 47ocm-‘,
517cm-’
intensities
increase with rising x.
and 672~~1~
as x > 1.6. Their
The low frequency and high frequency Raman spectra of the system NBBT are shown in Fig. 6 and Fig. 7. They are also similar to that of NBLT system. The difference is that the frequency of the 856cm-’
600
(Cm-‘)
Fig. 6. The low frequency Raman spectra of NBBT at room temperature for different composition x. on the Ba concentration. The frequency of the 856cm-’ mode increases nearly 3Ocm-’ from x = 0 to x = 1.0. In addition, the lowest frequency mode splits at x = 0.1 and becomes damped with increasing x. mode strongly depends
3.3. The system (NaBi)o.s(,_,)Ba,Ti4016 (NBBT)
400
Raman frequency
800
Raman frequency (cm-‘) Fig. 5. The low high Raman spectra of NBGT at room temperature for different composition n.
200
400
600
800
1000
Raman frequency (cm-‘) Fig. 7. The high frequency Raman spectra of NBBT at room temperature for different composition x.
1656
J. LIU et al. 4. DISCUSSION
The structure of NBT(n = 4) consist of four perovskite-like layers between the two (Bi20#+ oxide layers. Armstrong and Newnham have already shown that the 1Zcoordinated site in the perovskitelike layer of the layered bismuth compounds can be occupied by large cations like alkali, alkaline earth or large size lanthanides, on the contrary the (Bi20$+ layers cannot accommodate any substitution [4]. Thus, only the ions in the A sites of the perovskitelike layer in NBT can be replaced. 4.1. The ferro- to para-electric phase transition in NBLT system The composition dependence of the Raman spectra of NBLT system shows that a phase transition occurs near x = 1.6. The phase transition is determined by the abrupt disappearance of the lowest frequency mode and some other Raman peaks. The phase transition seems to be ferro- to para- electric and related to a soft mode. The room temperature structure of layered bismuth compounds can be described in terms of relatively small amplitude displacive perturbations away from a high symmetry, parent structure (space group symmetry 14/mmm). Withers et al. [S] have identified that atomic displacements with F2mm symmetry give rise to the observed spontaneous polarization along a- axis in all the layered bismuth compounds. An F2mm type displacive distortions can be generated by a rigid shift along the II direction of the B,,Ob+l subunits (i), a motion back in the opposite direction of the B cations with respect to their surrounding oxygen octahedral frame-work (ii), and finally a rigid shift along a direction of the oxygen ions within the (B&O,)‘+ layers (iii). The quenching of the F2mm distortions should be correlated with the ferroelectric to paraelectric phase transitions. Though there are two kinds of cations Na+ ions and Bi3+ ions distributed randomly in the A sites of the NBT [6], it is reasonable to assume that the major distortion results from the displacement of the asymmetric Bi3+ ions along the a axis. The Bi3+ ion has a pair of 6s-electrons beyond the closed shell. The possible hybridization of 6s- and 6p- orbitals causes a tendency to an antisymmetric distortion in a cubically coordinated ion. Bi3+ ions strongly deviate from the center of dodecahedral coordination sites of ideal perovskite structure leading to the distortion of the lattice. The replacement of the asymmetric Bi3’ ions by symmetric La3+ ions should logically cancel the distortions. The symmetric, nonpolarizable La3+ ion tends to occupy the center of the 1Zcoordinated polyhedron and reduce the lattice distortion causing the decrease of the Curie temperature
[7]. Thus, the ferro- to para-electric phase transition is induced by the substitution of La3+ ion for Bi3+ ion. The phase transformation from orthorhombic to tetragonal due to replacement of Bi3+ ion by La3+ ion has also been observed in other layered bismuth compounds [8,9]. Displacive ferroelectrics normally have at least one low-energy TO mode which softens as the ferroelectric T, is approached. Similarly, one or more vibrational modes in a solid solution may soften when a structural phase transition is approached by varying x at constant temperature. In NBLT system the Raman mode at 27 cm-’ is likely to be the soft TO mode. We have also observed this Raman mode softens under both high temperature and high pressure [lo]. It is likely a ferroelectric soft mode. Hence this mode appears to be responsible for the concentration dependent ferro- to para-electric phase transition near x = 1.6. The “softening” of the soft mode due to the substitution of the La3+ ions for the Bi3+ ions indicates that it might originate from the vibrations of the Bi3+ ions in the A sites of the perovskite-like layers. A plot of the squared frequency of the soft mode before splitting vs composition is presented in Fig. 8. The experimental results are well fitted by the following linear relation, w(x)’ = ~(0)~ - AX where w(O) is the soft mode frequency at x = 0, A = 211.9. It is consistent with the results of theoretical studies on the lattice dynamics of perovskite crystals with substitutional dopants [ 111. From the well known soft mode theory [12], the soft mode frequency is determined by the difference between a short-range forces and the Coulomb forces.
550
I 0
I
0.2
I
0.4
I
0.6
1
0.8
II
La concentration (x) Fig. 8. Composition dependence of the square of the soft mode frequency in NBLT system.
Raman scattering study The phase transition occurs when short-range forces cancel the Coulomb forces leading to the vanishing (or near vanishing for a first order transition) of the soft mode frequency. The isovalent and nonpolarizable dopant ions only change the short-range forces [ 111, hence, the substitution of La3+ ion for Bi3+ ion alters the short-range forces to reduce the frequency of the soft mode. The soft mode suddenly disappears when the first order phase transition takes place. The splitting of the soft mode has not been well understood. Perhaps it might be related to a subtle structural phase transition. The splitting of the soft mode has also been observed under high pressure [lo]. 4.2. The structural phase transition in NBGT sys tern The composition dependence of the Raman spectra of NBGT system shows that a first order phase transition occurs at about x = 1.6. The phase transition is mainly characterized by the abrupt splitting of the 6Ocm-’ band and appearance of some new high frequency bands. Fundamental studies of ionic substitutions in perovskite-like layers with various number of m were made by Subarrao [2]. It was shown that the tolerance factors for the perovskite-like layer were in the range 0.81-0.93 and were in a narrower range than 0.770.99 for perovskites. Furthermore, the range of the tolerance factors decreases with increasing m. Armstrong and Newnham found that the Bi3+ ions in the perovskite-like layer can be replaced by di- and trivalent ions with ionic radii between 0.11 and 0.13 nm and that the lower limit of the ionic radii is determined by the stability of the perovskite-like layers and the upper limit by mismatch between the (B&Or)*+ and perovskite-like layers [4]. Thus, in the NBGT system because the radius of Gd3+ ion is smaller than that of Bi3+ ion the replacement of Bi3+ ions by Gd3+ ions results in the reduction of tolerance factor of the perovskite-layers. The stability of perovskite-like layers reaches its limit near x = 1.6 leading to the structural phase transition. The 60 cm-’ band seems to be a mode which marks the symmetry of the layered structure. The splitting of the 60cm-’ band reveals that a change of layered structure has taken place. The substitution of Gd3+ ions for Bi3+ ions also decreases the lattice distortion as La3+ ions in NBLT system. The softening of the 27cm-’ band, the gradual disappearance of the 43cm-’ and 109cm-’ bands show that the NBGT system is indeed transforming from the orthorhombic phase to the tetragonal structure. However, the instability of the perovskite-like layers stops this process and leads to other kind of phase transition.
1657
The Raman bands in high frequency region primarily come from the internal vibrations of Ti06 octahedra as discussed below. The appearance of the new Raman bands results from the distortion of Ti06 octahedra. The lowering of the symmetry of the Ti06 octahedra can activate the modes which are originally Raman inactive in ideal Ti06 octahedra. 4.3. Mode assignments of NBT Raman spectra The room temperature structure of NBT can be described as an orthorhombic distortion of a tetragonal structure with a space group M/mmm. The ferroelectric structure of NBT has not been precisely determined. However, the symmetry of the tetragonal phase should give a reasonable approximation. Accordingly, using nuclear site group analysis [ 131, we may classify the optical phonons into the following irreducible representation: I’ = 8At, + 3Bb + 1IEg + lOA*, + 2Br, + 125, Of these, all gerude modes are Raman active and all ungerade mode except BzUare IR active. In practice it would be impossible to identify unequivocally all observed Raman bands in the absence of single-crystal data. However, one can obtain significant information regarding the dynamics of the basic structural units and the soft modes from studies of polycrystalline material. We try to identify the nature of the observed Raman vibrations on the basis of an internal and external mode analysis. It is well known that in metal oxides with Ti06 octahedral symmetry, the intragroup binding energy within the Ti06 octahedra is large compared with the intergroup or crystal binding energy. Therefore, to a first approximation, we may interpret the vibrational spectra as arising from the internal vibration of Ti06 octahedra plus the external vibrations. The latter occur at lower frequencies and arise from cationoctahedral and intergroup oscillations. The composition dependences of the Raman spectra in the NBT solid solution systems show that 27cm-t, 43 cm-’ and 109cm-’ bands are strongly dependent on the concentration of Ba*+, La3+ and Gd3+ ions. They can be attributed to the external lattice modes in which the Bi3+ ions move with respect to the Ti06 groups. 60 cm-’ band is hardly affected by the dopant at low concentration. This mode exists even in the tetragonal phase. The splitting occurs only at high Gd concentration. It is likely a mark of the layered structure. The splitting of that peak means that the layered structure is destroyed. It might relate to the vibration between the (Bi202)*+ layer and pervoskite-like layers or between the two perovskitelike layers. Because in the layered structure crystals
1658
J. LIU et al.
with more than one layer in the unit cell, there occur very low frequency zone-center optical modes in which a layer moves very much like a rigid unit [14]. The 6Ocm-’ band seems to be this rigid-layer mode which marks the symmetry of the layered structure. The Raman bands at high frequency region primarily come from the internal vibrations of Ti06 octahedra. The vibrational modes of a perfect ‘isolated’ Ti06 octahedron can be decomposed into two pure bond stretching vibrations of symmetry Ais and Er(~z), two interbond angle bending vibrations us and v6 of symmetry F2g and Fzu, respectively, and finally, two remaining vibrations y and ~4, considered as combinations of stretching and bending, both of F,, symmetry. The g modes are Raman active and only the F,, are infrared active, the Fz,, being silent. By comparison with the assignment of the case of the PbTiO,, we assign the high frequency Raman bands to the following vibrations. The broad band around 71Ocm-’ is related to the v1 mode; the vz(Er) mode occurs at a lower energy and corresponds to the 564 and 548 cm-‘. The splitting into two components is due to the orthorhombic distortion. In NBLT system this two peaks gradually become one peak. It reveals that the lattice distortion decreases with increasing x. The vs(Fzg) Raman mode is expected to be intense and may be identified at 272 cm-‘. The v4(FtU) bending mode appears at 339cm-‘. The band near 240 cm- * belongs to the V~(Fzu) mode v4(Fl ,) and ug(Fzu) modes are activated by the distortion of the Ti06 octahedra. The decrease of the lattice distortion diminishes both of the two Raman band as shown in Fig. 1 and Fig. 2. The strong dependence of the frequency of 856cm-’ mode on the Ba2+ ion concentration in the NBBT system reveals that 856cm-’ mode is associated with the vibrations of the ions in the A sites of the perovskite-like layers. It might arise from the vibration of Bi-0 bonds in the
perovskite-like layers. Above assignments are tentative and require confirmation from detailed analysis of single-crystal polarized Raman spectra.
5. CONCLUSIONS The composition dependences of Raman spectra in NBT solid solution systems indicate that a Iirst order transition occurs near x = 1.6 in NBLT and NBGT system. The phase transition in NBLT system is a ferro- to para-electric phase transition and related to a soft mode. The phase transition in NBGT system is due to the instability of the pervoskite-like layers. The low frequency modes of NBT can be assigned to the vibrations of Bi3+ ions in the A sites of the perovskitelike layers and the rigid-layer mode. The high frequency Raman bands mainly come from the internal vibrations of TiOd octahedra.
REFERENCES 1. 2. 3. 4. 5. 6. I. 8. 9. 10. 11. 12. 13. 14.
Aurivillius B. Arkiv Kei., 1,463 (1949); 1,499 (1949). Subbarao E. C., J. Amer. Cera. Sot., 45, 166 (i962). Subbarao E. C.. J. Phvs. Chem. Solids. 23.665 (1962). Armstrong R. A. and &vnham R. E., ko;er. Rk. Buil., 7,1025 (1972). Withers R. L., Thompson J. G. and Rae A. D., J. Solid State Chem., 94,404 (1991). Newnham R. E., Mater. Res. Bull., 2, 1041 (1967). Takenaka T. and Sakata K. Ferroelecrrics, 38, 769 (1981). Mercurio J. P. Souirti A., Manier M. and Frit B. Mat. Res. Bull., 27, 123 (1992). Shimazu M. Tanaka J. Muramatsu K. and Tsukioka M., J. Solid State Chem., 35,402 (1980). Liu J. Ph.D. Thesis, Jilin University, Changchun, China, (1994). Dvorak V. and Glogar P., Phys. Rev., 143,344 (1966). Co&ran W., Adv. Phys. 9,387 (1960). Rousseau D. L., Bauman R. P. and Port0 S. P. S., J. Raman Spectrosc. 10,253 (1981). Zallen R. and Slade M., Phys. Rev., 9, 1627 (1974)
J. Phys. Chem Solids
PII: 80022~36!q!%)ooO42-x
Pergamon
RAMAN
SCATTERING
Printed
1996 Ltd in Great Britain. AU rights reserved 0022-3697/96 $15.00 + 0.00
STUDY OF Na0.5Bi4,5Ti4015 AND ITS SOLID SOLUTIONS
JIANJUN LIU*, GUANGTIAN ZOU and YANRONG JIN State Key Laboratory for Superhard Materials, Jilin University, Changchun 130023,P. R. China (Received
11 November 1995; accepted 25 January 1996)
Abstract-The Raman spectra of N%,5Bi4.sTi40 is and its solid solutions Nao.sBi4,5_,La,Ti40,5, Na,,sB&,S_,Gd,T&015and (NaBi)o.su_,~Ba,Bi.,Ti.,Oiswere investigated. It was found that a first order phase transition takes place in the systems Na,,sB&.s_,La,Ti401S and Na,,SBi4.5_xGdxTi4015 at room temperature. The microscopic origins of the phase transitions are analysed. On the basis of an internal and external mode analysis, the nature of the various vibrations is discussed. Keywork
C.
Raman spectroscopy, D. lattice dynamics, D. phase transition.
1. INTRODUCTION
2.
The layered bismuth compounds first studied by Aurivillius [l] are known to possess a structure expressed by the general formula (Bir02)2+ (A,_, 2-, where A is Naf, Pb2+, Bi3+, . . .; B is 4n03m+l) Ti4+ NbSf Ta5+ W6+ , . . . ; m is an integer between 1 9 9 , and 5. These compounds are built up by regular intergrowth of (Bi202)2+ layers and perovskite-like layers (A,- 1k@3m+1 I *- along the c-axis. Most members of this family are displacive ferroelectrics. Nao,sBi4,sTi4015 (NBT) is one member of the layered bismuth compounds with m = 4. The structure of NBT consists of four perovskite-like layers between the two (Bi202)*+ oxide layers. One half of the unit cell of NBT is depicted in Fig. 1. The lattice parameters and ferroelectric activity of NBT have been studied by Subbarao [2,3]. At room temperature the crystal structure of NBT is orthorhombic with a = 0.5427 nm, b/a = 1.006, c = 4.065 nm. A ferro- to para-electric phase transition occurs at 655°C. Above Curie temperature the crystal structure is tetragonal with a space group Z4/mmm, and the dielectric constant (E) follows the Curie-Weiss law E = C/ (T - 0). The Curie constant C and the extrapolated Curie-Weiss temperature 0 are 0.79 x 1O’“C and 610°C. The vibrational spectra of NBT have not been investigated up to now. In this paper we report for the first time Raman scattering results on NBT and its solid solution systems.
*Present address: Department of Physics, Nankai University, Tianjin 300071, China
EXPERIMENTAL
In the present experiments polycrystalline samples Nao,sBi4,5_xLa,Ti4015, (x = 0,0.2,. . . ,2.0); NassBi4,5-xGd,Ti40i5, (x = 0,0.2,. . . ,2.6) and (NaBQss(r_,) BaxBi4Ti40r5, (x = O,O.l, . , 1.0) were prepared by the solid state reaction. Stoichiometric amounts of high purity Bi203, La20s, Gd203, Ti02, NarC03 and BaC03 were thoroughly mixed, pressed and fired between 800-1300°C for 48 h. The grinding, pressing and heating of mixed powders were repeated until their X-ray diffraction and Raman spectra remained unchanged. The Raman scattering experiments have been performed with a Jobin-Yvon T64000 system in a backward scattering geometry. The exciting source was the 514.5nm line from an argon ion laser which was filtered by a premonochromator in order to remove unwanted plasma lines. The incident power focused into a spot by an x40 objective lens was less than 1 mW. 3. RESULTS This paper reports the systematic study of composition dependence of lattice dynamical properties on the solid solution systems Nas.sBi4,s_xLa,Ti4015, Nao.sBi4,5_xGd,Ti4015, and (NaBi)o.S(l_,) Ba,Bi4 Ti40r5. The evolutions of the Raman spectra in the frequency range of lo-lOOOcm-’ with dopant concentration are illustrated in Figs 2-7. 3.1. Thesystem Nao,sBi4,5_xLa,Ti4015 (NBLT) Figure 2 shows the composition dependence of low frequency Raman spectra of NBLT. The lowest underdamped mode (27cm-‘) exhibits marked softening
1653
J. LIU et
1654
al.
the La concentration. 212cm-’ band becomes wider as La concentration increases. The high frequency Raman spectra of NBLT is shown in Fig. 3. It was found that the main high frequency bands hardly change. The intensities of the bands at 240 cm-’ and 347 cm-’ decrease. The two bands at Wcm-’ and 562 cm-’ gradually becomes one band. The frequency and linewidth of 856cm-’ mode nearly remain unchanged. The results show that the 27cm-‘, 43 cm-’ and 109 cm-’ bands are drastically influenced by the substitution of La3+ ion for Bi3+ ion. ’ 3.2. The system Na,,sB4,5_,G~Ti4015
eNaBi
l
Ti C 0
Fig. 1. One half of the pseudo-tetragonal unit cell of NBT. (A) perovskite layer (Nae.5Bi2.5T~019)2-. (B) unit of the hypothetical perovskite structure (NaBi)TiOr. (C) (BirO$+ layer. x. It obviously splits into two peaks at x = 1.0. After splitting the new lowest frequency mode continues to soften and suddenly disappears at x = 1.6. The 43cm-’ band rapidly submerges in the shoulder of the 60crn-’ band. 109cm-’ broad band becomes two bands at x = 0.4 and disappears at x = 1.6. 60cm-’ band seems not to be affected by
with increasing
I
IO
60
120
180
240
300
I
Raman frequency (cm-‘) Fig. 2. The low frequency Raman spectra of NBLT at room temperature for different composition x.
(NBGT)
The low frequency and high frequency Raman spectra of the system NBGT are shown in Fig. 4 and Fig. 5, respectively. At low Gd concentration (x < 1.6) the composition dependence of Raman spectra in the system NBGT are similar to that in the system NBLT except the splitting of the lowest frequency mode occurring at lower Gd concentration. However, when x > 1.6 some new features appear in NBGT system. Firstly, an abrupt splitting of 6Ocm-’ mode occurs at x = 1.6 as shown in Fig. 4. Secondly, the behavior of lowest frequency mode is also different from that of NBLT when x > 1.6. It still exists and its frequency begins to increase at x = 1.6. Finally, three new high frequency Raman bands emerge at
I
420
560
700
840
c
98’
Raman frequency (cm-‘) Fig. 3. The high frequency Raman spectra of NBLT at room temperature for different composition x.
Raman scattering study
I
10
1655
I
40
80
160
120
Raman
frequency
200
(cm-‘)
Fig. 4. The low frequency Raman spectra of NBGT at room temperature for different composition x. 47ocm-‘,
517cm-’
intensities
increase with rising x.
and 672~~1~
as x > 1.6. Their
The low frequency and high frequency Raman spectra of the system NBBT are shown in Fig. 6 and Fig. 7. They are also similar to that of NBLT system. The difference is that the frequency of the 856cm-’
600
(Cm-‘)
Fig. 6. The low frequency Raman spectra of NBBT at room temperature for different composition x. on the Ba concentration. The frequency of the 856cm-’ mode increases nearly 3Ocm-’ from x = 0 to x = 1.0. In addition, the lowest frequency mode splits at x = 0.1 and becomes damped with increasing x. mode strongly depends
3.3. The system (NaBi)o.s(,_,)Ba,Ti4016 (NBBT)
400
Raman frequency
800
Raman frequency (cm-‘) Fig. 5. The low high Raman spectra of NBGT at room temperature for different composition n.
200
400
600
800
1000
Raman frequency (cm-‘) Fig. 7. The high frequency Raman spectra of NBBT at room temperature for different composition x.
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J. LIU et al. 4. DISCUSSION
The structure of NBT(n = 4) consist of four perovskite-like layers between the two (Bi20#+ oxide layers. Armstrong and Newnham have already shown that the 1Zcoordinated site in the perovskitelike layer of the layered bismuth compounds can be occupied by large cations like alkali, alkaline earth or large size lanthanides, on the contrary the (Bi20$+ layers cannot accommodate any substitution [4]. Thus, only the ions in the A sites of the perovskitelike layer in NBT can be replaced. 4.1. The ferro- to para-electric phase transition in NBLT system The composition dependence of the Raman spectra of NBLT system shows that a phase transition occurs near x = 1.6. The phase transition is determined by the abrupt disappearance of the lowest frequency mode and some other Raman peaks. The phase transition seems to be ferro- to para- electric and related to a soft mode. The room temperature structure of layered bismuth compounds can be described in terms of relatively small amplitude displacive perturbations away from a high symmetry, parent structure (space group symmetry 14/mmm). Withers et al. [S] have identified that atomic displacements with F2mm symmetry give rise to the observed spontaneous polarization along a- axis in all the layered bismuth compounds. An F2mm type displacive distortions can be generated by a rigid shift along the II direction of the B,,Ob+l subunits (i), a motion back in the opposite direction of the B cations with respect to their surrounding oxygen octahedral frame-work (ii), and finally a rigid shift along a direction of the oxygen ions within the (B&O,)‘+ layers (iii). The quenching of the F2mm distortions should be correlated with the ferroelectric to paraelectric phase transitions. Though there are two kinds of cations Na+ ions and Bi3+ ions distributed randomly in the A sites of the NBT [6], it is reasonable to assume that the major distortion results from the displacement of the asymmetric Bi3+ ions along the a axis. The Bi3+ ion has a pair of 6s-electrons beyond the closed shell. The possible hybridization of 6s- and 6p- orbitals causes a tendency to an antisymmetric distortion in a cubically coordinated ion. Bi3+ ions strongly deviate from the center of dodecahedral coordination sites of ideal perovskite structure leading to the distortion of the lattice. The replacement of the asymmetric Bi3’ ions by symmetric La3+ ions should logically cancel the distortions. The symmetric, nonpolarizable La3+ ion tends to occupy the center of the 1Zcoordinated polyhedron and reduce the lattice distortion causing the decrease of the Curie temperature
[7]. Thus, the ferro- to para-electric phase transition is induced by the substitution of La3+ ion for Bi3+ ion. The phase transformation from orthorhombic to tetragonal due to replacement of Bi3+ ion by La3+ ion has also been observed in other layered bismuth compounds [8,9]. Displacive ferroelectrics normally have at least one low-energy TO mode which softens as the ferroelectric T, is approached. Similarly, one or more vibrational modes in a solid solution may soften when a structural phase transition is approached by varying x at constant temperature. In NBLT system the Raman mode at 27 cm-’ is likely to be the soft TO mode. We have also observed this Raman mode softens under both high temperature and high pressure [lo]. It is likely a ferroelectric soft mode. Hence this mode appears to be responsible for the concentration dependent ferro- to para-electric phase transition near x = 1.6. The “softening” of the soft mode due to the substitution of the La3+ ions for the Bi3+ ions indicates that it might originate from the vibrations of the Bi3+ ions in the A sites of the perovskite-like layers. A plot of the squared frequency of the soft mode before splitting vs composition is presented in Fig. 8. The experimental results are well fitted by the following linear relation, w(x)’ = ~(0)~ - AX where w(O) is the soft mode frequency at x = 0, A = 211.9. It is consistent with the results of theoretical studies on the lattice dynamics of perovskite crystals with substitutional dopants [ 111. From the well known soft mode theory [12], the soft mode frequency is determined by the difference between a short-range forces and the Coulomb forces.
550
I 0
I
0.2
I
0.4
I
0.6
1
0.8
II
La concentration (x) Fig. 8. Composition dependence of the square of the soft mode frequency in NBLT system.
Raman scattering study The phase transition occurs when short-range forces cancel the Coulomb forces leading to the vanishing (or near vanishing for a first order transition) of the soft mode frequency. The isovalent and nonpolarizable dopant ions only change the short-range forces [ 111, hence, the substitution of La3+ ion for Bi3+ ion alters the short-range forces to reduce the frequency of the soft mode. The soft mode suddenly disappears when the first order phase transition takes place. The splitting of the soft mode has not been well understood. Perhaps it might be related to a subtle structural phase transition. The splitting of the soft mode has also been observed under high pressure [lo]. 4.2. The structural phase transition in NBGT sys tern The composition dependence of the Raman spectra of NBGT system shows that a first order phase transition occurs at about x = 1.6. The phase transition is mainly characterized by the abrupt splitting of the 6Ocm-’ band and appearance of some new high frequency bands. Fundamental studies of ionic substitutions in perovskite-like layers with various number of m were made by Subarrao [2]. It was shown that the tolerance factors for the perovskite-like layer were in the range 0.81-0.93 and were in a narrower range than 0.770.99 for perovskites. Furthermore, the range of the tolerance factors decreases with increasing m. Armstrong and Newnham found that the Bi3+ ions in the perovskite-like layer can be replaced by di- and trivalent ions with ionic radii between 0.11 and 0.13 nm and that the lower limit of the ionic radii is determined by the stability of the perovskite-like layers and the upper limit by mismatch between the (B&Or)*+ and perovskite-like layers [4]. Thus, in the NBGT system because the radius of Gd3+ ion is smaller than that of Bi3+ ion the replacement of Bi3+ ions by Gd3+ ions results in the reduction of tolerance factor of the perovskite-layers. The stability of perovskite-like layers reaches its limit near x = 1.6 leading to the structural phase transition. The 60 cm-’ band seems to be a mode which marks the symmetry of the layered structure. The splitting of the 60cm-’ band reveals that a change of layered structure has taken place. The substitution of Gd3+ ions for Bi3+ ions also decreases the lattice distortion as La3+ ions in NBLT system. The softening of the 27cm-’ band, the gradual disappearance of the 43cm-’ and 109cm-’ bands show that the NBGT system is indeed transforming from the orthorhombic phase to the tetragonal structure. However, the instability of the perovskite-like layers stops this process and leads to other kind of phase transition.
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The Raman bands in high frequency region primarily come from the internal vibrations of Ti06 octahedra as discussed below. The appearance of the new Raman bands results from the distortion of Ti06 octahedra. The lowering of the symmetry of the Ti06 octahedra can activate the modes which are originally Raman inactive in ideal Ti06 octahedra. 4.3. Mode assignments of NBT Raman spectra The room temperature structure of NBT can be described as an orthorhombic distortion of a tetragonal structure with a space group M/mmm. The ferroelectric structure of NBT has not been precisely determined. However, the symmetry of the tetragonal phase should give a reasonable approximation. Accordingly, using nuclear site group analysis [ 131, we may classify the optical phonons into the following irreducible representation: I’ = 8At, + 3Bb + 1IEg + lOA*, + 2Br, + 125, Of these, all gerude modes are Raman active and all ungerade mode except BzUare IR active. In practice it would be impossible to identify unequivocally all observed Raman bands in the absence of single-crystal data. However, one can obtain significant information regarding the dynamics of the basic structural units and the soft modes from studies of polycrystalline material. We try to identify the nature of the observed Raman vibrations on the basis of an internal and external mode analysis. It is well known that in metal oxides with Ti06 octahedral symmetry, the intragroup binding energy within the Ti06 octahedra is large compared with the intergroup or crystal binding energy. Therefore, to a first approximation, we may interpret the vibrational spectra as arising from the internal vibration of Ti06 octahedra plus the external vibrations. The latter occur at lower frequencies and arise from cationoctahedral and intergroup oscillations. The composition dependences of the Raman spectra in the NBT solid solution systems show that 27cm-t, 43 cm-’ and 109cm-’ bands are strongly dependent on the concentration of Ba*+, La3+ and Gd3+ ions. They can be attributed to the external lattice modes in which the Bi3+ ions move with respect to the Ti06 groups. 60 cm-’ band is hardly affected by the dopant at low concentration. This mode exists even in the tetragonal phase. The splitting occurs only at high Gd concentration. It is likely a mark of the layered structure. The splitting of that peak means that the layered structure is destroyed. It might relate to the vibration between the (Bi202)*+ layer and pervoskite-like layers or between the two perovskitelike layers. Because in the layered structure crystals
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with more than one layer in the unit cell, there occur very low frequency zone-center optical modes in which a layer moves very much like a rigid unit [14]. The 6Ocm-’ band seems to be this rigid-layer mode which marks the symmetry of the layered structure. The Raman bands at high frequency region primarily come from the internal vibrations of Ti06 octahedra. The vibrational modes of a perfect ‘isolated’ Ti06 octahedron can be decomposed into two pure bond stretching vibrations of symmetry Ais and Er(~z), two interbond angle bending vibrations us and v6 of symmetry F2g and Fzu, respectively, and finally, two remaining vibrations y and ~4, considered as combinations of stretching and bending, both of F,, symmetry. The g modes are Raman active and only the F,, are infrared active, the Fz,, being silent. By comparison with the assignment of the case of the PbTiO,, we assign the high frequency Raman bands to the following vibrations. The broad band around 71Ocm-’ is related to the v1 mode; the vz(Er) mode occurs at a lower energy and corresponds to the 564 and 548 cm-‘. The splitting into two components is due to the orthorhombic distortion. In NBLT system this two peaks gradually become one peak. It reveals that the lattice distortion decreases with increasing x. The vs(Fzg) Raman mode is expected to be intense and may be identified at 272 cm-‘. The v4(FtU) bending mode appears at 339cm-‘. The band near 240 cm- * belongs to the V~(Fzu) mode v4(Fl ,) and ug(Fzu) modes are activated by the distortion of the Ti06 octahedra. The decrease of the lattice distortion diminishes both of the two Raman band as shown in Fig. 1 and Fig. 2. The strong dependence of the frequency of 856cm-’ mode on the Ba2+ ion concentration in the NBBT system reveals that 856cm-’ mode is associated with the vibrations of the ions in the A sites of the perovskite-like layers. It might arise from the vibration of Bi-0 bonds in the
perovskite-like layers. Above assignments are tentative and require confirmation from detailed analysis of single-crystal polarized Raman spectra.
5. CONCLUSIONS The composition dependences of Raman spectra in NBT solid solution systems indicate that a Iirst order transition occurs near x = 1.6 in NBLT and NBGT system. The phase transition in NBLT system is a ferro- to para-electric phase transition and related to a soft mode. The phase transition in NBGT system is due to the instability of the pervoskite-like layers. The low frequency modes of NBT can be assigned to the vibrations of Bi3+ ions in the A sites of the perovskitelike layers and the rigid-layer mode. The high frequency Raman bands mainly come from the internal vibrations of TiOd octahedra.
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