7950
J. Phys. Chem. B 2001, 105, 7950-7953
Vibrational Study of Layered Perovskites M2La2Ti3O10 (M ) Li, Na, K, Rb): Raman Spectra and Normal Mode Analysis R. Nozaki, J. N. Kondo, C. Hirose, K. Domen, and A. Wada* Chemical Resources Laboratory,† Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Y. Morioka Department of Chemistry, Faculty of Science, Saitama UniVersity, Shimo-Okubo 255, Urawa 338-8570, Japan ReceiVed: March 5, 2001; In Final Form: June 5, 2001
Vibrational study by Raman spectroscopy was made of the layered perovskite compounds of the formula M2La2Ti3O10 (M ) H, Li, Na, K, Rb), K2La2Ti3O10‚2H2O, and H2La2Ti3O10‚nH2O. The spectral features above 400 cm-1 were similar in all of the unhydrated samples suggesting that the bands originated from the vibrational modes of the La2Ti3O10 perovskite layer. Lattice dynamical calculations were performed with the use of a potential field derived from the molecular dynamics on the M2La2Ti3O10 (M ) Li, Na, K). The assignment of the Raman bands higher than 400 cm-1 was made on the basis of the results of lattice dynamical calculations and was confirmed by comparing the spectral feature of K2La2Ti3O10 with that of H2La2Ti3O10, K2La2Ti3O10‚2H2O, and H2La2Ti3O10‚nH2O.
Introduction M2La2Ti3O10 (M ) H, Li, Na, K, Rb) belong to layered perovskite compounds expressed by general formula Am[A′n-1BnO3n+1], where A′n-1BnO3n+1 is the perovskite-type layer and A is interlayer cation. Recently, a family of perovskite compounds, MCa2Nb3O10, M2Ln2Ti3O10, and MLnNb2O7 (M ) H, Na, K, Rb; Ln ) lanthanides), has been attracting profound interest because of its unique chemical properties which are associated with the interlayer space such as ion-exchange ability,1,2 intercalation reactivity of organic compounds,3 and photocatalytic activity.4-6 To understand the origin of these chemical properties, the knowledge of interaction between the reaction sites in the interlayer space and the intercalated molecules is essential. The slight structural change originating from the interaction will have an effect on the Raman spectrum, because the Raman spectral features are very sensitive to the crystal structure and bond order of metal oxides.7-9 Raman spectroscopy, therefore, is an effective method to obtain the knowledge of the chemical properties of layered perovskite compounds. Although the assignment of the observed vibration bands is needed to study the chemical properties on the basis of the results of vibration spectroscopy, the detailed assignment of the Raman bands by normal coordinate analysis has not been reported yet. The Raman spectra of Na2Ln2Ti3O10 (Ln ) Gd, Sm, Nd, La) have been studied by Byeon et al.,10 and several prominent Raman bands have been assigned on the basis of the structural data for Na2Ln2Ti3O10 and the assignment of the Raman bands of NaLnTiO4,11 which is a similar layered perovskite compound. However, careful consideration would be needed on the assignment for Na2Ln2Ti3O10, because the symmetry of the crystal structure of Na2Ln2Ti3O10 (space group D4V17),12 which has an inversion symmetry, is different from that of NaLnTiO4 (space group C4V1),13 which has no inversion symmetry. * To whom correspondence should be addressed. E-mail: awada@ csd.res.titech.ac.jp. † Renamed from Research Laboratory of Resources Utilization.
In this study, the Raman spectra of layered perovskites M2La2Ti3O10 (M ) H, Li, Na, K, Rb) and hydrated derivatives of M2La2Ti3O10 (M ) H, K) were measured. The measurements revealed that the Raman bands in the frequency region higher than 400 cm-1 originate from the vibrational motion in the perovskite layer. The assignment of the Raman bands of M2La2Ti3O10 (M ) Li, Na, K) in this frequency region was made on the basis of the normal-mode analysis using the potential field, which has been optimized to reproduce the observed Raman shifts in lattice dynamical calculations on the basis of the molecular dynamics. The assignment is confirmed by comparing the spectrum of K2La2Ti3O10 with those of K2La2Ti3O10‚2H2O, H2La2Ti3O10, and H2La2Ti3O10‚nH2O. Experiments The powder samples of M2La2Ti3O10 (M ) K, Rb) were prepared by conventional solid-state reaction.1 The stoichiometric amounts of La2O3, TiO2, and 20% excess of M2CO3 (M ) K, Rb) were mixed and kept at the temperature of 900 °C for 5 h in air. The obtained powders of M2La2Ti3O10 (M ) K, Rb) were ground, heated at 1050 °C in the furnace with intermittent grinding for 40 h, and cooled. After the treatments, the products were washed with distilled water and dried at 100 °C. Li2La2Ti3O10 and Na2La2Ti3O10 were prepared by ionexchange reaction which was carried out by adding K2La2Ti3O10 to a molten salt of NaNO3 and Na2La2Ti3O10 to a molten salt of LiNO3 at 350 °C for 50 h, respectively. The products were filtered out, washed with distilled water, and dried for 24 h at 100 °C. H2La2Ti3O10 was obtained by stirring K2La2Ti3O10 in 0.1 N HNO3 solution for 24 h at room temperature. The products were filtered out, washed with distilled water, and dried at 100 °C for 1 h. Hydrated forms of H2La2Ti3O10 and K2La2Ti3O10 were obtained by leaving the samples in water and air of 100% humidity for 24 h, respectively. The Raman spectra of the powder samples were measured at room temperature using a Raman spectrometer (JASCO NRS-
10.1021/jp010839g CCC: $20.00 © 2001 American Chemical Society Published on Web 07/26/2001
Layered Perovskites M2La2Ti3O10
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7951 TABLE 1: Correlation Diagram for Na2La2Ti3O10, K2La2Ti3O10, and Li2La2Ti3O10 (Space Group I4/mmm-D4h17)a
Figure 1. Schematic illustrations of the structures of (a) Li2La2Ti3O10, (b) M2La2Ti3O10 (M ) Na, K), and (c) K2La2Ti3O10‚2H2O. (d) Environment around titanium atoms in the perovskite layer.
2100) consisting of an argon ion laser, a triple monochromator, and a CCD detector cooled with liquid nitrogen. The 514.5 nm line of an argon ion laser was used as the excitation source. Results and Discussion Figure 1a-c shows schematic illustrations of the structures of Li2La2Ti3O10 (a),14 Na2La2Ti3O1012 or K2La2Ti3O10 (b),14 and K2La2Ti3O10‚2H2O (c),14 respectively. Figure 1d depicts the environment around the titanium atoms in the perovskite layers of M2La2Ti3O10 (M ) Li, Na, K). While Li2La2Ti3O10 belongs to the same space group D4h17 (I4/mmm) as Na2La2Ti3O10 and K2La2Ti3O10, the interlayer site of the Li ion is different from those of the Na and K. It should be noted that the hydration leads to the relative displacement of the La2Ti3O10 layers and the crystal structure of K2La2Ti3O10‚2H2O is transformed to a simple tetragonal lattice belonging to the space group D4h1 (P4/mmm). The symmetry species of vibrational modes are derived from the correlation diagrams shown in Tables 1 and 2. Raman-active modes are 6A1g + B1g + 7Eg in Na2La2Ti3O10 and K2La2Ti3O10, 5A1g + 2B1g + 7Eg in Li2La2Ti3O10, and 6A1g + B1g + 7Eg in K2La2Ti3O10‚2H2O. In the case of K2La2Ti3O10‚2H2O, additional Raman bands due to the internal and external modes of the water molecule are expected. In all of these compounds, the primitive unit cell contains one formula unit and the factor group symmetry is D4h. Among 14 Raman-active modes except the water modes, two alkaline metal modes (A1g and Eg in Na2La2Ti3O10, K2La2Ti3O10, and K2La2Ti3O10‚2H2O and B1g and Eg in Li2La2Ti3O10) and two lanthanum modes (A1g and Eg) are the external modes and expected in the low-frequency region. Figure 2 shows the observed Raman spectra of nonhydrated M2La2Ti3O10 (M ) (a) Li, (b) Na, (c) K, (d) Rb). It is seen from Figure 2 that the spectral features in the frequency region higher than 400 cm-1 are very similar to each other, although those in the frequency region lower than 400 cm-1 strongly depend on the kind of interlayer cation. The results suggest that the Raman bands observed at higher than 400 cm-1 correspond to the vibrational modes of the perovskite layer. Bands below 400 cm-1 would be dominated by two alkaline metal modes, two lanthanum modes, and two deformational modes of the perovskite layer including vibrational displacements of titanium ions. The difference among the spectra in Figure 2 must arise from the mass difference of alkali metal ions, which will affect the frequency not only of the alkali metal modes but also the
aΓ optical ) 6A1g + B1g + 7Eg + 7A2u + 2B2u + 9Eu for Na2La2Ti3O10 and K2La2Ti3O10, and Γoptical ) 5A1g + 2B1g + 7Eg + 7A2u + 2B2u + 9Eu for Li2La2Ti3O10. Γacoustic ) A2u + Eu.
TABLE 2: Correlation Diagram for K2La2Ti3O10‚2H2O (Space Group P4/mmm-D4h1)a
a Γoptical ) 6A1g + B1g + 7Eg + 7A2u + 2B2u + 9Eu for K2La2Ti3O10‚2H2O, and Γacoustic ) A2u + Eu. Internal and external modes of H2O are not taken into account.
lanthanum modes and the low-frequency perovskite layer modes through the vibrational coupling. Figure 3 shows the Raman spectra of the unhydrated and hydrated derivatives of M2La2Ti3O10 (M ) H, K). It is seen from Figure 3a,b that the hydration causes a remarkable upshift of the band at 875 cm-1 of K2La2Ti3O10. Furthermore, the bands at 875 and 514 cm-1 of K2La2Ti3O10 become broadened by the hydration. The results suggest that the bands correspond to the vibrational modes which involve the motion of the Ti-O(4) bond, because the Ti-O(4) bond projects into the interlayer space as shown in Figure 1d and the O(4) atom would interact with the intercalated water molecules in the hydrated structure.
7952 J. Phys. Chem. B, Vol. 105, No. 33, 2001
Nozaki et al. TABLE 3: Potential Parameters Used in This Study ion
z (e)
a (Å)
b (Å)
Ti O La K Na Li
2.4 -1.2 1.8 0.6 0.6 0.6
1.079 1.926 1.366 1.758 1.457 1.024
0.10 0.16 0.09 0.09 0.09 0.09
ion pair
Dijb
βij (Å-1)
rij* (Å)
24.0
2.0
1.8
Ti-O a
kcal1/2
Å3
mol-1/2. b
kcal
ca 0 20 5 15 10 5
mol-1.
potential between ith and jth atoms Uij is expressed as16
[
]
zizje2 cicj ai + aj - rij + (bi + bj) exp - 6+ rij bi + b j rij Dij(exp[-2βij(rij - rij*)] - 2 exp[-βij(rij - rij*)]) (1)
Uij(rij) )
Figure 2. Raman spectra of M2La2Ti3O10 (M ) (a) Li, (b) Na, (c) K, (d) Rb).
Figure 3. Raman spectra of (a) K2La2Ti3O10, (b) K2La2Ti3O10·2H2O, (c) H2La2Ti3O10, and (d) K2La2Ti3O10‚nH2O.
It should be noted from Figure 3a,c,d that the feature of the 445 cm-1 band is almost the same as those of H2La2Ti3O10 and H2La2Ti3O10‚nH2O, while the features of other bands of K2La2Ti3O10 are remarkably different from those of the H2La2Ti3O10 and H2La2Ti3O10‚nH2O bands. The normal-mode analysis using a central force field was carried out for M2La2Ti3O10 (M ) Li, Na, K) to give assignment of the observed Raman bands, but the analysis on Rb2La2Ti3O10 and H2La2Ti3O10 could not be done due to the lack of data on atomic positions. The frequencies and displacement vectors of each normal mode were calculated on the basis of the procedure described in ref 15. The atomic positions used in the calculations are determined by the XRD measurements.14 For the interatomic potential, a partially ionic model (PIM) was adopted. In this potential model, the potential energy of the crystal is expressed by the sum of pair potentials that depend on the interatomic distance. Within the pair potential approximation, the interatomic
The first term represents the Coulomb interaction, where zi and zj are the effective charges of the ions i and j, respectively, and rij is the interatomic distance between the ions. The second term describes the short-range repulsion potential. The parameters ai and aj reflect the radii, and bi and bj reflect the hardness of the ions i and j, respectively. The third term corresponds to the dipole-induced dipole dispersion potential based on the van der Waals interaction. The last term is the Morse potential. As we consider covalency only for the Ti-O bond, Dij, βij, and rij* are the specific parameters for this bond. For the parameters associated with the Ti-O bond, the values which reproduce the lattice parameters of TiO2 rutile17,18 with molecular dynamics (MD) simulation were employed. The parameters associated with K and La of K2La2Ti3O10 were determined as follows. The parameter z for K was changed from 0.4 to 1.0 with a step of 0.1, and the parameter z for La was also changed to maintain the total charge in the unit cell of K2La2Ti3O10 equal to zero. The parameters (a and b for K and a-c for La) were fitted by a least-squares method to the frequencies of Raman bands observed in the spectral range over 400 cm-1 for each value of the parameter z for K. For the parameter c for K, the value reported in ref 18 was adopted. It was found that z ) 0.6 yielded the best fit for K. For Li2La2Ti3O10 and Na2La2Ti3O10, the parameters b and z for Li and Na were set equal to those obtained for K. For the parameter c of Li and Na, the values reported in ref 18 were adopted. The parameter a for Li (Na) was determined by least-squares fitting to the observed Raman bands of Li2La2Ti3O10 (Na2La2Ti3O10) in the spectral range over 400 cm-1. The values of parameters used in the calculation are listed in Table 3 where the italic type letters indicate the values obtained by the procedure described above. The vibrational frequencies of the bands higher than 400 cm-1 obtained by the calculation and the Raman measurements are listed in Table 4. The assignment of each band is also presented in the right column of Table 4. The main displacement vectors for each normal mode of K2La2Ti3O10 are shown in Figure 4. In the frequency region, the vibrational frequencies obtained from the fitting agree well with the observed frequencies for the potassium compound (with average error of 5.5%) and the sodium compound (with average error of 8.0%), while the agreement is rather poor (with average error of 12%) for the Li compound. We consider, however, that the fits are sufficient for the calculation and fitting procedures adopted in this study, and further improvements of calculation are beyond the aim of this study. In the following,
Layered Perovskites M2La2Ti3O10
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7953
Figure 4. Displacement vectors for the calculated normal modes of K2La2Ti3O10: (a) A1g mode at the calculated frequency of 874 cm-1; (b) A1g mode at 874 cm-1; (c) Eg mode at 713 cm-1; (d) A1g mode at 638 cm-1; (e) Eg mode at 553 cm-1; (f) B1g mode at 469 cm-1.
TABLE 4: Results of Raman Measurements and Calculation frequency (cm-1) Na2La2Ti3O10 obsd calcd
K2La2Ti3O10 obsd calcd
865 782 682 562 520 454
902 770 672 576 516 450
875 770 688 556 514 445
907 773 720 686 427 433
Acknowledgment. This work was supported by Grant-inAids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 10304060). References and Notes
Li2La2Ti3O10 obsd calcd 795 712 715 608 679 414
The bands observed at around 520 cm-1 on the Raman spectra of M2La2Ti3O10 (M ) Li, Na, K) are assigned to the Eg mode which involves the large bending motion of the Ti-O(4) bond as presented in Figure 4e. This vibrational mode would also be strongly influenced by hydration due to the same reason of the blue shift and broadening of the band at around 900 cm-1 described above. The assignment is supported by the experimental results; hydration caused remarkable broadening of the band observed at 514 cm-1 on the spectrum of K2La2Ti3O10 in comparison with the neighboring bands which are assigned to the vibrational modes involving mainly the motions of O atoms inside the perovskite layer. The bands observed at around 445 cm-1 are assigned to the B1g mode which involves only the bending motion of Ti-O(3) bond as presented in Figure 4f. It is expected that such a vibrational mode is little influenced by the interlayer condition because the motions of the interlayer cations and O(4) atoms are not included in the vibrational mode and the interaction between the O(3) atom and the interlayer species would be weak due to large interatomic distance between them. As a matter of fact, the features of the bands observed at around 445 cm-1 on the spectra of K2La2Ti3O10, K2La2Ti3O10‚2H2O, H2La2Ti3O10, and H2La2Ti3O10‚nH2O are almost the same as seen from Figure 3. In summary, the Raman spectra of layered perovskite compounds, M2La2Ti3O10 (M ) H, Li, Na, K, Rb), K2La2Ti3O10‚ 2H2O, and H2Ti3O10‚nH2O, were measured. It was found that the Raman bands appearing in the frequency region higher than 400 cm-1 correspond to the vibrational modes of the perovskite layer. The assignment of the Raman bands in this spectral region was made on the basis of the normal-mode analysis using the central force field derived from the lattice dynamics calculation for the first time.
874 760 713 638 553 469
sym of vibrationl mode A1g A1g Eg A1g Eg B1g
the assignment was confirmed by comparing the features of the spectrum of K2La2Ti3O10 with those of K2La2Ti3O10‚2H2O, H2La2Ti3O10, and H2La2Ti3O10‚nH2O. The bands observed at around 900 cm-1 on the Raman spectra of M2La2Ti3O10 (M ) Li, Na, K) are assigned to the A1g mode which involves the large stretching motion of the Ti-O(4) bond which projects into the interlayer as shown in Figure 4a. The vibrational mode is expected to be strongly influenced by hydration, because the environment of the O(4) atom is dramatically changed by hydration and the O(4) atom is close to the intercalated water molecules in the hydrated structure as seen from Figure 1c,d. As a matter of fact, the band observed at 875 cm-1 on the Raman spectrum of K2La2Ti3O10 showed a large blue shift and broadening by hydration as seen from Figure 3a,b. Byeon et al. assigned the band at around 900 cm-1 to a stretching motion of a Ti-O(4) bond in highly distorted TiO6 octahedra. Both assignments agree that the bands correspond to the stretching motion of the Ti-O(4) bond.
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