A. Maaej a, M. Bahri b, Y. Abid a, N. Jaidane b, Z. B. Lakhdar b & A. Lautié c a Département de Physique, Faculté des

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1 This article was downloaded by: [University of Regina] On: 02 October 2014, At: 17:02 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: Raman study of low temperature phase transitions in the cubic perovskite CH 3 NH 3 PbCI 3 A. Maaej a, M. Bahri b, Y. Abid a, N. Jaidane b, Z. B. Lakhdar b & A. Lautié c a Département de Physique, Faculté des Sciencesde, B.P. 763, Sfax, Tunisia b Laboratoire de Physique moléculaire, Faculté des Sciences de Tunis, Tunisia c CNRS, Lasir, 2 Rue Henry Dunant, 94320, Thiais, France Published online: 01 Sep To cite this article: A. Maaej, M. Bahri, Y. Abid, N. Jaidane, Z. B. Lakhdar & A. Lautié (1998) Raman study of low temperature phase transitions in the cubic perovskite CH 3 NH 3 PbCI 3, Phase Transitions: A Multinational Journal, 64:4, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of

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3 Phaw Transirions, Vul. 64, pp Reprints available directly from the puhlisher Photocopying permitted by licensc only Q 1998 OPA (Overscas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint. Printed in India. RAMAN STUDY OF LOW TEMPERATURE PHASE TRANSITIONS IN THE CUBIC PEROVSKITE CH3NH3PbC13 A. MAAEJ", M. BAHRIb, Y. ABIDa,*, N. JAIDANEb, Z.B. LAKHDAR and A. LAUTIE a Dkpartement de Physique, Facultb des Sciencesde, B.P. 763, Sfax, Tunisia; Laboratoire de Physique rnolbculaire, FacultP des Sciences de Tunis, Tunisia; CNRS, Lasir, 2 Rue Henry Dunant, Thiais, France (Received 21 March 1997; In find form 30 June 1997) Raman spectra of polycrystalline samples of CH3NH3PbC13 were studied at between 80 and 300K. An assignment of most of the observed bands is proposed. The firstorder phase transitions previously detected at 172 and 179 K were characterized: Phase 111 t (I72 K) + Phase 11 t (179 K) + Phase I (P2221) (P4jmmm) (Pm3m) The spectral evolution and the mechanism proposed are in agreement with the theoretical analysis of normal vibrations and with previous X-ray diffraction studies and dielectric measurements. The I ++ I1 transition involves mainly conformational and orientational motions of the organic cation, whereas the antiferroelectric transition at 172K is connected with a complex mechanism involving the freezing of the cation motions coupled with the distortion of the PbC16 octahedra. A systematic measurement of the CH3NH: torsional vibration (at 483cm-') was performed, and the activation energy E, = 10.1 kjmol-' was extracted from the half-width versus temperature curve. Keywords: Perovskite compound; CH3NH3PbC13; SCF-calculation INTRODUCTION Structural phase transitions and electrical properties in cubic perovskite type compounds have gained much interest in recent years. Many studies have been published on substances with formula AMX3, *Corresponding author. 179

4 180 A. MAAEJ et al. such as BaTi03, PbTiO,, SrTi03, KTa03, LiTa03 (Sundram, 1990; Sinclair and West, 1989; Grenier et al., 1992; Balkansky et al., 1975). This family of compounds is characterized by a perovskite cubic structure, built up by a three-dimensional array of MX6 octahedra. The observed phase transitions involve displacive mechanisms associated with various rotations of the relatively rigid MXs octahedra. Extensive investigation has also been made in the new series of perovskite compounds CH3NH3PbX3 (X = C1, Br, I) (Poglitisch and Weber, 1987; Knop et al., 1990; Yamamoura et al., 1990) and CsPbC13 (Hua, 1991; Xu, 1991). All these - compounds - exhibit similar - phase - sequences: 111 (TI) I1 (7.2) I (Orthorhombic) (Tetragonal) (Cubic) As far as we know from all the previous works on these compounds, the methylammonium cations are believed to be responsible for the observed phase transitions. Their conformational and rotational motions were extensively studied and discussed (Poglitisch and Weber, 1987; Knop et al., 1990; Matsuo et al., 1992; Yamamoura et al., 1990), but the participation of PbX6 octahedra was often omitted or neglected. In order to learn more about the transitional behaviour of these compounds and to look for the participation of the PbC16 octahedra we have undertaken a spectroscopic study on CH3NH3PbC13 (MAPbC13 for short). EXPERIMENTAL CH3NH3PbC13 crystals were prepared by mixing CH3NH3Cl and PbClz in HCl solution with stoichiometric ratio, and colourless crystals were obtained by slow evaporation at ambient temperature. Raman spectra were recorded on a Dilor RTI.30 spectrometer equipped with an argon ion laser (514.5 nm). Low-temperature measurements were performed with a liquid nitrogen cryostat and a thermocouple was used to estimate the sample temperature. RESULTS AND DISCUSSION At room temperature (Phase I), CH3NH3PbC13 crystallizes in the cubic perovskite structure (Wasylishen et al., 1985; Poglitisch and

5 RAMAN STUDY OF CH3NH3PbC Weber, 1987) (Fig. 1); the space group is Pm3m, Z= 1. The inorganic sublattice is built up by a three-dimensional array of PbC16 regular octahedra and the methylammonium (MA+) cations (with C3" molecular symmetry) are situated in the cubic cavities between the octahedra. To satisfy the site symmetry these cations have to execute complex rotational and orientational motions and possess almost spherical statistical symmetry. Previous investigations have revealed two low-temperature phase transitions according - to the phase - sequence: TTT(P2221) c (172K).+ II(P4/mmm) (179K) I(Pm3m) The structure of phase I has been determined by single-crystal X-ray diffraction, while the space groups of the low-temperature phases I1 and I11 were determined by X-ray experiments on polycrystalline II I\ I I II I I CsPbCh FIGURE 1 Low-wavenumber Raman spectra of polycrystalline samples of CsPbC13 and CH3NH3PbC13 in their orthorhombic phases.

6 182 A. MAAEJ e/ at samples (Wasylishen et al., 1985; Poglitisch and Weber, 1987; Knop et al., 1990; Matsuo et al., 1992). The structure determination of the low-temperature phase I11 is in progress and some structural information should be mentioned here for the description of the transitional mechanism. In the orthorhombic phase (below 170 K), the preliminary structural determination confirms the P222, space group and shows that the PbC16 octahedra undergo a slight distortion with respect to phase I (rotations around the four-fold axis). Indeed, the PbCl bonds in the (a,b) plane are built with an angle ClPbCl=86" instead of 90". In addition, the organic parts occupy a two-fold disordered position. Structure and Group Theoretical Analysis (a) Phase I: In this structure the MA+ cation and the Pb atom (one of each per unit cell) occupy Oh sites and the three chlorine atoms lie on D4h sites. From group theory analysis the 18 lattice modes including PbC16 octahedra are distributed as: rtranslation = 3Flu + Fzu, racoustic = FI, rma'libration = Fig. Therefore one should notice that in phase I all lattice modes are inactive in Raman scattering. (b) Phase ZZ: Phase I1 is tetragonal with the space group P4/mmm (Dih)Z= 1, the MA+ cations, the Pb atom and one chlorine atom occupy D4h sites, the two remaining chlorine atoms occupy DZh sites. The lattice modes are distributed as follows: The low-wavenumber Raman spectra should include only the librational mode of MA+ cation with E, symmetry. (c) Phase IZI: In phase I11 the space group is P2221 (D:) with Z=2. Factor group analysis predicts 36 Raman active modes, and their symmetry is: All of them are Raman active.

7 RAMAN STUDY OF CH3NHjPbC13 I83 Raman Band Assignments (a) Lattice modes and PbC& frequencies calculation: The bands which appear in the low wavenumber spectra ( cm-') are associated to translational and librational motions of different groups in the unit cell. From the group theory analysis the translational modes are Raman active only in the orthorhombic phase 111. We have, therefore, proposed a tentative assignment below 170K (phase 111). As there are many vibrations in a relatively narrow wavenumber range ( cm-'), the overlapping of lattice modes makes a detailed analysis impossible and leads to a smaller number of external modes than that expected theoretically (Fig. 2). But by comparison with the CsPbC13 (Fig. 1) and other Perovskite compounds (Couzi, Daoud and Perret, 1977) we can tentatively propose an approximate assignment. In order to obtain more reliable assignment and unambigous interpretation of the spectra we have additionally undertaken an SCF semi-empirical calculation of the frequencies of both anionic and cationic vibrations. In this calculation the anionic sublattice and cations have been treated as separate entities. wave lllmlbdm loo0 W-UIdUkI-1 FIGURE 2 Raman spectra of polycrystalline sample of CH3NHjPbCI, at different temperatures: (a) in the low-wavenumber range (5-250 cm-i); (b) in the wavenumber range ( cm-i).

8 184 A. MAAEJ et ul. From the dynamical point of view the anionic vibrator entities is not a separate PbCI3 unit formula, the inorganic sublattice consists on infinite three dimensional array built up from PbC16 cornersharing octahedra. We have therefore calculated the vibrational frequencies of a PbC& octahedra taking into account the interaction with the six neighbouring ones with which is connected. The geometry optimization and the frequencies calculation were carried out using the PM3 method (Stewart, 1990) implemented in MOPAC series of programs (version 6). Initial structural parameters (Pb-CI distances and Cl-Pb-C1 angles) were obtained from X-ray diffraction studies at ambient temperature (cubic structure). All structural parameters are allowed to vary simultaneously and energy minima with respect to the nuclear coordinates were obtained using the method of Newton Raphson. Optimal geometry was obtained with final gradient less than l.iop4. Table I presents the assignments of the observed modes of PbCI6 sublattices in CsPbC13 and CH3NH3PbC13 together with calculated wavenumbers. We also present an approximate description of the modes as combinations of mass weighted Cartesian coordinates given by the out put of the program. We are resonably confident that this assignment is correct, based on three reasons. TABLE I Wavenumbers of the observed Raman bands in CH3NH3PbC13 and CsPbCI3 at 80 K. Assignments and comparison with calculated wavenumbersf CH3NH3PbC13 CSbClA Calculured Ten iu i ive ussignmen t (u/cm-i) (uicm-') (u/cm-') I Q, (PbC16) Rotation Qz (PbC16) sym. deform Q3 G(PbC16) asym. deform Q4 G(PbC16) asym. deform 1 I Qs u4(c1 Pb Cl) asym. deform Qh u2(c1-pb-c1) sym. deform Q7 ul(pb-c1) sym. stretch *(XO, YO, 2,) designate the Cartesian coordinates of the Pb atom. (Xi, Yi. Z,) designate the Cartesian coordinates of the chlorine atom number i. *QI =0.54 YI-0.29 Y Xz+0.46 Xa Q,=O.42 & Z,+O.2 Zz Xg+O.26 X6 Q)=-0.27 Yo+O.22 Y,-O.29 Yi+O.12 Xz+O.12 XI Q4=0.34 Z , X Xb Qs =0.68 & fo.1 Z5-0.1 Xt, Q6z-0.45 XI X Y Yq+O Zt, Q7~0.39 XI f0.33 Y2-0.32X Y Zt,

9 RAMAN STUDY OF CH3NH3PbC (i) The structural parameters (Pb-C1 distances and Cl-Pb-Cl angles) in the optimized geometry are in well agreement with the experimental crystallographic results in both compounds. Indeed, the Cl-Pb-Cl angles are perfectly close to those of cubic structure (90 and 180 ), whereas the error on the Pb-Cl distances is of about 3% which may be due to the fact that we have not taken into account the interaction with the cation situated in the cubic cavities. (ii) The observed and calculated frequencies agree with an average error of about 5% which should be considered very good as the calculation is restricted to the interaction with the first and second neighbouring atoms only. (iii) We can easily identify v2 and v4 modes of PbCli- ion observed respectively at 115 and 21Ocm-' in CsPbCI3 and at 119 and K FIGURE cm-'. Temperature variation of the full width at the half-height of T(MA+) at

10 186 A. MAAEJ et al. 203 cm-' in CH3NH3PbC13. The calculated values are respectively 129 and 233cm-'. The difference between the observed and calculated frequencies is due to the approximations mentioned above. Their assignment is confirmed by the infrared spectra of PbCli- in Nakamoto (1970). The low frequency spectra ( cm-') includes also translational and rotational vibrations of the organic cation as a whole (the so called external modes) we do not intend to give here a detailed assignment, but by comparison with homologous compounds (e.g.; CH3NH3- CdC14) (Couzi et a[., 1977), we can distinguish the translational and the rotational motions. The bands at 145 and 165 cm-' may be assigned to the translation modes whereas the band below 60cm-' (17,'28, 38, 45 and 54cm-I) are associated to the rotational motions. (b) Internal modes of CH3NH; cation: In contrast, the internal mode observed between 400 and 1600cm-l were easily and unambiguously identified and assigned by comparison with the infrared spectra of the title compound and Raman spectra of the homologous compounds (Miniewicz et al., 1994) (Figs. 43). The observed Raman lines assigned to internal and external modes of the organic cation and their assignments are listed in Table 11. Crystal Dynamics and Phase Transitions The spectral evolution shown in Fig. 2 characterizes clearly the two phase transitions. In the limit of temperature accuracy the temperature transitions are close to those found previously by X-ray diffractions and DSC calorimetry (TI = 172 K and 7'2 = 179 K) (Wasylishen et al., 1985; Poglitisch and Weber, 1987). (a) I (Pm3m)+II (P4/mmm) trarzsifion: As shown in Fig. 3, at room temperature the low-wavenumber Raman spectra characterizes a typically cubic disordered phase. In fact in this phase, the MA+ cations (with C3" symmetry) are situated in the cubic cavities between the octahedra (Fig. 1). To satisfy the site symmetry (Oh) these cations have to execute complex rotational and orientational motions around their three-fold axis and around their mass centre (isotropic rotations). Their non-correlated reorientations within the adjacent unit cells mean that the translational symmetry is missing. As a natural

11 RAMAN STUDY OF CH3NH3PbC TABLE I1 External and internal modes of the organic cation observed at selected temperatures and their assignments Phase III Phase II Phase I Teniative (77 K) ujcm-' (175K) vjcm-' (300K) vjcm-' assignment 17 vw 28 vs 38 vw 45 s 54 s 145 w 165 w 483 m 923 s 983 s 1265 w 1420w 1460 sh 1472s 1580sh 1596 s 1604s External mode (R') External mode (R') External mode (R') External mode (R') External mode (R') External mode (T') External mode (T') NH: torsion MA + roclung C-N stretch CH3 stretch and bending 6s NH3 6, NH3 vw = very weak; vs = very strong; m = medium; s = strong and sh = shoulder. consequence of this, the Raman spectrum shows not only the phonons with zero wave vector (k=0) but also phonons from the whole Brillouin zone. This description is in agreement with previous X-ray studies and dielectric measurements (Poglitisch and Weber, 1987) and may explain the feature of the low-frequency Raman spectra in phase I: one weak band is observed about 70cm-' while no mode is expected Raman active. The spectrum of phase I1 has almost the same feature as the one of phase I and characterizes almost the same degree of disorder, although, one notices the appearance of a strong and broad Raman line at 66cm-'. According to group theoretical analysis this line may be associated with the MA+ libration mode, which is a lone lattice mode which becomes active in phase 11. However, other weak bands also appear at about cm-'. On lowering the temperature across the I -+ I1 transition, some internal modes are also affected. One notices, for example, that the Raman line at 923cm-' associated with the rocking of MA+ cation becomes well resolved and increases in intensity. Such behaviour characterizes the beginning of an ordering process.

12 I88 A. MAAEJ et al. (b) I1 (P4/mmm) + I11 ( P2221) transition: This transition is characterized by substantial spectral changes in all the spectral regions studied. Lattice modes: The bands which appear in this region ( cm- ') are associated with translational and librational motions of different groups in the unit cell. This spectrum becomes relatively well resolved, with narrow bands and shows substantial changes. The sudden appearance of 12 lattice modes cannot be explained by a simple temperature effect and ordering process, but it must be related to important changes in the selection rules, due to the break-down of the symmetry and to the distortion of the PbC16 octahedra. Internal modes: The internal modes of MAf cation are also sensitive to this ordering process: all observed Raman lines become well resolved, increase in intensity and characterize an ordered phase. This spectral behaviour may be related to the distortions of the PbCI6 octahedra which generate potential wells for NHT polar heads of the MA+ cations and produce spontaneous polarization along the c-othorhombic axis. This description is in agreement with the previous dielectric measurements and X-ray studies (Poglitisch and Weber, 1987). From the above discussion it clearly appears that the internal modes of MA+ cation play a dominant role in the ordering process across both transitions. In particular, the line assigned to the torsional motion (at 483 cm-') has a remarkable evolution across the transitions: it shows an important broadening (Fig. 3) when increasing the temperature. From a dynamical point of view, such a mode is well known in all the alkylammonium perovskite compounds (Abid et a/., 1990; Abid, 1994; Miniewicz et al., 1994; Sourisseau and Lucazeau, 1979; Prasad ef af., 1987). It is believed to be induced by the oscillation of the cation between equivalent orientations. According to Andrade and Porto (1973), over a broad range of temperature, the dependence of the line width of a non-dispersive phonon associated with an order-disorder mechanism and with a self-diffusive process is a function of correlation time, r,, given by (Andrade and Porto, 1973):

13 RAMAN STUDY OF CH3NH3PbC where w is the phonon frequemy. The time-dependent statistical behaviour of the disorder mechanism is described by a time correlation of the form T~ = TO exp(eo/kt) where Eo is the activation energy corresponding approximately to the height of the potential barrier between equivalent molecular positions governing the disorder mechanism. According to the previous works on the homologous compounds (Abid et al., 1990; Miniewicz et al., 1994; Sourisseau and Lucazeau, 1979) the order of magnitude of the correlation time T~ is of s and one can assume that W ~ >> T 1. ~ By combining the above equations we obtain where C'= C/W~T~. The first term represents the influence of the anharmonicity, whereas the second term characterizes a thermally actived process of a diffusive nature. Using the least-squares method, the experimental curve Av,/~(T(MA+)) (Fig. 3) was fitted by the above equation in the K temperature range. The estimated parameters are: a = 30cm-'; b = 0.09crn-'K-'; and Eo = 10.1 kjmol-'. C' = 2376cm-' The activation energy has the same order of magnitude as those found for homologous compounds (Abid et al., 1990; Sourisseau and Lucazeau, 1979; Prasad et al., 1987). References Abid, Y., M. Kamoun, A. Daoud and F. Romain (1990). Vibrational study of phase transitions in (C3H7NH3)PbC14. J. Rarnun Spectrosc., 21, 709. Abid, Y. (1994). X-ray and Raman studies of re-entrant phase transition in the perovskite-type layer compound bis(n-propy1ammonium)lead tetrabromide. J. Phys.: Condens. Matter, 6, Andrade, P.R. and S.P.S. Porto (1973). Hard core phonon frequency at transition temperature. Solid State Corn., 13, 1249.

14 190 A. MAAEJ et al. Balkansky. M.. R.C.C. Leite, and S.P.S. Porto (1975). Light Scattering in Solids. Flanimarion Sciences, pp Couzi, M.. A. Daoud and P. Perret (1977). Low frequency Raman spectra and phase transitions in (CH3NH3)2MnC14 and in (CH3NH3)2CdC14. Phys. Status. Solidi., A41, Grenier, P., S. Jandl. M. Blouin and L.A. Boatner (1992). Study of ferroelectric microdomains due to oxygen vacancies in KTa03. Ferroelectrics, 137, I. Hua, G.L. (1991). Normal vibration modes and the structural phase transitions in cesium trichloroplumbate Cs PbC13. J. Phys.: Condens. Matter, 3, Knop, V., R.E. Wasylishen, M.A. White. S. Cameron and M.J.M Van Vort (1990). Alkylammonium lead halides. Part 2. CH3NH3 PbX3 (X = CI, Br, I) perovskites: cubohedral cages with isotropic cation reorientation. Can. J. Chem., 68, Miniewicz, A.. R. Jakubus, C. Ecolinet and A. Girard (1994). Raman scattering in ferroelectric (CH3NH3)l BiBr9 crystals. J. Raman Specrrosc., 25, 371. Nakamoto, K. (1970). Infrared Spectra of Inorganic Compounds, 2nd ed., pp , Wiley-Interscience, New York. Poglitisch. A. and D. Weber (1987). Dynamic disorder in methylammonium-trihalogenoplumbate (11) observed by millimeter-wave spectroscopy. J. Chem. Phys. 87, Prasad, S.P.R.. S. Sathaiah and H.D. Bist (1987). Raman spectroscopy study of (CH3NH3)- HgCI4. Chem. Phys. Letters. 5, 341. Sinclair, D.C. and A.R. West (1989). Electrical properties of LiTaO3 single crystal. Phy. Rev. B Sourisseau. C. and G. Lucazeau (1979). Vibrational study of phase transitions in NH3-(CH2)3NH3 MnCI4. J. Raman Spectrosc., 8, Stewart, J.J.P. (1990). A semi-empirical molecular orbital program. J. Comp. aided molecular design., Vol. 4. Sundram, S.K. (1990). Complex-plane impedance analysis and dielectric properties of BaTi03 based surface-layer capacitors. Ferroelectrics, 102, Xu. Y. (1991). Ferroelectric Materials and their Application, pp , North Holland. Amsterdam. London, New York, Tokyo. Yamamuro, N.O., 0. Yamamuro. T. Matsuo and H. Suga (1992). P.T. phase relations of CH3NH3PbX3 (X = CI. Br. I) crystals. J. Phys Chem. Solids, 53, 277. Yamamuro. O., N.O. Yamamuro, H. Suga, T. Kamiyama, T.R. Ishigaki and H. Asano ( 1994). Neutron diffraction and calorimetric studies of methylammonium chloride. J. Phys. Chem. Solids

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