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Dilatometric, dielectric and NMR studies of structural phase transitions of the (CH3NH3)3Bi2Cl9 (MACB) crystals
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 385 (1996) 145-151
Dilatometric, dielectric and NMR studies of structural phase transitions of the (CH3NH3)3Bi2C19(MACB) crystals R. Jakubas a'*, G. Bator a, W. Medycki h, N. Piglewski b, R. Decressain c, J. Lefebvre c, P. Franqois d alnstitute of Chemistry, University of Wroctaw, F. Joliot-Curie 14, PL-50383 Wroctaw, Poland blnstitute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17/19, 60 179 Poznah, Poland CLaboratoire de Dynamique et Structures des Mat&iaux Molbculaires (ULA. No 801) UFR de Physique, Universit~ de Lille 1, F-59655 Villeneuve d'Ascq C~dex, France dLaboratoire de Structures et Proprietes de l'Etat Solide, (U.A. No. 234), UFR de Physique, Universite de Lille L 59 655 Villeneuve d'Ascq Cedex, France
Received 14 July 1996; accepted 6 August 1996
Abstract
The phase transitions and molecular motions of the methylammonium cations were investigated in the (CH3NH3)3Bi2CI 9 (MACB) crystal by dilatometric and dielectric measurements, and by the measurements of the ~H spin-lattice relaxation times and second moment of the 1H NMR absorptions over a wide temperature range. Structural phase transitions, weakly first order at 247 K (III *---,II) and continuous at 352 K (II 4--*I), were detected by the dilatometric technique. The IH NMR measurements revealed the presence of the uniaxial reorientations of the three non-equivalent methylammonium cations in the lowest temperature phase (III). Keywords: NMR spectroscopy; Dilatometric measurements; Dielectric measurements; Phase transitions; MACB crystal
I. Introduction
Compounds of general formula [NH4_n(CH3)n]3M2X9 (where M = Sb, Bi; X = CI, Br, I; n = 1-4) have been the subject of many investigations due to their interesting physical properties. It was postulated that the unusual electric properties of the crystal were governed by the dynamics of the alkylammonium cations [1-3]. Some of the crystals, particularly those with small alkylammonium cations, reveal the ferroelectricity, e.g. (CH3NH3)3M2Br9 [4], [(CH3)2NHe)]3Sb2X9 [5] and [(CH3)3NH]3Sb2C19 * Corresponding author.
[6]. It has turned out that cationic dynamics of these compounds is affected by the type of anionic structure. From the methylammonium subgroup, (CH3NH3)3M2X9, the chlorine derivatives form onedimensional double chains of polyanions [7], the bromine analogs are characterized by two-dimensional layers of polyanions [8] whereas in the case of iodine crystals simple double face-sharing octahedra are present in the lattice [9]. The iodine analogs reveal at low temperatures antiferro- or antiferrielectric order [10], the bromine derivatives reveal ferroelectric order [4], whereas in the case of chlorine derivatives no longrange order is found [11,12]. The methylammonium salts are characterized by a presence of the high
0022-2860/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PI1 S0022-2860(96)09437-9
146
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
temperature, "plastic-like" phases. In these phases the cations undergo isotropic rotation on decreasing temperature, the motion is consecutively frozen at the phase transition temperatures. Two main representatives of this subgroup, i.e. (CH3NH3)3Sb2C19 (MACA) [11] and (CH3NH3)3Bi2C19(MACB) [12], appear to be isomorphous at room temperature and crystallize in the orthorhombic system, space group Pmna, Z = 4. MACA exhibits a structural phase transition of first order type at 208 K. It was shown that the transition is connected with the rapid freezing of two of three non-equivalent methylammonium cations. The bismuth analog, MACB, displays a more complex sequence of phase transitions: at 247 K, first order, and at about 349 K, probably continuous transition without any change in the symmetry (isomorphous phase transition). The low temperature phase transition in MACB has been detected by nuclear quadrupole resonance (NQR) studies of Ishihara et al. [3] and differential scanning calorimetry (DSC) measurements of Belkyal et al. [13]. The high temperature transition (349 K) studied by dielectric [12] and DSC [13] techniques was interpreted in terms of a diffused phase transition. A sensitive indicator of structural phase transitions appears to be a linear thermal expansion. The shapes of the thermal anomalies can also give insight into the nature of the structural phase transitions in crystals. We have used this technique to measure the relative thermal expansion along all three crystallographic axes of MACB in the temperature range where the phase transitions are postulated. We refer also to evidence of the low temperature transition at 247 K, re-examining previously reported dielectric measurements [12,13]. The aim of this paper is also to clarify the dynamics of methylammonium cations in the MACB crystal and to obtain information about the mechanism of the phase transitions using the results of the measurements of the second moment (M2) of 1H-NMR absorption and of the tH spin-lattice relaxation time (TI) over a wide temperature range.
2. Experimental The (CH3NH3)3Bi2C19 crystals were obtained in a reaction of the stoichiometric amounts of (BiO)2CO3
and (CH3NH3)CI with excess of HC1. Single crystals were grown from an aqueous solution at constant room temperature. Linear thermal expansion was measured using a thermomechanical analyzer, Perkin-Elmer TMS-2. The single crystals used in the measurements were prepared in the form of thin plates (5 x 5 x 2 mm3). The values of anomalies near the phase transition temperature were reproducible to 10% in sample dimensions. The accuracy of thermal expansion determination was about 2%. Differential scanning calorimetry (DSC) measurements were performed using a Perkin-Elmer. DSC-7 calorimeter with a heating/cooling rate of 10 K min -1. For dielectric measurements samples of dimensions 10 x 5 x 1 mm 3 and 4 x 3 x 1 mm 3 for the low and high frequency region, respectively, were cut perpendicularly to the a crystallographic axis of the MACB crystal. The plates were silver painted. The complex electric permittivity, e -- e' - ie", was measured using an HP 4284A Precision LCR meter in the frequency range 1 kHz-1 MHz and an HP 4191A RF Impedance Analyzer in the frequency range 3 0 900 MHz. The measurements were performed in the temperature range 140-400 K. The temperature was changed with the rate of 0.1 K min -1 in the vicinity of Tc and 0.5 K elsewhere in the case of the measurements in the low frequency region. For the microwave frequency region the temperature was stabilised and controlled by a UNIPAN Temperature Controller type 650 with fluctuation less than -+ 0.1 K. The overall error for the real part of complex electric permittivity, e', in the microwave frequency region was less than 5%. The overall error for the imaginary part of complex electric permittivity, e", was less than 7%. The NMR measurements were carried out in the high temperature region (110-400 K) at 100 MHz on the BRUCKER CXP 100 spectrometer. In the low temperature region (20-140 K) experiments were also performed on the BRUCKER SXP 4/100 spectrometer operating at 55.2 MHz. Proton T1 relaxation times were measured by employing the conventional (90 °, r, 90°) pulse sequence for time shorter than 1 s and by the saturation sequence of fifteen 90 ° pulses followed by the variable time interval ~"and an inspecting 90° pulse. The proton second moment M2 was estimated from
147
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
the shape dependence of the FID signal obtained by a solid echo pulse sequence [(90)x, r, (90)y], FID [14]. A value of 7" = 15 #s was used in order to avoid pole dead time problem and a Gaussian lineshape was assumed. The errors in Tt determination were estimated to be about 5%, those in M2 approximately 15%.
3. Results and discussion 3.1. Dilatometric measurements
The temperature dependencies of the linear thermal expansion (AL/L) measured along the a, b and c directions in the vicinity of the low temperature phase transition are shown in Fig. 1. Phase transition anomaly is clearly seen near 247 K along the a direction (notation of the directions was taken from Ref. [12]). For the c direction close to the 247 K only a change in the linear part of the AL/L(T) curve is noticed whereas
for the b direction no anomaly is visible. The change in the linear thermal expansion, AL a/L a, near the phase transition is of the order of + 0.2 × 10 -3. Neglecting the contribution of the remaining two axes to the resultant change in the transition volume (AV/V = 0.2 x 10 -3) it is expected that a pressure coefficient is positive (using the Clausius-Clapeyron relation dTc/dp = AV/AS) and relatively small in value. The presented dilatometric measurements suggest that the transition at 247 K is weakly first order and corresponds to the NQR [3] and DSC [13] anomaly earlier found at the same temperature. We should also notice that the thermal expansion coefficient, ac, changes its sign at about 265 K and becomes negative in the high temperature region. The temperature dependencies of dilatation of the MACB crystal along the a, b and c directions on heating around the high temperature phase transition are illustrated in Fig. 2. Both below and above room temperature a significant differences in the linear thermal behaviour of the corresponding axes can be noted. 10
0.5
00 0
--05
-10 T¢2
T
-1 L 210
Tel
220
230
240
250
-12 260
270
T Irl Fig. 1. The linear thermal expansion of the (CH3NH3)3Bi2CI 9 crystals along the a, b and c axes in the vicinity of the low temperature phase transition (To ~ 247 K) (by thermomechanicai analyzer).
-15
.
320
.
.
340
.
360
380
400
T0~ Fig. 2. The linear thermal expansion of (CH3NH3)3Bi2CI9crystals along the a, b and c axes in the vicinity of the high temperature phase transition (To = 352 K) (by thermomechanical analyzer).
148
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
18
17
.;;
,,' •
•
•
;e
•o ; ;
;e
,"
,
,
16
15
180
• 5 kHz
,"
~*
.50kHz
,"
T~
*100kHz 800kHz
,
i
,
,
200
,
L
,
,
,
220
i
,
,
,
J
,
,
,
260
240
280
T [K]
Fig. 3. Temperature dependence of the electric permittivity measured, ~', along the a axis, on cooling, in the frequency range 5 kHz-800 kHz for (CH3NH3)3Bi2C19. that this transition is very subtle, continuous and exhibits also the features of a diffused transition. Since the phase transition from phase I to phase II is postulated to appear without any change in the spacegroup symmetry (isomorphous phase transition) such a thermal anomaly in dilation is expected.
The thermal expansion coefficient, or, is negative only for the c direction between 265 K and about 375 K. The anomaly visible at 352 K ( ___ 2 K) for the c direction is continuous in temperature. This is in a very good agreement with previous dielectric measurements suggesting the continuous nature of this transition [12,13]. From the earlier X-ray temperature studies it has been reported by Belkyal et al. [13] that the GuinierLanne photographs between room temperature and 423 K showed a sharp continuous shift of diffraction lines at about 349 K. Our dilatometric studies reveal
3.2. D i e l e c t r i c m e a s u r e m e n t s
Previously reported by us [12] and Couzi's group [13] dielectric results obtained at the frequency i kHz did not reveal any visible anomaly in the vicinity of
26 ~:t a
22
tftttt Ittt
18
• 100
MHz
• 900
MHz
14 Tot
IH
T
H
lO 140
180
220
260
300
340
380
T [K]
Fig. 4. Temperature dependenceof the electric permittivity measured along the a axis, on cooling, at the frequencies 100 MHz and 900 MHz for (CHaNH3)3Bi2CI9.
149
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
the structural phase transition at Tc -- 247 K. Very careful dielectric re-examination for the a direction using a high precision LCR meter (HP 4284A) allowed us, however, to confirm this structural anomaly. The dielectric permittivity as a function of temperature in the frequency range 5 kHz-800 kHz, on cooling, is shown in Fig. 3. Crossing the 247 K phase transition point a small but clearly visible dip on the c'(T, o~) curves is found. Because no relaxation process is visible in the low frequency region below 1 MHz we decided to extend our studies up to a frequency of 900 MHz. The real part of the complex electric permittivity in the microwave frequency region between 140 K and 400 K is presented in Fig. 4. It is seen that regretfully no dielectric dispersion is found both in the high (To = 352 K) and the low temperature region (To = 247 K) up to the frequency 900 MHz. These dielectric measurements pointed out that the most apparently reorientations of the C - N bond of the methylammonium cation, to be identified with the orientational disorder in MACB crystal, are very fast and described by the relaxation times shorter than 10 -10 S. Recently, a similar dielectric response has been found in the microwave frequency region in mixed crystals, (CH3NH3)3Sb2(I_x)Bi2xCI9 (x = 0.22) [15], which are isomorphous to the title compound. One can state that the (CH3NH3)3M2CI9 crystals are characterized by the rapid reorientations of the methylammonium cations contributing to the electric polarizability. 3.3. 1H N M R studies
The experimental proton NMR second moment is presented as a function of temperature in Fig. 5. Below 150 K an almost constant M2 value of the order of 5.6 x 10 -8 T z is obtained. Then the second moment starts to decrease reaching in the vicinity of 240 K the value of 3.2 x 10 -8 T 2 (just below Tc2) which is then maintained approximately constant up to 290 K. Above this temperature a decrease to 1 x 10 -8 T 2 is observed on passing through the phase transition temperature Tel. The M2 is constant up to 390 K. The identification of the motional process responsible for the spectral narrowing is usually accomplished by comparing the observed value of the second moment plateau with those theoretically predicted for various possible motions. The precise
" • A A A
5
A• • •
• ••
~3 2 T¢2
Tel
!
L
0 50
150
I
i
i
i
T 250
J
i
i
i
350
TIKI Fig. 5. Temperaturedependenceof the secondmomentof the proton resonance lines, M2, for (CH3NH3)3Bi2CI9. calculation of M 2 can, therefore, be performed only if the data on crystal structure of the studied compound are available. Although no crystallographic data in low temperature phases have so far been reported for (CH3NH3)3Bi2C19, satisfactory analysis can be made using the M2 data on related molecules. Then by analogy with other methylammonium ions the theoretical values of M2 were assumed to be 30 x 10 -8, 19 x 10 -8 and 8 x 10 -8 T 2 for the rigid cation, for the C3 rotation of the CH3 groups with rigid NH] ones and for both groups reorienting around the C - N bond axis, respectively [16]. The value of the M2 plateau below 8 x 10 -8 T 2 found in MACB even in the low temperature region (for T < 130 K) means that there is another possible mechanism, which causes such narrowing of the 1H NMR line. This mechanism is probably due to the rotational tunnelling of the three-proton group of the monomethylammonium cations. Such a process is undoubtedly expected to appear in the helium temperature range whereas another mechanism like a precessional motion of the C - N bond axis is preferred at higher temperatures. When the tunneling process takes place the contribution of the small satellite 1H NMR line (lying far away from the central line) to the value of the second moment has been probably missed. The further detailed study of deuterated title compound should give an answer to this question.
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R. Jakubas et aL/Journal of Molecular Structure 385 (1996) 145-151
The second plateau of 3.2 x 10 -8 T 2 c a n be interpreted in terms of some intermediate modes between the overall and C3 rotation of the cation as it has already been reported for similar compounds [17,18]. The assumption that two cations undergo overall rotation and only one the C3 rotation gives a theoretical M2 value of 3 x 10 -8 T 2 which is in good agreement with the experimental value. This means that the I I I - I I phase transition is associated with a release of one cation becoming completely disordered in phase II. On going to phase I the M2 decrease from 3.1 to 1 x 10 -8 T 2 can be interpreted as a result of a new motion the frequency of which is high enough to narrow the resonance line. Since the value of M2 = 0.4 x 10 -8 T 2, corresponding to a complete overall rotation of the cations, is not reached at 370 K this allows us to assume that the release of the cations rotational motion is progressive. 3.3.1. Spin-lattice relaxation The temperature dependence of T1 in (CH3NHa)3Bi2C19 at 55.2 MHz and 100 MHz is '
\
1 T1
~11 i-1 nT{
l=dz 9y4h2( ri 4ri T~ o 20 r 6 \ 1 + ¢02~ + 1 + 4O~o2~J
(1)
(2)
where index i represents the different CH3NH~ cations from the view-point of uniaxial rotation, •, rl and OJo are the gyromaguetic ratio of a proton, the correlation time of this motion and the resonance frequency, respectively, and r is the distance between the methyl (or ammonium) protons. The parameter do = 1 for the classical limit and do < 1 for tunneling rotation. Assuming an Arrhenius-type relationship between ri and the activation energy Eai of the motional process one obtains:
101
Z
shown in Fig. 6. From T1 data observed at 55.2 MHz three minima could be distinguished at 35, 39 and 50 K. Upon increasing the temperature another minimum is observed at 85 K. In the high temperature range of phase III, T1 is almost independent of the spectrometer frequency. In phase I, the T1 value decreases rapidly on increasing the temperature; this probably indicates the onset of self-diffusion of the cations. To interpret the T1 data observed in phase III we assumed that the T1 dependence is explainable in terms of the summation of the BPP-type equation [19]. From crystallographic experiments on MACB it results that in the lattice of this crystal one can distinguish three nonequivalent methylammonium cations. That is why it was assumed the uniaxial rotation of the CH3NH~ cations as a whole about the molecular axis for all three observed minima of Tt was connected with three cations. In this case T11 is described by the following relation:
10 0
10-1
ri = roi exp(Eai/kT)
10 2
.......
i ...... 10
, . ,
......... 20
, ......... 3O
, ........ 40
50
1000/T [K"t]
Fig. 6. Temperaturedependenceof the protonspin-latticerelaxation between 400 K and 20 K for (CH3NH3)3Bi2CIg: I , 55.2 MHz; T, 100 MHz.The fittingcurvesof the protonspin-lattice relaxation time T1obtainedaccordingto the Eq. (1) are presentedas solid lines.
times T1
where roi is the correlation time at the limit of infinite temperature. We performed a fitting calculation of the TI data below 80 K using n = 3. The obtained motional parameters are listed in Table 1 and the best fitted curve using these parameters is given by a solid line in Fig. 6. Upon increasing temperature another minimum is observed at 86 K. A similar minimum has already
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
Table 1 Activation energies and correlation times for three non-equivalent methylammonium cations (A, B and C type) of (CH3NH3)3Bi2C19 r (K)
CH3NH ~ cation
E, (kJ mole -1)
ro (s)
T < 85 K
A B C C - N axis
2.02 -+ 0.024 2.46 -+ 0.03 3.6 -+ 0.014 8.32 +_ 0.02
0.105 x 10 -11 0.102 x 10 -11 0.51 x 10 -x2 0.162 x 10 -13
T > 85 K
been observed in the same temperature range for (CH3NH3)3Sb219 [20]. From the analysis we have found that one cation should perform an additional motion (C3 rotation), i.e. a motion of the C - N axis (uncorrelated reorientation of the CH3 and NH~ groups). Thus we can assign the minimum at 86 K to this motion and the high temperature data were fitted with Eqs. (1) and (2) using n -- 1. The activation energy Ea and correlation time 70 are equal to 8.82 kJ mo1-1 and 1.2 x 10 -12 s, respectively. These values are characteristic for this type of motion [21]. There exists, however, another explanation (less probable) that this minimum may be connected with an additional structural phase transition in the MACB crystal.
4. Conclusions The shape of the anomalies in the linear thermal expansion curves indicates that (CH3NH3)3Bi2CI9 undergoes a weakly first-order phase transition at 247 K and continuous transition at about 352 K, exhibiting the features of diffused transition. Dielectric dispersion studies performed between 140 and 400 K in the frequency range 1 kHz-900 MHz, did not reveal any relaxation process. This indicates that the methylammonium cations, which are believed to contribute to the mechanism of the phase transition in MACB crystal, perform very rapid reorientational motion with the macroscopic relaxation time shorter than 1 x 10 -1° to 1 x 10 -11 S. The low temperature T1 data at 55 MHz revealing well-separated minima were interpreted in terms of
151
uniaxial rotation of methylammonium cations. Around the phase transition at 247 K and 352 K no anomaly in the Tt measurements was detected. On the other hand the phase transition at 352 K is accompanied by a visible continuous increase in the second moment value by about 1 x 10 -8 T 2. The 1H NMR studies indicate that the changes in the motional state of the methylammonium cations at the phase transitions are rather subtle.
References [1] R. Jakubas and L. Sobczyk, Phase Trans., 20 (1990) 163. [2] V. Vama, R. Bhattarcharjee, H.N. Vasan and C.N.R Rao, Spectrochim. Acta, 48A (1992) 1631. [3] H. Ishihara, K. Watanabe, A. lwata, K. Yamada, Y. Kinoshita, T. Okuda, V.G. Krishnan, S. Dou and A. Weiss, Z. Natufforsch., 47a (1992) 65. [4] R. Jakubas, U. Krzewska, G. Bator and L. Sobczyk, Ferroelectrics, 77 (1988) 129. [5] R. Jakubas, L. Sobczyk and J. Matuszewski, Ferroelectrics, 80 (1987) 339. [6] R. Jakubas, Z. Czapla, Z. Galewski and L. Sobczyk, Ferroelectrics Lett., 5 (1988) 69. [7] K. Kihara and T. Sudo, Acta Cryst., B30 (1974) 1088. [8] F. Lazarini, Acta Crystallogr., B33 (1977) 2961. [9] B. Chabot and E. Parthe, Acta Crystallogr., B34 (1978) 645. [10] J. Zaleski, R. Jakubas, L. Sobczyk and J. Mr6z, Ferroelectrics, 103 (1990) 83. [11] R. Jakubas, Z. Czapla, Z. Galewski, L. Sobczyk, O.J. Zogal and T. Lis, Phys. Status Solidi (a), 93 (1986) 449. [12] R. Jakubas, P. Tomaszewski and L. Sobczyk, Phys. Status Solidi (a), 111 (1989) K27. [13] I. Belkyal, R. Mokhlisse, B. Tanouti, N.B. Chanh and M. Couzi, J. Alloys Compounds, 188 (1992) 186. [14] J.G. Powles, J.H. Strange, Proc. Phys. Sot., 82 (1963) 5. [15] R. Jakubas, G. Bator, J. Zaleski, A. Pietraszko and R. Decressain, J. Phys. Condens. Matter, 8 (1995) 367. [16] E.R. Andrew and P.C. Canepa, J. Magn. Reson., 7 (1972) 429. [17] R. Decressain, E. Cochon. J. Lefebvre, B. Meurer and R. Jakubas, J. Phys. Chem. Sol., 55 (1994) 139. [18] R. Decressain, R. Jakubas and J. Lefebvre, Phys. Status Solidi (a), 147 (1995) K73. [19] N.E. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev., 73 (1948) 676. [20] P. Koziol, Y. Furukawa. D. Nakamura and R. Jakubas, Bull. Chem. Soc. Jpn., 65 (1992) 1707. [21] R. Ikeda, Y. Kume, D. Nakamura, Y. Furukawa and H. Kiriyama, J. Magn. Reson., 24 (1976) 9.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 385 (1996) 145-151
Dilatometric, dielectric and NMR studies of structural phase transitions of the (CH3NH3)3Bi2C19(MACB) crystals R. Jakubas a'*, G. Bator a, W. Medycki h, N. Piglewski b, R. Decressain c, J. Lefebvre c, P. Franqois d alnstitute of Chemistry, University of Wroctaw, F. Joliot-Curie 14, PL-50383 Wroctaw, Poland blnstitute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17/19, 60 179 Poznah, Poland CLaboratoire de Dynamique et Structures des Mat&iaux Molbculaires (ULA. No 801) UFR de Physique, Universit~ de Lille 1, F-59655 Villeneuve d'Ascq C~dex, France dLaboratoire de Structures et Proprietes de l'Etat Solide, (U.A. No. 234), UFR de Physique, Universite de Lille L 59 655 Villeneuve d'Ascq Cedex, France
Received 14 July 1996; accepted 6 August 1996
Abstract
The phase transitions and molecular motions of the methylammonium cations were investigated in the (CH3NH3)3Bi2CI 9 (MACB) crystal by dilatometric and dielectric measurements, and by the measurements of the ~H spin-lattice relaxation times and second moment of the 1H NMR absorptions over a wide temperature range. Structural phase transitions, weakly first order at 247 K (III *---,II) and continuous at 352 K (II 4--*I), were detected by the dilatometric technique. The IH NMR measurements revealed the presence of the uniaxial reorientations of the three non-equivalent methylammonium cations in the lowest temperature phase (III). Keywords: NMR spectroscopy; Dilatometric measurements; Dielectric measurements; Phase transitions; MACB crystal
I. Introduction
Compounds of general formula [NH4_n(CH3)n]3M2X9 (where M = Sb, Bi; X = CI, Br, I; n = 1-4) have been the subject of many investigations due to their interesting physical properties. It was postulated that the unusual electric properties of the crystal were governed by the dynamics of the alkylammonium cations [1-3]. Some of the crystals, particularly those with small alkylammonium cations, reveal the ferroelectricity, e.g. (CH3NH3)3M2Br9 [4], [(CH3)2NHe)]3Sb2X9 [5] and [(CH3)3NH]3Sb2C19 * Corresponding author.
[6]. It has turned out that cationic dynamics of these compounds is affected by the type of anionic structure. From the methylammonium subgroup, (CH3NH3)3M2X9, the chlorine derivatives form onedimensional double chains of polyanions [7], the bromine analogs are characterized by two-dimensional layers of polyanions [8] whereas in the case of iodine crystals simple double face-sharing octahedra are present in the lattice [9]. The iodine analogs reveal at low temperatures antiferro- or antiferrielectric order [10], the bromine derivatives reveal ferroelectric order [4], whereas in the case of chlorine derivatives no longrange order is found [11,12]. The methylammonium salts are characterized by a presence of the high
0022-2860/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PI1 S0022-2860(96)09437-9
146
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
temperature, "plastic-like" phases. In these phases the cations undergo isotropic rotation on decreasing temperature, the motion is consecutively frozen at the phase transition temperatures. Two main representatives of this subgroup, i.e. (CH3NH3)3Sb2C19 (MACA) [11] and (CH3NH3)3Bi2C19(MACB) [12], appear to be isomorphous at room temperature and crystallize in the orthorhombic system, space group Pmna, Z = 4. MACA exhibits a structural phase transition of first order type at 208 K. It was shown that the transition is connected with the rapid freezing of two of three non-equivalent methylammonium cations. The bismuth analog, MACB, displays a more complex sequence of phase transitions: at 247 K, first order, and at about 349 K, probably continuous transition without any change in the symmetry (isomorphous phase transition). The low temperature phase transition in MACB has been detected by nuclear quadrupole resonance (NQR) studies of Ishihara et al. [3] and differential scanning calorimetry (DSC) measurements of Belkyal et al. [13]. The high temperature transition (349 K) studied by dielectric [12] and DSC [13] techniques was interpreted in terms of a diffused phase transition. A sensitive indicator of structural phase transitions appears to be a linear thermal expansion. The shapes of the thermal anomalies can also give insight into the nature of the structural phase transitions in crystals. We have used this technique to measure the relative thermal expansion along all three crystallographic axes of MACB in the temperature range where the phase transitions are postulated. We refer also to evidence of the low temperature transition at 247 K, re-examining previously reported dielectric measurements [12,13]. The aim of this paper is also to clarify the dynamics of methylammonium cations in the MACB crystal and to obtain information about the mechanism of the phase transitions using the results of the measurements of the second moment (M2) of 1H-NMR absorption and of the tH spin-lattice relaxation time (TI) over a wide temperature range.
2. Experimental The (CH3NH3)3Bi2C19 crystals were obtained in a reaction of the stoichiometric amounts of (BiO)2CO3
and (CH3NH3)CI with excess of HC1. Single crystals were grown from an aqueous solution at constant room temperature. Linear thermal expansion was measured using a thermomechanical analyzer, Perkin-Elmer TMS-2. The single crystals used in the measurements were prepared in the form of thin plates (5 x 5 x 2 mm3). The values of anomalies near the phase transition temperature were reproducible to 10% in sample dimensions. The accuracy of thermal expansion determination was about 2%. Differential scanning calorimetry (DSC) measurements were performed using a Perkin-Elmer. DSC-7 calorimeter with a heating/cooling rate of 10 K min -1. For dielectric measurements samples of dimensions 10 x 5 x 1 mm 3 and 4 x 3 x 1 mm 3 for the low and high frequency region, respectively, were cut perpendicularly to the a crystallographic axis of the MACB crystal. The plates were silver painted. The complex electric permittivity, e -- e' - ie", was measured using an HP 4284A Precision LCR meter in the frequency range 1 kHz-1 MHz and an HP 4191A RF Impedance Analyzer in the frequency range 3 0 900 MHz. The measurements were performed in the temperature range 140-400 K. The temperature was changed with the rate of 0.1 K min -1 in the vicinity of Tc and 0.5 K elsewhere in the case of the measurements in the low frequency region. For the microwave frequency region the temperature was stabilised and controlled by a UNIPAN Temperature Controller type 650 with fluctuation less than -+ 0.1 K. The overall error for the real part of complex electric permittivity, e', in the microwave frequency region was less than 5%. The overall error for the imaginary part of complex electric permittivity, e", was less than 7%. The NMR measurements were carried out in the high temperature region (110-400 K) at 100 MHz on the BRUCKER CXP 100 spectrometer. In the low temperature region (20-140 K) experiments were also performed on the BRUCKER SXP 4/100 spectrometer operating at 55.2 MHz. Proton T1 relaxation times were measured by employing the conventional (90 °, r, 90°) pulse sequence for time shorter than 1 s and by the saturation sequence of fifteen 90 ° pulses followed by the variable time interval ~"and an inspecting 90° pulse. The proton second moment M2 was estimated from
147
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
the shape dependence of the FID signal obtained by a solid echo pulse sequence [(90)x, r, (90)y], FID [14]. A value of 7" = 15 #s was used in order to avoid pole dead time problem and a Gaussian lineshape was assumed. The errors in Tt determination were estimated to be about 5%, those in M2 approximately 15%.
3. Results and discussion 3.1. Dilatometric measurements
The temperature dependencies of the linear thermal expansion (AL/L) measured along the a, b and c directions in the vicinity of the low temperature phase transition are shown in Fig. 1. Phase transition anomaly is clearly seen near 247 K along the a direction (notation of the directions was taken from Ref. [12]). For the c direction close to the 247 K only a change in the linear part of the AL/L(T) curve is noticed whereas
for the b direction no anomaly is visible. The change in the linear thermal expansion, AL a/L a, near the phase transition is of the order of + 0.2 × 10 -3. Neglecting the contribution of the remaining two axes to the resultant change in the transition volume (AV/V = 0.2 x 10 -3) it is expected that a pressure coefficient is positive (using the Clausius-Clapeyron relation dTc/dp = AV/AS) and relatively small in value. The presented dilatometric measurements suggest that the transition at 247 K is weakly first order and corresponds to the NQR [3] and DSC [13] anomaly earlier found at the same temperature. We should also notice that the thermal expansion coefficient, ac, changes its sign at about 265 K and becomes negative in the high temperature region. The temperature dependencies of dilatation of the MACB crystal along the a, b and c directions on heating around the high temperature phase transition are illustrated in Fig. 2. Both below and above room temperature a significant differences in the linear thermal behaviour of the corresponding axes can be noted. 10
0.5
00 0
--05
-10 T¢2
T
-1 L 210
Tel
220
230
240
250
-12 260
270
T Irl Fig. 1. The linear thermal expansion of the (CH3NH3)3Bi2CI 9 crystals along the a, b and c axes in the vicinity of the low temperature phase transition (To ~ 247 K) (by thermomechanicai analyzer).
-15
.
320
.
.
340
.
360
380
400
T0~ Fig. 2. The linear thermal expansion of (CH3NH3)3Bi2CI9crystals along the a, b and c axes in the vicinity of the high temperature phase transition (To = 352 K) (by thermomechanical analyzer).
148
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
18
17
.;;
,,' •
•
•
;e
•o ; ;
;e
,"
,
,
16
15
180
• 5 kHz
,"
~*
.50kHz
,"
T~
*100kHz 800kHz
,
i
,
,
200
,
L
,
,
,
220
i
,
,
,
J
,
,
,
260
240
280
T [K]
Fig. 3. Temperature dependence of the electric permittivity measured, ~', along the a axis, on cooling, in the frequency range 5 kHz-800 kHz for (CH3NH3)3Bi2C19. that this transition is very subtle, continuous and exhibits also the features of a diffused transition. Since the phase transition from phase I to phase II is postulated to appear without any change in the spacegroup symmetry (isomorphous phase transition) such a thermal anomaly in dilation is expected.
The thermal expansion coefficient, or, is negative only for the c direction between 265 K and about 375 K. The anomaly visible at 352 K ( ___ 2 K) for the c direction is continuous in temperature. This is in a very good agreement with previous dielectric measurements suggesting the continuous nature of this transition [12,13]. From the earlier X-ray temperature studies it has been reported by Belkyal et al. [13] that the GuinierLanne photographs between room temperature and 423 K showed a sharp continuous shift of diffraction lines at about 349 K. Our dilatometric studies reveal
3.2. D i e l e c t r i c m e a s u r e m e n t s
Previously reported by us [12] and Couzi's group [13] dielectric results obtained at the frequency i kHz did not reveal any visible anomaly in the vicinity of
26 ~:t a
22
tftttt Ittt
18
• 100
MHz
• 900
MHz
14 Tot
IH
T
H
lO 140
180
220
260
300
340
380
T [K]
Fig. 4. Temperature dependenceof the electric permittivity measured along the a axis, on cooling, at the frequencies 100 MHz and 900 MHz for (CHaNH3)3Bi2CI9.
149
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
the structural phase transition at Tc -- 247 K. Very careful dielectric re-examination for the a direction using a high precision LCR meter (HP 4284A) allowed us, however, to confirm this structural anomaly. The dielectric permittivity as a function of temperature in the frequency range 5 kHz-800 kHz, on cooling, is shown in Fig. 3. Crossing the 247 K phase transition point a small but clearly visible dip on the c'(T, o~) curves is found. Because no relaxation process is visible in the low frequency region below 1 MHz we decided to extend our studies up to a frequency of 900 MHz. The real part of the complex electric permittivity in the microwave frequency region between 140 K and 400 K is presented in Fig. 4. It is seen that regretfully no dielectric dispersion is found both in the high (To = 352 K) and the low temperature region (To = 247 K) up to the frequency 900 MHz. These dielectric measurements pointed out that the most apparently reorientations of the C - N bond of the methylammonium cation, to be identified with the orientational disorder in MACB crystal, are very fast and described by the relaxation times shorter than 10 -10 S. Recently, a similar dielectric response has been found in the microwave frequency region in mixed crystals, (CH3NH3)3Sb2(I_x)Bi2xCI9 (x = 0.22) [15], which are isomorphous to the title compound. One can state that the (CH3NH3)3M2CI9 crystals are characterized by the rapid reorientations of the methylammonium cations contributing to the electric polarizability. 3.3. 1H N M R studies
The experimental proton NMR second moment is presented as a function of temperature in Fig. 5. Below 150 K an almost constant M2 value of the order of 5.6 x 10 -8 T z is obtained. Then the second moment starts to decrease reaching in the vicinity of 240 K the value of 3.2 x 10 -8 T 2 (just below Tc2) which is then maintained approximately constant up to 290 K. Above this temperature a decrease to 1 x 10 -8 T 2 is observed on passing through the phase transition temperature Tel. The M2 is constant up to 390 K. The identification of the motional process responsible for the spectral narrowing is usually accomplished by comparing the observed value of the second moment plateau with those theoretically predicted for various possible motions. The precise
" • A A A
5
A• • •
• ••
~3 2 T¢2
Tel
!
L
0 50
150
I
i
i
i
T 250
J
i
i
i
350
TIKI Fig. 5. Temperaturedependenceof the secondmomentof the proton resonance lines, M2, for (CH3NH3)3Bi2CI9. calculation of M 2 can, therefore, be performed only if the data on crystal structure of the studied compound are available. Although no crystallographic data in low temperature phases have so far been reported for (CH3NH3)3Bi2C19, satisfactory analysis can be made using the M2 data on related molecules. Then by analogy with other methylammonium ions the theoretical values of M2 were assumed to be 30 x 10 -8, 19 x 10 -8 and 8 x 10 -8 T 2 for the rigid cation, for the C3 rotation of the CH3 groups with rigid NH] ones and for both groups reorienting around the C - N bond axis, respectively [16]. The value of the M2 plateau below 8 x 10 -8 T 2 found in MACB even in the low temperature region (for T < 130 K) means that there is another possible mechanism, which causes such narrowing of the 1H NMR line. This mechanism is probably due to the rotational tunnelling of the three-proton group of the monomethylammonium cations. Such a process is undoubtedly expected to appear in the helium temperature range whereas another mechanism like a precessional motion of the C - N bond axis is preferred at higher temperatures. When the tunneling process takes place the contribution of the small satellite 1H NMR line (lying far away from the central line) to the value of the second moment has been probably missed. The further detailed study of deuterated title compound should give an answer to this question.
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R. Jakubas et aL/Journal of Molecular Structure 385 (1996) 145-151
The second plateau of 3.2 x 10 -8 T 2 c a n be interpreted in terms of some intermediate modes between the overall and C3 rotation of the cation as it has already been reported for similar compounds [17,18]. The assumption that two cations undergo overall rotation and only one the C3 rotation gives a theoretical M2 value of 3 x 10 -8 T 2 which is in good agreement with the experimental value. This means that the I I I - I I phase transition is associated with a release of one cation becoming completely disordered in phase II. On going to phase I the M2 decrease from 3.1 to 1 x 10 -8 T 2 can be interpreted as a result of a new motion the frequency of which is high enough to narrow the resonance line. Since the value of M2 = 0.4 x 10 -8 T 2, corresponding to a complete overall rotation of the cations, is not reached at 370 K this allows us to assume that the release of the cations rotational motion is progressive. 3.3.1. Spin-lattice relaxation The temperature dependence of T1 in (CH3NHa)3Bi2C19 at 55.2 MHz and 100 MHz is '
\
1 T1
~11 i-1 nT{
l=dz 9y4h2( ri 4ri T~ o 20 r 6 \ 1 + ¢02~ + 1 + 4O~o2~J
(1)
(2)
where index i represents the different CH3NH~ cations from the view-point of uniaxial rotation, •, rl and OJo are the gyromaguetic ratio of a proton, the correlation time of this motion and the resonance frequency, respectively, and r is the distance between the methyl (or ammonium) protons. The parameter do = 1 for the classical limit and do < 1 for tunneling rotation. Assuming an Arrhenius-type relationship between ri and the activation energy Eai of the motional process one obtains:
101
Z
shown in Fig. 6. From T1 data observed at 55.2 MHz three minima could be distinguished at 35, 39 and 50 K. Upon increasing the temperature another minimum is observed at 85 K. In the high temperature range of phase III, T1 is almost independent of the spectrometer frequency. In phase I, the T1 value decreases rapidly on increasing the temperature; this probably indicates the onset of self-diffusion of the cations. To interpret the T1 data observed in phase III we assumed that the T1 dependence is explainable in terms of the summation of the BPP-type equation [19]. From crystallographic experiments on MACB it results that in the lattice of this crystal one can distinguish three nonequivalent methylammonium cations. That is why it was assumed the uniaxial rotation of the CH3NH~ cations as a whole about the molecular axis for all three observed minima of Tt was connected with three cations. In this case T11 is described by the following relation:
10 0
10-1
ri = roi exp(Eai/kT)
10 2
.......
i ...... 10
, . ,
......... 20
, ......... 3O
, ........ 40
50
1000/T [K"t]
Fig. 6. Temperaturedependenceof the protonspin-latticerelaxation between 400 K and 20 K for (CH3NH3)3Bi2CIg: I , 55.2 MHz; T, 100 MHz.The fittingcurvesof the protonspin-lattice relaxation time T1obtainedaccordingto the Eq. (1) are presentedas solid lines.
times T1
where roi is the correlation time at the limit of infinite temperature. We performed a fitting calculation of the TI data below 80 K using n = 3. The obtained motional parameters are listed in Table 1 and the best fitted curve using these parameters is given by a solid line in Fig. 6. Upon increasing temperature another minimum is observed at 86 K. A similar minimum has already
R. Jakubas et al./Journal of Molecular Structure 385 (1996) 145-151
Table 1 Activation energies and correlation times for three non-equivalent methylammonium cations (A, B and C type) of (CH3NH3)3Bi2C19 r (K)
CH3NH ~ cation
E, (kJ mole -1)
ro (s)
T < 85 K
A B C C - N axis
2.02 -+ 0.024 2.46 -+ 0.03 3.6 -+ 0.014 8.32 +_ 0.02
0.105 x 10 -11 0.102 x 10 -11 0.51 x 10 -x2 0.162 x 10 -13
T > 85 K
been observed in the same temperature range for (CH3NH3)3Sb219 [20]. From the analysis we have found that one cation should perform an additional motion (C3 rotation), i.e. a motion of the C - N axis (uncorrelated reorientation of the CH3 and NH~ groups). Thus we can assign the minimum at 86 K to this motion and the high temperature data were fitted with Eqs. (1) and (2) using n -- 1. The activation energy Ea and correlation time 70 are equal to 8.82 kJ mo1-1 and 1.2 x 10 -12 s, respectively. These values are characteristic for this type of motion [21]. There exists, however, another explanation (less probable) that this minimum may be connected with an additional structural phase transition in the MACB crystal.
4. Conclusions The shape of the anomalies in the linear thermal expansion curves indicates that (CH3NH3)3Bi2CI9 undergoes a weakly first-order phase transition at 247 K and continuous transition at about 352 K, exhibiting the features of diffused transition. Dielectric dispersion studies performed between 140 and 400 K in the frequency range 1 kHz-900 MHz, did not reveal any relaxation process. This indicates that the methylammonium cations, which are believed to contribute to the mechanism of the phase transition in MACB crystal, perform very rapid reorientational motion with the macroscopic relaxation time shorter than 1 x 10 -1° to 1 x 10 -11 S. The low temperature T1 data at 55 MHz revealing well-separated minima were interpreted in terms of
151
uniaxial rotation of methylammonium cations. Around the phase transition at 247 K and 352 K no anomaly in the Tt measurements was detected. On the other hand the phase transition at 352 K is accompanied by a visible continuous increase in the second moment value by about 1 x 10 -8 T 2. The 1H NMR studies indicate that the changes in the motional state of the methylammonium cations at the phase transitions are rather subtle.
References [1] R. Jakubas and L. Sobczyk, Phase Trans., 20 (1990) 163. [2] V. Vama, R. Bhattarcharjee, H.N. Vasan and C.N.R Rao, Spectrochim. Acta, 48A (1992) 1631. [3] H. Ishihara, K. Watanabe, A. lwata, K. Yamada, Y. Kinoshita, T. Okuda, V.G. Krishnan, S. Dou and A. Weiss, Z. Natufforsch., 47a (1992) 65. [4] R. Jakubas, U. Krzewska, G. Bator and L. Sobczyk, Ferroelectrics, 77 (1988) 129. [5] R. Jakubas, L. Sobczyk and J. Matuszewski, Ferroelectrics, 80 (1987) 339. [6] R. Jakubas, Z. Czapla, Z. Galewski and L. Sobczyk, Ferroelectrics Lett., 5 (1988) 69. [7] K. Kihara and T. Sudo, Acta Cryst., B30 (1974) 1088. [8] F. Lazarini, Acta Crystallogr., B33 (1977) 2961. [9] B. Chabot and E. Parthe, Acta Crystallogr., B34 (1978) 645. [10] J. Zaleski, R. Jakubas, L. Sobczyk and J. Mr6z, Ferroelectrics, 103 (1990) 83. [11] R. Jakubas, Z. Czapla, Z. Galewski, L. Sobczyk, O.J. Zogal and T. Lis, Phys. Status Solidi (a), 93 (1986) 449. [12] R. Jakubas, P. Tomaszewski and L. Sobczyk, Phys. Status Solidi (a), 111 (1989) K27. [13] I. Belkyal, R. Mokhlisse, B. Tanouti, N.B. Chanh and M. Couzi, J. Alloys Compounds, 188 (1992) 186. [14] J.G. Powles, J.H. Strange, Proc. Phys. Sot., 82 (1963) 5. [15] R. Jakubas, G. Bator, J. Zaleski, A. Pietraszko and R. Decressain, J. Phys. Condens. Matter, 8 (1995) 367. [16] E.R. Andrew and P.C. Canepa, J. Magn. Reson., 7 (1972) 429. [17] R. Decressain, E. Cochon. J. Lefebvre, B. Meurer and R. Jakubas, J. Phys. Chem. Sol., 55 (1994) 139. [18] R. Decressain, R. Jakubas and J. Lefebvre, Phys. Status Solidi (a), 147 (1995) K73. [19] N.E. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev., 73 (1948) 676. [20] P. Koziol, Y. Furukawa. D. Nakamura and R. Jakubas, Bull. Chem. Soc. Jpn., 65 (1992) 1707. [21] R. Ikeda, Y. Kume, D. Nakamura, Y. Furukawa and H. Kiriyama, J. Magn. Reson., 24 (1976) 9.