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NMR and dilatometric studies of the structural phase transitions of (CH3NH3)3Sb2I9 and (CH3NH3)3Bi2I9 crystals
1. Phys. Chem. Solids Vol. 53. No. 6. pp. 75S-7S9, 1992 Printed in Great Britain.
0022.3697192 55.00 + 0.00 Pcrgamon Press Ltd
NMR AND DILATO~ETR~C STUDIES OF THE STRUCTURAL PHASE TRANSITIONS OF (CH,NH,)$b21, AND (~H~NH~)~Bi~I~CRYSTALS R. JAKUBAS,? R. DECRESSAINand J. LEFEBVRE Laboratoire de Dynamique et Structures des Materiaux Molktlaires, (U.A. n”gOl), UFR de Physique, Universite de Lille I, 59 655 Villeneuve d’Ascq Cedex, France (Received 7 November 1991; accepted 6 December 1991) Abstract-Linear thermal expansions of single crystals of (CH~NH,)$b,I,(MAIA) and (CH,NH&Bi& (MAIB) were measured in the 100-295K temperature range. Both crystals display a large anisotropy in the dilatation measured along the a- and c-axes in the vicinity of all phase transitions (PI). The pressure coefficients at the first-order phase transitions for MAIA (149 K) and MAIB (145K) were -t-6.0 and +5.7 x 1O-2K MPa-‘, respectively. The temperature dependence of the ‘H spin-lattice relaxation time 7’,, was measured for solid MAIB. Discontinuities in the spin-lattice relaxation plot corroborate the first-order PT at 145 K and the second-order one at 218 K. Molecular motions occurring in the respective phases are identified and their activation energies evaluated. Keyword: iodobismuthate,
iodoaRtimonate, phase transitions, thermal expansion, NMR.
1. INTRODUCTION
have been recently reported by Kozioi et al. fl I J. Both iodine compounds are isomorphous with Cs,Sb,I, [12] at room tem~rature, space group P6~/~c, and the methylammonium cations are dynamically disordered. The dielectric and DSC measurements [lo] showed that MAIB undergoes two PTs at T, = 142 K and at T, = 223 K. The former transition is apparently of first order whereas the latter one is of second order. The dielectric and pyroelectric behaviour in the vicinity of the first order PT [12] suggests a ferrielectric ordering below 142 K. The calorimetric and dielectric studies on MAIA [9] revealed the existence of two structural PTs at T, = 147 K and at II-,= 111 K. Both transitions are of first order. The pyroelectric response in MAIA near 147 K seems to indicate an antiferroelectric ordering below this point. In this paper we report thermal expansion measurements from 100 K to room temperature on MAlA and MAIB. To obtain detailed information about dynamical properties of the cations in the different phases, investigations of the temperature dependence of ‘H spin lattice relaxation time (T,,) have also been undertaken on MAIB.
Methylammonium crystals of the halogenoantimonates (III) and halogenobismuthates (III) family, with the general formula (CH,NHj),M,XP (where M = Sb, Bi and X = Cl, Br, I), have been the subject of many investigations [l-7]. It has been shown that the structure of anions M,XG- and the cationic dynamics in these compounds are affected by the type of halogen atoms. The common feature of all methylammonium crystals is that in the high-temperature phases they show considerable freedom for the rotational motion of the CH,NH, + cations. All these crystals undergo phase transitions (PTs) mainly resulting from a change of the motional state of the cations when they are cooled. Up to now, the thermal expansion measurements have been performed only on (CH,NH3),Sb,Br9 (MABA) and (CH,NH,),Bi,Br, (MABB) (81. Because the dilatometric measurement is a very sensitive technique to detect and determine the nature of phase transitions, it seemed justified to extent such studies to iodine analogues; (CH,NH@b&, (MAIA) and (CH,NH&Bi,I, (MAIB). The observed sequence of the PTs in MAIA [9] and MAIB IlO] was found to be complex and ambiguous, this is why we were motivated additionally to investigate one of the iodine crystals, i.e. MAIB, using the Proton Magnetic Resonance technique. ‘H NMR studies for MAIA ton
2. EXPERIMENTAL (C~~NH~)~Sb~I~ and ~CH~NH~}~Bi~I~were obtained in a reaction of Sb,O, or (BiO)$O,, respectively, in a hot concentrated aqueous solution of HI. The single crystals were grown by slow evaporation of a saturated acetonitriie solution at room temperature.
leave of absence from the Institute of Chemistry, University of Wroclaw, 50-383 Wroclaw, Poland. 755
R.
156
JAKUBAS et
Thermal expansion along the a- and c-axes of the compounds was measured by a dilatomoeter of the difference transformer type (thermomechanical analyzer Perkin-Elmer TMS-2). For the measurements presented in this paper, samples in the form of thin plates (3 x 3 x 1 mm’) with parallel faces perpendicular to the axis of measurement were chosen. Under dynamic temperature conditions the measurements were performed on heating and cooling at rates of 0.15-2 K min-‘. Several specimens of each crystal were examined. Measurements on each sample were repeated several times and the results were reproducible. The anomalies always appeared at the same temperature for the same direction of temperature change, and the size of the anomalies varied by less than 10% over several measurements on both iodine crystals. Proton pulsed NMR experiments were carried out at 100 MHz on a Bruker CXP 100 spectrometer. The longitudinal relaxation time T,, was measured by employing the inversion recovery pulse sequence (n/2, 5, n). The sample temperature, changing between 110 and 310 K was controlled with either precooled nitrogen or preheated gas flow system within + 1 K. All the measurements were taken at decreasing and increasing temperature on polycrystalline samples. 3. RESULTS
3.1.
Dilatometric
AND DISCUSSION
studies
The temperature dependence of dilation of MAIA along the a- and c-axes (a and c correspond to the crystallographic axes of the hexagonal room temperature phase) is shown in Fig. 1. An anomalous change in dilation observed in the vicinity of 149 K confirms the first-order PT reported by dielectric and calorimetric studies [9]. The mean value of the linear by expansion coefficient 6, (6 is defined
al.
i
‘1
120
160
200
2
0
Fig. 2. Thermal expansion of (CH,NH,),Bi,I, (MAIB) crystal along the a-axis on warming. The arrows show the transition points.
(A~I-AW(~O(300K~ x VI-T,)), where &300Kjis the length of the sample at room temperature) of this along a-axis, from crystal, the changed 9.1 x lo-‘K-’ in phase (I) to 4.04 x 10m5K-’ in phase (II). For the c-axis the observed dilatation is much smaller and the corresponding coefficient, o?,, changes from 2.18 x 10e5 K-’ (phase (I)) to 0.6 x lo-‘K-l (phase (II), below 130K). In the low-temperature phase (II), below T+,,), 6 possesses negative value approaching to zero at about 130 K. It should be noticed, that along only this axis does a small but reproducible anomaly appear at about 3-4 K above T,,,_,,,. No thermal anomaly was observed in the vicinity of the expected (II-III) PT at 111 K recorded previously by DSC experiment [9]. The thermal expansion of the second analogueMAIB, is more complicated. Figures 2 and 3 show the temperature dependence of dilation of MAIB along the a- and c-axes, respectively, on heating. Starting from 115 K the thermal expansion along the u-axis increases and then changes its sign around 135 K. Below 140 K the sample exhibits a sharp anomaly consistent with the first-order PT confirmed
I
IL0 Fig. 1. Thermal expansion of (CH,NH,),Sb,I, (MAIA) crystal along the c-axis (upper part) and a-axis (lower part) on warming. The arrow shows the transition point.
T/K
180
/
220 T/K
Fig. 3. Thermal expansion of (CH,NH,),Bi,I, (MAIB) crystal along the c-axis on warming. The arrows show the transition points.
NMR and dilatometric studies
previously by other techniques [IO]. Similarly, as it was seen in MAIA (see Fig. 1, along the c-axis), also this PT is followed by a small dip. Above 145 K the thermal expansion increases linearly with a7,= 5.1 x 10mSKm4 up to 165 K where a distinct change in the slope is visible. Above this point OS, remains relatively unchanged through phases (II) and (I) (071: 7.3 x 10m5K-‘). In Fig. 3 the thermal expansion curve along the c-axis clearly evidences the (I-II) PT at 218 K and the (III-IV) PT at 145 K the latter as of first-order. The insert in Fig. 3 shows in detail the changes in expansion curve near the (I-II) PT and confirms the second-order nature of this transformation. Above 218 K cc, has a value equal to 1.75 x 10m5K-‘. Below the transition point the CI, coefficient changes its sign reaching zero at about 188 K. In the vicinity of 165 K a relatively large change in the slope of the dilation curve is in good agreement with that observed for the u-axis and suggests the continuous PT in the MAIB crystal. The structural transformation at 165 K was detected for the first time. The spontaneous changes in the volume and lengths along the a- and c-axes recorded for the first-order PT in both crystals are collected in Table 1. Summarizing the thermal study, one can state that the present dilatometric measurements confirm both the sequence and the nature of the PTs reported previously by DSC and dielectric methods for MAIB, i.e. at 145 K a first order type and at 218 K a second-order one. The latter corresponds to that found at 223 K by DSC and the temperature shift is probably caused by the relatively high speed of the temperature change of the DSC scan-5 K mini. In the case of MAIA only a first-order PT at 149 K (147 K on cooling) was detected, while the second one visible by DSC at 111 K was not confirmed. On the other hand the present dilatometric studies revealed an additional continuous PT at 165 K in MAIB. Comparing the temperature behaviour of the electric permittivity, cc, for MAIB (see Fig. 3 in [lo]) with the dilatometric curve along the c-axis, a large similarity in the temperature characteristic of these two parameters is to be noticed. Both clc and thermal expansion within a temperature range comprising phases
757
(II) and (III) change only slightly and irregularly with respect to the other phases. One can see that L’, and (AL/L) along the c-axis reveal the obvious changes in the slope in the temperature range of about 170-165 K. It would be interesting to re-examine, in detail, the dielectric response on MAIB especially above 145 K to confirm the structural PT at 165 K. It is also interesting to notice that a small dip overlapping with a large anomaly at 145 K for MAIB and an additional peak close to (I-II) PT in MAIA confirm the complex mechanism of these transformations visible in the pyroelectric measurements [ 10, 121. According to the pyroelectric response in MAIB close to 147 K, on approaching T, from above, a very narrow polar intermediate phase precedes the ferroelectric one. We recall that, from the pyroelectric and dielectric point of view, the PT at 147K in MAIA and at 142 K in MAIB correspond to each other showing an essential similarity. However, our dilatometric measurements reveal that the thermal anisotropy in the vicinity of the above-mentioned PTs is reversed. Such unexpected behaviour makes the picture more complicated. The best agreement between the dielectric and dilatometric measurements for MAIB is observed below the room temperature phase (I). The dielectric anisotropy (see Fig. 3 in [lo]) changes its sign below 220 K and a nearly constant value of L, is observed down to about 170 K, below this point the decrease in the E~ function becomes distinct. The dielectric properties seem to be reflected in the thermal expansion along this axis. It is known from preliminary crystallographic studies [9, lo], that at room temperature MAIB and MAIA are isomorphous, but by cooling, the difference in the dynamic disorder of the methylammonium cations becomes more and more distinct leading to the breaking of the isomorphic of both crystals. As a result a new sequence of PTs appears in MAIB in comparison to MAIA. Next let us estimate the values of the pressure coefficients, (dT,/dp), for the first-order PTs. Dilatations along the a- and c-axes and volume changes at the first order PT are listed in Table 1. AS is the entropy change corresponding to the latent heat at T,. Values of AS were taken from [9, lo]. The pressure
Table 1. Spontaneous changes of lengths along the a- and c-axes, and volume and pressure coefficients (dTC/dp) at the transition temperatures of MAIA and MAIB. The a’- and c’-axes refer to the corresponding axes of the room temperature phase (I) (T-)
Axis
AL/L (x IO-‘)
AVIV (x 10-q
AS (J mol-i K-i)
d Tldp (x 10d2 K MPa-‘)
MAIA
149
+2.5 -2.38
+1.95
13.4
6.0 f 1.0
MAIB
145
-2.74 +7.5
+2.75
20.0
5.7 f 1.2
R.
758
JAKULIAS
et al.
coefficient of the transition temperature can be estimated by the Clausius-Clapeyron relation: dT, _ AV dp -AS
where AV is the change in molar volume at T,. In order to calculate the value of A V for MAIB at 145 K it was assumed that the departure of crystal symmetry at low temperature from hexagonal symmetry will be small. The calculated pressure coefficients for both analogues (see Table 1) are positive and comparable. Our results would appear to indicate that the same type of motion of the methylammonium cations is involved in the mechanism of the first-order PT of iodine crystals. It should be also noted, that the calculated values of (dT,/dp) for all methylammonium crystals studied in our family (see also MABA and MABB [S]) are rather small, neglecting the sign of these changes. Thus, this confirms the fact that, in such systems having high-temperature, plastic-like phases even in the low-temperature region, the methylammonium cations exhibit still a large freedom of motion. The methylammonium cations, usually occupying large cavities of the inorganic sublattice, do not seem to be sensitive to the contraction of the lattice cell. At present, however, the molecular mechanism of these PTs is not understood. 3.2. Proton NMR The ‘H linewidth FWHM (full width at half maximum) for MAIB in the temperature range 110-310 K (on cooling) is presented in Fig. 4. Below 142 K (phase (IV)) one can observe a linear increase of FWHM with decreasing temperature starting with the value of 14 kHz at 142 K and finishing with 18.5 kHz at 115 K. In the high-temperature region, above 142 K two plateaus are observed (11.5, 10.5 kHz) corresponding to phases (III-II) and (I).
‘+
18 i
i
100
!
200
lo-
(1)
300
TK I
Fig. 4. Temperature dependence of the FWHM (full width at half maximum) for (CH,NH,),Bi,I, (MAIB).
2
2
l-
tI 3
4
tI 5
8
t
I
7
8
9
q/K-’
Fig. 5. A semilog plot of the spin-lattice relaxation time T,, vs the inverse temperature for (CH,NH,),Bi,I, (MAIB)
We indicate by (IItrIII) the temperature range comprising phases (II) and (III). The second moment analyses on different salts containing methylammonium cations have shown that usually in the lowest temperature phases the C-N bond axis is fixed in the lattice whereas the CH, and NH3 groups perform C, reorientations. Such motions give rise to M, (‘H) of 8G2[14-171. The overall rotation of CH,NH, + cations can diminish the value of M, markedly below 1 G*. The value of the second moment estimated by us under the assumption of a Gaussian line-shape amounts to about 6.5 G* at 115 K, whereas within phases (I&+111) and (I) it decreases to ca 2.5 and 2.0 G*, respectively. The above results are consistent with the proton relaxation time measurements vs temperature (see Fig. 5). Two distinct anomalies observed in the vicinity of 220 and 142 K correspond to the PTs found by DSC, dielectric and dilatometric techniques. Both the (I-II) and (III-IV) PT induces a rapid decrease in T,, in the vicinity of the transition points and an important change in the slope of T,, vs temperature curve within each phase is observed. In particular, no changes have been noticed in the vicinity of the 165 K structural PT suggested by the present dilatometric measurements. In the temperature range studied no T,, minimum in the log T,, vs T-’ curve was observed and the correlation time for the methylammonium motion is expected to be very short (w,,r <<1). Assuming the Arrhenius relation (T = r. exp EJRT), the activation parameters are evaluated separately from the gradient of the linear portion of the log T,, vs T-’ curve within phases (I) and (II-III). The activation
NMR and dilatometric studies energies and
for these
Em (II++III)
phases
are E,(Z) = 8.09 kJ mol-’
= 2.2 k.I mol-‘,
former value is comparable ent methylammonium forming an overall
respectively.
The
to those found for differ-
salts, where cations are perrotation. For example in
(CH,NH,),Sb,Br9 [6] and (CH,NH,),Bi,Br, [ 1 l] crystals having the plastic-like phase (I) E, was found to be 7.5 and 8.5 kJ mol-‘, respectively. The relatively small value of E, obtained in phases (IIc*III) of MAIB indicates that the methylammonium cations are still considerably free. Because the magnetization recovery curve becomes nonexponential between 145 and 115 K we were not able to estimate the E, (IV) value. We should also stress that the activation energies express the motional parameters of average methylammonium cations. At present the structure of MAIB is not solved and a detailed discussion of the disordering of methylammonium cations would be fruitless. However, from the ‘H NMR results obtained, one can state that the two types of methylammonium in phase (I) perform overall rotation. An increase of the second moment of the order of 0.5 G2 when the (I-II) PT is crossed may be related to a slight changing two CH,NH:
in the motional state for one of the cations. The small value of E, in
phases (IIcrIII) indicates, that the possible N-H . I hydrogen bonds influencing the motion of the methy lammonium cations are quite weak as compared with those in other iodine crystals [17, 181. The motional mode of the cation in the lowest temperature phase (IV) below 142 K seems to be complex. the continuous strong decrease in FWHM with increasing temperature suggests an essential change in the motional state of the methylammonium cations. In the low-temperature range the relaxation was found to be slightly nonexponential. In these cases, the T,= reported in Fig. 5 characterize the initial slope of the relaxation
curve which was well defined. 4. CONCLUDING
REMARKS
The main conclusions that may be drawn from our dilatometric and ‘H NMR studies on MAIA and MAIB are given below. (i) Good agreement was found between the dielectric, DSC and thermal expansion measurements with respect to the nature of the PTs in MAIA and MAIB. (ii) Significant measured along
differences in thermal expansion the a- and c-axes are seen in the
759
vicinity of the lowest temperature MAIA
(at 147 K) and MAIB
PT of first-order
in
(at 145 K). Addition-
ally, a more complex sequence of the PTs found in MAIB in comparison to MAIA suggests different motional states of the methylammonium cations for these two crystals within all the phase modifications. (iii) A new structural PT continuous in nature was found around 165 K in MAIB. (iv) The distinct discontinuities in the log r’, vs l/r curve in the vicinity of 220 K and 145 K suggest that these PTs are largely
affected
by the dynamic
properties of the CH,NH: cations. (v) The rigid C, reorientation of the CH, and NH, groups about their C-N bond axis fixed in the lattice is suggested to exist in the lowest temperature phase of MAIB (Below 145 K). Both the E, value (8.09 kJ mol-‘) and the second moment M,, equal to 2 G2, within the high-temperature phase (I) are characteristic of the overall rotation of methylammonium cations. REFERENCES 1. Jakubas R., Czapla Z., Galewski Z., Sobczyk L., Zogal 0. J. and Lis T.. Phvs. Status Solidi fa) 93. 449 (1986). 70, 145 2. Jakubas R. and Miniewicz A., Fekokecirics (1986).
3. Jakubas R., Krzewska U., Bator G. and Sobezyk L., Ferroelectrics
77, 129 (1988).
4. Jakubas R., Tomaszewski P. E. and Sobezyk L., Phys. Status Solidi fa) 111. K21 (1989). 5. Jakubas R. and Sobczyk L:, Ph&e Transitions 20, 163 (1990).
6. Maekowiak M., Weiden N. and Weiss A., Phvs. Status Solidi (a) 119, 77 (1990).
7. Kozioi P.. Furukawa Y. and Nakamura D.. J. Phvs. Sot. Japan 60, 3850 (1991).
8. Jakubas R., Lefebvre J., Fontaine H. and Francois P., Sol. State Commun. 81, 139 (1992). 9. Zaleski J., Jakubas R., Sobczyk L. and Mroz J., Ferroelectrics
103, 83 (1990).
10. Jakubas R., Zaleski J. and Sobczyk L., Ferroelectrics 108, 109 (1990). 11. Kozioi P. et al. (unpublished results). 12. Miniewicz A. and Jakubas R., Ferroelectrics 110, 261 (1990).
13. Ishida H., Ikeda R. and Nakamura D., Bull. Chem. Sot. Jap. 55, 3116 (1982). 14. Ishida H., Ikeda R. and Nakamura D., Bull. Gem. Sot. Jan 60, 467 (1987). 15. Yamada K., Nose s., Umehara T., Okuda T. and Ichiba S., Bull. Chem. Sot. Jap. 61, 4265 (1988). 16. Furukawa Y. and Nakamura D., Z. Naturf 448, 1122 (1989). 17. Furukawa Y., Kiriyama H., Ikeda R. and Ishida H., Bull. Chem. Sot. Jap. 54, 103 (1981). 18. Ishida H., Ikeda R. and Nakamura D., Bull. Chem. Sot. Jap. 59, 915 (1986).
0022.3697192 55.00 + 0.00 Pcrgamon Press Ltd
NMR AND DILATO~ETR~C STUDIES OF THE STRUCTURAL PHASE TRANSITIONS OF (CH,NH,)$b21, AND (~H~NH~)~Bi~I~CRYSTALS R. JAKUBAS,? R. DECRESSAINand J. LEFEBVRE Laboratoire de Dynamique et Structures des Materiaux Molktlaires, (U.A. n”gOl), UFR de Physique, Universite de Lille I, 59 655 Villeneuve d’Ascq Cedex, France (Received 7 November 1991; accepted 6 December 1991) Abstract-Linear thermal expansions of single crystals of (CH~NH,)$b,I,(MAIA) and (CH,NH&Bi& (MAIB) were measured in the 100-295K temperature range. Both crystals display a large anisotropy in the dilatation measured along the a- and c-axes in the vicinity of all phase transitions (PI). The pressure coefficients at the first-order phase transitions for MAIA (149 K) and MAIB (145K) were -t-6.0 and +5.7 x 1O-2K MPa-‘, respectively. The temperature dependence of the ‘H spin-lattice relaxation time 7’,, was measured for solid MAIB. Discontinuities in the spin-lattice relaxation plot corroborate the first-order PT at 145 K and the second-order one at 218 K. Molecular motions occurring in the respective phases are identified and their activation energies evaluated. Keyword: iodobismuthate,
iodoaRtimonate, phase transitions, thermal expansion, NMR.
1. INTRODUCTION
have been recently reported by Kozioi et al. fl I J. Both iodine compounds are isomorphous with Cs,Sb,I, [12] at room tem~rature, space group P6~/~c, and the methylammonium cations are dynamically disordered. The dielectric and DSC measurements [lo] showed that MAIB undergoes two PTs at T, = 142 K and at T, = 223 K. The former transition is apparently of first order whereas the latter one is of second order. The dielectric and pyroelectric behaviour in the vicinity of the first order PT [12] suggests a ferrielectric ordering below 142 K. The calorimetric and dielectric studies on MAIA [9] revealed the existence of two structural PTs at T, = 147 K and at II-,= 111 K. Both transitions are of first order. The pyroelectric response in MAIA near 147 K seems to indicate an antiferroelectric ordering below this point. In this paper we report thermal expansion measurements from 100 K to room temperature on MAlA and MAIB. To obtain detailed information about dynamical properties of the cations in the different phases, investigations of the temperature dependence of ‘H spin lattice relaxation time (T,,) have also been undertaken on MAIB.
Methylammonium crystals of the halogenoantimonates (III) and halogenobismuthates (III) family, with the general formula (CH,NHj),M,XP (where M = Sb, Bi and X = Cl, Br, I), have been the subject of many investigations [l-7]. It has been shown that the structure of anions M,XG- and the cationic dynamics in these compounds are affected by the type of halogen atoms. The common feature of all methylammonium crystals is that in the high-temperature phases they show considerable freedom for the rotational motion of the CH,NH, + cations. All these crystals undergo phase transitions (PTs) mainly resulting from a change of the motional state of the cations when they are cooled. Up to now, the thermal expansion measurements have been performed only on (CH,NH3),Sb,Br9 (MABA) and (CH,NH,),Bi,Br, (MABB) (81. Because the dilatometric measurement is a very sensitive technique to detect and determine the nature of phase transitions, it seemed justified to extent such studies to iodine analogues; (CH,NH@b&, (MAIA) and (CH,NH&Bi,I, (MAIB). The observed sequence of the PTs in MAIA [9] and MAIB IlO] was found to be complex and ambiguous, this is why we were motivated additionally to investigate one of the iodine crystals, i.e. MAIB, using the Proton Magnetic Resonance technique. ‘H NMR studies for MAIA ton
2. EXPERIMENTAL (C~~NH~)~Sb~I~ and ~CH~NH~}~Bi~I~were obtained in a reaction of Sb,O, or (BiO)$O,, respectively, in a hot concentrated aqueous solution of HI. The single crystals were grown by slow evaporation of a saturated acetonitriie solution at room temperature.
leave of absence from the Institute of Chemistry, University of Wroclaw, 50-383 Wroclaw, Poland. 755
R.
156
JAKUBAS et
Thermal expansion along the a- and c-axes of the compounds was measured by a dilatomoeter of the difference transformer type (thermomechanical analyzer Perkin-Elmer TMS-2). For the measurements presented in this paper, samples in the form of thin plates (3 x 3 x 1 mm’) with parallel faces perpendicular to the axis of measurement were chosen. Under dynamic temperature conditions the measurements were performed on heating and cooling at rates of 0.15-2 K min-‘. Several specimens of each crystal were examined. Measurements on each sample were repeated several times and the results were reproducible. The anomalies always appeared at the same temperature for the same direction of temperature change, and the size of the anomalies varied by less than 10% over several measurements on both iodine crystals. Proton pulsed NMR experiments were carried out at 100 MHz on a Bruker CXP 100 spectrometer. The longitudinal relaxation time T,, was measured by employing the inversion recovery pulse sequence (n/2, 5, n). The sample temperature, changing between 110 and 310 K was controlled with either precooled nitrogen or preheated gas flow system within + 1 K. All the measurements were taken at decreasing and increasing temperature on polycrystalline samples. 3. RESULTS
3.1.
Dilatometric
AND DISCUSSION
studies
The temperature dependence of dilation of MAIA along the a- and c-axes (a and c correspond to the crystallographic axes of the hexagonal room temperature phase) is shown in Fig. 1. An anomalous change in dilation observed in the vicinity of 149 K confirms the first-order PT reported by dielectric and calorimetric studies [9]. The mean value of the linear by expansion coefficient 6, (6 is defined
al.
i
‘1
120
160
200
2
0
Fig. 2. Thermal expansion of (CH,NH,),Bi,I, (MAIB) crystal along the a-axis on warming. The arrows show the transition points.
(A~I-AW(~O(300K~ x VI-T,)), where &300Kjis the length of the sample at room temperature) of this along a-axis, from crystal, the changed 9.1 x lo-‘K-’ in phase (I) to 4.04 x 10m5K-’ in phase (II). For the c-axis the observed dilatation is much smaller and the corresponding coefficient, o?,, changes from 2.18 x 10e5 K-’ (phase (I)) to 0.6 x lo-‘K-l (phase (II), below 130K). In the low-temperature phase (II), below T+,,), 6 possesses negative value approaching to zero at about 130 K. It should be noticed, that along only this axis does a small but reproducible anomaly appear at about 3-4 K above T,,,_,,,. No thermal anomaly was observed in the vicinity of the expected (II-III) PT at 111 K recorded previously by DSC experiment [9]. The thermal expansion of the second analogueMAIB, is more complicated. Figures 2 and 3 show the temperature dependence of dilation of MAIB along the a- and c-axes, respectively, on heating. Starting from 115 K the thermal expansion along the u-axis increases and then changes its sign around 135 K. Below 140 K the sample exhibits a sharp anomaly consistent with the first-order PT confirmed
I
IL0 Fig. 1. Thermal expansion of (CH,NH,),Sb,I, (MAIA) crystal along the c-axis (upper part) and a-axis (lower part) on warming. The arrow shows the transition point.
T/K
180
/
220 T/K
Fig. 3. Thermal expansion of (CH,NH,),Bi,I, (MAIB) crystal along the c-axis on warming. The arrows show the transition points.
NMR and dilatometric studies
previously by other techniques [IO]. Similarly, as it was seen in MAIA (see Fig. 1, along the c-axis), also this PT is followed by a small dip. Above 145 K the thermal expansion increases linearly with a7,= 5.1 x 10mSKm4 up to 165 K where a distinct change in the slope is visible. Above this point OS, remains relatively unchanged through phases (II) and (I) (071: 7.3 x 10m5K-‘). In Fig. 3 the thermal expansion curve along the c-axis clearly evidences the (I-II) PT at 218 K and the (III-IV) PT at 145 K the latter as of first-order. The insert in Fig. 3 shows in detail the changes in expansion curve near the (I-II) PT and confirms the second-order nature of this transformation. Above 218 K cc, has a value equal to 1.75 x 10m5K-‘. Below the transition point the CI, coefficient changes its sign reaching zero at about 188 K. In the vicinity of 165 K a relatively large change in the slope of the dilation curve is in good agreement with that observed for the u-axis and suggests the continuous PT in the MAIB crystal. The structural transformation at 165 K was detected for the first time. The spontaneous changes in the volume and lengths along the a- and c-axes recorded for the first-order PT in both crystals are collected in Table 1. Summarizing the thermal study, one can state that the present dilatometric measurements confirm both the sequence and the nature of the PTs reported previously by DSC and dielectric methods for MAIB, i.e. at 145 K a first order type and at 218 K a second-order one. The latter corresponds to that found at 223 K by DSC and the temperature shift is probably caused by the relatively high speed of the temperature change of the DSC scan-5 K mini. In the case of MAIA only a first-order PT at 149 K (147 K on cooling) was detected, while the second one visible by DSC at 111 K was not confirmed. On the other hand the present dilatometric studies revealed an additional continuous PT at 165 K in MAIB. Comparing the temperature behaviour of the electric permittivity, cc, for MAIB (see Fig. 3 in [lo]) with the dilatometric curve along the c-axis, a large similarity in the temperature characteristic of these two parameters is to be noticed. Both clc and thermal expansion within a temperature range comprising phases
757
(II) and (III) change only slightly and irregularly with respect to the other phases. One can see that L’, and (AL/L) along the c-axis reveal the obvious changes in the slope in the temperature range of about 170-165 K. It would be interesting to re-examine, in detail, the dielectric response on MAIB especially above 145 K to confirm the structural PT at 165 K. It is also interesting to notice that a small dip overlapping with a large anomaly at 145 K for MAIB and an additional peak close to (I-II) PT in MAIA confirm the complex mechanism of these transformations visible in the pyroelectric measurements [ 10, 121. According to the pyroelectric response in MAIB close to 147 K, on approaching T, from above, a very narrow polar intermediate phase precedes the ferroelectric one. We recall that, from the pyroelectric and dielectric point of view, the PT at 147K in MAIA and at 142 K in MAIB correspond to each other showing an essential similarity. However, our dilatometric measurements reveal that the thermal anisotropy in the vicinity of the above-mentioned PTs is reversed. Such unexpected behaviour makes the picture more complicated. The best agreement between the dielectric and dilatometric measurements for MAIB is observed below the room temperature phase (I). The dielectric anisotropy (see Fig. 3 in [lo]) changes its sign below 220 K and a nearly constant value of L, is observed down to about 170 K, below this point the decrease in the E~ function becomes distinct. The dielectric properties seem to be reflected in the thermal expansion along this axis. It is known from preliminary crystallographic studies [9, lo], that at room temperature MAIB and MAIA are isomorphous, but by cooling, the difference in the dynamic disorder of the methylammonium cations becomes more and more distinct leading to the breaking of the isomorphic of both crystals. As a result a new sequence of PTs appears in MAIB in comparison to MAIA. Next let us estimate the values of the pressure coefficients, (dT,/dp), for the first-order PTs. Dilatations along the a- and c-axes and volume changes at the first order PT are listed in Table 1. AS is the entropy change corresponding to the latent heat at T,. Values of AS were taken from [9, lo]. The pressure
Table 1. Spontaneous changes of lengths along the a- and c-axes, and volume and pressure coefficients (dTC/dp) at the transition temperatures of MAIA and MAIB. The a’- and c’-axes refer to the corresponding axes of the room temperature phase (I) (T-)
Axis
AL/L (x IO-‘)
AVIV (x 10-q
AS (J mol-i K-i)
d Tldp (x 10d2 K MPa-‘)
MAIA
149
+2.5 -2.38
+1.95
13.4
6.0 f 1.0
MAIB
145
-2.74 +7.5
+2.75
20.0
5.7 f 1.2
R.
758
JAKULIAS
et al.
coefficient of the transition temperature can be estimated by the Clausius-Clapeyron relation: dT, _ AV dp -AS
where AV is the change in molar volume at T,. In order to calculate the value of A V for MAIB at 145 K it was assumed that the departure of crystal symmetry at low temperature from hexagonal symmetry will be small. The calculated pressure coefficients for both analogues (see Table 1) are positive and comparable. Our results would appear to indicate that the same type of motion of the methylammonium cations is involved in the mechanism of the first-order PT of iodine crystals. It should be also noted, that the calculated values of (dT,/dp) for all methylammonium crystals studied in our family (see also MABA and MABB [S]) are rather small, neglecting the sign of these changes. Thus, this confirms the fact that, in such systems having high-temperature, plastic-like phases even in the low-temperature region, the methylammonium cations exhibit still a large freedom of motion. The methylammonium cations, usually occupying large cavities of the inorganic sublattice, do not seem to be sensitive to the contraction of the lattice cell. At present, however, the molecular mechanism of these PTs is not understood. 3.2. Proton NMR The ‘H linewidth FWHM (full width at half maximum) for MAIB in the temperature range 110-310 K (on cooling) is presented in Fig. 4. Below 142 K (phase (IV)) one can observe a linear increase of FWHM with decreasing temperature starting with the value of 14 kHz at 142 K and finishing with 18.5 kHz at 115 K. In the high-temperature region, above 142 K two plateaus are observed (11.5, 10.5 kHz) corresponding to phases (III-II) and (I).
‘+
18 i
i
100
!
200
lo-
(1)
300
TK I
Fig. 4. Temperature dependence of the FWHM (full width at half maximum) for (CH,NH,),Bi,I, (MAIB).
2
2
l-
tI 3
4
tI 5
8
t
I
7
8
9
q/K-’
Fig. 5. A semilog plot of the spin-lattice relaxation time T,, vs the inverse temperature for (CH,NH,),Bi,I, (MAIB)
We indicate by (IItrIII) the temperature range comprising phases (II) and (III). The second moment analyses on different salts containing methylammonium cations have shown that usually in the lowest temperature phases the C-N bond axis is fixed in the lattice whereas the CH, and NH3 groups perform C, reorientations. Such motions give rise to M, (‘H) of 8G2[14-171. The overall rotation of CH,NH, + cations can diminish the value of M, markedly below 1 G*. The value of the second moment estimated by us under the assumption of a Gaussian line-shape amounts to about 6.5 G* at 115 K, whereas within phases (I&+111) and (I) it decreases to ca 2.5 and 2.0 G*, respectively. The above results are consistent with the proton relaxation time measurements vs temperature (see Fig. 5). Two distinct anomalies observed in the vicinity of 220 and 142 K correspond to the PTs found by DSC, dielectric and dilatometric techniques. Both the (I-II) and (III-IV) PT induces a rapid decrease in T,, in the vicinity of the transition points and an important change in the slope of T,, vs temperature curve within each phase is observed. In particular, no changes have been noticed in the vicinity of the 165 K structural PT suggested by the present dilatometric measurements. In the temperature range studied no T,, minimum in the log T,, vs T-’ curve was observed and the correlation time for the methylammonium motion is expected to be very short (w,,r <<1). Assuming the Arrhenius relation (T = r. exp EJRT), the activation parameters are evaluated separately from the gradient of the linear portion of the log T,, vs T-’ curve within phases (I) and (II-III). The activation
NMR and dilatometric studies energies and
for these
Em (II++III)
phases
are E,(Z) = 8.09 kJ mol-’
= 2.2 k.I mol-‘,
former value is comparable ent methylammonium forming an overall
respectively.
The
to those found for differ-
salts, where cations are perrotation. For example in
(CH,NH,),Sb,Br9 [6] and (CH,NH,),Bi,Br, [ 1 l] crystals having the plastic-like phase (I) E, was found to be 7.5 and 8.5 kJ mol-‘, respectively. The relatively small value of E, obtained in phases (IIc*III) of MAIB indicates that the methylammonium cations are still considerably free. Because the magnetization recovery curve becomes nonexponential between 145 and 115 K we were not able to estimate the E, (IV) value. We should also stress that the activation energies express the motional parameters of average methylammonium cations. At present the structure of MAIB is not solved and a detailed discussion of the disordering of methylammonium cations would be fruitless. However, from the ‘H NMR results obtained, one can state that the two types of methylammonium in phase (I) perform overall rotation. An increase of the second moment of the order of 0.5 G2 when the (I-II) PT is crossed may be related to a slight changing two CH,NH:
in the motional state for one of the cations. The small value of E, in
phases (IIcrIII) indicates, that the possible N-H . I hydrogen bonds influencing the motion of the methy lammonium cations are quite weak as compared with those in other iodine crystals [17, 181. The motional mode of the cation in the lowest temperature phase (IV) below 142 K seems to be complex. the continuous strong decrease in FWHM with increasing temperature suggests an essential change in the motional state of the methylammonium cations. In the low-temperature range the relaxation was found to be slightly nonexponential. In these cases, the T,= reported in Fig. 5 characterize the initial slope of the relaxation
curve which was well defined. 4. CONCLUDING
REMARKS
The main conclusions that may be drawn from our dilatometric and ‘H NMR studies on MAIA and MAIB are given below. (i) Good agreement was found between the dielectric, DSC and thermal expansion measurements with respect to the nature of the PTs in MAIA and MAIB. (ii) Significant measured along
differences in thermal expansion the a- and c-axes are seen in the
759
vicinity of the lowest temperature MAIA
(at 147 K) and MAIB
PT of first-order
in
(at 145 K). Addition-
ally, a more complex sequence of the PTs found in MAIB in comparison to MAIA suggests different motional states of the methylammonium cations for these two crystals within all the phase modifications. (iii) A new structural PT continuous in nature was found around 165 K in MAIB. (iv) The distinct discontinuities in the log r’, vs l/r curve in the vicinity of 220 K and 145 K suggest that these PTs are largely
affected
by the dynamic
properties of the CH,NH: cations. (v) The rigid C, reorientation of the CH, and NH, groups about their C-N bond axis fixed in the lattice is suggested to exist in the lowest temperature phase of MAIB (Below 145 K). Both the E, value (8.09 kJ mol-‘) and the second moment M,, equal to 2 G2, within the high-temperature phase (I) are characteristic of the overall rotation of methylammonium cations. REFERENCES 1. Jakubas R., Czapla Z., Galewski Z., Sobczyk L., Zogal 0. J. and Lis T.. Phvs. Status Solidi fa) 93. 449 (1986). 70, 145 2. Jakubas R. and Miniewicz A., Fekokecirics (1986).
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77, 129 (1988).
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