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J. Phys. Chem. 1996, 100, 888-890
Temperature-Dependent Raman Study of Ethylammonium Chloride P. S. R. Prasad Laser Programme, Centre For AdVanced Technology, Indore 452 013, India ReceiVed: April 13, 1995; In Final Form: October 10, 1995X
Temperature-dependent Raman spectra of ethylammonium chloride have been studied in the range 80-346 K. Spectral variations around the 221 K transition are marginal. However, the transition at 345 K produces many spectral anomalies. Many internal modes became weaker and broader. The entire lattice mode structure disappears in the prototype phase. Our results indicate that the ethylammonium ions undergo enhanced hindered rotations in prototype phase.
Introduction
Experimental Section
In disordered materials, the molecular groups on certain lattice sites undergo hindered rotations in the prototype phase. On lowering the temperature these hindered rotations freeze and the system undergoes an order-disorder transition. The Raman spectroscopy in such transition region provides useful information. Anomalous variations in the Raman spectra occur in the band intensity and width near the order-disorder transition.1,2 The perovskite layered type compounds with the general formula (CnH2n+1NH3)2MX4 (M ) Cd, Mn, Pd, Sn, etc.; X ) Cl, Br, or I; and n ) 1, 2, 3 etc.) have attracted attention because they undergo many structural transitions and show twodimensional magnetic interactions and excitonic nonlinearities.3-8 The structural transitions in these compounds are mainly due to hindered rotations of alkylammonium chain.3-5 In order to understand the dynamics of the transition, a prior knowledge of structural variations in the parent alkylammonium halides is needed. Previous differential scanning calorimetry (DSC) and infrared spectroscopic studies in ethylammonium chloride have shown that there are at least two phase transitions, at 221 and 345 K, respectively.9 However, the earlier DSC report showed no transitions in this compound.10 We have undertaken a systematic Raman spectroscopic study in ethylammonium chloride (EAC) in view of this controversy. Our earlier results on the temperature-dependent Raman modes in the internal mode regime showed an evidence for an order-disorder type change at 345 K.11 Furthermore, we report a detailed temperature-dependent Raman study of a few thermosensitive modes to throw some light on the phase transitions. Temperature evolution of lattice modes and broadening of a few characteristic internal modes clearly demonstrate a phase change near 345 K.
EAC was obtained from Fluka AG and was used without further purification. An earlier report has described the difficulities in obtaining good quality single crystals.13 We also made an attempt to grow good quality single crystals but failed. Hence we used finely ground powder in the experiment. The Raman spectra were recorded by exciting with an argon laser (514.5 nm) with about 80 mW power. The temperature of the sample was varied using a homemade cell.3 A copperconstantan thermocouple was kept close to the sample to measure temperature. The temperature stability was better than (2 K. All the spectral changes could be reproduced within an accuracy of (2 cm-1, under similar experimental conditions. It is known that the finite slit width of the monochromator produces a measurable contribution to the actual line width, which can be corrected by the procedure described elsewhere.14,15 In brief, we estimated the true widths by adopting the following procedure: First we recorded the laser line by using the same slit conditions and carefuly measured the contribution to the linewidth due to slit (Γslit). The true widths were estimated by measuring the ratio of line widths between the Raman lines and that of slit, as described elesewhere.14,15 It has been observed that if this ratio is more than 3, the slit contribution to the measured Raman lines is marginal.
Crystal Structure and Phase Transitions Ethylammonium chloride (EAC) is isostructural with corresponding bromide and iodide salts in its ambient phase. It crystallizes into C52h space group with two molecules per unit cell. The lattice parameters are a ) 8.36, b ) 5.19, and c ) 4.51 Å and β ) 93.2°.12 The DSC experiments by Tasu and Gilson in 196810 in EAC claim no phase transitions upto its melting. However, in a subsequent study by Rao et al.9 observed two transitions at 221 and 345 K. The transition enthalopies are estimated to be 0.3 and 7.4 kJ mol-1, respectively. The crystal structures in the other phases are not known. X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0888$12.00/0
Results and Discussion EAC is monoclinic with a space group of C52h (Z ) 2) at ambient temperature. The free ion and site group symmetry of ethylammonium ion is Cs. Thus, under Cs f Cs f C52h correlation, each of the Cs(A′) mode is distributed as C52h(Ag + Bu), and that of Cs(A′′ ) as C52h(Bg + Au). Consequently, every internal mode of ethylammonium ion has a corresponding infrared and Raman active component in the lattice. Our studies show a broad agreement with the previous report.16 In the following we briefly describe mode identification and temperature-induced variations. Five Raman modes at 80, 86, 119, 155, and 192 cm-1 appear in the region below 200 cm-1. The modes at 80 and 86 cm-1 are assigned to the translational modes of chloride as they are inert to deuteration.16 Other three modes at 119, 155, and 192 cm-1 are librations of ethylammonium. These modes are intense in the lattice mode range and carry important information about dynamics. Other modes which provide vital information about phase changes namely ν(CC) and ν(CN) are found in the Raman spectra at 870 and 1048 cm-1, respectively, whereas, the corresponding IR components are at 885 and 1050 cm-1. The skeleton bending mode is observed at 416 and 425 cm-1 in © 1996 American Chemical Society
Raman Study of Ethylammonium Chloride
J. Phys. Chem., Vol. 100, No. 2, 1996 889
Figure 1. Temperature evolution of lattice modes in ethylammonium chloride.
Raman and IR spectra, respectively. A detailed mode identification is given elsewhere.3,16 In the following we describe temperature-induced variations in the lattice mode region. The temperature evolution of lattice modes is shown in Figure 1. The five lattice modes are clearly seen at ambient temperature (299 K). The temperature at which the spectra is recorded is indicated on the top of each curve. The first four curves (from the bottom) are during the warming cycle, whereas the other two are during the cooling cycle. The spectra comprising of the translational modes of chloride ions are recorded at a full scale sensitivity of 2000 counts, while the other portion is recorded at a higher sensitivity (1000 counts). On increasing the temperature, these modes gradually became weaker and broader and abruptly disappear near 346 K, a phase change regime. The modes reappear on decreasing the temperature, clearly indicating the reversibility. However, the modes corresponding to the translations of chloride ions recover its intensity from the prototype phase at a much slower rate. Furthermore, there is an appreciable increase in the Rayleigh wing in the high-temperature phase. Such an increase in the Rayleigh wing has previously been observed in these perovskite type layered compounds of the series and has been attributed to the disorder induced scattering.17,18 Moreover, the spectra look similar to that of solution state indicating that the prototype phase could be highly disordered. This became even clearer by the abrupt variations in line width of some internal modes.11 It is important to note that the peak frequency of these modes does not show softening as shown in Figure 2. However, the line widths of two low librations at 119 and 155 cm-1 show a nonlinear increase at higher temperature as shown in Figure 3. These vibrations show a striking similarity with those of the ν(CC) and ν(CN) modes (see Figure 3B). The abrupt disappearance of the entire lattice modes and broadening of some Raman modes around 346 K are the basic features indicating the transition. Further evidence of phase change is found by an anomalous variation in Raman strength and line width in ν(CH2 ) and 2δ(CH3 ) modes.11 It is wellknown that some of the Raman modes became weaker and broader near the order-disorder transition.1,2 It has been observed that the line widths of some modes in similar compounds (trimethylammonium chloride) show divergent behavior near the transition.19 Further, the lattice modes were replaced by a strong Rayleigh wing in perovskite type layed compounds near the order-disorder transition which was attributed to the disorder-induced mode.17,18 The abrupt disappearance of the strong lattice mode structure has also been observed near the order-disorder transition in some alkali
Figure 2. Raman frequency variation with temperature in ethylammonium chloride.
Figure 3. Line width changes of (A) lattice modes at 119 cm-1 (triangles) and 155 cm-1 (circles). Part B depicts the same for ν(CC) (triangles) and ν(CN) (circles) modes.
thiocyanides and has been attributed to cubic proto type phase.20 Such a detailed analysis in our case is not possible as no crystallographic information is available in the high temperature phase. Furthermore, all our attempts to grow good single crystals resulted in negative, confirming the earlier observation.13 This further prevents us to extend single-crystal Raman study to throw more light on the nature of the transition. It is interesting to note that Raman spectra at lower temperature regime (80 K) do not produce anomalous variations near 221 K which could be attributed to phase change.3 The line widths of all thermosensitive modes, as shown in Figure 3, vary linearly down to 80 K, indicating that this could be due to anharmonicity. The observed abrupt changes around 346 K are, however, due to disordering of ethylammonium ions. The Raman spectral variations clearly corroborate large entropy variations associated with the transition. It was difficult to record the Raman spectra deep in the disorder phase as the compound melts and dissociates at 383 K. Conclusions The Raman spectra of ethylammonium chloride in 80-346 K region have been investigated for phase changes. Reported
890 J. Phys. Chem., Vol. 100, No. 2, 1996 transitions at 345 K produce many spectral variations, like the disappearance of well-resolved lattice modes and abrupt broadening of ν(CC) and ν(CN) modes. All these variations elucidate that the phase change could be an order-disorder type. Acknowledgment. The author sincerely acknowledges many helpful discussions with Dr. Kailash C. Rustagi. Financial support through the scientist’s pool scheme from Council for Scientific and Industrial Research, New Delhi, is acknowledged with thanks. References and Notes (1) Bruce, A. D.; Taylor, W.; Murray, A. F. J. Phys. C 1980, 13, 483. (2) Dultz, W. J. Chem. Phys. 1976, 65, 2812. (3) Prasad, P. S. R. Ph.D dissertation, Department of Physics, Indian Institute of Technology, Kanpur, India, 1990. (4) Kind, R. Ferroelectrics, 1980, 24, 81. (5) Couzi, M. In Vibrational Spectra and Structures: Sixty Years on Raman Spectroscopy; Bist, H. D., Durig, J. R., Sullevan, J. A., Eds.; Elsevier Publ. Co.: Amsterdam, 1989; Vol 17A, p 49. (6) de Jongh, L. J.; Miedema, A. R. AdV. Phys. 1974, 23, 1. (7) Ishihara, T.; Takahashi J.; Goto, T. Solid State Commu. 1989, 69, 933.
Prasad (8) Mitzi, D. B.; Feild, C. A.; Harrison W. T. A.; Guloy, A. M. Nature 1994, 369, 467. (9) Rao, C. N. R.; Ganguli, S.; Ramachandra Swamy, H.; Oxton, I. A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1825. (10) Tasu, J.; Gilson, D. F. R. J. Phy. Chem. 1968, 72, 4082. (11) Prasad, P. S. R.; Bist, H. D. Proc. XI Int. Conf. Raman Spectrosc., London 1988, 457. (12) Jellinek, F. Acta. Crystallogr. 1958, 11, 626. (13) Ratcliffe, C. I.; Sherman, W. I.; Wilkinson, G. R. J. Raman Spectrosc. 1982, 13, 189. (14) Asthana, B. P.; Keifer, W. Appl. Spectrosc. 1982, 36, 250. (15) Prasad, P. S. R.; Sathaiah, S.; Bist, H. D. Chem. Phys. Lett. 1987, 142, 341. (16) Hagemann, H.; Bill, H. J. Chem. Phys. 1984, 80, 111. (17) Mokhlisse, R.; Couzi M.; Loyzance, P. L. J. Phys. C 1983, 16, 1367. (18) Mokhlisse, R.; Couzi, M.; Wang, C. H. J. Chem. Phys. 1982, 77, 1138. (19) Schlaak, M.; Couzi M.; Huong, P. V. Ber. Bunsenges. Phys. Chem. 1976, 80, 881. (20) Sathaiah, S.; Sarin, V. N.; Bist, H. D. J. Phys. Condens. Matter 1989, 1, 7829.
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