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Raman scattering study of pressure induced phase transition in paraelectric thiourea
~Volume 37, number 3
MAN
CHEMICAL
SCATTERING
IN PARAELECTRIC 2. IQBAL
1 February
PHYSICS LETTERS
1976
STUDY OF PRESSURE INDUCED PHASE TRANSITION
THIOiJREA
and C.W. CI-IRISTOE
FdttMtl Research Laboratories, Dover. New Jersey. USA Received
15 August 1975
The pressure dependence of Raman scattering due to the internal and external modes of vibration in pxaelcctric thiourca have been invwigatcd in relation to its pressure induced phase transition. The pressure induced changes in the internal mode spectrum suggest ihat a chmphg of the disordering molecule fluctuations coupled to changes in the S..H
hydrogen. bonds trigger the high pressure phase transition
1, Introduction Thiourea is a molecular type crystal which is unique among knc,wn ferroelectrics because its ferroelectric behaviour is associated with the relative displacements of the entire molecule rather than of the ions or protons within the crystal [I]. In view of its interesting properties extensive measurements using spectroscopic (infrared absorption [2J, &man and neutron scattering [2-51 and NMR [6]) and diffraction (X-ray [7 j and neutron [B]) techniques have been made on this material in both its paraelectric (PE) and ferroelectric (FE) phases. Pressure dependent measurements on this material have, however, been of a limited nature. A hi& pressure (HP:) phase was first noted by Bridgman [9! using volumetric techniques. Recently Figuiere et al. [IO] reported a detailed phase diagram
study of thiourea using the low frequency Raman peaks as the experimental probe. The HP phase has also been investigated by Lecmidova [ 111 using electric polarization and capacitance measurements and by Kabalkina [12] using X-ray diffraction. In this communication w: report on the interesting features of the pressure dependence of the external and internal mode Raman spectra of thiourea and the relationship of this data to the nature of the pressure Induced, Structural transition. We have paid particular attetitiok
to vibrations
whicil
could be affected
wing scattering arising from proton motions have anoma!ous pressure dependences as shown recently for KDP by Peercy and Fritz [14]. The crystal structure of thiourea in the PE phase at 293 K and 1 bar is orthorhombic with space group Pnma and iattice parameters a= 7.655, b = 8.537, c = 5.520 .& and 2 = 4 [7,8].
The neutrcn
diffraction
study of Elcombe role of H-bonding t!!e crystal. These bonding along the
and Taylor [B] has clarified the in the ferroelectric properties of results indicate strong S..H hydrogen b axis and large amplitude (11”) molectJar Iibration about the b axis in the
non-rigid PE phase. At 153 K (in the FE.phase)
the libration
becomes rigid and the amplitude about b is reduced to 5”. This suggests a probable correlation between H-bonding and the ferroelectric phase change consistent with the upward deuterium shift of T, by ca. 16 K [ 11. In the HP phase the results of Kabalinka [12] and Figuiere et al. [IO] are consistent with an orthbrhombic unit cell with strict orientational connection between the lattices of the PE and HP phases.
2. Experimental
by
changes in the strong S..M h:fdrogen bonding alcng _,
the b crystal axis. We have also looked for the KDP type wing scattering noted in the bb scattering data of Schrader et al. [3] by Scott [13]. Quasi-elastic
The Raman scattering measurements
were carried
Volume
37, number 3
CHEMICAL PHYSICS LETTERS
out using a modified Cary 81 double monochromator [15] coupled to a stabilized Coherent Radiation argon laser oscillating at 488.0 nm (300 mW power). Polarized Raman studies were also carried out with a Coderg triple monochromator Reman system. High pressures at 293 K were maintained to within +0.&I kbar using a 10 kbar hydrostatic pressure cetl described elsewhere [ 161. rz.hexane of spectral grade was used as the pressure transmission fluid. The data reported were taken with 2.0 cm-l or less spectral slit widths and calibrated (via Ar emissions) frequency accuracy off 1 .O cm-l. The single crys:als used were grown from methanol solution and oriented by Laue back reflection.
3. Results
The unpolarized Raman spectrum of a single crystal at 293 K and 1 bar showed the following peak frequen-
cies (assignments due to Bleckman et al. [17] and Schrader et al. [3] are given in parentheses and the frequencies are given in cm-r): 44.60.99.5 and 116 (lattice modes); 475 (6 N-C-N); 495 (rNH, + S..H stretch); 731 (~3 SCN2Hq); 1090 (v~SCNZH~); 1370, 1383 (ul SCN;H,); 3180 (v,NH) and 3370(v,,NH).
34cx’
AM
I
3232
,
i
.’
II.IOL
1976
In addition, there are peaks at 3235 and 3280 cm-l, which can be tentatively identified as second order modes in Fermi resonance with v,NH. The lattice modes are likely to have mixed librational and translational character, as pointed out by Bandy et al. [2]: We have also recorded the polarized Raman spectra to see if the wing scattering for the bb tensor component could be observed. Although we observed the N-H stretching modes increasing remarkably in intensity for this scattering component, in agreement with the data of Schrader et al. [3 1, we did not observe the quasielastic wing extending from O-100 cm-l in our crystals. It is also worth noting here that the quasi-elastic wing is also not evident in the data published by Bandy et al. [2]. The unpolarized Raman spectrum of thiourea in its
PE (at 3.3 kbar) and HP (at 5 kbar) phases are shown in fig. 1. The crystal was oriented in such a way that the unpclarized spectrum corresponds closely to that of the powder spectrum of thiourea. In the lattice mode region the strong peak at 60 cm-l (1 bar value) is replaced by a strong peak at 52 cm-r and a weak line at 93 cm-l (5 kbar values). The PE to HP transi-
and discussion
I
1 February
tion at 293 K was monitored by observing the intensity of the 60 cm-l peak as a function of pressure. The transition begins at 3.5 1 kbar in agreement with
I,I,IIIi,I
!-mllfOiweu~M7d0
,,,/I!
,,,,I
,,I,
‘,
ma0
,.,_&#$@j-
Fk. 1. Unpolarized hman spectra of thiourea crystal at 3.3 and 5.0 kbu pressure znd 295 K. Arrows HP phase and asterisks show peaks due to laser emissions.
indicate new peaks in the
461
Volume 37, number
3
CHEMICAL
PHYSICS LET-I-EM
the previous observations [9, lo]. Hbwever, both the PE and HP phases were found to co-exist 1-l~ to approximately 3.70 kbar. The reverse transition was found to begin at 3.27 kbar. Although the hysteresis is relatively small (0.24 kbar) compared with that observed for pressux induced transitions in ionic crystals [ 181, the transition is still likely to be largely first order since both the FE and HP phases co-exist at certain pressures. The relativeiy
small
change
in the lattice
mode
spec-
trum ifindicates that the HP and PE phase lattices are closely related in agreement with the X-ray data [12]. The HP phase lattice mode spectr:Jm, however, does not correspond to the spectrum in the FE phase below 160 K, indicating that the HP and FE phases are different. The pressure dependences of the external and internal modes of thiourea are plotted in fig. 2. The mode
FRESWRE,
Fig. 2. Pressure dependences HT phases of thiourea.
Htar
cf Bamm peaks in the PE 2nd
1 Februm
Griineisen parameters yi(k) for the lattice modes PE and HP phases calculated via the relation
YjCk)=[1/XTwi(k)l[6wi(k)/GplT 1
1976
in the
0)
where xT is the isothermal compressibility and wj(Jc) the mode frequency, are listed in table 1. [swi(k)/GP] T was determined from the slopes of the essentially linear pressure variation of the mode frequencies while xT was determined from the compression data of Bridgman [9] to be 5.25 X 10e3 kbar-l. The pressure coefficients of the extemai modes in the PE phase are roughly equal and much larger than those of the internal modes. The pressure coefficients and the Tj’S for the external modes in the HP phase are smaller compared with the values in the PE phase. These results indicate &at the HP phase iattice is less anharmohic than the PE phase structure. The internal modes show interesting changes on compression which suggest 2 possible mechanism for the phase change. Fran; fig. 2 it can be seen that the NH, stretching modes in the 3000 cm-l region and the S N-C-N mode at 475 cm-l soften slightly with increasing pressure in ‘LhePE phase. Also, as shown in fig. 3, the peaks at 475 aad 495 cm-l rapidly decrease in line-width in the vicinity of the transition pressure. For example, the deconvoluted liner,width (at half maximum) of the 495 cm-l line decreases from 32.0 cm -I at 3.50 kbar to 10 cm-l at 3.62 kbar. The large line-width of the 495 cm-l peak in the PE phase (assigned to the mixed 7NH, and S..H stretching vibration) could be due to anharmonic coupling with the damped molecular &rations about the b axis in particular. As the HP phase is approached the Iibrational oscillations probably decrease in amplitude resulting in the observed line-width narrowing of the
Table 1
Pressure coefficients 2nd Griineisen parameters
of external
modes in the PE and HP phases of thiourea
PE phase a)
HP phasea)
wi
16jrJ6PlT
Wi
(cm-‘)
(Lrn-‘kbar-‘) -1.73 2.OG 2.08 2.29
(cm-l)
44.0 60.0 .99.5 .. 116.0 --_,;. -.. a’ Freqcensics
.’ .467,..‘.
-PA-7.5 6.3 4.0 3.8
52.0 93.0 110.0 130.0
in the PE phase are at 1 bar and in the HP phase at 5 kbu.
,. .-:-:. -,...I
: -1
_:
..
[a Wj/6plT
(cm-‘kbzr-‘) 0.47 1.84 1.32 1.18
1.7 3i7 2.3 1.7
CHEMICAL
PHYSICS LETTERS
1 Febmvy
1976
and usNH2 modes, suggesting that two crystallographically nonequivalent thiourea sites are created in the HP phase.
~3 SCN2H4
References [l] C. Cdvo, J. Chem. Phys. 33 (1960) 1721, and references
therein [2] A. Bandy,.G.L. Cessx and E.R. Lippincott, Spectrochim.
Fig. 3. Raman spectra of thiourea between in the vicinity of the transition pressure.
450 and 550 cm-’
495 cm-l line. The S..H bonding interactions also increase as the HP phase is approached as shown by the decrease in the frequency of the NH2 stretching motions with pressure in the PE phase. The clamping of the molecular oscillations coupled to enhanced H-bonding with increasing pressure, therefore, appears to trigger the structural phase transition. hnother interesting feature which emerges from the interna!
mode spectra in the HP phase is the observation of splitting (of the order of 10 ~rn-~) of the v2,
Acta 28A (1972) 1807, and references therein. 131 B. Schnder, W. hleier, K;. Gottlieb, H. Agatha, H. Barentzcn. and P. Bleckmann, Ber. Bunsenges. Physik. Chem. 75 (1971) 1263. I41 J.J. Rush, J. Gem. Phys. 47 (1967) 4278. iSI A. Bajarek, D. Parlinski, ?.I. Sudnik, M. Hrynkieivicy and J.A. Jmik, Physica 35 (1969) 469. 161 I.W. ELnsley and J.A.S. Smith, Trans.. Faraday Sot. 57 (1961) 893, 1233,. I71 G-l. Goldsmith and J.G. White, 5. Chem. Phys. 31 (1959) 1175. I81 XI. Elcombe and J.C. Taylor, Acta Cryst. A24 (1968) 410. 191 P.W. Bridgman, Proc. Am. Acad. Arr Sci. 72 (1937) 227. I101 P. Figuiere, M. Ghelfenstein and H. Szwarc, Chem. Phys. Letters 33 (1975) ?9. [ill C.G. Lconidova, Fiz. Tvcrd. Tela 5 (1963) 3430. [121 S.S. Kabalkina, Zh. Fiz. Khim. 35 (1961) 276. [131 J.F. Scott, Rev. ?.lod. Phys. 46 (1974) 83. I141 PS. Peercy and I.J. Fritz, Fhys. Rev. Letters 32 (1974) 466. [I51 G.R. Elliott and Z. Iqbal, J. Chem. Phys. 63 (15 August, 1975) to be published. 1161 Z. Iqbal and C.W. Christoe, J. Chcm. Phys. 62 (1975) 3246. Il.71 P. Bleckmann, B. Schrader, W. hieier and H. Takahashi, Ber. Bunsenges. Physik. Chem. 75 (1971) 1279. 15 [I81 C.W. Chrirtoe and Z. Iqbal, Solid State Commun (1974) 859.
463
MAN
CHEMICAL
SCATTERING
IN PARAELECTRIC 2. IQBAL
1 February
PHYSICS LETTERS
1976
STUDY OF PRESSURE INDUCED PHASE TRANSITION
THIOiJREA
and C.W. CI-IRISTOE
FdttMtl Research Laboratories, Dover. New Jersey. USA Received
15 August 1975
The pressure dependence of Raman scattering due to the internal and external modes of vibration in pxaelcctric thiourca have been invwigatcd in relation to its pressure induced phase transition. The pressure induced changes in the internal mode spectrum suggest ihat a chmphg of the disordering molecule fluctuations coupled to changes in the S..H
hydrogen. bonds trigger the high pressure phase transition
1, Introduction Thiourea is a molecular type crystal which is unique among knc,wn ferroelectrics because its ferroelectric behaviour is associated with the relative displacements of the entire molecule rather than of the ions or protons within the crystal [I]. In view of its interesting properties extensive measurements using spectroscopic (infrared absorption [2J, &man and neutron scattering [2-51 and NMR [6]) and diffraction (X-ray [7 j and neutron [B]) techniques have been made on this material in both its paraelectric (PE) and ferroelectric (FE) phases. Pressure dependent measurements on this material have, however, been of a limited nature. A hi& pressure (HP:) phase was first noted by Bridgman [9! using volumetric techniques. Recently Figuiere et al. [IO] reported a detailed phase diagram
study of thiourea using the low frequency Raman peaks as the experimental probe. The HP phase has also been investigated by Lecmidova [ 111 using electric polarization and capacitance measurements and by Kabalkina [12] using X-ray diffraction. In this communication w: report on the interesting features of the pressure dependence of the external and internal mode Raman spectra of thiourea and the relationship of this data to the nature of the pressure Induced, Structural transition. We have paid particular attetitiok
to vibrations
whicil
could be affected
wing scattering arising from proton motions have anoma!ous pressure dependences as shown recently for KDP by Peercy and Fritz [14]. The crystal structure of thiourea in the PE phase at 293 K and 1 bar is orthorhombic with space group Pnma and iattice parameters a= 7.655, b = 8.537, c = 5.520 .& and 2 = 4 [7,8].
The neutrcn
diffraction
study of Elcombe role of H-bonding t!!e crystal. These bonding along the
and Taylor [B] has clarified the in the ferroelectric properties of results indicate strong S..H hydrogen b axis and large amplitude (11”) molectJar Iibration about the b axis in the
non-rigid PE phase. At 153 K (in the FE.phase)
the libration
becomes rigid and the amplitude about b is reduced to 5”. This suggests a probable correlation between H-bonding and the ferroelectric phase change consistent with the upward deuterium shift of T, by ca. 16 K [ 11. In the HP phase the results of Kabalinka [12] and Figuiere et al. [IO] are consistent with an orthbrhombic unit cell with strict orientational connection between the lattices of the PE and HP phases.
2. Experimental
by
changes in the strong S..M h:fdrogen bonding alcng _,
the b crystal axis. We have also looked for the KDP type wing scattering noted in the bb scattering data of Schrader et al. [3] by Scott [13]. Quasi-elastic
The Raman scattering measurements
were carried
Volume
37, number 3
CHEMICAL PHYSICS LETTERS
out using a modified Cary 81 double monochromator [15] coupled to a stabilized Coherent Radiation argon laser oscillating at 488.0 nm (300 mW power). Polarized Raman studies were also carried out with a Coderg triple monochromator Reman system. High pressures at 293 K were maintained to within +0.&I kbar using a 10 kbar hydrostatic pressure cetl described elsewhere [ 161. rz.hexane of spectral grade was used as the pressure transmission fluid. The data reported were taken with 2.0 cm-l or less spectral slit widths and calibrated (via Ar emissions) frequency accuracy off 1 .O cm-l. The single crys:als used were grown from methanol solution and oriented by Laue back reflection.
3. Results
The unpolarized Raman spectrum of a single crystal at 293 K and 1 bar showed the following peak frequen-
cies (assignments due to Bleckman et al. [17] and Schrader et al. [3] are given in parentheses and the frequencies are given in cm-r): 44.60.99.5 and 116 (lattice modes); 475 (6 N-C-N); 495 (rNH, + S..H stretch); 731 (~3 SCN2Hq); 1090 (v~SCNZH~); 1370, 1383 (ul SCN;H,); 3180 (v,NH) and 3370(v,,NH).
34cx’
AM
I
3232
,
i
.’
II.IOL
1976
In addition, there are peaks at 3235 and 3280 cm-l, which can be tentatively identified as second order modes in Fermi resonance with v,NH. The lattice modes are likely to have mixed librational and translational character, as pointed out by Bandy et al. [2]: We have also recorded the polarized Raman spectra to see if the wing scattering for the bb tensor component could be observed. Although we observed the N-H stretching modes increasing remarkably in intensity for this scattering component, in agreement with the data of Schrader et al. [3 1, we did not observe the quasielastic wing extending from O-100 cm-l in our crystals. It is also worth noting here that the quasi-elastic wing is also not evident in the data published by Bandy et al. [2]. The unpolarized Raman spectrum of thiourea in its
PE (at 3.3 kbar) and HP (at 5 kbar) phases are shown in fig. 1. The crystal was oriented in such a way that the unpclarized spectrum corresponds closely to that of the powder spectrum of thiourea. In the lattice mode region the strong peak at 60 cm-l (1 bar value) is replaced by a strong peak at 52 cm-r and a weak line at 93 cm-l (5 kbar values). The PE to HP transi-
and discussion
I
1 February
tion at 293 K was monitored by observing the intensity of the 60 cm-l peak as a function of pressure. The transition begins at 3.5 1 kbar in agreement with
I,I,IIIi,I
!-mllfOiweu~M7d0
,,,/I!
,,,,I
,,I,
‘,
ma0
,.,_&#$@j-
Fk. 1. Unpolarized hman spectra of thiourea crystal at 3.3 and 5.0 kbu pressure znd 295 K. Arrows HP phase and asterisks show peaks due to laser emissions.
indicate new peaks in the
461
Volume 37, number
3
CHEMICAL
PHYSICS LET-I-EM
the previous observations [9, lo]. Hbwever, both the PE and HP phases were found to co-exist 1-l~ to approximately 3.70 kbar. The reverse transition was found to begin at 3.27 kbar. Although the hysteresis is relatively small (0.24 kbar) compared with that observed for pressux induced transitions in ionic crystals [ 181, the transition is still likely to be largely first order since both the FE and HP phases co-exist at certain pressures. The relativeiy
small
change
in the lattice
mode
spec-
trum ifindicates that the HP and PE phase lattices are closely related in agreement with the X-ray data [12]. The HP phase lattice mode spectr:Jm, however, does not correspond to the spectrum in the FE phase below 160 K, indicating that the HP and FE phases are different. The pressure dependences of the external and internal modes of thiourea are plotted in fig. 2. The mode
FRESWRE,
Fig. 2. Pressure dependences HT phases of thiourea.
Htar
cf Bamm peaks in the PE 2nd
1 Februm
Griineisen parameters yi(k) for the lattice modes PE and HP phases calculated via the relation
YjCk)=[1/XTwi(k)l[6wi(k)/GplT 1
1976
in the
0)
where xT is the isothermal compressibility and wj(Jc) the mode frequency, are listed in table 1. [swi(k)/GP] T was determined from the slopes of the essentially linear pressure variation of the mode frequencies while xT was determined from the compression data of Bridgman [9] to be 5.25 X 10e3 kbar-l. The pressure coefficients of the extemai modes in the PE phase are roughly equal and much larger than those of the internal modes. The pressure coefficients and the Tj’S for the external modes in the HP phase are smaller compared with the values in the PE phase. These results indicate &at the HP phase iattice is less anharmohic than the PE phase structure. The internal modes show interesting changes on compression which suggest 2 possible mechanism for the phase change. Fran; fig. 2 it can be seen that the NH, stretching modes in the 3000 cm-l region and the S N-C-N mode at 475 cm-l soften slightly with increasing pressure in ‘LhePE phase. Also, as shown in fig. 3, the peaks at 475 aad 495 cm-l rapidly decrease in line-width in the vicinity of the transition pressure. For example, the deconvoluted liner,width (at half maximum) of the 495 cm-l line decreases from 32.0 cm -I at 3.50 kbar to 10 cm-l at 3.62 kbar. The large line-width of the 495 cm-l peak in the PE phase (assigned to the mixed 7NH, and S..H stretching vibration) could be due to anharmonic coupling with the damped molecular &rations about the b axis in particular. As the HP phase is approached the Iibrational oscillations probably decrease in amplitude resulting in the observed line-width narrowing of the
Table 1
Pressure coefficients 2nd Griineisen parameters
of external
modes in the PE and HP phases of thiourea
PE phase a)
HP phasea)
wi
16jrJ6PlT
Wi
(cm-‘)
(Lrn-‘kbar-‘) -1.73 2.OG 2.08 2.29
(cm-l)
44.0 60.0 .99.5 .. 116.0 --_,;. -.. a’ Freqcensics
.’ .467,..‘.
-PA-7.5 6.3 4.0 3.8
52.0 93.0 110.0 130.0
in the PE phase are at 1 bar and in the HP phase at 5 kbu.
,. .-:-:. -,...I
: -1
_:
..
[a Wj/6plT
(cm-‘kbzr-‘) 0.47 1.84 1.32 1.18
1.7 3i7 2.3 1.7
CHEMICAL
PHYSICS LETTERS
1 Febmvy
1976
and usNH2 modes, suggesting that two crystallographically nonequivalent thiourea sites are created in the HP phase.
~3 SCN2H4
References [l] C. Cdvo, J. Chem. Phys. 33 (1960) 1721, and references
therein [2] A. Bandy,.G.L. Cessx and E.R. Lippincott, Spectrochim.
Fig. 3. Raman spectra of thiourea between in the vicinity of the transition pressure.
450 and 550 cm-’
495 cm-l line. The S..H bonding interactions also increase as the HP phase is approached as shown by the decrease in the frequency of the NH2 stretching motions with pressure in the PE phase. The clamping of the molecular oscillations coupled to enhanced H-bonding with increasing pressure, therefore, appears to trigger the structural phase transition. hnother interesting feature which emerges from the interna!
mode spectra in the HP phase is the observation of splitting (of the order of 10 ~rn-~) of the v2,
Acta 28A (1972) 1807, and references therein. 131 B. Schnder, W. hleier, K;. Gottlieb, H. Agatha, H. Barentzcn. and P. Bleckmann, Ber. Bunsenges. Physik. Chem. 75 (1971) 1263. I41 J.J. Rush, J. Gem. Phys. 47 (1967) 4278. iSI A. Bajarek, D. Parlinski, ?.I. Sudnik, M. Hrynkieivicy and J.A. Jmik, Physica 35 (1969) 469. 161 I.W. ELnsley and J.A.S. Smith, Trans.. Faraday Sot. 57 (1961) 893, 1233,. I71 G-l. Goldsmith and J.G. White, 5. Chem. Phys. 31 (1959) 1175. I81 XI. Elcombe and J.C. Taylor, Acta Cryst. A24 (1968) 410. 191 P.W. Bridgman, Proc. Am. Acad. Arr Sci. 72 (1937) 227. I101 P. Figuiere, M. Ghelfenstein and H. Szwarc, Chem. Phys. Letters 33 (1975) ?9. [ill C.G. Lconidova, Fiz. Tvcrd. Tela 5 (1963) 3430. [121 S.S. Kabalkina, Zh. Fiz. Khim. 35 (1961) 276. [131 J.F. Scott, Rev. ?.lod. Phys. 46 (1974) 83. I141 PS. Peercy and I.J. Fritz, Fhys. Rev. Letters 32 (1974) 466. [I51 G.R. Elliott and Z. Iqbal, J. Chem. Phys. 63 (15 August, 1975) to be published. 1161 Z. Iqbal and C.W. Christoe, J. Chcm. Phys. 62 (1975) 3246. Il.71 P. Bleckmann, B. Schrader, W. hieier and H. Takahashi, Ber. Bunsenges. Physik. Chem. 75 (1971) 1279. 15 [I81 C.W. Chrirtoe and Z. Iqbal, Solid State Commun (1974) 859.
463