Phase transition in triglycine sulfate crystals by 1H and 13C nuclear magnetic resonance in the rotating frame

Article abstract of DOI:10.1016/j.molstruc.2013.06.010

The ferroelectric phase transition in triglycine sulfate ((NH ₂CH₂COOH)₃·H₂SO₄, TGS)) crystals, occurring at T_C of 322 K, was studied using ¹H and ¹³C CP/MAS NMR. From the spin-lattice relaxation time in the rotating frame, T_1ρ, of ¹H and ¹³C, we found that the slopes of the T_1ρ versus temperature curve changed near T_C. In addition, the change of intensities for the protons and carbons NMR signals in the ferroelectric and the paraelectric phases led to the noticeable changes in the environments of proton and carbon in the carboxyl groups. The carboxyl ordering was the dominant factor driving the phase transition. Our study of the ¹H and ¹³C spectra showed that the ferroelectric phase transition of TGS is of the order-disorder type due to ordering of the carboxyl groups.

Full text of DOI:10.1016/j.molstruc.2013.06.010

Journal of Molecular Structure 1048 (2013) 471–475
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
1
13
Phase transition in triglycine sulfate crystals by H and C nuclear
magnetic resonance in the rotating frame
Ae Ran Lim a, , Se-Young Jeong b
⇑
a
Department of Science Education, Jeonju University, Jeonju 560-759, Republic of Korea
School of Nanoscience & Technology, Pusan National University, Miryang 627-706, Republic of Korea
b
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
The ferroelectric phase transition in triglycine sulfate ((NH
The spin–lattice relaxation time in the rotating frame, T , of H and C.
The ferroelectric phase transition of TGS by ordering of the carboxyl groups.
2
CH
2
COOH)
3
ꢁH
2 4
SO , TGS)) crystals.
1
13
1q
a r t i c l e i n f o
a b s t r a c t
Article history:
The ferroelectric phase transition in triglycine sulfate ((NH CH COOH) ꢁH SO , TGS)) crystals, occurring
1 13
2
2
3
2
4
Received 5 November 2012
Received in revised form 18 May 2013
Accepted 4 June 2013
at T
rotating frame, T
near T . In addition, the change of intensities for the protons and carbons NMR signals in the ferroelectric
C
of 322 K, was studied using H and C CP/MAS NMR. From the spin–lattice relaxation time in the
1
13
1q 1q
, of H and C, we found that the slopes of the T versus temperature curve changed
C
Available online 10 June 2013
and the paraelectric phases led to the noticeable changes in the environments of proton and carbon in the
carboxyl groups. The carboxyl ordering was the dominant factor driving the phase transition. Our study
Keywords:
1
13
of the H and C spectra showed that the ferroelectric phase transition of TGS is of the order–disorder
type due to ordering of the carboxyl groups.
(
NH
TGS
Ferroelectrics
Order–disorder type
2
CH
2
COOH)
3
ꢁH SO
2 4
Ó 2013 Elsevier B.V. All rights reserved.
Nuclear magnetic resonance
Phase transition
1
. Introduction
The most significant effects of the ferroelectric transition were a
large change in the time-averaged value of the CꢂN bond direction
Numerous experimental studies have investigated the relaxation
behavior of ferroelectric crystals of triglycine sulfate [(NH
COOH) SO ; TGS] [1,2]. Nonlinearity of real ferroelectric crystals
in ac electric fields has been observed in various studies [3–6]. The
nonlinear properties of ferroelectric crystals remain poorly under-
stood, and constitute a priority problem in ferroelectric physics
of glycine I, and the onset of the chemical inequivalence of glycines
II and III due to ordering of the protons in the short OꢂHꢁ ꢁ ꢁO bond.
The cited authors concluded that the phase transition was of the
order–disorder type, on the basis of their pulsed proton-nitrogen
double resonance study [15]. The temperature dependence of the
2
CH2-
ꢁH
3 2
4
1
proton spin–lattice relaxation time, T , in TGS was investigated
[
7]. The electrical properties and domain structures of ferroelectric
near the transition temperature by Blinc et al. [16] and Tsujimi
et al. [17]. Despite the important role of these materials in various
applications, unresolved problems remain regarding the mecha-
TGS crystals have been discussed in recent publications [8–13].
TGS is a ferroelectric material that shows second-order phase tran-
sitions, with a Curie point at around room temperature [14]; the Cur-
1
nism of the ferroelectric phase transition. Although H spin–lattice
1
4
ie temperature is 322 K (=T
C
). The ferroelectric phase transition in
1
relaxation time T and N NMR spectrum investigations of TGS in
TGS crystals has been extensively investigated, as it is a model exam-
ple of a continuous second-order phase transition.
the laboratory frame have been conducted over the last years, the
protons and carbons in the ferroelectric and paraelectric phases
have not been discussed. For instance, there is a lack of information
obtained from dynamic measurements, such as measurements of
Blinc et al. [15] used nuclear magnetic pulsed double resonance
in the rotating frame to study changes in the 1 N quadrupole inter-
actions in the ferroelectric and paraelectric phases of a TGS crystal.
4
1q
the spin–lattice relaxation time T in a rotating frame for each
proton and carbon.
To investigate the influence of structural changes on the phase
transition properties of TGS, we studied the nature of each proton
⇑
Corresponding author. Tel.: +82 (0) 63 220 2514; fax: +82 (0) 63 220 2053.
E-mail addresses: aeranlim@hanmail.net, arlim@jj.ac.kr (A.R. Lim).
0
022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.06.010

4
72
A.R. Lim, S.-Y. Jeong / Journal of Molecular Structure 1048 (2013) 471–475
and carbon. Specifically, to better elucidate the nature of the ferro-
electric phase transition in these crystals, and, in particular, to ob-
tain a better understanding of the internal molecular motion in the
ferroelectric and paraelectric phases, we measured the tempera-
ture dependence of the nuclear magnetic resonance (NMR) spec-
trum and the nuclear spin–lattice relaxation times in the rotating
Cross-polarization, magic angle spinning (CP/MAS) 1H and 13
C
NMR experiments were performed at a Larmor frequency of
400.12 MHz and 100.61 MHz, respectively. The samples were
placed in the 4 mm CP/MAS probe as powders. The magic angle
1
13
spinning rate was set at 14 kHz and 4–6 kHz for H and C CP/
MAS, respectively, to minimize spinning sideband overlap. The
p
/
1
13
1
13
frame T
1
q
for H and C in TGS. Based on these results, we will dis-
2 pulse time for H and C was 10
ls and 16.8 ls, respectively,
cuss the roles for the each proton and carbon.
corresponding to a spin-locking field strength of 27.77 kHz and
1
13
4
1q
6.66 kHz. The H and C T measurements were performed by
1
13
applying H and C spin-locking pulses after the CP preparation
period.
2
. Crystal structure
TGS crystals with the phase transition temperature of 322 K are
monoclinic in ferroelectric phase, and belong in the high-tempera-
ture paraelectric phase of the centro-symmetrical space group P2
m. Below the Curie point, the mirror plane disappears and the
space group is P2 . The ferroelectric transition is of second-order,
and the spontaneous polarization is parallel to the monoclinic b-
axis. The crystal structure of TGS projected along the c-axis is
shown in Fig. 1 [18]. The lattice constants in ferroelectric phase
are a = 9.1666 Å, b = 12.6436 Å, c = 5.7339 Å, and b = 105.5°
4
. Experimental results and discussion
1
/
Structural analysis of the protons in TGS was carried out by a
1
1
solid state NMR method. Fig. 2a shows the H MAS NMR spectrum
of a TGS single crystal at room temperature. The NMR spectrum
consists of three peaks, at chemical shift of d = 7.60, 15.15, and
1
8.22 ppm. The spinning sidebands are marked with asterisks.
The signal at chemical shift of 7.60 ppm is assigned to the ammo-
nium hydrogen, methylene proton, and amine hydrogen. The sig-
nals at chemical shifts of 15.15 ppm and 18.22 ppm are assigned
to the carboxyl hydrogen and hydrogen sulfate, respectively.
The intensities in the each proton signals as a function of tem-
perature for TGS are shown in Fig. 2b. As the temperature is in-
creased, the intensity of the signal due to the ammonium
hydrogen, methylene proton, and amine hydrogen (labeled as 1,
[
(
18,19]. There are two formula units per unit cell:
þ
ꢂ
NH CH
2
COO ) and (NH CH
2 2
2 2 4
COOH) ꢁH SO [20]. In the ferroelec-
3
tric phase, the two monoprotonated glycine groups I and III (NH2-
CH
þ
ꢂ
2
COOH and NH CH
3
2
COO , hereinafter designated G(I) and
G(III)) are completely planar, whereas the zwitterions
þ
ꢂ
(
NH CH
2
COO , hereinafter designated G(II)) are only partially pla-
3
nar [15,21]. The nitrogen atoms form NꢂHꢁ ꢁ ꢁO hydrogen bonds of
the usual strength, whereas the strong OꢂHꢁ ꢁ ꢁO hydrogen bond
incorporating the oxygen atom of the carboxyl group of the zwit-
terion glycine has a distance of 2.43 Å [21]. The bond distances
and angles in G(I) change only slightly with temperature, whereas
the variations in G(II) and G(III) are significant [18].
2
, and 3) is near constant. The intensities of the signals due to
the carboxyl hydrogen (labeled as 4) and hydrogen sulfate (la-
beled as 5) are abruptly decrease with temperature, whereas
those of the carboxyl hydrogen and hydrogen sulfate is nearly
C
constant above T , indicating that the carboxyl hydrogen and
hydrogen sulfate play an important role in the phase transition.
Usually, the dynamic ordering of hydrogen occurs in one of the
hydrogen bonds as the material transforms into the ferroelectric
phase [22].
3
. Experimental method
TGS single crystals were grown at room temperature by slowly
evaporating an aqueous solution containing NH
2
2
CH COOH and
1q
The spin–lattice relaxation times in the rotating frame, T ,
H
2
SO
4
in the stoichometric ratio of 3:1. The chemical formula of
were taken at several temperatures for the each proton in TGS.
The nuclear magnetization recovery traces obtained for all
protons were described by the following single exponential
function [23]:
þ
ꢂ
TGS is (NH CH
2
COO ) and (NH
2
CH COOH) ꢁH
2 2 2
SO
4
. The resulting
3
single crystals were transparent and colorless with dimensions of
3
1
0 ꢃ 10 ꢃ 5 mm .
Solid-state NMR experiments were performed using a Bruker
MðtÞ ¼ M
o
expðꢂt=T
1
q
Þ
ð1Þ
DSX 400 FT NMR spectrometer at the Korea Basic Science Institute.
Fig. 1. The crystal structure of ferroelectric (NH
2
CH
2
3
COOH) ꢁH
2 4
SO projected along the c-axis. Dashed lines show the hydrogen bonds [18].

A.R. Lim, S.-Y. Jeong / Journal of Molecular Structure 1048 (2013) 471–475
473
is no evidence of an anomalous decrease in T
1
q
C
near T . The relaxa-
tion time increased as the temperature was increased above the
temperature at which a minimum was observed (i.e., 190 K). The
minimum in T
of protons is present. The form of the proton T
perature curve indicates that the relaxation process occurs via
molecular motion. The T values can be related to the rotational
correlation time, , which corresponds to the length of time that
a molecule remains in a given state before undergoing reorienta-
tion. Thus, is a direct measure of the rate of rotational motion.
The experimental value of T can be expressed in terms of the iso-
1
q
at 190 K indicates that distinct molecular motion
1q
versus inverse tem-
1q
s
C
s
C
1q
tropic correlation time for molecular motion using the Bloember-
gen-Purcell-Pound (BPP) function [24]. According to the BPP
1q
theory, the T value for a spin–lattice interaction in the case of ran-
dom motion is given by
ꢂ1
3
HꢂC
2
ðnT1q Þ ¼ 0:05ðcHcC ꢀh =r Þ ½4A þ B þ 3C þ 6D þ 6Eꢄ
ð2Þ
where
2
1
2
A ¼
B ¼
C ¼
D ¼
E ¼
s
C
=½1 þ
x
s
ꢄ
C
2
2
s
C
=½1 þ ð
x
H
ꢂ
x
C
Þ
s
ꢄ
C
2
H
2
s
C
=½1 þ
x
sC Þ
2
2
s
C
=½1 þ ð
x
H
þ
ꢄ
x
C
Þ
s
ꢄ
C
2
H
2
s
C
=½1 þ
x
s
C
Here,
c
H
and
c
C
are the gyromagnetic ratio for the 1H and 13
C
nuclei, respectively, n is the number of directly bound protons, r
is the H–C internuclear distance, ⁄ = h/2 (where h is Planck’s con-
stant), and are the Larmor frequencies of H and C, respec-
tively, and is the spin-lock field. The analysis of our data was
carried out assuming that T would show a minimum when
= 1, and that the BPP relation between T and the character-
istic frequency of motion could be applied. Since the T curves
p
1
13
x
H
x
C
x
1
1
q
Fig. 2. (a) Solid state 1H MAS NMR spectrum for TGS at room temperature and (b)
the intensity variations of each proton in TGS as a function of temperature (1, 2, and
x
1
s
C
1q
x
1
1q
3
: ammonium hydrogen, methylene proton, and amine hydrogen; 4: carboxyl
hydrogen; 5: hydrogen sulfate).
were found to exhibit minima, it was possible to determine the
coefficient in the BPP formula. Having determined this coefficient,
we were then able to calculate the parameter
temperature. The temperature dependence of sC followed a simple
Arrhenius expression, /RT), where is the pre-
exp (ꢂE
exponential factor, T is the temperature, R is the gas constant,
and E is the activation energy. Thus, the slope of the straight-line
portion of the semilog plot could be used to determine the activa-
tion energy, E . The activation energy for the molecular motion can
C
s as a function of
where M(t) is the magnetization at time t, and M
o
is the total nucle-
1
ar magnetization of H at thermal equilibrium. The recovery traces
are shown in Fig. 3 for delay times ranging from 0.001 ms to 300 ms
at room temperature. The recovery traces showed a single exponen-
tial decay at all temperatures. The slopes of the recovery traces are
s
C =
s
o
a
s
o
a
1
different at each temperature. The H spin–lattice relaxation time in
the rotating frame is shown in Fig. 4 as a function of inverse temper-
a
1q C
ature. While the proton T data show changes in slope at T , there
Fig. 3. The saturation recovery traces of 1H in TGS as a function of delay time at
Fig. 4. The 1H spin–lattice relaxation time, T
, in the rotating frame for the TGS as a
1q
room temperature.
function of inverse temperature.

4
74
A.R. Lim, S.-Y. Jeong / Journal of Molecular Structure 1048 (2013) 471–475
be obtained from the log
The pre-exponential factors for each proton were as shown in Ta-
ble 1. Below and above T , the activation energies were obtained,
and observed that the change of E for the hydrogen sulfate (5)
C
s versus 1000/T curve, as shown in Fig. 5.
C
a
was larger than that of other protons (1, 2, 3, and 4).
Structural analysis of the carbons in TGS was carried out using
1
3
13
C NMR spectroscopy. Fig. 6a shows the solid-state C CP/MAS
1
3
NMR spectrum. The C NMR spectrum for TGS shows four signals,
at chemical shifts of d = 41.04, 42.59, 170.43, and 173.97 ppm. The
spinning sidebands are marked with asterisks. The signals at chem-
ical shifts of d = 41.04 ppm and d = 42.59 ppm represent the meth-
þ
ꢂ
ylene carbons in (NH CH
2
COO ) and (NH
2
CH
2
COOH)
2
ꢁH
2 4
SO ,
3
respectively, while the signals at d = 170.43 ppm and d =
73.97 ppm represent the carboxylate in (NH CH
þ
ꢂ
1
2
COO ) and the
3
carboxylic acid in (NH
2
CH
2
COOH)
2
ꢁH
2 4
SO , respectively.
The variations in the carbon peak intensities as a function of
temperature for a TGS are shown in Fig. 6b. As the temperature
is increased, the intensities of the signals due to the methylene car-
þ
ꢂ
bons (NH CH
2
COO and NH
2
CH
2
COOH) remain constant, whereas
3
the intensities of the signals due to the carboxyl groups in
þ
ꢂ
NH CH
2
COO and NH
2
2
CH COOH decrease near T
C
, indicating that
3
the carboxyl groups play an important role in the phase transition.
Based on these results, we attribute the temperature-dependent
decrease in the intensity of the carboxyl group signals to the order-
ing of the carboxyl groups. Prompted by the result reported in the
literature [22] that dynamic ordering of hydrogen occurs in one of
the hydrogen bonds as the material goes into the ferroelectric
phase, we propose that the decrease of the intensity of the signal
from carbons with temperature is related to the ordering of such
protons.
Fig. 6. (a) Solid state 13C CP/MAS NMR spectrum for TGS at room temperature and
b) the intensity variations of each carbon in TGS as a function of temperature (1
(
and 2: methylene carbons; 3 and 4: carboxyl groups).
1q
The spin–lattice relaxation times in the rotating frame, T ,
were taken for each carbon in the TGS at several temperatures,
with variable spin locks on the carbon channel following cross
1
3
polarization. The C magnetization was generated by cross-polar-
ization, after spin locking of the protons. The proton field was then
1
3
turned off for a variable time t, while the C rf field remained on.
1
3
Finally, the C free induction decay was observed under high
power proton decoupling, and was subsequently Fourier trans-
1q
formed. Values of T could be selectively obtained by the Fourier
transformation of the FID, following the end of spin locking and
Fig. 5. Arrhenius plot of the natural logarithm of the correlation times for proton as
a function of the inverse temperature.
repetition of the experiment with variations in the time t. All of
Table 1
The pre-exponential factor
s
o
and activation energy E
a
for 1H below and above T
C
of ammonium hydrogen (1), methylene proton (2), amine hydrogen (3), carboxyl hydrogen (4),
and hydrogen sulfate (5) in TGS.
Label for protons
s
o
E
a
(kJ/mol)
E
a
(kJ/mol)
E
a
(kJ/mol)
a
E (kJ/mol)
Brosowski et al. [25]
a
a
b
b
b
Present work
Present work
Blinc et al. [16]
Tsujimi et al. [17]
T < T
C
ꢂ12
1
4
5
,2, and 3
9.00 ꢃ 10
19.19 ± 1.49
18.30 ± 1.49
22.21 ± 1.60
22.18
20.25
24.11
ꢂ11
1.45 ꢃ 10
ꢂ12
1.62 ꢃ 10
T > T
C
ꢂ10
1
4
5
,2, and 3
1.49 ꢃ 10
11.73 ± 1.37
12.64 ± 0.88
13.47 ± 0.76
22.18
17.36
21.22
ꢂ11
8.78 ꢃ 10
ꢂ11
4.48 ꢃ 10
a
The data obtained by 1H T
The data obtained by 1H T
1
q
.
b
1
.

A.R. Lim, S.-Y. Jeong / Journal of Molecular Structure 1048 (2013) 471–475
475
ylene proton, amine hydrogen, carboxyl hydrogen, and hydrogen
sulfate, respectively. Additionally, methylene carbons and carboxyl
1
3
groups from C CP/MAS NMR spectrum were also distinguished.
The molecular motions of the carboxyl groups in the ferroelectric
phase were very large, whereas those of the methylene carbons
1
3
1q
were not large. Changes in C T associated with the carboxyl
groups were observed at the transition from the ferroelectric to
the paraelectric phase. The changes in the environments of proton
and carbon in the carboxyl groups in the ferroelectric and the para-
electric phases led to the noticeable change in the proton and car-
bon magnetic resonance spectra, respectively. The intensities of
the signals due to the carboxyl groups decreased with increasing
temperature, consistent with ordering of the carboxyl groups. In-
deed, carboxyl ordering was the dominant factor driving the phase
transition. The ferroelectric phase transition observed in the pres-
ent work was of order–disorder type in terms of the proton and
1
13
carbon in the carboxyl groups as determined by H and C NMR.
These results shed new light on phase transitions involving the
H and C in TGS.
Fig. 7. Temperature dependences of the 13C spin–lattice relaxation time, T
, in the
1q
rotating frame for the TGS.
1
13
the traces obtained for the carbons were described by a single
exponential function of Eq. (1). The T values for methylene and
the carboxyl groups are shown in Fig. 7. In the case of the carboxyl
Acknowledgement
1
q
This research was supported by Basic Science Research program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology
(2012001763).
groups (labeled as 3 and 4), the 1 C T
3
increased rapidly with
q
for the methylene car-
1
q
increasing temperature, whereas the 13C T
1
bons (labeled as 1 and 2) increased very slowly with increases in
temperature.
References
5
. Conclusion
[
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electric to the ferroelectric phase at 320 K was indeed accompa-
[
[
[
nied by an anomalous decrease in the proton T
significant decrease of the proton motion in the ferroelectric phase.
From this result, E was shown to have same values in both phases
as shown in Table 1. Brosowski et al. [25] have reported a nonmon-
otonic anomaly of proton T near T , whereas the Tsujmi et al. [17]
did not observe the anomalous behavior in T . However, the E of
this hindered rotation appeared to have similar value. The E ob-
tained from T here were consistent with those obtained from
reported by Blinc et al. [16], Tsujimi et al. [17], and Brosowski
et al. [25], as shown in Table 1; the E in T < T is much larger than
those in T > T . The values of T and E previously reported were not
1
, indicating a
a
[
[
[18] K. Itoh, T. Mitsui, Ferroelectrics 5 (1973) 235.
1
C
[
[
[
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a
a
1
q
[22] O.V. Rozanov, Yu.N. Moskvich, A.A. Sukhovskii, Sov. Phys. Solid State 25 (1983)
12.
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[24] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679.
2
T
1
[
a
C
1
C
1
a
[
25] G. Brosowski, W. Buchheit, D. Muller, J. Petersson, Phys. Stat. Sol. (b) 62 (1974)
93.
distinguishable in each proton sites. However, we were able to re-
1
solve H MAS NMR spectrum into the ammonium hydrogen, meth-