Journal of the Physical Society of Japan
Vol. 70, No. 7, July, 2001, pp. 1937–1941
Ferroelastic Domain Switching Behaviour of [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4
Single Crystals Studied by External Stress
Ae Ran Lim1,* , Dae Keun Park2 , Jin-Hae Chang2 and Se-Young Jeong3
1 Department
of Physics, Jeonju University, Jeonju 560-759, Korea
of Polymer Science & Engineering, Kumoh National University of Technology,
Kumi 730-701, Korea
3 Department of Physics, Pusan National University, Pusan 609-735, Korea
2 Department
(Received February 13, 2001)
We studied the temperature dependence of the stress-strain hysteresis in [N(CH3 )4 ]2 CuCl4
and [N(CH3 )4 ]2 ZnCl4 single crystals grown by the slow evaporation method. From the obtained
results, we determined that [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 single crystals have ferroelastic characteristics in phase IV and III, and phase V and IV, respectively. Also, the ferroelastic
domain switching stress due to saturation effect is approximately 0.9 MPa in [N(CH3 )4 ]2 CuCl4
and 0.4 MPa in [N(CH3 )4 ]2 ZnCl4 crystals. In order words, the ferroelastic phase transition
due to external mechanical stress applied to these crystals causes a transformation from the
twin-domain state to the single-domain state.
KEYWORDS: ferroelectricity, transition and Curie point, domain structure and effects; hysteresis
§1.
Introduction
It has been reported that [N(CH3 )4 ]2 CuCl4 and
[N(CH3 )4 ]2 ZnCl4 single crystals with high optical quality undergo phase transitions at −8, 20, and 25◦ C, and
at −112, −92, 3.3, 6, and 20◦ C, respectively.1–3) Previous reports also clarified that in [N(CH3 )4 ]2 CuCl4 and
[N(CH3 )4 ]2 ZnCl4 single crystals, four phases and six
phases occur, separated by three and five transition temperatures, respectively, as summarized in Table I. In the
case of [N(CH3 )4 ]2 CuCl4 , the highest temperature phase
(phase I) belongs to the orthorhombic structure with the
space group P mcn. The orthorhombic lattice constants
are a = 9.039 Å, b = 15.515 Å and c = 12.268 Å, which
are slightly deformed from the hexagonal ones.4) As temperature increases, phase IV transforms to another monoclinic phase III at −8◦ C. The space group of phase IV is
P 1121 /n, and the unique axis is parallel to the c-axis of
phase I. At 20◦ C, phase III transforms to the incommensurate phase II. The structure of phase III is monoclinic
P 121 /c1, with a unique axis parallel to the b-axis of phase
I. Phase II transforms to a commensurate phase I at
25◦ C. The three phase transitions in [N(CH3 )4 ]2 CuCl4
are ferroelastic → ferroelastic (IV–III), ferroelastic →
paraelastic (III–II), and paraelastic → paraelastic (II–
I).5) In the case of [N(CH3 )4 ]2 ZnCl4 single crystals, the
phase between 20 and 6◦ C is incommensurate and the
phase between 6 and 3.3◦ C is the orthorhombic commensurate ferroelectric phase. Phase IV between 3.3 and
−92◦ C is monoclinic, phase V between −92 and −112◦ C
is monoclinic or triclinic, and phase VI, which is stable
below −112◦ C, is orthorhombic. The highest temperature phase, phase I, has P mcm symmetry. In this phase,
∗ To
whom all correspondence should be addressed, Tel: +8263-220-2514, Fax: +82-63-220-2362 (work), +82-42-483-6155
(home), E-mail: [email protected]
one unit cell contains four formula units consisting of two
inequivalent types of tetramethylammonium ions. The
transitions at −112, −92 and 3.3◦ C are of the first-order
and the transition at 20◦ C is continuous. The order of
the transition at 6◦ C was not determined.6) The space
group at room temperature is P mcm with a = 8.946 Å,
b = 15.515 Å and c = 12.268 Å.7) In this respect, the
structures of the two crystals are similar (see Fig. 1).
Ferroelasticity was first recognized as a new property
by Aizu in 1970.8) A crystal is ferroelastic if it has two
or more stable orientation states in the absence of mechanical stress and if it can be reversibly transformed
from one to another of these states by the application of
Fig. 1. Crystal structure of [N(CH3 )4 ]2 ZnCl4 in the paraelectric
phase. The bold face large and small tetrahedrons represent
ZnCl4 and a-[N(CH3 )4 ], respectively, while the light face small
tetrahedron represents b-[N(CH3 )4 ].
1937
Ae Ran Lim et al.
1938
Table I.
Successive phase transitions in [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 single crystals.
T3
−8◦ C
Transition
Temp.
phase
space group
IV
Monoclinic
Ferroelastic
T5
−112◦ C
space group
VI
Orthorhombic
P 21 21 21
T2
20◦ C
III
Monoclinic
Ferroelastic
Commensurate
P 121 /c1
P 1121 /n
Transition
Temp.
phase
(Vol. 70,
II
P 121 /c1
I
Orthorhombic
Paraelastic
Prototype
P mcn
Paraelastic
Incommensurate
T4
−92◦ C
V
Monoclinic
or Triclinic
Ferroelastic
T1
25◦ C
T3
3.3◦ C
T2
6◦ C
IV
Monoclinic
III
Orthorhombic
Ferroelastic
Commensurate
P 1121 /n
Ferroelastic
Commensurate
P 21 cn
T1
20◦ C
II
I
Orthorhombic
Incommensurate
P mcm
mechanical stress. When a ferroelastic crystal is heated,
the ferroelastic effect usually disappears at a well defined
temperature. At this temperature a structural phase
transition between a ferroelastic and a paraelastic phase
takes place, the main feature of which is that a ferroelastic hysteresis exists in one phase but not the other. Also,
the ferroelastic domain occurs in all ferroelastic crystals
as a consequence of the reduction in symmetry between
the paraelastic and ferroelastic phase.
Until now [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4
crystals have been studied using various experimental
techniques such as neutron diffraction,5) Raman scattering,9) heat-capacity measurements,9) dielectric and ultrasonic measurements,10–14) optical birefringence and
optical-microscope observations of the domain structure.9, 10, 12, 13) Recently, the new phase transition near
−73◦ C in the [N(CH3 )4 ]2 CuCl4 single crystal was reported by our group.15)
This paper describes the ferroelastic domain switching behaviour due to stress in [N(CH3 )4 ]2 CuCl4 and
[N(CH3 )4 ]2 ZnCl4 single crystals grown by the slow evaporation method. From the strain recorded in the stressstrain hysteresis, the phase transition was also determined as a function of temperature. This study appears
to be the first report of ferroelastic domain switching
stress in these single crystals.
§2.
Experimental Section
Single crystals were grown by slow evaporation at
room temperature from an aqueous solution containing a stoichiomeric proportion of N(CH3 )4 Cl and CuCl2
for [N(CH3 )4 ]2 CuCl4 , and N(CH3 )4 Cl and ZnCl2 for
[N(CH3 )4 ]2 ZnCl4 . The [N(CH3 )4 ]2 CuCl4 crystals are
transluscent and bright orange, and [N(CH3 )4 ]2 ZnCl4
crystals are transparent and colorless.
The stress-strain hysteresis was measured with a thermal mechanical analyzer (TMA, 2940 module of Instron
type) using a temperature controller. Figure 2 shows the
penetration probe inside the TMA chamber. The pen-
Fig. 2. Schematic of a DuPont TMA 2940 thermomechanical analyzer.
etration probe has a small tip that permits it to sink
into the crystal at a given stress, and is used to measure
the strain. The advantage of this probe is that it can
be used with a small crystal. Stress is applied to the
sample by means of a calibrated electromagnet located
around the bottom of the probe allowing versatile control over stress between 1 kPa and 1 MPa. A computer
2001)
Ferroelastic Domain Switching Behaviour of [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 . . . .
1939
Fig. 3. Domain walls in [N(CH3 )4 ]2 CuCl4 as a function of temperature (a) at 30◦ C, (b) at 10◦ C, and (c) −20◦ C by optical
polarizing microscope.
controller also makes it possible to change the stress and
loading rate during experiments in some cases. All heating runs were performed at 5◦ C/min. Samples were air
cooled by the instrument. The flat looking samples had
a cross sectional dimension of 4 × 4 mm2 , and samples
were mounted inside the temperature controlled chamber as shown in Fig. 2. The stress with the loading rate
of 1.67 × 10−3 N/s was applied perpendicular to the flat
sample. The phase transition temperature was investigated as a function of temperature at fixed stress and
loading rate. The strain as a function of stress was also
measured for several different stresses [N(CH3 )4 ]2 CuCl4
in phase III and [N(CH3 )4 ]2 ZnCl4 in phase IV, respectively. The strain was recorded with changing amount of
stress.
§3.
Results and Discussion
In order to confirm the ferroelastic property in the
two crystals, the domain structures were observed by
Fig. 4. Domain walls in [N(CH3 )4 ]2 ZnCl4 as a function of temperature (a) at 25◦ C, (b) at 5◦ C, (c) at −80◦ C, and (d) −98◦ C
by optical polarizing microscope.
employing an optical polarizing microscope. The samples were thin plates and were polished with rouge. A
sample specimen was mounted on a transparent cooling
stage in order to observe the domain structure as a function of temperature. The domain patterns in the ferroelastic phases of [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4
crystals are shown in Figs. 3 and 4. In the case of
1940
Ae Ran Lim et al.
(Vol. 70,
[N(CH3 )4 ]2 CuCl4 , the domain pattern at 30◦ C did not
appear as the paraelastic phase [see Fig. 3(a)]. Also, the
appearance of microscopic domain walls of many parallel lines with decreasing temperature is a property of the
ferroelastic phase [see Figs. 3(b) and 3(c)]. In the case
of the [N(CH3 )4 ]2 ZnCl4 , the domain walls at 25◦ C and
5◦ C did not appeared. According to the decreasing temperature, the domain walls in the ferroelastic V and IV
phases are occurred. From these results, we can see that
phases (III, IV) and (IV, V) in [N(CH3 )4 ]2 CuCl4 and
[N(CH3 )4 ]2 ZnCl4 , respectively, have ferroelastic properties.
The stress-strain hysteresis was measured at several
temperatures. Figures 5 and 6 illustrate the temperature dependences of the strain obtained from the analysis of the hysteresis curves for the loading rate of
1.67 × 10−3 N/s. The stress-strain curves were measured
at a stress of 0.4 MPa. In [N(CH3 )4 ]2 CuCl4 , the strain
in the stress-strain hysteresis between phases III and IV
did not recover to its initial value when the stress was
removed. The strain decreased as the temperature approached T2 , and when T reached T2 the hysteresis disappeared; the strain by the stress-strain hysteresis was
not observed for phases I and II. This confirms that the
[N(CH3 )4 ]2 CuCl4 crystals are ferroelastic in phases III
and IV. The strain caused by the ferroelastic property
in phase III was weaker than that of phase IV. In the
case of the [N(CH3 )4 ]2 ZnCl4 crystals, the strain caused
by the stress-strain hysteresis in each phase is different,
as shown in Fig. 6. The strain obtained by the stressstrain hysteresis turns out to decrease with increasing
temperature. However, the strains in phases I, II, and
III are close to zero. From these results, we can conclude that the [N(CH3 )4 ]2 ZnCl4 single crystal undergoes
a ferroelastic phase transition about T3 = 3◦ C. The
[N(CH3 )4 ]2 ZnCl4 crystal is confirmed to be ferroelastic
in phase IV. The stress-strain hysteresis below −50◦ C
could not be observed in the present work because the
status of the crystal by air cooled inside the TMA is not
good.
In order to obtain the ferroelastic domain switching stress of [N(CH3 )4 ]2 CuCl4 in phase III and
[N(CH3 )4 ]2 ZnCl4 in phase IV, respectively, the variation
of strain (%) as a function of stress was measured using a
loading rate of 1.67 × 10−3 N/s. The variations in strain
due to stress are shown in Figs. 7 and 8, respectively.
In [N(CH3 )4 ]2 CuCl4 , the initial response of the strain
was relatively small, but the strain at a stress greater
than about 0.7 MPa increased largely. Near a stress of
0.9 MPa, the strain reached the saturation effect corresponding to a single-domain state. In [N(CH3 )4 ]2 ZnCl4 ,
the strain changed greatly and reached saturation effect
at 0.4 MPa. Therefore, the ferroelastic domain switching
stresses by the saturation effect are about 0.9 MPa and
Fig. 5. The variation of the strain from the stress-strain hysteresis as a function of temperature in [N(CH3 )4 ]2 CuCl4 .
Fig. 7. Strain (%) measured as a function of stress at phase III
in [N(CH3 )4 ]2 CuCl4 .
Fig. 6. The variation of the strain from the stress-strain hysteresis as a function of temperature in [N(CH3 )4 ]2 ZnCl4 .
Fig. 8. Strain (%) measured as a function of stress at phase IV
in [N(CH3 )4 ]2 ZnCl4 .
2001)
Ferroelastic Domain Switching Behaviour of [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 . . . .
0.4 MPa in [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 crystals, respectively. Ferroelastic domain switching means
that when an external mechanical stress is applied to
these crystals they are transformation from one phase
to another. [N(CH3 )4 ]2 CuCl4 crystals with a ferroelastic domain switching stress of 0.9 MPa were stronger
than [N(CH3 )4 ]2 ZnCl4 crystals with switching stress of
0.4 MPa. [N(CH3 )4 ]2 ZnCl4 , therefore, can be more easily
transformed.
§4.
Conclusion
The stress-strain curves in the several phases show different in strain, and are temperature dependent. The
transition temperatures previously reported15, 16) agree
with the results obtained from the stress-strain hysteresis. However, this stress-strain hysteresis experiment cannot be used to classify T1 for [N(CH3 )4 ]2 CuCl4 ,
and T2 and T1 for [N(CH3 )4 ]2 ZnCl4 because the phase
transition is not ferroelastic. Phase transition means
that when an external mechanical stress is applied to
[N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4 crystals there is
a transformation from the twin-domain state to singledomain state. [N(CH3 )4 ]2 CuCl4 and [N(CH3 )4 ]2 ZnCl4
crystals have characteristics of ferroelasticity: the ferroelastic character of the crystals was established by direct
observation of a stress-strain hysteresis curve. Also, the
twin configuration can easily be changed by a mechanical stress and the crystals can be transformed to a single
domain state by external stress.
1941
Acknowledgments
One of the authors (J. H. Chang) was supported by
the Center for Research of High Quality and Automated
Processes in Electronic Parts Industry in K.N.U.T, assigned by the Korea Science Foundation (RRC).
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