Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Journal of Physics and Chemistry of Solids 73 (2012) 890–895

Contents lists available at SciVerse ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Polymorphic phase transition and thermal stability in squaric acid (H2C4O4) Kwang-Sei Lee a, Jin Jung Kweon b, In-Hwan Oh c, Cheol Eui Lee b,n a b c

Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Gimhae 621-749, Korea Department of Physics and Institute for Nano Science, Korea University, Seoul 136-713, Korea Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2011 Received in revised form 29 December 2011 Accepted 14 February 2012 Available online 1 March 2012

Phase transformations in squaric acid (H2C4O4) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates b. The mass loss in TG apparently begins at onset temperatures Tdi ¼ 245 7 5 1C (b ¼5 1C min  1), 262 7 5 1C (b ¼ 10 1C min  1), and 275 7 5 1C (b ¼ 20 1C min  1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 1C (Tcþ ). Further heating with b ¼ 10 1C min  1 in DSC revealed deviation of the baseline around 310 1C (Ti), and a large unusual exothermic peak around 355 1C (Tp), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon. & 2012 Elsevier Ltd. All rights reserved.

Keywords: B: Crystal growth C: Differential scanning calorimetry (DSC) C: Thermogravimetric analysis (TGA) C: X-ray diffraction D: Phase transitions

1. Introduction Squaric acid molecule (H2C4O4), its complexes, and hydrogen squarates have been a long subject of the spectral and structural studies [1–5]. Squaric acid compound is a hydrogen-bonded molecular crystal undergoing a structural phase transition at Tc E101 1C at ambient pressure [6–19]. Below the transition temperature Tc, it has a monoclinic system with a space group P21 =mC 22h in the antiferroelectric/ferroelastic phase (Fig. 1) [7,8]. In the paraelectric/ paraelastic phase above Tc, it is a tetragonal system with a space group I4=mC 54h . Molecules of squaric acid form two-dimensional (2D) networks of hydrogen bonds in the crystal. The polar molecules with hydrogen bonds are aligned within each molecular sheet. The polar molecular sheets stack along the b axis in an antiferroelectric manner, alternating sheet polarizations. Squaric acid shows a large isotope effect, the protons playing an important role in the phase transition [8]. The mechanism of polymorphic phase transition at Tc is attributed to coexistence of order-disorder and displacive features [14–17]. In addition to the structural phase transition at 101 1C, a second phase transition around 147 1C was suggested by 1H NMR measurements on squaric acid [18]. Lee et al. reported that there is no evidence for a second phase transition near 147 1C from Raman spectroscopy [19]. Further heating to much higher temperatures gives rise to thermal instability. Cohen et al. reported a decomposition temperature of squaric acid crystal around about 293 1C [20]. Brown et al. showed an onset of a sharp but complex exotherm at

n

Corresponding author. Tel.: þ82 2 3290 3098; fax: þ82 2 927 3292. E-mail address: [email protected] (C. Eui Lee).

0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2012.02.013

307 1C with some sublimation [21]. While these groups reported thermal decomposition near 293 1C or 307 1C, many investigators have erroneously identified this temperature as the melting point of squaric acid. However, as observed in many hydrogen-bonded crystals [22–24], one should differentiate sublimation and melting from thermal decomposition. Considering thermal decomposition at high temperatures, a tentative phase diagram of squaric acid was proposed in Ref. [19]. The theoretical works have also been devoted to the description of the structural phase transition of squaric acid at Tc E101 1C, based on the Ising-type pseudo-spin Hamiltonian with or without proton tunneling terms [25–30]. To the authors’ knowledge, there has been no report on the DSC trace of the polymorphic phase transition for squaric acid at 101 1C. Besides, no systematic investigation concerning the heating-rate-dependent thermal behavior of squaric acid has been reported until now. In this work, we have carried out a systematic thermal study by employing heatingrate-dependent thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements, capable of providing valuable information on the thermal properties of squaric acid. X-ray diffraction (XRD) was also conducted to analyze the final residue material of squaric acid in order to investigate thermal decomposition of the crystal at high temperatures.

2. Experimental Squaric acid powder (99%), purchased from Aldrich, was purified by recrystallization from deionized water, followed by drying at 80 1C for removal of water molecules included in the bulk or adsorbed at the surface of the grains. A sample was then

K.-S. Lee et al. / Journal of Physics and Chemistry of Solids 73 (2012) 890–895

891

Tdi

100 N2 100 mL min -1

Mass (%)

80

100.0 % H2C4O4

Heating o

5 C min -1 o 10 C min -1 o 20 C min -1

60 40 20

15.8 % C Tdf

0 0 Fig. 1. The monoclinic crystal structure (lattice þbasis) of the low-temperature antiferroelectric/ferroelastic phase of squaric acid (H2C4O4) with a  b  c unit cell in the space group P21 =mC 22h at room temperature and morphology of bulk crystal with Miller indices of crystal faces [7,8].

crushed into fine powder for thermal analysis. A thermal analyzer (TA Instruments SDT 2960) and a differential scanning calorimeter (DSC 2010) were used for TG and DSC measurements, respectively. The measurements were made from room temperature to 400 1C with heating rates b ¼ dT=dt of 5 1C min  1, 10 1C min  1 and 20 1C min  1, with nitrogen flow rates of 100 mL min  1 for TG and 60 mL min  1 for DSC, respectively. Squaric acid samples of about 4.3 mg and about 1.9 mg were put into a platinum pan and a copper pan for TG and DSC measurements, respectively. In a TG experiment, the platinum pan containing the sample is open to allow the volatilization of possible product gas, while in the present DSC experiment the sample is encapsulated by a metal lid to form nearly closed system. The different thermodynamic conditions may give rise to different thermal stability. The residue material obtained as a result of thermal decomposition was examined by X’Pert Philips MPD X-ray diffractometer, with the data being collected at room ˚ temperature by using Cu Ka X-ray beam of wavelength 1.5405 A.

50

100

150 200 250 Temperature (°C )

300

350

Fig. 2. The effect of heating rate b on the TG curves for the thermal decomposition of squaric acid under N2 flux. The yellow blocks are the spread regions of the initial temperature Tdi and final temperature Tdf of thermal decomposition, depending on the heating rate b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

27575 1C (b ¼ 20 1C min  1). A new baseline shows up, apparently ending up with final temperatures Tdf ¼35075 1C (b ¼5 1C min  1), 36075 1C (b ¼10 1C min  1), and 37075 1C (b ¼20 1C min  1). There is no intermediate baseline except for the initial baseline of 100.0% below Tdi and final baseline of about 10–14% above Tdf, indicating that the kinetics of thermal decomposition of squaric acid follows a single stage without appreciable formation of an intermediate product. The inflection point lies near 290–335 1C, depending on the heating rates. Thermal decomposition of squaric acid yields CO, CO2 and H2O according to the evolved gas analysis (EGA) [21]. If the solid residue from the decomposition of squaric acid in N2 is assumed to be carbon only, two types of thermal decomposition of squaric acid are likely to occur. One possible chemical reaction is H2 C4 O4 ðsÞ-H2 O ðgÞ þ CO ðgÞ þ CO2 ðgÞ þ 2CðsÞ,

The TG curve provides information on whether a high-temperature phase transformation is a polymorphic phase transition or a thermal decomposition [31–34]. In an ideal thermal decomposition with a single stage, the fraction as a function of temperature or time is sigmoid in the TG curve. Thus, we can definitely define the initial temperature or onset temperature of thermal decomposition, Tdi, and the extrapolated onset temperature of thermal decomposition, Tde [32]. However, in real systems Tdi is not easy to pinpoint and it depends on the sensitivity of the balance and the amount of drift or ‘‘noise’’ seen. The thermal stability of squaric acid was characterized by TG and the mass loss curves as functions of temperature and heating rate b ¼dT/dt are shown in Fig. 2. The initial baseline of mass is best maintained up to 24575 1C with a heating rate b ¼5 1C min  1, while with higher heating rates of b ¼10 1C min  1 and b ¼ 20 1C min  1 mass seems to gradually lose from room temperature to 260 1C. Therefore, pinpointing the initial decomposition temperature is difficult, and the beginning of mass loss was determined as an apparent initial or onset temperature Tdi ¼24575 1C (b ¼5 1C min  1), 26275 1C (b ¼ 10 1C min  1), and

ð2  1Þ

i.e., 2H2 C4 O4 ðsÞ-2H2 OðgÞ þ 4COðgÞ þCO2 ðgÞ þ 3CðsÞ,

3.1. Thermogravimeric analysis

ð1Þ

while the other as proposed in Ref. [21] is H2 C4 O4 ðsÞ-H2 O ðgÞ þ 2COðgÞ þ0:5CO2 ðgÞ þ 1:5CðsÞ,

3. Results and discussion

400

ð2  2Þ

For comparison with experimental TG results and possible chemical reactions, the solid product, carbon, is calculated on the basis of Eq. (1) and (2-2): Case ð1Þ : H2 C4 O4 ðsÞ-H2 OðgÞ þ COðgÞ þ CO2 ðgÞ þ 2CðsÞ Residue :

Mð2CÞ 24:0214 ¼ 0:21061 ffi 21:1% ¼ MðH2 C4 O4 Þ 114:05628

Case ð2-2Þ : 2H2 C4 O4 ðsÞ-2H2 OðgÞ þ 4COðgÞ þCO2 ðgÞ þ 3CðsÞ Resiue :

Mð3CÞ 3  18:01605 ¼ 0:15796ffi 15:8% ¼ Mð2H2 C4 O4 Þ 2  114:05628

Thus, the stoichiometry 15.8% of the residual carbon in the second case is more compatible with the experimental results of about 10–14% above Tdf, the difference in the residue mass with the theoretical value being attributed to H2O or O2 included in the N2 flux. Eq. (1) indicates that one squaric acid molecule at the surface is decomposed, while two squaric acid molecules at the surface participate in the thermal decomposition in Eq. (2-2).

K.-S. Lee et al. / Journal of Physics and Chemistry of Solids 73 (2012) 890–895

nMH2 XO4 ðsÞ-Mn H2 Xn O3n þ 1 ðsÞ þðn1ÞH2 OðgÞ

ð3Þ

After heating in air up to 400 1C, a black residue was found. The residue material, supposed to be carbon, was analyzed by XRD in order to identify the composition and crystallinity (Fig. 3). The thermal decomposition of Eq. (2-2) for interpreting Fig. 2 is the carbonization process, or pyrolysis, in which the organic precursor is transformed into a material that is essentially all carbon. Carbon-yielding graphitization can be defined as the transformation of a turbostratic graphitic material (i.e., a carbon) into a well-ordered graphitic structure. This occurs in a series of steps which begins as the temperature passes the carbonization temperature of  1200 1C. The degree of graphitization of carbon precursors varies considerably, depending on whether a coke or a char is formed. As with all crystalline materials, a sharp diffraction pattern is obtained with single-crystal graphite. Pronounced crystallinity is indicated by the development of a peak at about 2y ¼ 26o arising from (002) diffraction [35]. The broad halo profiles at about 2y ¼26o in Fig. 3 are indicative of an amorphous structure of carbon. The residue after heating of H2C4O4 in air up to 400 1C was identified as amorphous carbon, indicating that amorphous carbon does not graphitize readily below the carbonization temperature of  1200 1C. 3.2. Polymorphic phase transition and kinetics of thermal decomposition The endothermic peaks in the DSC curve, indicative of the solid-solid phase transition temperature on heating, are not accompanied by any mass loss in the TG curve. Fig. 4 shows the DSC curve of squaric acid with various heating rates b. Apparent polymorphic phase transition temperature on heating, Tcþ , was revealed as a weak endothermic peak: 101.8 1C (b ¼5 1C min  1), 102.8 1C (b ¼10 1C min  1), and 105.5 1C (b ¼20 1C min  1). As the heating rate b decreases, Tcþ is shifted towards lower temperature

Intensity (a.u.)

3000

2000

1000

0 0

10

20

30

40

50

60

2 (deg.) Fig. 3. X-ray diffraction pattern of the final residue of the thermal decomposition at the heating rate of b ¼ 5 1C min  1 under N2 flux. Inset: optical image of the final residue material.

1.5 Endo

Squaric acid (H 2 C4O4)

Ti

Tf

300

350

N2 60 mL min -1 o

Heat flow (W/g)

The experimental results support that the hydrogen bond in O – H y O between two squaric molecules in the intralayer in Fig. 1 is broken as a first step of thermal decomposition. Three or four molecules at the surface may take part in the thermal decomposition such as the high-temperature phase transformations (solid state polymerization) proposed in MH2XO4-type (M¼Li, Na, K, Rb, Cs, Tl; X ¼P, As) crystals [22–24]:

5 C min -1 o 10 C min -1 o 20 C min -1

1.0

Heating +

Tc

0.5

Exo

892

0.0 0

50

100

150

200

250

400

Temperature (°C) Fig. 4. The effect of heating rate b on the DSC curves for the polymorphic phase transition and thermal decomposition of squaric acid under N2 flux. The solidsolid phase transition at Tcþ is revealed as a small endothermic anomaly, while the dashed lines are the extrapolated lines of solid baseline of squaric acid. The yellow blocks are the spread regions of the solid-solid phase transition Tcþ , the initial temperature Ti and final temperature Tf of exotherm corresponding to thermal decomposition, depending on the heating rate b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with less sensitivity of thermal detection. The extrapolated temperature of the DSC endothermic peaks at various heating rates, in Fig. 5, shows that if the squaric acid samples are heated at a very slow heating rate (b \0 1C min  1), the polymorphic phase transition on heating would approach the thermal equilibrium temperature of 100.8 1C. The phase transition temperature is in good agreement with near-equilibrium temperature reported by many authors, especially adiabatic specific heat [10] and thermal expansion measurements with very slow rates [11]. 1 H NMR measurements of squaric acid suggested a transition anomaly around 147 1C [18], but there is no thermal anomaly for a phase transition around 147 1C in the present DSC curve, in agreement with Raman spectroscopic measurements [19]. This is also consistent with the 17O nuclear-quadrupole-relaxation study which showed that one-dimensional in-plane correlations of the proton motion persist in squaric acid over  50 molecular units up to 140 1C [12]. An anomaly observed around 147 1C in the 1H NMR measurements [18] may be due to the moisture included in the measured sample. Upon further heating in DSC, the baselines of high-temperature tetragonal phase of squaric acid show deviation at Ti (see Fig. 4): 300 1C (b ¼ 5 1C min  1), 310 1C min  1 (b ¼10 1C min  1), and 317 1C (b ¼20 1C min  1), which may be closely related to an onset temperature of thermal decomposition, Tdi in TG. The onset of deviation from the baseline in DSC was also dependent on the heating rate. As is well known, onset of thermal decomposition of solids begins at the surface and depends on many factors. Therefore, experimental determination of Tdi varies according to the sample preparation (purity, thermal treatment, particle size, etc.), heating rate, environmental atmosphere, measurement systems, and so on. The very different values of Tdi in TG and Ti in DSC curves may be due to one of these factors. Another possible reason is the experimental condition in TG and DSC. In the TG experiments, the pan containing the sample is open to allow the volatilization of possible product gas, while in the present DSC experiments the sample is hermetically sealed by metal lid to form a nearly closed system. These different thermodynamic conditions may give rise to different thermal stabilities. Unusually large exothermic peaks at Tp were

K.-S. Lee et al. / Journal of Physics and Chemistry of Solids 73 (2012) 890–895

893

110 Squaric acid (H 2 C4O4)

Tp

350

Temperature (°C)

Temperature (°C)

PE PEL 105 + Tc

AFE FEL

100

Ti 300

Tdi 250

Squaric acid (H 2C 4O 4 ) PE PEL

200

95 0

5

10 15 Heating rate,  (°C min-1)

20

25

Fig. 5. Variation of the polymorphic phase transition temperature, Tcþ , of squaric acid with heating rate b under N2 flux (PE: Paraelectric, PEL: Paraelastic, AFE: Antiferroelectric, FEL: Ferroelastic). The dashed line represents the extrapolation of the Tcþ .

2

Tf

Endo

Squaric acid (H 2 C4 O4 ) 0

Ti

Heat flow (W/g)

+ Tc

-2

N2 60 mL min -1 Heating

-4

o

5 C min-1 o 10 C min-1 o 20 C min-1

Exo

-6

-8 Tp 0

50

100

150

200

250

300

350

400

Temperature (°C) Fig. 6. The effect of heating rate b on the DSC curves for the polymorphic phase transition and thermal decomposition of squaric acid under N2 flux. The yellow blocks are the spread regions of the solid-solid phase transition Tcþ , the initial temperature Ti, peak temperature Tp and final temperature Tf of exotherm corresponding to thermal decomposition, depending on the heating rate b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

seen at 341.6 1C (b ¼5 1C min  1), 354.5 1C min  1 (b ¼10 1C min  1), and 365.4 1C (b ¼20 1C min  1) in Fig. 6, corresponding to temperatures with fastest reaction. As in the case of structural phase transition temperatures, different heating rates have to do with the chemical reaction of solids. At higher heating rates, the time Dt during which the system stays at the temperature being shorter needed for the system to reach thermal and chemical equilibria at the thermal decomposition temperature is shorter. Therefore, thermal decomposition occurs at a higher temperature for a shorter Dt [31–34], and at least two measurements with different heating rates should be conducted for the phase transition temperature in thermal equilibrium. Fig. 7 shows the high-temperature DSC exothermic peaks at different heating rates and the extrapolation of the exothermic peaks, the onset temperature (Tdi) approaching 296 1C and the peak temperature (Tdp) approaching 336 1C. Above 380 1C, the DSC curves in Fig. 4 return to the baselines and a

0

5

10

15

20

25

Heating rate,  (°C min-1) Fig. 7. Variation of the onset temperature of the thermal decomposition, Tdi, the onset temperature Ti, and the peak temperature Tp of the thermal decomposition of squaric acid with heating rate b under N2 flux (PE: Paraelectric, PEL: Paraelastic). The dashed lines represent the extrapolation of the Tdi, Ti, and Tp.

constant mass is obtained as shown in the TG curves of Fig. 2, indicating a complete decomposition of the squaric acid samples. In the thermal decomposition of phosphates and arsenates in Eq. (3), pristine MH2XO4 solids are crystallized via ionic bonds and hydrogen bonds. The hydrogen-bond breaking takes place by a multistage solid state polymerization, forming new oxygen bridging [22,24]. In contrast, in the thermal decomposition of squaric acid in Eq. (2-2), pristine squaric acid H2C4O4 is crystallized through the hydrogen bonds in the intralayer and van der Walls bonds between the interlayers, looking much like graphite or graphene. The TG, DSC and XRD results of squaric acid show that thermal decomposition occurs via a single stage without formation of intermediates. Self-generated gas may evolve as indicated in Eq. (2-2), and sublimation may occur simultaneously [21]. Thus, thermal decomposition of squaric acid is a very complex phenomena, being very sensitive to the initial surface conditions of the pristine crystals [22–24,36,37]. The activation energy for the thermal decomposition of squaric acid may be obtained from the dependence of the exothermic peak temperature on the heating rate by using the Kissinger method, lnðb=T p 2 Þ ¼ E=RT p þC, or the Ozawa method, lnb ¼ E=RT p þ C, where b is the heating rate, Tp is the peak temperature of the thermal decomposition (Tp in Fig. 6), and R is the universal gas constant [38,39]. Fig. 8 shows the fits of the decomposition peak temperatures and heating rates according to the two different methods. The value of the activation energy obtained from the slopes of the fits is respectively 179711 kJ/mol by the Kissinger method, and 189 711 kJ/mol by the Ozawa method. The activation energy may be considered as the thermal energy for the decomposition of squaric acid in nitrogen according to the second case of the stoichiometries given in Eq. (2-2). 3.3. Symmetry of the H2C4O4 molecule and phase diagram Crystallographic data suggest that the squaric acid molecule itself has a point symmetry 4/m  C4h in the PE phase [7,8]. Symmetry as high as this is not, however, confirmed by Raman spectroscopy, although Raman and Brillouin scattering studies

894

K.-S. Lee et al. / Journal of Physics and Chemistry of Solids 73 (2012) 890–895

-9

4

Amorphous carbon

600

Thermal decomposition

Kissinger method Ozawa method -10

ln 

2

ln ( / T p )

3

-11

2

Temperature (K)

500 HT ? IT

-12

1.56

1.58

1.60 1.62 1000 / Tp (K-1)

1.64

Fig. 8. The activation energy fits of the decomposition peak temperatures and heating rates according to the Kissinger and Ozawa methods. The data were fitted by the least-squares method.

have been performed to investigate the dynamics of the transition [13,19]. Some new Raman lines become clear in the AFE phase and are ascribed to unit-cell doubling along the b axis due to the transition. This discrepancy in the molecular symmetry could be resolved by supposing that X-ray diffraction provides structural data related to the average of the possible orientations of the molecule in a disordered structure, while vibrational spectroscopy is mainly influenced by the symmetry of the molecules themselves [13,19]. With increasing pressures beyond a critical pressure (Pc), a new type of structural change was shown to occur. The squaric acid molecules show a symmetry change from the asymmetric (m  C1h) to the centrosymmetric (4/m  C4h) form [13,19]. According to 13C NMR and 17O NQR studies, the shortrange order of AFE phase persisted far above Tc [12]. Referring to previous experimental results, the tentative temperature-pressure phase diagram for a squaric acid crystal was constructed by Lee et al. [19]. Taking present thermal analytic results into consideration, modified phase diagram of squaric acid crystal is shown in Fig. 9. Under hydrostatic pressures, the PE phase composed of polar (m C1h -distorted) molecules is realized, even near 0 K, due to a joint quantum motion and deformation of the molecular skeletons C4O24  . The possibility of a ‘quantum paraelectrics’ or ‘incipient ferroelectrics’ at low temperatures was discussed [19]. The theoretical works have been devoted to the description of the structural phase transition of squaric acid at Tc E101 1C, based on the Ising-type pseudo-spin Hamiltonian with or without the proton tunneling terms [25–30]. Ishizuka et al. [30] have investigated the geometrically frustrated transverse-field Ising model. Through the unbiased quantum Monte Carlo simulations they have identified an intermediate liquidlike state between the ferroelectrically ordered state and the completely disordered paraelectric state, in which molecular polarizations are well preserved but are globally disordered due to the frustration. In this work, our thermal analysis of squaric acid clarified why we did not observe HT phase owing to thermal decomposition. A future problem is to investigate the molecular symmetry of H2C4O4 at temperatures above 200 1C on approaching the onset of thermal decomposition. While theoretical works of the squaric acid crystal have been concentrated on the polymorphic phase transition at Tc E 101 1C at ambient pressure [25–30], density functional calculation is necessary for deeper understanding the

(4)

Squaric acid (H2C4O4)

300

HP

200

(3)

IP

LT LP Monoclinic P21/ m− C22h (m - C1h ) Antiferroelectric Ferroelastic

100

1

?

400

Tetragonal I 4 / m − C45h (m - C1h ) Paraelectric Paraelastic

(2)

Tetragonal I 4 / m − C 45h (4/m - C4h) Paraelectric Paraelastic

(1)

0 0

1

2

3

4

5

6

7

Pressure (GPa) Fig. 9. Modified phase diagram of squaric acid (H2C4O4) crystal. The red hatched zone is the region of Tdi determined by TG and Ti determined by DSC curves. The reason of different values of Tdi in TG and Ti in DSC curves are discussed in the text. Circles below 3.0 GPa show the transition temperature determined by using the dielectric constant under various hydrostatic pressures [19]. Rectangles above 3.0 GPa show the transition pressure determined by using Raman spectroscopy at 290, 200, and 100 K [13]. The linear extrapolation of dTc/dp¼  106 K/GP indicates a transition pressure of  3.3 GPa at absolute zero, as plotted by the straight dashed line (1). Quantum fluctuations at low temperatures induce the change from dashed line (1) to the curved solid line (2). LT, IT, HT, LP, IP, and HP denote, respectively, low temperature, intermediate temperature, high temperature, low pressure, intermediate pressure, and high pressure. The hatched zone (3) is a boundary between the IP and the HP phases. The hatched zone (4) is a tentative extrapolation of (3) between the IP and the HP phases or between the IT and the HT phases, suggesting that the true existence of HT phase may not be realized due to thermal decomposition. The point groups in the parentheses are point groups of H2C4O4 molecules themselves, not factor groups of H2C4O4 crystallographic unit cells.

molecular symmetry of H2C4O4 and its broken symmetry at high temperatures as well as at high pressures.

4. Conclusions Polymorphic phase transition and thermal decomposition in squaric acid crystal were examined by systematic heating-ratedependent thermal analysis. Thus, the polymorphic phase transition temperature on heating close to thermal equilibrium was determined from DSC measurements by employing different heating rates. The onset temperature and the peak temperature were obtained for the high-temperature thermal decomposition, and the corresponding activation energy was obtained by employing the relation between the peak temperature of the thermal decomposition and the heating rate. The XRD measurements revealed the final residue of the thermal decomposition to be amorphous carbon.

Acknowledgments This work was supported by the 2008 Inje University research grant. References [1] O. Angelova, R. Petrova, V. Radomirska, T. Kolev, Acta Crystallogr. C 52 (1996) 2218. [2] O. Angelova, V. Velikova, T. Kolev, V. Radomirska, Acta Crystallogr. C 52 (1996) 3252.

K.-S. Lee et al. / Journal of Physics and Chemistry of Solids 73 (2012) 890–895

[3] T. Kolev, H. Preut, P. Bleckmann, V. Radomirska, Acta Crystallogr. C 53 (1997) 805. [4] T. Kolev, Z. Glavcheva, R. Petrova, O. Angelova, Acta Crystallogr. C 56 (2000) 110. [5] T. Kolev, R. Wortmann, M. Spiteller, W.S. Sheldrick, M. Heller, Acta Crystallogr. E 60 (2004) o956. [6] D. Semmingsen, J. Feder, Solid State Commun. 15 (1974) 1369. [7] D. Semmingsen, F.J. Hollander, T.F. Koetzle, J. Chem. Phys. 66 (1977) 4405. [8] F.J. Hollander, D. Semmingsen, T.F. Koetzle, J. Chem. Phys. 67 (1977) 4825. [9] I. Suzuki, K. Okada, Solid State Commun. 29 (1979) 759. ¨ [10] E. Barth, J. Helwig, H.-D. Maier, H.E. Muser, J. Petersson, Z. Physik B 34 (1979) 393. [11] T.H. Johansen, J. Feder, T. Jossang, Z. Physik B 56 (1984) 41. [12] J. Seliger, V. Zagar, R. Blinc, J. Magn. Reson. 58 (1984) 359. [13] Y. Moritomo, Y. Tokura, H. Takahashi, N. Mori, Phys. Rev. Lett. 67 (1991) 2041. [14] N. Dalal, A. Klymachyov, A. Bussmann-Holder, Phys. Rev. Lett. 81 (1998) 5924. [15] R. Fu, A.N. Klymachyov, G. Bodenhausen, N.S. Dalal, J. Phys. Chem. B 102 (1998) 8732. [16] S.P. Gabuda, S.G. Kozlova, N.S. Dalal, Solid State Commun. 130 (2004) 729. [17] A. Bussmann-Holder, N. Dalal, Struct. Bond. 124 (2007) 1. [18] C.E. Lee, C.H. Lee, M.W. Park, Solid State Commun. 129 (2004) 565. [19] K.-S. Lee, J.-A. Seo, Y.-H. Hwang, H.K. Kim, C.E. Lee, K. Nishiyama, J. Korean Phys. Soc. 54 (2009) 853. [20] S. Cohen, J.R. Lacher, J.D. Park, J. Am. Chem. Soc. 81 (1959) 3480. [21] M.E. Brown, H. Kelly, A.K. Galwey, M.A. Mohamed, Thermochim. Acta 127 (1988) 139. [22] K.-S. Lee, J. Phys. Chem. Solids 57 (1996) 333.

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

[37]

[38] [39]

895

J.-H. Park, K.-S. Lee, J.-N. Kim, J. Korean Phys. Soc. 32 (1998) S1149. K.-S. Lee, Ferroelectrics 268 (2002) 369. L. Zhou, Y. Zhang, L. Wu, J. Li, J. Mol. Struct.: Theochem. 497 (2000) 137. M. Spassova, T. Kolev, I. Kanev, D. Jacquemin, B. Champagne, J. Mol. Struct.: Theochem. 528 (2000) 151. C. Rovira, J.J. Novoa, P. Ballone, J. Chem. Phys. 115 (2001) 6406. X. Xue, C. Wang, W. Zhong, J. Mater. Sci. Technol. 20 (2004) 206. S.P. Dolin, A.A. Levin, T.Y. Mikhailova, M.V. Solin, N.V. Zinova, Int. J. Quant. Chem. 111 (2011) 2671. H. Ishizuka, Y. Motome, N. Furukawa, S. Suzuki, Phys. Rev. B 84 (2011) 064120. M.E. Brown, Introduction to Thermal Analysis: Techniques and Applications, Chapman and Hall, New York, 1988 Chaps. 2 & 13. P.J. Haines (Ed.), Principles of Thermal Analysis and Calorimetry, Cambridge, 2002, pp. 77–79. B. Wunderlich, Thermal Analysis of Polymeric Materials, Springer, Berlin, 2005. M.E. Brown, P.K. Gallagher (Eds.), Handbook of Thermal Analysis and Calorimetry, Vol. 5, Elsevier, Amsterdam, 2008. H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, Noyes Publications, Park Ridge, New Jersey, 1993. W.P. Gomes, W. Dekeyser, Factors Influencing the Reactivity of Solids, in Treatise on Solid State Chemistry, in: N.B. Hannay (Ed.), Reactivity of Solids, Vol. 4, Plenum Press, New York, 1976, pp. 61–113. F.C. Tompkins, Decomposition Reactions, in Treatise on Solid State Chemistry, in: N.B. Hannay (Ed.), (Reactivity of Solids), Vol. 4, Plenum Press, New York, 1976, pp. 193–231. H.E. Kissinger, Anal. Chem. 29 (1957) 1702. T. Ozawa, J. Thermal Anal. 2 (1970) 301.