LI Jun-Yi CHEN Xiang JIA Qiang-Qiang LU Hong-Fei ZHENG Shao-Jun CHEN Li-Zhuang

(School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China)

ABSTRACT A novel organic-inorganic hybrid phase transition compound [(C3H7)2dabco]CoBr4 with switchable dielectric property is synthesized, and the phase transition behavior was measured by DSC measurements. The DSC curves display a pair of endothermic and exothermic peaks at 230 and 212 K, confirming a reversible first-order phase transition by the calculation of entropy change and the 18 K thermal hysteresis. The structural change from P42/m at room-temperature to P42 at 100 K accompanied by a deletion of mirror plane m was determined by variable temperature single-crystal X-ray diffractions. Furthermore, switchable step-like dielectric anomalies can be observed around 230 K by the real part of dielectric constant curves. The 10-fold difference of ε' between high ε' and low ε' states make compound 1 a potential candidate for sensor and detection devices at appropriate temperature and frequencies.

Keywords: phase transition, switchable dielectric; DOI: 10.14102/j.cnki.0254-5861.2011-2517

1 INTRODUCTION

Researches on reversible solid-solid phase transitions and the ferroelectric, dielectric, and optical properties have received great attention in last few decades, which have been extensively used in data communication, sensing, and optical data storage[1-7]. Especially, the dielectric anomalies around the phase transition temperature make it possible that the compounds can be switchable between high dielectric state and low dielectric state, which are widely applied in phase shifters, dielectric switches, high permittivity capacitors,etc[8-11].

Organic-inorganic hybrid phase transition compounds are more environmentally friendly and easier to conduct molecular modification compared to traditional inorganic compounds. In addition, the ordered-disordered phase transitions are usually triggered by the twisting motion of organic cations and different anions lead to different phase transition temperature and physical properties. Dabco (1,4-diazabicyclo[2.2.2]octane) is a potential organic mole- cule for the synthesis of organic-inorganic phase transition compounds because the high symmetrical structure is easy to twist at various temperature[14-18]. For instance, [Cu(Hdabco)(H2O)Cl3] has been reported previously, and the reversible phase transition and sharp-like dielectric anomalies are observed around 237 K. Two organic-inorganic hybrid compounds were synthesized and dielectric curves display wide switchable peaks around phase transition temperature. Hereby, one novel switchable dielectric phase transition compound [(C3H7)2dabco]CoBr4is synthesized by solution method. Compound 1 undergoes reversible phase transitions at 230 and 212 K under heating-cooling cycles, respectively. The structure of 1 is measured and compared at 100 and 298 K. The result shows that the mirror plane is broken with the temperature decreasing to the phase transition temperature. Besides, the dielectric curves display step-like anomalies around 230 K, which makes the compound a potential material for dielectric switch.

2 EXPERIMENTAL

2. 1 Materials and measurements

All reagent-grade chemicals and solvents employed throughout experiments were obtained from commercial sources and used directly without further purification. Infrared (IR) spectra were recorded on a SHIMADZU IR prestige-21 FTIR-8400S spectrometer in the range of 4000～400 cm-1by using potassium bromide pellets. Elemental analyses were taken on a Perkin-Elmer 240C elemental analyser. Powder X-ray diffraction (PXRD) data were collected by using a Rigaku SA-HFM3 diffraction system from 2θ= 5o to 2θ= 50o at 6 o/min with an increment of 0.02o. Thermogravimetric analyses (TGA) were conducted on a TGA Q500 V20.13 Build 39 thermogravimeter at the heating rate of 10 K/min in a N2atmosphere.

2. 2 Synthesis of compound 1

For 1, as shown in Scheme 1, 1,4-diisopropyl-1,4-diaza- bicyclo[2.2.2]octane (1 mmol) was solubilized in deionized water (5 mL), and then the prepared solution was added to an aqueous solution of CoBr2.4H2O (1 mmol). After five days, blue flake single crystals of 1 were obtained by slow evaporation of a mixture of aqueous solutions at room temperature.

Scheme 1. Synthesis of 1 by solution method

2. 3 DSC measurements

Differential scanning calorimetry (DSC) experiments were performed in the temperature range of 110～380 K on a Perkin-Elmer Diamond DSC instrument. Under a nitrogen atmosphere, the powder samples of 1 (4.7 mg) were placed in aluminium crucibles and were heated and cooled at a heating rate of 10 K/min.

2. 4 Single-crystal X-ray diffraction analysis

A Bruker SMART APEX-II CCD diffractometer equipped with Mo-Kαradiation (λ= 0.71073 ?) was used to gather the single-crystal X-ray crystallography data of compound 1 at different temperature. Empirical absorption corrections were performed by using SADABS. The crystal structure of 1 was solved by direct methods and refined by full-matrix least-squares method onF2by means of SHELXL software package. All non-hydrogen atoms were refined anisotro- pically and all hydrogen atoms were located and refined geometrically. Besides, the calculations of distances and angles between some atoms were performed by DIAMOND or SHELXL[12,13]. Crystallography data and refinement of 1 are as shown in Table S1.

2. 5 Dielectric measurements

The temperature dependence of the permittivity of 1 was measured by using a Tonghui TH2828A instrument in a heating and cooling cycle over the frequency range of 5 kHz to 1 MHz with an applied alternating electric field of 1 V. The pressed-powder pellets made by the samples of 1 were covered with silver conducting glue to act as the electrodes for dielectric measurements.

3 RESULTS AND DISCUSSION

3. 1 Phase transition of 1

As shown in Fig. 1, DSC curves of 1 were measured in a cooling-heating run and showed significant differences of phase transitions behavior because of the different halogen. For 1, DSC curves display a pair of endothermic and exothermic peaks at 230 and 212 K with a hysteresis loop of 18 K, which indicates the occurrence of a reversible phase transition. The entropy changes ΔSof 1 are estimated to be 9.87 and 10.64 J.mol-1.K-1in endothermic and exothermic procedures, respectively. The big values of ΔSand the 18 K thermal hysteresis show that the phase transition of 1 belongs to the first order reversible phase transitions[19-21]. According to Boltzmann equation ΔS=RlnN,where R is the gas constant and N represents the ratio of possible orientations in the whole disordered system, the value ofNis calculated to be 3.6 and 3.27 for heating and cooling cycles in 1. The large N values suggest that the phase transitions in 1 show representative characteristics of ordered-disordered type of phase transition.

3. 2 Crystal structures of 1

To further explore the mechanism of phase transition and the causes of different phase transition behaviors between 1 and 2, crystal structures of 1 and 2 were measured at 298 K (the room-temperature phase, RTP) and 100 K (the low-temperature phase, LTP) by the results of DSC curves.

Fig. 1. DCS curves of 1 under heating-cooling cycles

Fig. 2. Asymmetric unit of 1-RTP

At 298 K, 1 crystallizes in the tetragonal crystal system and space groupP42/mwitha= 21.188(3),b= 21.188(3),c= 10.1124(14) ? andV= 4539.6(14) ?3. As shown in Fig. 2, the asymmetric unit of 1-RTP consists of two separate 1,4-diisopropyl-1,4-diazabicyclo[2.2.2]octane cations and two discrete [CoBr4]2-anions. Each Co atom is four-coor- dinated by Br atoms, one of which is in a disordered state with the occupancy of 0.5 and 0.5. The Co-Br bond distances range from 2.267(7) to 2.557(14) ?. Without regards the disordered Br atoms and the angles involved, the angles of Br-Co-Br are in the range from 105.92(12)° to 112.98(8)°, forming a distorted tetrahedron. As to the torsion angles, the biggest N-C-C-N angle equals to 23°. The bond lengths and angles are well matching with the values in other [CoBr4]2-compounds. For the organic 1,4-diisopropyl-1,4-diaza- bicyclo[2.2.2]octane cations, the lengths of C-C bonds vary from 1.42(2) to 1.58(3) ? and C-N from 1.47(3) to 1.56(3) ?. In addition, the maxima of the angle difference of C-C-N and C-N-C are 5.5° and 7.5°, referring to the large N and disordered [CoBr4]2-anions. Clearly, 1-RTP is in a disordered state.

With the temperature decrease to 100 K, the space group of 1 changes fromP42/mtoP42and the crystal system is still tetragonal with similar cell parameters:a= 21.015(3),b= 21.015(3),c= 10.057(14) ?,V= 4441(2) ?3. As Fig. 3 shows, the most remarkable difference of the asymmetric unit structure between 1-RTP and 1-LTP is the break ofmplane and is in ordered state. Besides, induced by the low temperature and unique space group, the cell units shrink slightly. To be specific, the maximum of the Co-Br distance equals to 2.425(4) ?, with a 0.132 ? decrease compared to that at room temperature. Analogously, the bond distances of N and C in isopropyl change from 1.56 to 1.53 ?. In addition, the largest torsion angle of N-C-C-N is equivalent to 2.6(16)o, which confirms that compound 1 is in an ordered state as well. Another significant difference between 1-RTP and 1-LTP results from the rearrangement of the compound. One cell unit contains eight asymmetric units in 1-LTP and four in 1-RTP on the contrary. Overall, it is evident that compound 1 undergoes a reversible structural phase tran- sition.

Fig. 3. Asymmetric unit of 1-LTP

3. 3 Dielectric properties of 1

The dielectric is highly related to external stimuli, such as pressure, temperature, light, electric field and so on. The step-like dielectric anomalies around phase transition temperature are driven by local dynamic changes of dabco. Particularly, the ordered state (LTP) and disordered state (RTP) result in the dipolar orientation transformation. The real part of the temperature-dependent dielectric constant of the powder samples that are measured at 500, 1, 5, 10, 100 and 1 MHz and different temperature ranges are shown in Fig. 4 and Fig. S3, Compound 1 is at low dielectric constant state below 220 K, the value of the real part (ε') of dielectric constant is circa 20. As the temperature increased, theε' jumps and curves display step-like dielectric anomalies, which reveals the character of the first-order phase transition and is also consistent with the integral computation of DSC curves peaks. Finally,ε' gets stable at 245 K and the value ofε' is 220 at 500 Hz, which is a high dielectric state. The minimum value of high dielectric constant state is about 10-fold compared to that of low dielectric constant state. The large difference of the value and the switchable dielectric property make compound 1 a potential selection in dielectric switch under appropriate frequencies[22-24].

4 CONCLUSION

Generally speaking, through solution method a novel organic-inorganic hybrid dielectric switchable phase transition compound with an excellent thermostability (Fig. 5) based on DABCO derivate is synthesized. The DSC curves demonstrate that compound 1 undergoes a reversible phase transition at 230 and 212 K with an 18 K thermal hysteresis. The most striking structural difference is the deletion of the mirror planemwith the temperature plunged from room-temperature to 100 K. What's more, the dielectric curves display step-like anomalies around the phase transition temperature, the large numerical difference of dielectric constant and switchable property make 1 a potential candidate for sensors and detection devi

Fig. 4. Real part of dielectric constant curves measured in heating run at selected frequencies

Fig. 5. Thermal gravity analysis of compound 1