Physica B 406 (2011) 426–429

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Thermal and dielectric studies of nickel malonate dihydrate single crystals

Varghese Mathew a, K.C. Mathai a, C.K. Mahadeven b, K.E. Abraham c,n

a Department of Physics, St. Aloysius’ College, Edathua 689573, Indiab Physics Research Centre, S.T. Hindu College, Nagercoil 629002, Indiac Department of Physics, S.B. College, Changanacherry 686101, India

a r t i c l e i n f o

Article history:

Received 18 September 2010

Received in revised form

30 October 2010

Accepted 2 November 2010

Keywords:

Nickel malonate

Crystal growth

Thermal properties

Dielectric constant

Dielectric loss

AC conductivity

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.11.004

esponding author. Tel.: +91 9447406915; fax

ail address: abrahamke@gmail.com (K.E. Abra

a b s t r a c t

Single crystals of nickel malonate dihydrate were grown by the gel technique, employing the single

diffusion method. Thermal dehydration of the crystal was investigated by thermogravimetric and

differential thermal analyses. The title compound exhibits a steady thermal behaviour at higher

temperature range of 350–800 1C. The dielectric properties of the prepared sample were analyzed as

a function of frequency in the range of 1 kHz–1 MHz and at temperatures between 40 and 140 1C.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Metal malonates arouse considerable interest amongst severalresearchers due to their potential applications in molecular elec-tronics, catalysts, biologically active compounds and molecular-based magnetic materials. Further, they provide the framework forsupramolecular crystal engineering [1–3]. Camps and Hirsch [4]showed that nucleophilic cyclopropanation of fullerenes (C60) inhigh yields is possible starting directly from malonates. Malonicacid is a dicarboxylic acid, the next higher homologue to oxalic acidwith a fairly active methylene group between the two carboxylategroups. Carboxylate complexes of 3d elements, including severalexamples of nickel derivatives play an important role in biochem-istry [5,6]. Growth of crystals by employing the gel technique isuseful for substances having low solubility and low dissociationtemperature. Gels, especially silica gel are the best and versatilegrowth media for crystals. The gel medium acts as a polymer gridpermitting the reactant to diffuse at desirable controlled rate [7].

2. Material and methods

Sodium meta silicate having a density of 1.033 g/cm3, mixed with1 M malonic acid to achieve a pH of 6, was allowed to set in straighttubes. An aqueous solution of nickel chloride of 0.5 M concentrationwas taken over the set gel. The outer electrolyte slowly diffuses intothe gel and the controlled reaction resulted in the crystallization of

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ham).

nickel malonate. Greenish crystals of nickel malonate were grownafter a long period of fifteen weeks. The growth details and spectralcharacterization of the crystal are reported elsewhere [8].

Thermogravimetric runs of the sample were taken on a PerkinElmer Diamond system. The TG–DTG–DTA patterns were obtainedusing 6.049 mg of the sample between 40 and 800 1C at a heatingrate of 10 1C per minute under nitrogen atmosphere.

The dielectric constant (er) and the dielectric loss (tan d) weremeasured using an AGILENT 4284-A model LCR meter by the parallelplate capacitor method [9]. The nickel malonate crystals werepowdered and pelletized into diameter of 13 mm. The opposite facesof the pellet were coated with fine graphite powder. Using the LCRmeter, the capacitance and dielectric loss of these crystals weremeasured for frequencies 1, 10, 100 kHz and 1 MHz at varioustemperatures ranging from 40 to 140 1C. The dielectric constant ofthe crystal was calculated using the relation, er¼Ccrys/Cair, where Ccrys

is the capacitance of the crystal and Cair the capacitance of the samedimension of air. From the dielectric constant er and the loss factor(tan d), the AC conductivity (sac) of the sample can be evaluated usingthe relation,sac¼2pfe0ertan d, where f is the frequency of the appliedfield and e0 the permittivity of free space.

3. Results and discussion

3.1. Thermal studies

Thermal decomposition studies of metal malonates were inves-tigated by a number of researchers [10–12]. Galwey et al. [13]

V. Mathew et al. / Physica B 406 (2011) 426–429 427

reported an elaborate kinetic and mechanistic study of the thermaldecomposition of nickel malonate synthesized by the chemicalmethod. The pyrolysis process is complex and often occurs in amulti-stepped mechanism. The shape of the curve is determined by

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Fig. 1. TG (a), DTG (b) and DTA (c) pattern

the kinetic parameters of the pyrolysis, such as reaction order,frequency factor and energy of activation [14].

The TG, DTG and DTA curves of the gel-grown nickel malonatecrystals recorded in the temperature range from ambient to 800 1C

(°C)

ture (°C)

500 600 700 803.1-2.047

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s of nickel malonate dihydrate crystal.

2.02.22.42.62.83.03.23.43.63.84.04.24.44.64.85.0

Die

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ric c

onst

ant

1 kHZ 10 kHz 100 kHZ 1 MHz

V. Mathew et al. / Physica B 406 (2011) 426–429428

are depicted in Fig. 1. The thermograms show a three-stage thermaldecomposition pattern. The first stage (190–245 1C) is attributed tothe dehydration stage of two water molecules and corresponds tothe weight loss of nearly 19%, which agrees with the calculatedvalue of 18.4%. The resulting anhydrous malonate follows a TGplateau up to 330 1C after which the curve exhibits a mass loss of50% extending up to 355 1C. This is due to the decomposing processof the crystal and becomes 30% of original weight and remainsconstant until the end of the analysis. The DTG pattern confirms thedehydration process.

The DTA plot exhibits a smooth endotherm at 238.7 and352.17 1C. These values correspond to the thermal decompositionprocess in the ranges 190–240 and 330–355 1C, respectively. Thesharpness of endothermic peaks shows a good degree of crystal-linity and purity of the sample [15]. The decomposition processagrees fairly well with other metal malonates.

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40Temperature (°C)

60 80 100 120 140

Fig. 2. Dielectric parameters of nickel malonate dihydrate crystal: (a) Variation of

dielectric constant with temperature; (b) Variation of dielectric loss with tempera-

ture and (c) Variation of AC conductivity with temperature.

3.2. Dielectric studies

Fig. 2 shows the variation in dielectric properties of nickelmalonate dihydrate crystals with temperature at four differentfrequencies, viz. 1, 10, 100 kHz and 1 MHz. The dielectric para-meters, viz. er, tan d and sac are found to increase as the tempera-ture increases. The variation in dielectric permittivity withtemperature is more pronounced at low frequencies than at higherfrequencies. The er and tan d values decrease with the increase infrequency. The sac value increases with the increase in frequency.This is considered to be a normal dielectric behaviour. It isinteresting to note that the er values are significantly less (o4.0)at near ambient temperatures (up to 60 1C for 1 kHz frequency; seeFig. 2(a)).

In the microelectronics industry, it is required to replace thedielectric materials in multilevel interconnect structures with lowdielectric constant materials as an interlayer dielectric (ILD), whichsurround and insulate interconnect wiring. Lowering the dielectricconstant values of the ILD decreases the RC delay, lowers powerconsumption and reduces ‘cross-talk’ between nearby intercon-nects [16]. Silica has the er value E4.0, in part as a result of Si–Obonds. Although innovative developments have been made forthe development of new low er value materials to replace silica,there is still a need for new low er value dielectric materials [16].Also, materials in the single crystal form would be very muchinteresting. Mahadevan and his co-workers have observed sig-nificant reduction in er value of potassium dihydrogen orthopho-sphate (KDP) single crystal when doped with L-arginine [17] andurea [18], which makes KDP a low er value dielectric material. Inaddition, Meena and Mahadevan [16] have found that L-arginineacetate and L-arginine oxalate are promising low er value dielectricmaterials. Considering the above facts, it can be understood thatthe title compound crystal is a promising low er value dielectricmaterial.

In order to understand the dielectric behaviour of the titlecompound crystal, the mechanism of polarization can be consideredsimilar to the conduction process. Electrical conductivity for thistype of a single crystal can be determined by the proton transportwithin the framework of hydrogen bonds due to the presence ofwater molecules [17]. Two possible mechanisms can be consideredfor the conductivity. The first mechanism is identical to theconductivity mechanism of ice, which also contains hydrogenbonds. According to the second mechanism, conductivity is asso-ciated with the incorporation of impurities into the crystal latticehaving different values and the formation of corresponding defectsin ionic crystals. Conductivity of ice is determined by the simulta-neous presence of positive and negative ions and orientationaldefects-vacant hydrogen bonds (L-defects) and doubly occupied

hydrogen bonds (D-defects). Other possible defects are vacanciesand defect associates. The temperature dependence of electricalconductivity observed in the present study indicates that the

V. Mathew et al. / Physica B 406 (2011) 426–429 429

conductivity of nickel malonate dihydrate single crystal is mainlydue to the thermally generated L-defects. Hence the increase inconductivity with the increase in temperature observed in thepresent study for the title compound crystal can be understood dueto the temperature dependence of the proton transport as itdepends on the generation of L-defects. In addition, the conductivityincreases smoothly through the temperature range (40–140 1C)considered; there is no sharp increase that would be characteristicof the super protonic phase transition [17].

4. Conclusions

Nickel malonate dihydrate single crystals were grown by thecontrolled reaction of aqueous solutions of malonic acid and nickelchloride in silica gel. From the TG–DTA curves it is inferred that thedecomposition of the material takes place in the vicinity of 350 1C.Also, the thermograms reveal the presence of two water moleculesin the title compound. The low er values observed (o4.0) atambient temperatures indicate that nickel malonate dihydratecrystal is a promising low er value dielectric material useful inmicroelectronics industry. The conductivity mechanism could beexplained as due to the proton transport within the framework ofhydrogen bonds.

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