Physica B 407 (2012) 222–226

Contents lists available at SciVerse ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

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

A study of thermal, dielectric and magnetic properties of strontiummalonate crystals

Varghese Mathew a,n, Sabu Jacob a, C.K. Mahadevan b, K.E. Abraham c

a Department of Physics, St. Aloysius’ College, Edathua 689 573, Indiab Physics Research Centre, S.T. Hindu College, Nagercoil 629 002, Indiac Department of Physics, S.B. College, Changanacherry 686 101, India

a r t i c l e i n f o

Article history:

Received 26 September 2011

Received in revised form

15 October 2011

Accepted 18 October 2011Available online 21 October 2011

Keywords:

Crystal growth

Thermal analysis

Dielectric properties

Magnetic studies

26/$ - see front matter & 2011 Elsevier B.V. A

016/j.physb.2011.10.032

esponding author. Tel.: þ91 9447104674; fa

ail address: varma57@gmail.com (V. Mathew

a b s t r a c t

Crystals of strontium malonate (SrC3H2O4) were grown in silica gel by the single diffusion technique.

The thermo gravimetric (TG), differential thermal analysis (DTA) and differential scanning calorimetric

(DSC) studies were carried out to investigate the thermal stability of the crystal. The dielectric behavior

of the title compound crystal was investigated by measuring the dielectric parameters – dielectric

constant, dielectric loss and AC conductivity as a function of four frequencies �1 kHz, 10 kHz, 100 kHz

and 1 MHz at temperatures ranging from 50 to 170 1C. Results indicate that the title compound is

thermally stable up to about 409 1C and is a promising low er-value dielectric material. The magnetic

behavior of the crystal was also explored using a vibrating sample magnetometer.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Strontium malonate and other strontium salts are widely usedas ingredients for pharmaceuticals, vitamins and nutritionalsupplements. Malonate complexes of strontium exhibit remark-able magnetic properties [1,2]. The three dimensional structuralnetwork aspects of strontium malonate anhydrate are reported byKenny Stahl and co-workers [3]. Strontium malonate and othermetal malonates are generally prepared by the precipitationtechnique. The authors have reported the growth and spectro-scopic characterization of strontium malonate crystals grown bythe gel technique [4]. Growth of crystals in gel is the mostversatile and inexpensive technique. Further, it is a self-purifyingprocess, free from thermal strains, which is common in crystalsgrown from the melt [5].

Malonate, the dianion of 1,3-propanedioic acid can function asa versatile bridging ligand. The intriguing structural complexity ofmalonate is associated with the simultaneous adoption of chelat-ing bidentate or unidentate and the different bridging coordina-tion modes like syn–syn, syn–anti and anti–anti. The ability of thebridging ligand to mediate magnetic coupling between the para-magnetic centers that it links plays a fundamental role in themagnetic behavior of metal malonates. Thus malonate ligand is apromising candidate in designing extended magnetic networks [6–9].

ll rights reserved.

x: þ91 477 2210564.

).

Thermal studies of transition metal malonates and malonic acid havebeen reported [10,11]. But the reports related to dielectric studies arescanty in literature [12,13]. In this report, we present the thermal,dielectric and magnetic studies of strontium malonate crystals grownby the gel method.

2. Materials and methods

Strontium malonate crystals were grown by the gel techniqueusing the single diffusion method. Silica gel was prepared by mixingan aqueous solution of sodium meta silicate of density 1.033 g/ccwith 1 M malonic acid solution. After the gel was set, strontiumchloride solution (0.5 M) was slowly poured over it. Transparentcrystals of strontium malonate were grown after a period of sixweeks. The growth details are discussed elsewhere [4]. Powderedsamples were used for X-ray diffraction studies using a Philips Pananalytical X’pert-Pro diffractometer with Cu-Ka radiation in 2yranging from 0 to 701. The measured X-ray diffraction (XRD) profileof the title compound is shown in Fig. 1.

Thermo gravimetric (TG) and differential thermal analysis(DTA) runs of the crystal were taken in nitrogen atmosphere ata heating rate of 10 1C per minute on Perkin Elmer Diamondthermal analyzer in the temperature range of 40 to 800 1C.Differential scanning calorimetric (DSC) curve of the title crystalwas recorded using a Mettler Toledo DSC 822e instrument in thetemperature range of 40–550 1C. The dielectric constant (er) anddielectric loss (tan d) were estimated using an Agilent 4284-A LCR

V. Mathew et al. / Physica B 407 (2012) 222–226 223

meter [14,15]. The crystal was powdered and pelletized to adiameter of 13 mm. The opposite faces of pellet were coated withgraphite in order to obtain good ohmic contact. Using the LCRmeter, the capacitance of the crystal was measured for frequen-cies 1 kHz, 10 kHz, 100 kHz and 1 MHz at various temperaturesranging from 50–170 1C. The dielectric constant of the crystalwas calculated using the expression er¼Ccrys/Cair, where Ccrys isthe capacitance of the crystal and Cair is the capacitance of samedimension of air.

10 20 30 40 50 60 700

500

1000

1500

2000

2500

3000

3500

(212)

(421)

(050)

(131)

(221)

(410)(110)

(310)

Cou

nts

Position (2θ)

Fig. 1. XRD pattern of strontium malonate crystals.

75

80

85

90

95

100

Wei

ght %

Temperature (°C)

6.9026.8

6.6

6.4

6.2

6.0

5.8

5.6

5.4

5.2

5.01836.99

Wei

ght (

mg)

Temperature (°C)100

-10

0

10

20

30

40

Hea

t Flo

w E

ndo

Dow

n (m

W)

200 300 400 500 600 700 800

100 200 300 400 500 600 700 800

Fig. 2. TG, DTG, DTA and DSC of strontium malonate crystal. (a)

The magnetic behavior of the material was studied using avibrating sample magnetometer, model Lakeshore 7300 VSM [16]at room temperature with a maximum applied magnetic fieldof 10 kOe.

3. Results and discussion

3.1. Thermal studies

Fig. 2 (a–c) shows TG, DTG and DTA curves for strontium malonatecrystal at a heating rate of 10 1C/min. The thermal studies indicatethat the title compound is thermally stable up to about 409 1C. Thereis only one stage of decomposition as the compound is anhydrousand crystalline [17]. DTA shows an endotherm at 412.36 1C corre-sponding to DTG peak at 409.39 1C with a shoulder in DTG at about420 1C. Since any intermediate formed could not be detected, so theshoulder is not attributed to any stable intermediate species but toa change in pyrolysis of gaseous decomposition products only or to aphase transformation. TG shows a change in slope at a mass lossof 15%. It may be due to the removal of one CO molecule. The endproduct formed is strontium carbonate with a mass loss of 22.15%.

Fig. 2d shows the DSC curve for strontium malonate in nitrogenatmosphere at a heating rate of 10 1C/minute. The endotherm at411.35 1C corresponds to the DTA curve at 412.36 1C.

3.2. Dielectric studies

Fig. 3 shows the variation of dielectric properties of strontiummalonate crystal with temperature at four different frequencies,viz. 1 kHz, 10 kHz, 100 kHz and 1 MHz.

Temperature (°C)100 200 300 400 500 600 700 803.4

-0.4575

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.000.02976

Der

ivat

ive

Wei

ght (

mg/

min

)

50

-10

-8

-6

-4

-2

0

2

4

mW

°C100 150 200 250 300 350 400 450 500 550

TG profile. (b) DTG profile. (c) DTA profile. (d) DSC profile.

V. Mathew et al. / Physica B 407 (2012) 222–226224

It can be seen that the dielectric constant increases with theincrease in temperature (except for the frequency of 1 MHz) anddecreases with the increase in frequency. When the frequency is1 MHz, the er value increases with the increase in temperature atlower temperatures and decreases with the increase in tempera-ture at higher temperatures. However the temperature depen-dence is found to be very less. The dielectric loss does not varysystematically with the temperature for the lower frequencies(1 kHz and 10 kHz) while it increases with the increase intemperature for the higher frequencies considered in the presentstudy (100 kHz and 1 MHz). For 1 kHz frequency, the tan d valuedecreases with increase in temperature up to 110 1C and thenincreases with increase in temperature. For 10 kHz frequency, thetan d value increases with increase in temperature up to 100 1C,decreases with increase in temperature up to 140 1C and thenincreases with increase in temperature. The tan d values arefound to be small, which indicates that the crystals grown inthe present study are of high quality. The variation of ACconductivity with temperature is observed to be small for thehigher frequencies (100 kHz and 1 MHz). For the lower frequen-cies, the sac increases significantly with the increase in tempera-ture. Further, the sac is found to increase with the increase infrequency.

For a normal dielectric behavior, the er and tan d valuesincrease with increase in temperature and decrease with increasein frequency. The sac value increases with increase in both thetemperature and frequency [18]. In the case of strontium mal-onate crystal considered in the present study, some deviationsfrom the above behavior occur at low frequencies, more signifi-cantly at 1 kHz frequency. However, lower frequencies up toabout 1 kHz frequency is normally considered to be static moreor less similar to the DC electrical behavior. Moreover, thesedeviations may be due to the anhydrous nature of strontiummalonate, thus having lower number of hydrogen bonds than forseveral other metal malonates having water molecules. Recently,we have found [13,19 and 20] less deviations from the normaldielectric behavior in the case of nickel malonate dihydrate,cobalt malonate dihydrate and copper malonate trihydrate crys-tals confirming this.

In normal dielectric behavior, the dielectric constant decreaseswith increasing frequency and reaches a constant value, depend-ing on the fact that beyond a certain frequency of the electricfield, the dipole does not follow the alternating field. Crystals withhigh dielectric constant lead to power dissipation. The materialhaving low dielectric constant will have less number of dipolesper unit volume. As a result it will have maximum losses whencompared to the material having high dielectric constant [21]. Theer values observed in the present study for strontium malonatecrystal at all temperatures and for all frequencies considered arewithin 4.2 only. The tan d values observed are also considerable,within 0.10. So, the results obtained in the present study are verynearly in accordance with the fact mentioned above.

The electrical conductivity of the title compound crystal can bedetermined by the proton transport within the framework ofhydrogen bonds [22]. Two possible mechanisms can be consid-ered for the conductivity: the first one is identical to theconductivity mechanism in ice also containing hydrogen bonds;as per the second mechanism, conductivity is associated with theincorporation into the crystal lattice of impurities having differentvalues and the formation of corresponding defects in ioniccrystals. The conductivity of ice is determined by the simulta-neous presence of positive and negative ions and orientationaldefects-vacant hydrogen bonds (L-defects) and doubly occupiedhydrogen bonds (D-defects). Other possible defects are vacanciesand defect associates. The temperature dependence of conductiv-ity observed in the present study allows us to state that the

conductivity of strontium malonate crystal is determined mainlyby the thermally generated L-defects. So, the proton transportdepends on the generation of L-defects. Hence the increase ofconductivity with the increase in temperature observed in thepresent study can be understood as due to the temperaturedependence of the proton transport. Also, conductivity increasessmoothly through the temperature range (50–170 1C) considered;there is no sharp increase that would be characteristic of thesuper protonic phase transition [23].

The dielectric constant of a material is generally composedof four types of contributions, viz. ionic, electronic, orientationaland space charge polarizations. All these may be active at lowfrequencies, the nature of variations of dielectric constant withfrequency and temperature indicates the type of contributionsthat are present in them. Variation of er with temperature isgenerally attributed to the crystal expansion, the electronic andionic polarizations and the presence of impurities and crystaldefects. The variation at low temperatures is mainly due to thecrystal expansion and electronic and ionic polarizations.The increase at higher temperatures is mainly attributed to thethermally generated charge carriers and impurity dipoles.Varotsos [24] has shown that the electronic polarizability practi-cally remains constant in the case of ionic crystals. The increase indielectric constant with temperature is essentially due to thetemperature variation of ionic polarizability.

A dielectric material is necessary to insulate two metalcomponents in interconnected structures. The capacitance (C)between interconnected structures is proportional to er andinversely proportional to the square of the line spacing. Thus, asline dimensions decrease there is a rapid increase in the signaldelay (known as RC delay), because of the combined increase inthe line resistance (R) and C. Lowering the er value of thedielectric material decreases the RC delay and lowers powerconsumption.

Single crystals are normally considered to have propertiessuch as, no pores, low dielectric loss, good chemical and thermalstability, good mechanical strength, etc. Even water solublematerials in the single crystal form would be very much inter-esting as electrical interconnects are with waterless condition.Large single crystals can be cut into thin slices and polished andused. Ohmic contact with metal components, if necessary, can beestablished by coating the surface with silver or graphite paint.So, low er value dielectric crystals are expected to be useful ininterconnected electrical structures.

Mahadevan and his co-workers have observed significantreduction in the er value of potassium dihydrogen orthophosphate(KDP) single crystal when doped with L-arginine [22] andurea [25], which makes KDP as a low er value dielectric material.Also, Meena and Mahadevan [23] have found that L-arginineacetate and L-arginine oxalate are promising low er value dielec-tric materials.

In the present study, it has been observed that the er values aresignificantly less (o4.0) at low temperatures up to 100 1C for1 kHz frequency and at all temperatures for all other frequenciesconsidered (see Fig. 3a). Thus, the present study indicates thatstrontium malonate crystal is a promising low er value dielectricmaterial.

3.3. Magnetic studies

The hysteresis loop (M–H curve) for the title compound crystalwas taken at room temperature using VSM, with a maximumapplied field of 10 kOe. Fig. 4 shows the variation of magneticmoment with the applied magnetic field.

Strontium malonate is 8 coordinated with four oxygen atomsof the malonate ligand and four strontium atoms, the bridging

40 60 80 100 120 140 160 1803.2

3.3

3.4

3.5

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1 kHz10 kHz100 kHz1 MHz

40 60 80 100 120 140 160 1800.00

0.02

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40 60 80 100 120 140 160 180-202468

101214161820222426283032343638

Die

lect

ric c

onst

ant

Die

lect

ric lo

ssA

C c

ondu

ctiv

ity

Temperature (°C)

Temperature (°C)

Temperature (°C)

1 kHz10 kHz100 kHz1 MHz

Fig. 3. (a) Variation of dielectric constant with temperature. (b) Variation of

dielectric loss with temperature. (c) Variation of AC conductivity (�10�7 mho/m)

with temperature. (a–c) Dielectric parameters of strontium malonate crystal.

-10000 -5000 0 5000 10000

-0.1

0.0

0.1

Mom

ent (

emu/

g)

Applied field (Oe)

Fig. 4. M–H curve at room temperature for strontium malonate crystal.

V. Mathew et al. / Physica B 407 (2012) 222–226 225

ligand mediates magnetic coupling between the paramagneticcenters that it links. In addition to the bidentate chelation one of theoxygen acts as a m-oxo bridge between two metal centers. Themagnetic coupling among the magnetic centers is given by J value,

J being negative or positive depending on the antiferro- or ferro-magnetic character of the coupling. The carboxylato bridge will beaffected by the possible syn–syn, syn–anti and anti–anti bridgingmodes that it can adopt. The nature of the coupling (ferro-orantiferro) is dependent upon the nature of the magnetic orbitals ofthe spin carriers connected by the bridging ligand. The two crystal-lographically independent strontium (II) ions [Sr(1) and Sr(2)] havedistorted square pyramidal surroundings. The four coplanar carbox-ylate oxygen atoms, which are coordinated to Sr(1), define the basalplane, whereas the apical position is occupied by another carbox-ylate O atom, which adopts bidentate bridging coordination modeswith Sr (II) center. Each malonate adopts bidentate and unidentatecoordination modes. The effectiveness of magnetic exchange cou-pling is related to the degree of protonation of the malonate group.Two slightly different carboxylate bridges, exhibit the anti–synconformation, regularly within each Strontium (II) chain. Due tostrict orthogonality of the magnetic orbitals of the spin carriers, anddue to the protonation of the malonate group the ferromagneticcontribution may dominate, leading to ferromagnetism [26–29].

4. Conclusions

The growth of strontium malonate crystals was accomplishedusing the silica gel technique, by the controlled reaction ofstrontium chloride with malonic acid. The TG–DTA–DSC thermo-grams reveal the thermal decomposition mechanism of the titlematerial. The decomposition is a single step process and the endproduct formed is carbonate. The higher thermal stability (up to409 1C) of strontium malonate crystal is ascribed to it beinganhydrous so that decomposition proceeds without restructuring.The conductivity behavior could be understood as due to protontransport within the framework of hydrogen bonds. The low er

values observed (o4.0) indicate that the title compound crystal isa promising low er value dielectric material expected to be usefulin interconnected electrical structures. The magnetic hysteresisprofile of the crystal was determined using a vibrating samplemagnetometer by measuring the magnetic moments for differentapplied fields. Due to the protonation of the malonate group anddue to strict orthogonality of the magnetic orbitals of the spincarriers the crystal exhibits ferromagnetism.

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