Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.15, pp.1487-1492, 2011 Printed in the USA. All rights reserved
Dielectric and Thermal Studies of Copper Doped Magnesium Hydrogen
Maleate Hexahydrate Single Crystals
B. Rajagopal
, A.V. Sarma
and M.V. Ramana
Department of Physics, Kavitha Degree College, Khammam, 507002 A.P., India
Department of Physics, Andhra University, Waltair, A.P., India
Department of Physics, SR&BGNR degree College, Khammam, 507002 A.P., India
Corresponding author:
The dielectric and thermal analysis of copper doped Magnesium Hydrogen Maleate
Hexahydrate single crystals
are studied for the first time. The dielectric constant, loss
tangent, a.c. conductivity and a.c. activation energy were measured with variation of
frequency in the range 50 Hz to 1 MHz. Dielectric studies reveal that the present crystal
under investigation show good optical quality. The melting point and final compound after
decomposition were determined using the DTA/TG analysis.
Keywords: Dielectric constant, loss tangent, TG, DTA
The maleates are of practical importance because of their use as coatings with specific
properties, efficient catalysts and are also of medicinal significance. Dielectric characteristics
of organo-metallic crystals are of increasing importance as the field of solid state electronics
continues to expand rapidly. For these applications the properties of most concern are the
dielectric constant, loss tangent and a.c. conductivity. New devices and applications are
continually increasing the frequency range and the range of environmental conditions,
particularly temperature, that are of practical interest. The frequency dependent conductivity
and dielectric constant provide important information on the ionic or electronic transport
mechanism. It gives an insight into the structure of the materials since the localised electronic
states within the material are created due to the presence of disorder in the atomic
configuration and/or the composition. The structure of MHMH crystal was studied by various
investigators [1,2]. Recently the synthesis and crystal structures on alkali metal maleates were
studied [3]. The thermal decomposition of the complexes formed between manganese, cobalt,
1488 B. Rajagopal, A.V. Sarma and M.V. Ramana Vol.10, No.15
nickel, copper and zinc with maleic acid was studied using thermo-gravimetry TG and
differential thermal analysis DTA [4]. Thermal analysis of transition metal carboxylates [5]
has been subject of recent interest due to technological importance. The final thermolysis
products (metal oxides) find extensive applications as catalysts, ceramic colorants,
photoconductors, etc. The present investigation was focussed on the dielectric studies and
thermal analysis of copper
doped magnesium hydrogen maleate hexahydrate single crystals.
Copper doped magnesium hydrogen maleate hexahydrate (hereafter Cu
:MHMH) single
crystal were grown from the aqueous solution containing magnesium carbonate and maleic
acid by slow evaporation method at room temperature. Cu
ions were introduced to an extent
of 0.1 mol %, by the addition of solution of copper sulphate, using AnalaR grade reagents.
Good quality Cu
:MHMH crystals were grown from aqueous solution within a period of two
weeks. The platy crystals grown were electroded with silver paste for electrical contact to
study the a.c. electrical characteristics using Multi-frequency Hioki 3532-50 LCR Hi-Tester.
In order to know the thermal behaviour, the Cu
:MHMH crystals was subjected to thermo-
gravimetric analysis. The thermo-gravimetric analysis (TGA) and differential thermal
analysis (DTA) were carried out using Netzsch STA 409 C/CD thermal analyzer at a heating
rate of 10 °C min
in the nitrogen atmosphere to determine the thermal stability of the
compound. The crucible used was of alumina (Al
), which served as a reference for the
3.1. Dielectric Studies
The dielectric constant ε' of Cu
:MHMH single crystal at different temperatures 308K,
323K, 343K, 363K and 383K with frequency range 50 Hz to 1 MHz was presented in Figure
1. It was evident from the Figure 1 that the dielectric constant decreases with increase in
frequency [6-10]. Also, the frequency dependence of dielectric constant ε' at different
temperatures showed that at high frequencies the dielectric constant values were almost
independent of temperature but as the frequency decreases the dielectric constant was more
temperature dependent. The dielectric constant of the sample was more dispersive below
1 kHz while decreases from 10 kHz. The high dielectric constant in the lower frequency
region of the sample under investigation can be assigned to the presence of interfacial i.e.,
space charge polarization mechanism [11] and the low value of dielectric constant at higher
frequencies may be due to the loss of significance of space charge polarisation gradually. The
increase in dielectric constant with increasing temperature can be attributed to space charge
polarisation near the grain boundary interfaces which depends on purity and perfection of the
sample. It was evident from the Figure 2 that the loss tangent has high values at low
frequencies and decreases with increasing frequency. The variation of loss tangent with
frequency can be assigned to the dipole alignment when the field is applied. At low
Vol.10, No.15 Dielectric and Thermal Studies 1489
frequencies the dipoles can easily switch alignment with the changing field. As the frequency
increases the dipoles are less able to rotate and maintain phase with the applied field, thus
they reduce their contribution to the polarization field. The low loss tangent at high frequency
is generally expected in the samples with good optical quality [12,13]. The loss tangent is
strongly frequency dependent similar to that in L-Valine single crystal [14].
Figure 1. Dielectric constant versus log f
Figure 2. Loss tangent versus log f
It is observed from the Figure 3 that the a.c. conductivity has low values at lower frequencies
and increases with increase in frequency and obeyed the empirical law
σ(ω) α ω
where “ω” is the angular frequency and the value of “n” is frequency exponent depends on
the temperature and frequency. The increase in a.c. conductivity could be due to the reduction
in the space charge polarization. At low frequencies the a.c conductivity can be assigned to
the trapping of some carriers at defect sites in the crystal. But at intermediate frequencies, it
depends progressively of the frequency. At high frequencies, the a.c. conductivity is almost
proportional to the frequency [15]. The behaviour of log(σ
) against 1000/T, at constant
frequencies, was presented for the Cu
:MHMH crystal in Figure 4. The activation energy of
conduction “E” is calculated at different frequencies from the slopes of plot log(σ
) against
1000/T. It is observed that the slopes are decreasing with increasing temperature. The
activation energy is 1.39 eV for 100 Hz frequency at 363K to 383K temperatures.
1490 B. Rajagopal, A.V. Sarma and M.V. Ramana Vol.10, No.15
Figure 3. a.c. conductivity versus log f Figure 4. log σ
versus 1000/T
3.2. Thermal Studies
Figure 5 depicts the TGA and DTA profile for Cu
:MHMH crystal. The thermo gravimetric
analysis of the present sample was carried out between 296K and 1673K in nitrogen
atmosphere. An endothermic peak at 413.8 K can be attributed to the melting point of the
sample [16]. The sample starts loosing water at 376.4 K.
Figure 5. TG/DTA of Cu
MHMH crystal
An exothermic peak was observed at 598K. Further, heating above melting point results in
the formation of volatile substances CO and CO
and rest of water molecule. The TG/DTA
trace shows origin of major weight loss immediately after loss of water of crystallization.
Therefore loss of water of crystallization, melting point and decomposition are expected to
occur in sequential manner. Prolonged heating upto 1159K does not produce any significant
endothermic or exothermic peaks in TG/DTA curves. The loss of weight on heating the
Vol.10, No.15 Dielectric and Thermal Studies 1491
sample started almost in a continuous manner attaining constancy in weight at different
Dielectric studies revealed that the sample under investigation possesses good optical quality.
The melting point of the sample from DTA curve is close to that determined using capillary
method. It is evident from the TGA that the final product of the sample after decomposition
at 1159K leaves a residue MgO whose molecular weight is approximately equal to its
theoretical calculated value.
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