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Dielectric and magnetic studies of gel-grown copper malonate trihydrate crystals
Materials Letters 65 (2011) 2142–2145
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dielectric and magnetic studies of gel-grown copper malonate trihydrate crystals Varghese Mathew a,⁎, Sabu Jacob a, C.K. Mahadevan b, K.E. Abraham c a b c
Dept. of Physics, St. Aloysius' College, Edathua, 689573, India Physics Research Centre, S.T. Hindu College, Nagercoil, 629002, India Dept. of Physics, S.B. College, Changanacherry, 686101, India
a r t i c l e
i n f o
Article history: Received 19 December 2010 Accepted 15 April 2011 Available online 21 April 2011 Keywords: Copper malonate Dielectric constant Dielectric loss AC conductivity Magnetic studies
a b s t r a c t Single crystals of copper malonate trihydrate were grown in silica gel by slow diffusion of copper nitrate by sodium metasilicate gel impregnated with malonic acid. The grown crystal was subjected to dielectric studies and magnetic studies. The dielectric properties of the crystal were analyzed as a function of frequency and temperature. A vibrating sample magnetometer was used to determine the hysteresis properties of the crystals by measuring the magnetic moment for different applied fields. The crystal is found to be weakly ferromagnetic. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Co-ordination Chemistry of copper complexes is of considerable attention, as their pharmacological and toxicological properties have been proven beneficial against several diseases such as tuberculosis, arthritis, gastric ulcers and cancers [1,2]. Copper malonate is quite an interesting metal dicarboxylate due to the structural diversity provided by a variety of malonate binding modes and the possibility for the tailoring of magnetic behavior [3]. The gel growth technique yields large and well ordered crystals of a myriad of materials, suitable for solid state experimentation. In this method, one reagent is incorporated in the gelling mixture (like silica gel) another is later diffused in to the gel, leading to very high super saturation and leads to nucleation and crystal growth. The gel medium acts as a three-dimensional crucible, its softness and uniform nature of the constraining forces which it exerts upon the growing crystals promote orderly growth, even in the presence of impurities [4]. Literature survey reveals the development of various super molecular structures of dicarboxylates by chemical synthesis with different dimensionalities, dielectric as well as magnetic properties. Carboxylate ligands play an important role in the formation of metallo carboxylate co-ordination frame work. Of the various carboxylates, oxalate and malonate have proved to have great versatility and structural complexity for its compounds with metallic ions like Cu II, Co II, etc [5–7]. The dielectric studies of malonates are scanty in literature [8]. The dielectric properties of nickel malonate dihydrate
⁎ Corresponding author. Tel.: + 91 9447104674, + 91 4772212789. E-mail address: [email protected] (V. Mathew). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.049
crystals were reported by the authors [9]. Malonate may be imagined as a dissymmetric ligand having bidendate and unidendate chelation. This results in ferromagnetic coupling in many Cu II complexes. In protonated malonates effective exchange of magnetic coupling is related to the degree of protonation of malonate between the magneto centers [10–12]. In this paper, the dielectric and magnetic properties of gel-grown copper malonate trihydrate crystals are presented. 2. Material and methods Crystals of copper malonate were grown by gel technique by single diffusion method. Analytical reagent (AR grade) malonic acid and copper nitrate were used as the reagents. Silica gel was prepared by mixing an aqueous solution of sodium meta silicate of specific gravity 1.033 with 1 M malonic acid solution in ratio such that the pH of the resulting solution is 7. The gel is set after one day. The feed solution of copper nitrate (.3 M) was then slowly poured above the gel down the walls of the tube. Blue crystals of copper malonate were obtained after a long crystallization period of four months. The growth details as well as spectroscopic and thermal characterization of the crystal are discussed in our previous paper [13]. The study reveals that the crystal contains three water molecules and starts decomposing at about 100 °C. Dielectric measurements (capacitance of the crystal, Ccrys and the dielectric loss factor, tan δ) on the copper malonate trihydrate crystal were carried out by the parallel plate capacitor method as a function of temperature for various frequencies using a Precision LCR meter (AGILENT 4284 A model) [14,15]. The crystals were powdered and pelletized (by applying a pressure of 4 tons) into 13 mm diameter and
V. Mathew et al. / Materials Letters 65 (2011) 2142–2145
ð1Þ
where Ccrys is the capacitance of the crystal and Cair is the capacitance of the same dimension of air. The AC electrical conductivity (σac) was calculated using the relation ðσac Þ = εr ε0 ω ðtanδÞ
frequency 7
6
5
4
3
ð2Þ
where ε0 is the permittivity of free space (8.85 × 10− 12 Farad/m) and ω is the angular frequency (ω = 2πf). The magnetic characterization of copper malonate trihydrate crystals was carried out using a Lakeshore 7300 model vibrating sample magnetometer (VSM) [16]. The VSM measures the difference of magnetic induction between one region of space with the sample and another without the sample, thus allowing the calculation of magnetic moment, m generally with sensitivity in the order of 5 × 10− 6 emu. The magnetic moments of the sample were measured at fields ranging from 0 to 10 kOe at 285 K. The various magnetic properties like saturation magnetization and coercivity are estimated from the hysteresis curve.
1 KHz 10 KHz 100 KHz 1 MHz
40
50
60
70
80
90
100
Temperature (°C)
(b) Variation of dielectric loss with temperature and frequency 0.5 1 KHz 10 KHz 100 KHz 1 MHz
0.4
0.3
tanδ
Ccrys εr = Cair
(a) Variation of dielectric constant with temperature and
Dielectric constant
2.43 mm thickness using a hydraulic press and annealed up to 100 °C to remove water molecules if present. To make contact with the electrodes, both the pellet surfaces were coated with fine graphite powder. The sample was placed between the electrodes and heated from room temperature to 100 °C using a thermostat. The observations were made while cooling the sample. Air capacitance (Cair) was also measured. The dielectric constant of the crystal was calculated using the relation
2143
0.2
3. Results and discussion 0.1
3.1. Dielectric studies
0.0 40
50
60
70
80
90
100
Temperature (°C)
(c) Variation of AC conductivity with temperature and frequency
ac conductivity (x 10-7mho/m)
The electrical conductivity study reveals wealth of information as regard the usefulness of the materials for various applications. Further, it sheds light on the behavior of charge carriers under an electric field, their mobility and mechanism of conduction. The study of the dielectric parameters like dielectric constant (εr), dielectric loss (tan δ) and AC conductivity (σac) as a function of frequency and temperature unveils the various polarization mechanisms in the material. Fig. 1 shows the variation of dielectric properties of copper malonate trihydrate crystals with temperature at four different frequencies, viz. 1 kHz, 10 kHz, 100 kHz and 1 MHz. The dielectric parameters — εr, tan δ and σac are found to increase with the increase in temperature. The variation of εr and tan δ values are more pronounced at low frequencies than at higher frequencies. Also, these values decrease with the increase in frequency. The variation of σac value is more pronounced at high frequencies than at lower frequencies. Also, this value increases with the increase in frequency. This behavior is considered to be a normal one for a dielectric material [17]. The εr values observed, in the present study, at lower temperatures are significantly less (b4.0 upto 60 °C for 1 kHz frequency and at all temperatures considered for higher frequencies) [see Fig. 1(a)]. The microelectronics industry needs to replace the dielectric materials in multilevel interconnect structures with low dielectric constant materials as an inter layer dielectric (ILD) which surrounds and insulates interconnect wiring. Silica has the εr value ≈ 4.0. Several innovative developments have been made for the development of new low εr value materials to replace silica. However, there is still a need for new εr value dielectric materials [18]. In addition, materials in the single crystal form would be very much interesting. Meena and Mahadevan [18] have found that L-arginine acetate and L-arginine oxalate are promising low εr value dielectric materials. Also, Mahadevan and his co-workers have observed significant reduction
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
1 KHz 10 KHz 100 KHz 1 MHz
40
50
60
70
80
90
100
Temperature (°C) Fig. 1. Dielectric properties of gel-grown copper malonate trihydrate crystals.
in εr value of potassium dihydrogen orthophosphate (KDP) single crystal when doped with urea [19] and with L-arginine [20] which makes KDP as a low εr value dielectric material. Considering the above facts, copper malonate trihydrate crystal can also be considered as a promising dielectric material with low εr value.
2144
V. Mathew et al. / Materials Letters 65 (2011) 2142–2145
The dielectric behavior of the copper malonate trihydrate crystal can be understood on the basis that the mechanism of polarization is similar to the conduction process. For this type of single crystals, the electrical conductivity can be determined by the proton transport within the frame work of hydrogen bonds due to the presence of water molecules [20]. Further, the conductivity observed in the present study increases smoothly through the temperature range considered (40–100 °C); there is no sharp increase that would be characteristic of a superprotonic phase transition [20]. So, two mechanisms can be considered. The first mechanism is identical to the conductivity mechanism in ice also containing hydrogen bonds. According to the second mechanism, conductivity is associated with the incorporation in the crystal lattice of impurities having different valences and the formation of corresponding defects in ionic crystals. The conductivity of ice is determined by the simultaneous presence of positive and negative ions and orientational defects — vacant hydrogen bonds (L-defects) and doubly occupied hydrogen bonds (D-defects). Other possible defects are vacancies and defect associates. The temperature dependence of conductivity observed in the present study for copper malonate trihydrate crystal allows us to mention that the conductivity of copper malonate trihydrate crystal is determined by both thermally generated L-defects and the foreign (natural) impurities incorporated with the lattice and generating Ldefects there. So, the proton transport in the crystal depends on the generation of L-defects. Hence, the increase of conductivity with the increase in temperature observed for the copper malonate trihydrate crystal can be explained as due to the temperature dependence of the proton transport. 3.2. Magnetic studies
4. Conclusions
Moment (emu/g)
The malonate bridging ligand can mediate significant magnetic interactions; hence the current work is focused on a gel-grown malonate bridged complex of Cu(II) using vibrating sample magnetometer (Lakeshore 7300 model) at a temperature of 285 K. In VSM, the sample is made to vibrate sinusoidaly in a uniform field and the signal generated proportional to the magnetization of the sample is induced to pickup coils yielding the magnetization vs. magnetizing field curve (the hysteresis loop). The hysteresis properties are closely related to the arrangement of magnetic domains in bulk materials. These magnetic domains are linked to relaxation times and the magnetic properties of the material. The variation of magnetic moment with external field is depicted in Fig. 2. Saturation magnetization Ms, remanence magnetization Mrs, coercivity Hc are the parameters linked with the hysteresis loop. The saturation magnetization obtained by extrapolating the high field magnetization curve to the y axis and the value of Ms for CuC3H2O4 3H2O is 0.16 emu/g and saturation remanant magnetization (Mrs) is 242.2 Oe and coercivity which is structure sensitive property that depends on crystal structure, grain size, preferred orientation, stress, defect density, thickness, etc is 0.00887 mT. 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -10000
The sample is expected to respond well to magnetic field without any permanent magnetization at ambient temperature. This means that in the absence of the external field, thermal energy can overcome preferential orientation. The low value coercivity in bulk sample whose size exceeds domain wall width, magnetism reversal occurs due to domain wall motion. As domain wall moves through the sample, they become pinned at the grain boundaries and additional energy is needed to continue the motion. If domain size is less than the ferromagnetic exchange length d, coercivity Hc will be close to zero making hysteresis loss trivial. The structure of CuC3H2O4·3H2O is reported in literature which consists of two neutral diaqua malonato of copper units co-existing with diaqua dimalonate of copper ; these three entities are connected by a three-dimensional network of hydrogen bonds [21]. The magnetic orbitals at each copper atom defined by the short equatorial bonds and the axial bonds of d orbitals, in indicating the out of plane exchange pathway. The carboxylato bridge connects the equatorial bonds to the Cu(1)–Cu(2) pair. The trinuclear cation is structurally independent in tetra acqua malonate of copper and is bridged by diaqua malonato of CuII and the basal CuO bonds reveal significant distortion from square pyramidal surroundings leading to out of plane exchange pathway. The malonato copper II unit acts as a bridging ligand in an anti-syn conformation mode. The triaqua dimalonate of copper features octahedral co-ordination mode. The two different carboxylates bridge anti-syn configuration. The magnetic moment of hydrated copper(II) malonate is 1.77BM at room temperature and is close to the ‘spin-only’ value 1.73BM which is in agreement with the earlier work of Ploquin [22].
-5000
0
5000
10000
Applied field (Oe) Fig. 2. Moment vs field curve for gel-grown copper malonate trihydrate crystals.
Single crystals of copper malonate trihydrate were grown in gel media. Dielectric studies of the title compound were carried out to understand the electrical behavior. Results obtained indicate a normal dielectric behavior. The temperature dependence of conductivity could be understood as due to the proton transport within the framework of hydrogen bonds. The low εr values observed (b4.0) at low temperatures (up to 60 °C for 1 kHz frequency) indicate that the title compound crystal is a promising low εr value dielectric material useful in microelectronics industry. Using vibrating sample magnetometer method, the magnetic properties of the material are studied. In malonate bridged Cu II compound CuC3H2O4·3H2O the anti-syn carboxylate bridges between copper(II) ions, the contributions from the 2p orbitals of the oxygen atoms belonging to the magnetic orbitals centered on Cu(II) ions are unfavorably oriented to give a significant overlap; and the reduction in the overlap between the carboxylate bridge and the magnetic 3 d orbital of Cu in Cu–O–C–O–Cu section in the out of plane exchange path way as well as proton transport contributes to ferromagnetic domination. References [1] Sorensen John RJ. J Med Chem 1976;19(1):135–48. [2] Wu Guangguo, Wang Guoping, Fu Xuchun, Zhu Longguan. Molecules 2003;8: 287–96. [3] Montney Matthew R, LaDuca Robert L. Inorg Chem Commun 2007;10:1518–22. [4] Halberstadt ES, Henisch HK. J Cryst Growth 1968;3:363–6. [5] Pasan Jorge, Sanchiz Joaquın, Lloret Francesc, Julve Miguel, Ruiz-Perez Catalina. Cryst Eng Comm 2007;9:478–87. [6] Delgado Fernando S, Sanchiz Joaquin, Ruiz-Perez Catalina, Lloret Francesc, Julve Miguel. Cryst Eng Comm 2004;6(73):443–50. [7] Delgado Fernando S, Hernandez-Molina Maria, Sanchiz Joaquin, Ruiz-Perez Catalina, Rodrıguez-Martin Yolanda, Lopez Trinidad, et al. Cryst Eng Comm 2004;6(22):106–11. [8] Tveekrem JL, Greer SC, Jacobs DT. Macromolecules 1998;2:147–53. [9] Mathew Varghese, Mathai KC, Mahadevan CK, Abraham KE. Physica B 2011;406: 426–9. [10] Mukherjee Partha Sarathi, Maji Tapas Kumar, Mostafa Golam, Hibbs Wendy, Chaudhuri Nirmalendu Ray. New J Chem 2001;25:760–3.
V. Mathew et al. / Materials Letters 65 (2011) 2142–2145 [11] Li Baolong, Wang Xinyi, Zhang Yuping, Gao Song, Zhang Yong. Inorg Chim Acta 2005;358:3519–24. [12] Makhankova Valeriya G, Beznischenko Asya O, Kokozay Vladimir N, Zubatyuk Roman I, Shishkin Oleg V, Jezierska Julia, et al. Inorg Chem 2008;47:4554–63. [13] Mathew Varghese, Jochan Joseph, Jacob Sabu, Lizymol Xavier, Abraham KE. Mod Phys Lett B 2010;24:1135–43. [14] Perumal S, Mahadevan CK. Physica B 2005;367:172–81. [15] Neelakanda Pillai N, Mahadevan CK. Mater Manuf Processes 2007;22:393–9. [16] Vineesh A, Bhargava Hina, Lekshmi N, Venugopalan K. J Appl Phys 2009;105 (07A309):1–3.
[17] [18] [19] [20] [21]
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Tareev B. Physics of Dielectric Materials. Moscow: Mir Publishers; 1979. Meena M, Mahadevan CK. Mater Lett 2008;62:3742–4. Goma S, Padma CM, Mahadevan CK. Mater Lett 2006;60:3701–5. Meena M, Mahadevan CK. Cryst Res Technol 2008;43:166–72. Naumov Pance, Ristova Mirjana, Soptrajanov Bojan, Drew Michael GB, Ng Seik Weng. Croatica Chem Acta 2002;75(3):701–12. [22] Ploquin J. Bull Soc Chim France 1951;18:757–9.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dielectric and magnetic studies of gel-grown copper malonate trihydrate crystals Varghese Mathew a,⁎, Sabu Jacob a, C.K. Mahadevan b, K.E. Abraham c a b c
Dept. of Physics, St. Aloysius' College, Edathua, 689573, India Physics Research Centre, S.T. Hindu College, Nagercoil, 629002, India Dept. of Physics, S.B. College, Changanacherry, 686101, India
a r t i c l e
i n f o
Article history: Received 19 December 2010 Accepted 15 April 2011 Available online 21 April 2011 Keywords: Copper malonate Dielectric constant Dielectric loss AC conductivity Magnetic studies
a b s t r a c t Single crystals of copper malonate trihydrate were grown in silica gel by slow diffusion of copper nitrate by sodium metasilicate gel impregnated with malonic acid. The grown crystal was subjected to dielectric studies and magnetic studies. The dielectric properties of the crystal were analyzed as a function of frequency and temperature. A vibrating sample magnetometer was used to determine the hysteresis properties of the crystals by measuring the magnetic moment for different applied fields. The crystal is found to be weakly ferromagnetic. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Co-ordination Chemistry of copper complexes is of considerable attention, as their pharmacological and toxicological properties have been proven beneficial against several diseases such as tuberculosis, arthritis, gastric ulcers and cancers [1,2]. Copper malonate is quite an interesting metal dicarboxylate due to the structural diversity provided by a variety of malonate binding modes and the possibility for the tailoring of magnetic behavior [3]. The gel growth technique yields large and well ordered crystals of a myriad of materials, suitable for solid state experimentation. In this method, one reagent is incorporated in the gelling mixture (like silica gel) another is later diffused in to the gel, leading to very high super saturation and leads to nucleation and crystal growth. The gel medium acts as a three-dimensional crucible, its softness and uniform nature of the constraining forces which it exerts upon the growing crystals promote orderly growth, even in the presence of impurities [4]. Literature survey reveals the development of various super molecular structures of dicarboxylates by chemical synthesis with different dimensionalities, dielectric as well as magnetic properties. Carboxylate ligands play an important role in the formation of metallo carboxylate co-ordination frame work. Of the various carboxylates, oxalate and malonate have proved to have great versatility and structural complexity for its compounds with metallic ions like Cu II, Co II, etc [5–7]. The dielectric studies of malonates are scanty in literature [8]. The dielectric properties of nickel malonate dihydrate
⁎ Corresponding author. Tel.: + 91 9447104674, + 91 4772212789. E-mail address: [email protected] (V. Mathew). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.049
crystals were reported by the authors [9]. Malonate may be imagined as a dissymmetric ligand having bidendate and unidendate chelation. This results in ferromagnetic coupling in many Cu II complexes. In protonated malonates effective exchange of magnetic coupling is related to the degree of protonation of malonate between the magneto centers [10–12]. In this paper, the dielectric and magnetic properties of gel-grown copper malonate trihydrate crystals are presented. 2. Material and methods Crystals of copper malonate were grown by gel technique by single diffusion method. Analytical reagent (AR grade) malonic acid and copper nitrate were used as the reagents. Silica gel was prepared by mixing an aqueous solution of sodium meta silicate of specific gravity 1.033 with 1 M malonic acid solution in ratio such that the pH of the resulting solution is 7. The gel is set after one day. The feed solution of copper nitrate (.3 M) was then slowly poured above the gel down the walls of the tube. Blue crystals of copper malonate were obtained after a long crystallization period of four months. The growth details as well as spectroscopic and thermal characterization of the crystal are discussed in our previous paper [13]. The study reveals that the crystal contains three water molecules and starts decomposing at about 100 °C. Dielectric measurements (capacitance of the crystal, Ccrys and the dielectric loss factor, tan δ) on the copper malonate trihydrate crystal were carried out by the parallel plate capacitor method as a function of temperature for various frequencies using a Precision LCR meter (AGILENT 4284 A model) [14,15]. The crystals were powdered and pelletized (by applying a pressure of 4 tons) into 13 mm diameter and
V. Mathew et al. / Materials Letters 65 (2011) 2142–2145
ð1Þ
where Ccrys is the capacitance of the crystal and Cair is the capacitance of the same dimension of air. The AC electrical conductivity (σac) was calculated using the relation ðσac Þ = εr ε0 ω ðtanδÞ
frequency 7
6
5
4
3
ð2Þ
where ε0 is the permittivity of free space (8.85 × 10− 12 Farad/m) and ω is the angular frequency (ω = 2πf). The magnetic characterization of copper malonate trihydrate crystals was carried out using a Lakeshore 7300 model vibrating sample magnetometer (VSM) [16]. The VSM measures the difference of magnetic induction between one region of space with the sample and another without the sample, thus allowing the calculation of magnetic moment, m generally with sensitivity in the order of 5 × 10− 6 emu. The magnetic moments of the sample were measured at fields ranging from 0 to 10 kOe at 285 K. The various magnetic properties like saturation magnetization and coercivity are estimated from the hysteresis curve.
1 KHz 10 KHz 100 KHz 1 MHz
40
50
60
70
80
90
100
Temperature (°C)
(b) Variation of dielectric loss with temperature and frequency 0.5 1 KHz 10 KHz 100 KHz 1 MHz
0.4
0.3
tanδ
Ccrys εr = Cair
(a) Variation of dielectric constant with temperature and
Dielectric constant
2.43 mm thickness using a hydraulic press and annealed up to 100 °C to remove water molecules if present. To make contact with the electrodes, both the pellet surfaces were coated with fine graphite powder. The sample was placed between the electrodes and heated from room temperature to 100 °C using a thermostat. The observations were made while cooling the sample. Air capacitance (Cair) was also measured. The dielectric constant of the crystal was calculated using the relation
2143
0.2
3. Results and discussion 0.1
3.1. Dielectric studies
0.0 40
50
60
70
80
90
100
Temperature (°C)
(c) Variation of AC conductivity with temperature and frequency
ac conductivity (x 10-7mho/m)
The electrical conductivity study reveals wealth of information as regard the usefulness of the materials for various applications. Further, it sheds light on the behavior of charge carriers under an electric field, their mobility and mechanism of conduction. The study of the dielectric parameters like dielectric constant (εr), dielectric loss (tan δ) and AC conductivity (σac) as a function of frequency and temperature unveils the various polarization mechanisms in the material. Fig. 1 shows the variation of dielectric properties of copper malonate trihydrate crystals with temperature at four different frequencies, viz. 1 kHz, 10 kHz, 100 kHz and 1 MHz. The dielectric parameters — εr, tan δ and σac are found to increase with the increase in temperature. The variation of εr and tan δ values are more pronounced at low frequencies than at higher frequencies. Also, these values decrease with the increase in frequency. The variation of σac value is more pronounced at high frequencies than at lower frequencies. Also, this value increases with the increase in frequency. This behavior is considered to be a normal one for a dielectric material [17]. The εr values observed, in the present study, at lower temperatures are significantly less (b4.0 upto 60 °C for 1 kHz frequency and at all temperatures considered for higher frequencies) [see Fig. 1(a)]. The microelectronics industry needs to replace the dielectric materials in multilevel interconnect structures with low dielectric constant materials as an inter layer dielectric (ILD) which surrounds and insulates interconnect wiring. Silica has the εr value ≈ 4.0. Several innovative developments have been made for the development of new low εr value materials to replace silica. However, there is still a need for new εr value dielectric materials [18]. In addition, materials in the single crystal form would be very much interesting. Meena and Mahadevan [18] have found that L-arginine acetate and L-arginine oxalate are promising low εr value dielectric materials. Also, Mahadevan and his co-workers have observed significant reduction
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
1 KHz 10 KHz 100 KHz 1 MHz
40
50
60
70
80
90
100
Temperature (°C) Fig. 1. Dielectric properties of gel-grown copper malonate trihydrate crystals.
in εr value of potassium dihydrogen orthophosphate (KDP) single crystal when doped with urea [19] and with L-arginine [20] which makes KDP as a low εr value dielectric material. Considering the above facts, copper malonate trihydrate crystal can also be considered as a promising dielectric material with low εr value.
2144
V. Mathew et al. / Materials Letters 65 (2011) 2142–2145
The dielectric behavior of the copper malonate trihydrate crystal can be understood on the basis that the mechanism of polarization is similar to the conduction process. For this type of single crystals, the electrical conductivity can be determined by the proton transport within the frame work of hydrogen bonds due to the presence of water molecules [20]. Further, the conductivity observed in the present study increases smoothly through the temperature range considered (40–100 °C); there is no sharp increase that would be characteristic of a superprotonic phase transition [20]. So, two mechanisms can be considered. The first mechanism is identical to the conductivity mechanism in ice also containing hydrogen bonds. According to the second mechanism, conductivity is associated with the incorporation in the crystal lattice of impurities having different valences and the formation of corresponding defects in ionic crystals. The conductivity of ice is determined by the simultaneous presence of positive and negative ions and orientational defects — vacant hydrogen bonds (L-defects) and doubly occupied hydrogen bonds (D-defects). Other possible defects are vacancies and defect associates. The temperature dependence of conductivity observed in the present study for copper malonate trihydrate crystal allows us to mention that the conductivity of copper malonate trihydrate crystal is determined by both thermally generated L-defects and the foreign (natural) impurities incorporated with the lattice and generating Ldefects there. So, the proton transport in the crystal depends on the generation of L-defects. Hence, the increase of conductivity with the increase in temperature observed for the copper malonate trihydrate crystal can be explained as due to the temperature dependence of the proton transport. 3.2. Magnetic studies
4. Conclusions
Moment (emu/g)
The malonate bridging ligand can mediate significant magnetic interactions; hence the current work is focused on a gel-grown malonate bridged complex of Cu(II) using vibrating sample magnetometer (Lakeshore 7300 model) at a temperature of 285 K. In VSM, the sample is made to vibrate sinusoidaly in a uniform field and the signal generated proportional to the magnetization of the sample is induced to pickup coils yielding the magnetization vs. magnetizing field curve (the hysteresis loop). The hysteresis properties are closely related to the arrangement of magnetic domains in bulk materials. These magnetic domains are linked to relaxation times and the magnetic properties of the material. The variation of magnetic moment with external field is depicted in Fig. 2. Saturation magnetization Ms, remanence magnetization Mrs, coercivity Hc are the parameters linked with the hysteresis loop. The saturation magnetization obtained by extrapolating the high field magnetization curve to the y axis and the value of Ms for CuC3H2O4 3H2O is 0.16 emu/g and saturation remanant magnetization (Mrs) is 242.2 Oe and coercivity which is structure sensitive property that depends on crystal structure, grain size, preferred orientation, stress, defect density, thickness, etc is 0.00887 mT. 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -10000
The sample is expected to respond well to magnetic field without any permanent magnetization at ambient temperature. This means that in the absence of the external field, thermal energy can overcome preferential orientation. The low value coercivity in bulk sample whose size exceeds domain wall width, magnetism reversal occurs due to domain wall motion. As domain wall moves through the sample, they become pinned at the grain boundaries and additional energy is needed to continue the motion. If domain size is less than the ferromagnetic exchange length d, coercivity Hc will be close to zero making hysteresis loss trivial. The structure of CuC3H2O4·3H2O is reported in literature which consists of two neutral diaqua malonato of copper units co-existing with diaqua dimalonate of copper ; these three entities are connected by a three-dimensional network of hydrogen bonds [21]. The magnetic orbitals at each copper atom defined by the short equatorial bonds and the axial bonds of d orbitals, in indicating the out of plane exchange pathway. The carboxylato bridge connects the equatorial bonds to the Cu(1)–Cu(2) pair. The trinuclear cation is structurally independent in tetra acqua malonate of copper and is bridged by diaqua malonato of CuII and the basal CuO bonds reveal significant distortion from square pyramidal surroundings leading to out of plane exchange pathway. The malonato copper II unit acts as a bridging ligand in an anti-syn conformation mode. The triaqua dimalonate of copper features octahedral co-ordination mode. The two different carboxylates bridge anti-syn configuration. The magnetic moment of hydrated copper(II) malonate is 1.77BM at room temperature and is close to the ‘spin-only’ value 1.73BM which is in agreement with the earlier work of Ploquin [22].
-5000
0
5000
10000
Applied field (Oe) Fig. 2. Moment vs field curve for gel-grown copper malonate trihydrate crystals.
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