Structural and thermoelectric power studies of citrate-gel-prepared cobalt-substituted lithium nanoferrites

XRD studies

The XRD patterns of the prepared samples are shown in Fig.  ¹ , which reveal the formation of a single-phase cubic spinel structure without any extra impurity peak as confirmed by comparing the same with the reference data from JCPDS card number 00-013-0207. From these patterns, it is concluded that there was no change in the basic crystal structure of Li ferrites on substitution of Co ²⁺ ions.

Fig. 1

The average crystallite sizes of the prepared Li–Co ferrites of different compositions have been estimated from the XRD line width of the most intense peak (311) using Debye–Scherrer’s formula and were found to be in the range of 36–43 nm as evidenced from Table  ¹ . Lattice parameter and hopping length of the prepared ferrites were calculated from the Eqs.  ² and ³ , and the values are tabulated in Table  ¹ . It is observed that the there is an increase in the lattice parameter (a) with the increase in cobalt content in Li ferrites as shown in Fig.  ² , which obeys Vegard’s law [ ¹⁶ ].

Fig. 2

The increase in the lattice parameter with the increasing Co concentration was attributed to the larger ionic radius of Co ²⁺ ion (0.82 Å) compared to those of Li ¹⁺ ion (0.74 Å) and Fe ³⁺ ion (0.64 Å) [ ¹⁶ ] in which substitution by the larger ions results in the expansion of the lattice. All the three ions are associated with the coordination number 6. Hopping length has also increased with Co composition as evidenced by Table  ¹ , because it is directly proportional to lattice parameter.

TEM studies

The morphology and particle size of the citrate-gel prepared nanocrystalline Li–Co ferrite sample powders were observed by transmission electron microscopy (TEM). The bright-field TEM images of [Li _0.5 Fe _0.5 ] _{1− x}  Co _x  Fe ₂ O ₄ powders where x  = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 calcined at 500 °C are shown in Fig.  ³ in which the average particle size observed was ~30 nm. Hence, TEM analysis and XRD analysis confirm the nanoscale nature of the prepared samples. TEM images show the homogeneous distribution of the particles, and a few of them are in agglomerated form. The fact that the agglomeration of particles lies in nanometric region was evidenced by TEM images.

Fig. 3

FTIR Studies

The room-temperature FTIR spectra of Li–Co series [Li _0.5 Fe _0.5 ] _{1− x}  Co _x  Fe ₂ O ₄ (where x  = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) are shown in Fig.  ⁴ . The FTIR spectra were found to exhibit two major bands υ ₁ and υ ₂ in range of 400–600 cm ⁻¹ . The well-defined sharp absorption bands in the FTIR spectrum establish the formation of ferrites in the sample [ ¹⁷ ].

Fig. 4

The first absorption band υ ₁ around 490–500 cm ⁻¹ has appeared due to the vibrations of Fe ³⁺ –O bond in the tetrahedral sites (A-sites). The second absorption band υ ₂ observed around 400–425 cm ⁻¹ was due to the vibration of Fe ³⁺ –O bond in the octahedral sites (B-sites). The third band υ ₃ observed at nearly 450 cm ⁻¹ can be attributed to the Li–O vibrations [ ¹⁸ ]. In Li–Co ferrites, cobalt ions are distributed randomly between the tetrahedral and octahedral sites; hence, υ ₁ and υ ₂ themselves indicate the Co–O band. Similar results were reported earlier [ ¹⁵ ]. The band υ ₃ can also show the presence of the lithium in the prepared sample, and upon increasing the Co concentration, the third band υ ₃ disappears due to less amount of lithium as evidenced in the samples with X  = 0.8 and X  = 1.0. The values of υ _1, υ _2, and υ ₃ are summarized in Table  ² .

Thermoelectric power studies

The thermoelectric power studies of the prepared nanocrystalline Li–Co ferrites were measured by the differential method in the temperature range of 200–600 °C. The thermo e.m.f of the samples was measured during cooling cycle, because during cooling the sample attains more thermal stability than during heating. The values of the Seebeck coefficient of the mixed Li–Co nanoferrites under study were calculated from the observed values of the thermo e.m.f and are summarized in Table  ³ . Carrier concentration of the prepared samples was calculated at Curie temperature. From the Table  ³ one can observe that Seebeck coefficient increases with the increasing Co concentration. This may be attributed to the fact that by increasing the Co concentration, cobalt ions occupy the B-sites and transfer the Fe ³⁺ ions to the A-site resulting in a decrease in Fe ³⁺ ions in the B-site. This fact leads to the decrease in Fe ²⁺ ions in the B-site. However, according to Srinivasan [ ¹⁹ ], Seebeck coefficient S for cubic system is given by with the increasing Co content as the number of Fe ²⁺ ions on B-sites decreases, S increases in the lithium ferrites which can be justified from the above expression of S .

Due to the above mentioned reason, with the increasing Co content, a large numbers of charge carriers are produced so that more energy is needed to transfer the charge carriers between the different cations. Hence, large emf is produced, which enhances the thermoelectric power.

Figure  ⁵ shows the variation of Seebeck coefficient with temperature for all the prepared samples.It can be seen from the Fig.  ⁵ that the sign of Seebeck coefficient was positive at low temperature. By increasing the temperature, Seebeck coefficient attained negative value for all the ferrites under investigation. This indicates that, at low temperature, these prepared samples behave like p-type semiconductors. By increasing the temperature, the predominant conduction mechanism in these ferrites was due to the electrons and the behave like n-type semiconductors. Such type of conductivity is attributed to the hopping of electrons between Fe ²⁺ and Fe ³⁺ ions at octahedral sites. The n-type of conducting mechanism can be shown as below:

Fig. 5

It can be seen that the values of the Seebeck coefficient for all mixed nanocrystalline Li–Co ferrites increases with the increasing temperature which indicates that more n-type charge carriers (electrons) were released with the increasing temperature. Further increase in the temperature results in a decrease in Seebeck coefficient which remains almost constant later on. This temperature at which there is a sudden change in S value was named T _c . From the Fig.  ⁵ , the T _c of all the ferrite samples were measured and are tabulated in Table  ³ . Ferrites after reaching the T _c become paramagnetic; hence, there is no probability for the hopping of electrons between A-site and B-site beyond T _c . Thus, S becomes constant after T _c [ ²⁰ ]. The T _c of pure lithium ferrite was around 630 °C (903 K) [ ²¹ ]. In the present case, the thermoelectric power measurements were carried out in the temperature range of 200–600 °C (473–873 K) using the differential method. Hence, the T _c is not observed in the Fig.  ⁵ , i.e., the T _c of the pure lithium ferrite was beyond the measured temperature range and is mentioned in the Table  ³ as >873 K.

It is clear that in case of Li–Co nanoferrites, the nonmagnetic property (thermoelectric power) under study was exhibiting a well-defined transition at the Curie temperature, like the magnetic properties, viz., susceptibility, permeability, and spontaneous magnetization. The fact that the value of Seebeck coefficient shows minimum values at T _c indicates that the magnetic ordering has a marked influence on the thermoelectric power of these ferrite samples under investigation. From the present study, considering the semiconducting behavior of investigated ferrites, the charge carriers are considered to be localized at ions or at vacant sites as a result of electron–phonon interaction [ ²² ].

The variation of T _c with Co composition of the prepared samples was shown in Fig.  ⁶ . From the figure, it is clear that the T _c of the samples decrease with the increasing cobalt content. This may be due to the fact that as Co concentration increases, the Fe ³⁺ ions decreases, which reduces the overall number of Fe ions in the B-Sites which in turn results in a decrease of A–B interaction Fe _A ³⁺ –O–Fe _B ³⁺ _. Hence, thermal energy required to offset the alignment of magnetic moment decreases leading to the decrease of T _c [ ²² ].

Fig. 6

Synthesis

The starting materials are Lithium nitrate [Li(NO ₃ )], Cobalt nitrate [Co(NO ₃ )6H ₂ O], Ferric nitrate [Fe(NO ₃ ) ₂ 9H ₂ O], Citric acid[C ₆ H ₈ O ₇ H ₂ O], and Ammonia [NH ₃ ] (98 % purity; SD Fine-chem limited, Mumbai, India). The chemicals were weighted in a stoichiometric proportion without any further purification. Nanocrystalline Li–Co ferrites were prepared by citrate-gel autocombustion method [ ¹³ ]. First of all, the stoichiometric quantities of metal nitrates were dissolved in a minimum quantity of distilled water, and an aqueous solution of citric acid was added to the metal nitrate solution. The pH of the solution was maintained at seven by addition of ammonia solution drop by drop and heated to 80 °C until a fluffy mixture was formed, and finally the fluffy mixture was dried, which results in a burn ash and which was calcinated at 500 °C for 4 h to ensure good densification and elimination of impurities. In this preparation method, the energy from the exothermic reaction between oxidant and reductant can be used to prepare the material in the nanoform with desirable phase in a short period of time.

Characterization

The crystallographic structure/micro structure was investigated by using XRD analysis, FTIR, and TEM.

X-ray studies on the obtained powders were carried out by X-ray powder diffractometer (Rigaku Geiger Flex 3 kW diffractometer) using Cu K _α radiation ( λ  = 1.5406 Å) in a wide range of Bragg’s angles 2 (10° < 2 <80°) with scanning rate of 2°/min.

FTIR spectra of the prepared ferrites were recorded on a Perkin Elmer RX-1 FTIR spectrometer using KBr pellets in the range of 600–400 cm ⁻¹ .

The size and shape of the synthesized lithium–cobalt ferrite particles were analyzed by TEM (TEM, Philips CM12, Holland).

Thermoelectric power of the prepared samples was measured using the differential method.