Structural analysis and polarization study of CoCr2O4 nanoparticles

We present the room temperature polarization study of the multiferroic compound CoCr2O4. The co-existence of two elements (Co and Cr) in partially filled d-blocks makes these compounds exhibit a wide range of physical properties. The Cr3+ions almost invariably cover the octahedral (Oh) sites as the crystal field stabilization energy (CFSE) is more than 224 kJ mol −1. The chemical based Sol-gel self-combustion method has been used to successfully synthesize CoCr2O4 chromite. Laboratory X-ray diffraction (LXRD) and synchrotron X-ray diffraction (SXRD) investigations approve the single-phase face centered crystalline structure. The obtained space group is Fd3m. The average crystallite size has been determined by Debye Scherrer’s method and the Williamson Hall [W-H] method. The obtained size is found to be ~ 27 and 17 nm respectively, for both methods when the data is measured by SXRD. The scanning electron micrograph (SEM) shows the agglomeration of particles, which is confirmed by surface topography. Room temperature polarization study exposes the weaker strength of ferroelectricity, which is principally due to charge reversal in the CoCr2O4 compound. The experimental value for retaining polarization is found to be ~ 65% at room temperature (300K).


Introduction
Multiferroics [the coexistence of ferromagnetism (FM) and ferroelectricity (FE) orders], materials with multibit memory phenomena, have been extensively studied and have led to significant technological advances.Magnetically produced ferroelectricity, spin-charge ordering, geometrical frustration, octahedral distortion, lone-pair electron effects, strain mediation, and other sources of ferroelectricity have all been investigated [1] .This compound contains d-elements with a wide range of physical characteristics.The occupancy of cations at the tetrahedral (T h ) and octahedral (O h ) sites is explained with the help of crystal field stabilization energy (CFSE).In CoCr 2 O 4, the Cr 3+ occupy the octahedral site (O h ) as their CFSE value is greater than 224 kJ mol -1 .As a result, Cr 3+ ions occupy the octahedral site (O h ) and Co 2+ ions occupy the tetrahedral (T h ) sites.The partially filled orbitals of d-block elements are responsible for the spin ordering of the electrons present in the incomplete ionic shells.The ferromagnetic behavior of this compound is due to this spin ordering in electrons.On the other side, ferroelectricity is observed in compounds where there is a shifting of charge carriers.This shifting in the charge carriers may be due to both positive and negative charges.This shifting also requires empty or partially filled d-orbitals [2] .
Spinel CoCr 2 O 4 [space group, Fd3m] chromite is a type-II multiferroic (antiferromagnetic / ferromagnetic, and ferroelectric properties) material below T C = 94 K known as the ferrimagnetic transition temperature.Chromites with a multiferroic nature can be very useful as functional materials in high-frequency memory device applications.The magnetism driven electric properties make them materials of great scientific use.Chromites with the general formula AB 2 O 4 belong to a class of complex oxides.where A-ions stand commonly for divalent cations (Co 2+ ) inhabiting tetrahedral (T h ) sites and B-ions stand for trivalent cations that occupy (Cr 3+ ) O h sites.
Nanoparticles are materials of great interest as they have a larger surface area and a smaller particle size.The surface-to-volume ratio in nanoparticles increases as the particle size reduces.The desired homogeneous and very fine nanoparticles can be obtained with the use of wet chemical synthesis methods like sol-gel auto combustion.This method required a lower temperature and calcination time as compared to the conventional solid-state method [3].CoCr2O4 nanoparticles possess electrically induced polarization properties at 300 K are very effectively used in memory applications [4].In the existing work, we thus intended to understand the synchrotron and laboratory based X-ray diffraction and polarization behavior of CoCr2O4 nanoparticles prepared via the low-temperature sol-gel selfcombustion method.

Chemicals Required
In the present study, we prepared the samples by using the nitrates of cobalt and chromium.The other main chemicals required for the synthesis process are citric acid, anhydrous ammonia solution, and polyvinyl alcohol (PVA).AR-grade chemicals were used for the synthesis process, and they were mixed in the distilled water.The chemicals used in the process are utilized without any further purification, while low-conductive water is preferred for all the experiments.

Synthesis
Multiferroic CoCr 2 O 4 nanoparticles were prepared by the sol-gel self-combustion method operated at a low temperature.For the synthesis process, we have dissolved the nitrate precursor into the distilled water.All the chemicals were weighted as per the required stoichiometry ration.The prepared solution is maintained at a ratio of 1:2 among the precursors and citric acids.To control the acidic nature of the solution and make the environment neutral, we have used ammonia solutions.Ammonia is mixed dropwise to avoid a sudden change in the solution environment and also to maintain the pH value at 10. Further, to enhance the reaction rate, we have maintained the solution temperature at 80°C and performed continuous stirring with the magnetic stirrer.The solution converted into gel after a few hours of stirring.The dried gel was then converted into loose powder following the process of selfcombustion.The temperature is maintained at 110°C until the complete solution is converted into powder.The calcination of the obtained sample powder is done at 800°C for 8 h so that we can get single phase.This calcined powder is then compressed with a hydraulic press to form the disc.The palletization of the loose powder is done under pressure of 8 tons, and for the binding of the powder to form a compact disk (pellet), polyvinyl alcohol (PVA) was used as binder material.The disk-shaped pellets were sintered at 900°C for 12 h.The prepared pellets have a thickness of 1 mm, while the disks' radius is 5 mm.

Characterizations
The structure of lattice reorientation and purity of phase of prepared samples were assessed by the Xray diffraction technique at room temperature.Laboratory X-ray diffraction (LXRD) data are noted by the Bruker D8 advance X-ray diffractometer (l = 1.5406).The synchrotron diffraction of incident Xrays (SXRD) has been executed on beam line BL-12 at RRCAT Indore.The beamline is installed at Indus-2 and has an X-ray source of wavelength 0.8042.The surface topography and the microstructural morphology of the synthesized nanoparticles have been explored via scanning electron microscope [SEM: JEOL-JSM-5600] with the help of Image-J software.The ferroelectric measurement of prepared samples was verified by Radiant Tech USA.
The following Williamson-Hall (W-H) relation is used to define the induced microstrain in the sample and the average crystalline size.The relationship is: where ε and k are the induced microstrain in the prepared sample and a constant, respectively.The following equation is used to determine the lattice parameter (a) of the prepared sample: where interplanar spacing is represented by the term d and Miller indices for the corresponding plane are represented by the symbols h, k, l [6] .Crystallite size (Dsch) was also determined using Scherrer's formula for the most intense diffraction plane (311).Crystallite size was found to have a smaller value as calculated from a synchrotron X-ray source compared to a lab X-ray source.Synchrotron diffraction allows us to select an appropriate 2θ value and a perfect fit.Differences between the crystallite sizes were observed in two different methods because in nanoparticles, induced microstrain is present.The W-H method is perfectly good for estimation, as shown in Figure 2 and in the structural data display in Table 1.The morphology of the nanoparticles is depicting by scanning electron microscopy (SEM).The micrograph of the SEM sintered nanoparticles is shown in Figure 3. From this micrograph, it is observed that CoCr 2 O 4 nanoparticles are not spherical in shape.Though the resolution of the system is not good due to some technical issues, one can still see the homogeneous agglomeration and porosity in the sample.The above statement is supported by the surface topography of the synthesized sample, which is represented in Figure 4.
As a multiferroic CoCr 2 O 4 nanoparticle, the ferroelectric property is promising and one of the most important properties of chromites.Electrically induced polarization studies show a non-linear connection between the applied electric field (E) and polarization (P) arising in a hysteresis loop.Room temperature ferroelectric (P-E) behavior of CoCr 2 O 4 nanoparticles was observed at 1100 V, as shown in Figure 5.
The percentage loss of retaining polarization (RP) is defined as RP (%) =, where remnant polarization is given by P R and maximum polarization is presented by P max at zero electric field.The value of RP is found to be 65.4 percentage points.This value may be obtained due to the partial reversal of effective charge carriers.The observed electric polarization mainly arises due to related electric dipole moments.This polarization can also be obtained from different states like P Co-Cr and P Cr- Cr [7, 8]   .The ferroelectric response observed in the negative polarization axis also indicates a small amount of leakage current.This leakage current is attributed to oxygen vacancies in different sites (most probably in the O h -site) of CoCr 2 O 4 nanoparticles.
In summary, multiferroic CoCr 2 O 4 nanoparticles were efficaciously prepared by the low-temperature sol-gel self-combustion method.The laboratory X-ray diffraction confirms that CoCr 2 O 4 nanoparticles form a single-phase face-centered cubic structure [space group, Fd3m].W-H methods are perfectly good estimation methods to calculate average crystallite size and induced microstrain in nanoparticles.Surface morphology studies homogeneous agglomeration.Room temperature electrically induced polarization confirms the weak ferroelectric nature.A very small amount of leakage current is also present in nanoparticles.An electrically induced polarization study proposed a possible application in charge storage memory devices.

Table 1 :
Average particle size and lattice (a) parameter calculated from Lab and Synchrotron XRD source.