Raman Spectroscopy
Raman spectroscopy is one of the prime tools being used to investigate the crystallographic phase details, lattice disorder, charge and spin-lattice coupling and local cation distributions [32]. Especially in ferrite materials, the transport properties are partially dependent on the cation distributions in the tetrahedral (A-site) and octahedral (B-site) sites. Hence, the Raman spectroscopic analysis (LabRAM HR Evolution, France) has been carried out for the samples of both the control and shocked. The obtained profiles of the Raman spectra are presented in Fig. 1.
According to the factor group analysis, cubic MnFe2O4 spinel has five Raman active modes as that of A1g, Eg and 3F2g. Among the Raman active modes, A1g and 3F2g Raman modes belong to the tetrahedral (A-site) and Eg Raman mode belongs to the octahedral (B-site). As seen in Fig. 1, the control sample has four Raman modes such as A1g, F2g(2), Eg and F2g (1) whose corresponding Raman positions are 650, 407, 293, and 221 cm− 1, respectively and the observed Raman bands are well-matched with the previous reports of the normal spinel MnFe2O4 NPs [32]. No considerable change is obtained under shock loading which is clearly evident for the mere naked eye.
But, while seeing the zoomed versions of the tetrahedral and the octahedral sites, slight changes are observed in terms of the ratio of Raman intensity that is between the tetrahedral and the octahedral sites in accordance with the number of shock pulses. As reflected in Fig. 2, the peak of F2g (1) for the control MnFe2O4NPs has maximum intensity than that of the other Raman peaks such as Eg and F2g (2) which remain the same for all the other shocked conditions. But, Eg Raman band intensity increases slightly on increasing the number of shock pulses whereas F2g (1) Raman intensity reduces on increasing the number of shock pulses. Moreover, the above-mentioned Raman bands have undergone a shift towards the lower frequency corresponding to the counts of shock pulses and the respective Raman shifts are depicted in Fig. 3.
The observed changes both in Raman band intensity and Raman shifts in accordance with the number of shock counts could have lead to lattice defects along with the redistribution of divalent and trivalent cations in the A and B sites wherein the interaction between oxygen and cations in the octahedral as well as the tetrahedral positions are strongly affected [36]. In the normal spinel structure, the divalent ions (Mn2+ in MnFe2O4) occupy the A site only whereas the trivalent ions (Fe3+ in MnFe2O4) occupy the B sites only. Considering the structure of inverse spinel, half of the B sites are occupied by the divalent ions while the rest of the B sites are occupied by the trivalent ions in addition to the occupancy of the entire A sites [32]. At high-pressure and high-temperature conditions, the migration of Mn2+ions from the A to the B sites can occur. Theoretically, this would mean an effective increase in unit cell volume with which a significant XRD peak shift would have occurred for the higher inversion degree for MnFe2O4 [37]. Even a minor number of Mn2+ changes to Mn3+ ions in the B-sites, it significantly affects the XRD peak positions [26]. But, such changes have not been observed in the XRD peak positions at shocked conditions. Hence, it is confirmed that there is no inverse spinel structure that has been formed at shocked conditions. Therefore, the observed changes in the Raman band intensity may be due to the local lattice distortions. But, the cubic normal spinal phase of MnFe2O4 remains the same even after 150 shocked conditions which are well corroborated with the previously reported XRD results [26]. It could be noted that, at shocked conditions, the technologically important materials such as TiO2 and Co3O4 NPs undergo significant Raman shifts due to the occurrence of the crystallographic phase transitions [21, 23].
Cyclic-voltametry Analysis
The Cyclic-voltammetric study has been performed to analyze the specific capacitance of the title material at shocked conditions which are presented in Fig. 4. The variation of the electrochemical performance of the control and shocked samples have been analyzed with different scan rates such as 5, 10, 20, 50, and 100 mV s− 1 in the potential range of 0 − 1.0 V. As seen in Fig. 4, while increasing the scan rate, the positive shifts are observed in the redox peaks with respect to the anodic and cathodic potentials as well as the increased loop area with respect to the scan rate which is the usual behavior found in materials of ideal capacitive behavior [38].
It could be noted that the CV loop shapes and the loop widths of the control and shocked samples are almost similar while at different rates of shocked conditions. From an initial viewpoint, it could be seen that there is no significant change that has been observed in the capacitance value at shocked conditions. Furthermore, we have calculated the specific capacitance of both the control and shocked MnFe2O4 NPs using the standard formulation and the obtained values of specific capacitance in accordance with the scan rates are portrayed in Fig. 5. As seen in Fig. 5, the values of specific capacitance are reduced while increasing the scan rate for all the samples. Even at a lower scan rate, the electrolyte ions have adequate time to penetrate the pores of the electrode material because of their higher value of specific capacitance.
For better understanding the shapes of CV loops and the values of specific capacitance with respect to the number of shock pulses, the comparison loops are presented in Fig. 6a. All the loops are similar in their shape for the samples such that no change is observed in their respective values of potential with respect to the number of shock pulses and the corresponding values of specific capacitance with respect to the number of shock pulses are plotted as presented in Fig. 6b. Hence, it is clear that no cationic re-distributions appear at shocked conditions and the observed slight changes may be due to the lattice defects and surface changes of the particles. It could be noted that the observed values of specific capacitance with respect to the number of shock pulses have not changed significantly.
Electrochemical Impedance Spectroscopic Analysis
Furthermore, we have performed the electrochemical impedance spectroscopic analysis to understand the behavior of electrical resistance under shock loading. Note that the variation of the microstructure, lattice disorder and cation redistribution can influence changes in the electronic transport properties in such a way that several high-pressure compression experiments have been performed on this theme providing several interesting results [11]. Hence, electrochemical impedance spectroscopic analysis (EIS) has been performed and the obtained Cole-Cole plots of the control and shocked samples are portrayed in Fig. 7. As per the normal spinel crystal structure of MnFe2O4 NPs, the interactions of the Mg2+ ion in the tetrahedral sites and Fe3+ ions in the octahedral sites are quite low hence it has lower electrical conductivity. As seen in Fig. 8, the control sample gives rise to an in-complete semi-circular arc and shocked samples also show a similar trend of semi-circular arcs with slight changes. Especially in the grain resistance portion (higher frequency region), the width of the first semi-circular arc is increased with respect to the number of shock pulses and the observed zoomed version of the Cole-Cole plots are depicted in Fig. 8. As the semi-circular arc width increases, the electrical resistance also increases. Hence, it is clear that the impact of applied shock waves has increased the values of electrical resistance (bulk grain resistance) with respect to the number of shock pulses which may be due to the formation of lattice defects in the cubic spinel, grain boundary region that has been disordered resulting in a tremendous increase in electron scattering [35, 38]. The mobility of charge carriers have a high dependency on temperature and pressure such that the reduction in mobility could have enforced a reduction in the electrical conductivity due to the change occurring in activation energy. Similar results of slight changes in the values of saturation magnetization have been witnessed in accordance with the counts of shock pulses [26].