arXiv subjects

Rishabh Shukla

Publications and source records attributed to Rishabh Shukla.

21 records · Page 2Linked to original sources

arXiv2018

Synthesis and physical properties of Na$_x$TO$_2$ (T=Mn, Co) nanostructures for cathode materials in Na--ion batteries

We prepared Na$_x$TO$_2$ (T = Mn, Co) nanostructures to use as cathode material in Na-ion batteries. The Rietveld refinement of x-ray diffraction data confirms the hexagonal symmetry. Scanning and transmission electron microscopy measurements show hexagonal and rod-shape morphology for T = Co and Mn, respectively. The magnetic measurements indicate the presence of variable oxidation states of Mn/Co ions, but no long range ordering till 5~K. We find that the Na$_{0.6}$MnO$_2$ is highly insulating whereas Na$_{0.7}$CoO$_2$ is semiconducting in nature. The conduction in Na$_{0.7}$CoO$_2$ takes place through both variable range hopping (VRH) as well as activation mechanisms in different temperature ranges. However, for Na$_{0.6}$MnO$_2$ the VRH prevails at elevated temperatures. Furthermore, the coin cells have been fabricated and tested the electrochemical behavior using cyclic voltammetry, which confirms the reversibility of Na--ions during intercalation/de-intercalation.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous magnetic and spin glass behavior in Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$

We report the structural, magnetic, transport and electronic properties of Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$ ($x =$ 0--0.2). The Rietveld analysis of x-ray diffraction data demonstrate structural phase transitions from rhombohedral to orthorhombic and further to monoclinic with increasing the Nb concentration up to $x \ge$ 0.2. Interestingly, we observed dramatic changes in the magnetization (M) with increasing the Nb concentration, as the M sharply increases below 10~K even at 2.5\% substitution. Furthermore, ac susceptibility data show the spin-glass behavior in $x=$ 0.1 sample. We find that the density of states near the Fermi level decreases and the activation energy increases, which results in the decreasing conductivity with higher Nb concentration. A significant shift in the peak position of A$_{2g}$ phonon mode has been observed using Raman spectroscopy, which indicates the change in the coupling due to the structural distortion with Nb substitution. The core-level photoemission study confirms that the Nb is present in 5+ valence state. Our study reveals that the nonmagnetic Nb$^{5+}$ ($d^0$) substitution converts Co$^{3+}$ ions to Co$^{2+}$ and stabilize both in the high-spin state. Our results suggest that structural and spin-state transitions as well as the difference in the ionic radii between Nb$^{5+}$ and Co$^{3+}$ are playing an important role in tuning the physical properties.

cond-mat.str-el↗

arXiv2017

Tuning ferromagnetism and spin state in La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca) nanoparticles

We use the x-ray diffraction, magnetic susceptibility, isothermal magnetization, and photoelectron spectroscopy to study the structural, magnetic and electronic properties of La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca; $x=$ 0 -- 0.2) nanoparticles. The Rietveld refinements of room temperature powder x-ray diffraction data confirm the single phase and the rhombohedral crystal structure with R$\bar{3}$C space group. We find drastic changes in the magnetic properties and spin-states with Sr/Ca substitution (hole doping). For $x=$ 0 sample, the magnetic measurements show a ferromagnetic transition at T$_{\rm C}$$\approx$85 K, which shifted significantly to higher temperatures with hole doping; simultaneously a significant increase in the spontaneous magnetic moment has been observed. Whereas, the coercive field H$_{\rm C}$ values are 7, 4.4 and 13.2~kOe for $x=$ 0, 0.2 (Sr), and 0.2(Ca) samples. Furthermore, the FC magnetization shows a ferromagnetic Brillouin function like behavior at low temperatures for Ca samples. We demonstrate that the Sr/Ca substitution increases the population of IS (Co$^{3+}$) and LS (Co$^{4+}$) states and tune the ferromagnetism in nanoparticles via double-exchange interactions between Co$^{3+}$-- Co$^{4+}$. Our results suggest an important role of hole carriers and nano-size effect in controlling the spin-state and magnetism in La$_{(1-x)}$$A_x$CoO$_3$ nanoparticles.

cond-mat.mes-hall↗