Chandan Upadhyay | Banaras Hindu University, Varanasi (original) (raw)

Papers by Chandan Upadhyay

Journal of Physics: Condensed Matter, May 23, 2023

Electronic and optical studies on Dy2Ti2−Mn x O7 (x = 0.00, 0.05, 0.10, 0.15, & 0.20) hav... more Electronic and optical studies on Dy2Ti2−Mn x O7 (x = 0.00, 0.05, 0.10, 0.15, & 0.20) have been presented through both, theoretical (density functional theory (DFT) calculations) and experimental (ultraviolet-visible absorption and photoluminescence emission spectroscopy) approaches. DFT calculations were employed considering the local density approximation (LDA) and LDA-1/2 for exchange-correlation interactions. Computed crystallographic parameters and energy band-gap using theoretical formulations are in good agreement with experimental results. The band-gap value obtained through the LDA-1/2 approach indicates insulated ground state of Dy2Ti2−x Mn x O7 (x = 0.00, 0.05, 0.10, 0.15, 0.20) system. Experimentally obtained band gap value reduces from 3.82 eV to 2.45 eV with increase in positive chemical pressure as x increases from 0 to 0.20. Reduction in band gap value is attributed to the fact that there exists a lack of hybridization between the O-2p orbital and Ti-3d orbital, which is well correlated with the crystallographic data. Jahn-Teller effect is likely to be responsible for the presence of a mixed state of Mn (explained using x-ray photoelectron spectroscopy results), resulting in the intermediate Mn state between the valence band and the conduction band with immediate inclusion of Mn at Ti site in Dy2Ti2−x Mn x O7 system.

PubMed, 2023

Objective: To evaluate in-vitro surface characteristics and frictional properties of orthodontic ... more Objective: To evaluate in-vitro surface characteristics and frictional properties of orthodontic stainless steel and beta-titanium archwires after surface modification with different concentrations and coating time of titanium oxide (TiO2) nanoparticles by Sol-gel dip coating method. Materials and methods: The experiment was carried out with 4 different concentrations (1:2, 1:4, 1:6, and 1:8) and three different dipping durations (24 hours, 48 hours, and 72 hours) over ten main test groups of SS and TMA archwires with uncoated wires acting as control in both dry and wet conditions. Phase analysis and surface characterization of TiO2 was analyzed by X-ray Diffractometry, surface evaluation with the help of scanning electron microscopy (SEM), and frictional characteristics were evaluated. Results: Among all the concentrations 1:6 ratio with 48 hours of dipping duration showed better surface characteristics. A statistically significant difference in frictional coefficient was observed in both SS and TMA wires than their respective controls (p = 0.001). Intragroup comparison among SS and TMA groups showed that groups with 1:6 ratio and 48 hours dipping duration had least frictional coefficient in both dry and wet conditions (p = 0.001). Intergroup comparison between SS and TMA showed that SS group had significantly reduced friction than TMA (p = 0.001) except in few groups. Conclusion: TiO2 nanoparticle with a concentration ratio of 1:6 and 48 hours dipping duration is recommended for surface modification of orthodontic archwires.

Springer eBooks, 2013

Fougèrite mineral responsible for the bluish-green shade of gleysols in aquifers was identified a... more Fougèrite mineral responsible for the bluish-green shade of gleysols in aquifers was identified as Fe II−III oxyhydroxycarbonate, [Fe II 6(1−x) Fe III 6x O 12 H 2(7−3x) ] 2+ • [CO 2− 3 • 3H 2 O] 2− where the average ferric molar fraction x = [Fe III /Fe total ] was restricted to the [1/3-2/3] range, up till now. In this paper, Mössbauer spectra of gleys extracted from the schorre of maritime marshes have values of x in the [2/3-1] range. Magnetic properties of homologous chemical compounds studied by Mössbauer

Advanced Materials Research, Jul 1, 2015

Highly stable silver nanoparticles synthesized in single-step green method by mixing silver nitra... more Highly stable silver nanoparticles synthesized in single-step green method by mixing silver nitrate and aqueous extract of Almond (Prunus amygdalus). Experiments were conducted to influence the change in the silver nitrate concentration and time on the synthesis of silver nanoparticles at room temperature under dispersed sun light. The almond extract acted both as the reducing and stabilizing agent for the synthesis of silver nanoparticles. The change in the color of the reaction mixture was monitored using UV-Visible spectrometry whereas particles synthesized were characterized using Scanning Electron Microscopy, Dynamic Light Scattering and Fourier Transform Infrared Spectroscopy. The synthesized nanoparticles were almost spherical in shape with an average size about 20 nm and they exhibited bacteriostatic property against E. coli.

Environmental Science & Technology, Jun 28, 2006

ABSTRACT Bacterial activity is commonly thought to be directly responsible for denitrification in... more ABSTRACT Bacterial activity is commonly thought to be directly responsible for denitrification in soils and groundwater. However, nitrate reduction in low organic sediments occurs abiotically by Fe-II ions within the fougerite mineral (IMA 2003-057), giving the bluish-green color of gleysols. Fougerite, the mineral counterpart of FeII-III oxyhydroxycarbonate, (Fe6(1-x)Fe6xO12H2(7-3x)CO3)-Fe-II-O-III, provides a unique in situ redox flexibility, which can adapt x = {[Fe-III]/[Fe-total]} between 1/3 and 2/3 as shown using Mossbauer spectroscopy. Chemical potential and E-h-pH diagrams for this system were determined from electrode potential monitored during deprotonation of hydroxycarbonate (Fe4Fe2III)-Fe-II(OH)(12)CO3 to assess the possibility of reducing pollutants in the field. Bioreduction of ferric oxyhydroxides in anoxic groundwater yields dissolved Fe-II, whereas HCO3- anions produced from organic matter are incorporated into fougerite layered double oxyhydroxide structure. Thus, fougerite is the solid-state redox mediator acting as electron shuttle that helps bacterial activity for reducing nitrate by coupling dissimilatory Fe-III reduction and oxidation of Fe-II with reduction of NO3-. It is proposed that this system could be used in the remediation of soils and nitrified waters.

Journal of Physics and Chemistry of Solids

$Research paper thumbnail of Emergence of Spin Ice freezing in Dy$_2$Ti$_{1.8}$Mn$_{0.2}$O$_7$$

arXiv (Cornell University), May 9, 2022

We herein present the spin freezing dynamics of stuffed polycrystalline compound Dy2Ti1.8Mn0.2O7.... more We herein present the spin freezing dynamics of stuffed polycrystalline compound Dy2Ti1.8Mn0.2O7. In Dy2Ti2O7, spin freezes with ice like spin relaxations at a temperature around 3 K (Ti) along with another spin freezing at a temperature around 0.7 K (T<Ti). These relaxations can be observed prominently with an application of varying DC magnetic field bias and applied AC-field. We show here that with fractional inclusion of Mn at Ti site in Dy2Ti2O7, there is a significant shift in these temperatures. In Dy2Ti1.8Mn0.2O7 the Ti shifts to a higher temperature around 5 K and freezing belonging to T<Ti shifts to 2.5 K without any application of external DC Bias and/or AC-field. The inclusion of Mn at Ti site also enhances the ferromagnetic interaction for Dy2Ti1.8Mn0.2O7 as compared to Dy2Ti2O7. Arrhenius fit of freezing temperature with frequency for Dy2Ti1.8Mn0.2O7 shows that these spin relaxations at Ti and T<Ti are thermally induced. Low-temperature structural change in lattice parameters and crystal field phonon coupling has been studied using synchrotron x-ray diffraction. Debye-Gruineisen analysis of temperaturedependent lattice volume shows the emergence of crystal field phonon coupling at a much higher temperature (70 K) in Dy2Ti1.8Mn0.2O7 in contrast to 40 K in Dy2Ti2O7. These findings make Dy2Ti1.8Mn0.2O7 a suitable system to explore the application of spin ice phenomenon at a workable temperature.

ChemistrySelect

Monomeric [Cu(DACH)(L)]2+ (DACH=1,2‐diaminocyclohexane) heteroleptic copper(II) complexes (1–4) o... more Monomeric [Cu(DACH)(L)]2+ (DACH=1,2‐diaminocyclohexane) heteroleptic copper(II) complexes (1–4) on reaction with diphenyl phosphate yielded dinuclear phosphate bridged copper(II) complexes [Cu2L2(dpp)3]+ (5–7), [dpp=diphenyl phosphate; L=bpy (5), Phen (6), 5, 6 dimethyl phenanthroline, (7)] and a tetra‐nuclear [Cu4(2,9‐dmp)2(dpp)4(OH)2]2+ complex (8) (2, 9 dmp=2, 9 dimethyl phenanthroline). Interestingly, isomeric ligands 5,6‐dmp and 2,9‐dmp yielded dinuclear complex 7 and tetra‐nuclear complex 8 triggered by steric effects. Even though the initial combination contains mixed bidentate ligands, the resulting complexes have only one type of bidentate ligand around the copper(II) ions. These reactions involve ligand expulsion to form the phosphate bridged complexes. All the phosphate bridged complexes 5–8 were characterized by elemental analysis, absorption spectrum, FT‐IR, ESI‐MS, and single‐crystal XRD. The molecular structures show Cu(II) in square pyramidal/trigonal bipyramidal geo...

Polyhedron, 2019

Di and tetranuclear Cu(II) complexes, [Cu 2 (2-AEP) 4 (µ-Cl)](ClO 4) 3 (1) and [Cu 4 (µ 3-OH) 2 (... more Di and tetranuclear Cu(II) complexes, [Cu 2 (2-AEP) 4 (µ-Cl)](ClO 4) 3 (1) and [Cu 4 (µ 3-OH) 2 (µ 2-OH) 2 (2-AEP) 4 (µ 2-ClO 4) 2 ](ClO 4) 2 (2), with the simple 2-aminoethylpyridine (2-AEP) ligand have been synthesized and characterized by different spectroscopic and analytical techniques. The X-ray structure reveals that complex 1 is a dimer with a monochloro bridge connecting the two copper atoms and complex 2 is a tetramer with µ 2 and µ 3 hydroxo bridges connecting the four copper atoms. Both copper centers in complex 1 have a distorted square pyramidal (sp) geometry, whereas two copper centres show a sp geometry and the other two copper centres show a distorted octahedral geometry in complex 2. Variable temperature magnetic susceptibility analysis reveals that complex 1 shows a ferromagnetic interaction with 2J = +1.73 cm-1 , whilst 2 shows predominantly antiferromagnetic interactions between the copper(II) ions with J 1 = +2.98 and J 2 =-16.91 cm-1. Both complexes 1 and 2 hydrolyze the phosphodiester BNPP with rate constants k = 9.65 × 10-3 and 1.42 × 10-2 s-1 in CH 3 CN/H 2 O, respectively. These complexes interact with DNA as evidenced by theoretical and experimental methods. The DNA in silico study suggests that complexes 1 and 2 bind with CT-DNA through minor groove interactions, which is further confirmed by UV-Vis spectroscopic titrations, CD measurements, viscosity studies and electrochemical methods. The binding interaction of both 2 complexes with calf thymus DNA shows efficient binding with K b values of 3.54 × 10 4 M-1 for 1 and 3.18 × 10 4 M-1 for 2. Both complexes proficiently cleave plasmid DNA (pBR322) to the linear form (form III) at 25 µM under oxidative conditions and both exhibit moderate cytotoxic activity on cervical cancer cell lines (ME-180 and SiHa). In addition, both complexes catalyze the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ), as studied by UV-Vis spectroscopic titrations. The rate of the reaction is 1.47 × 10-3 M s-1 for 1 and 3.45 × 10-3 M s-1 for 2. The present complexes, with the simple 2-AEP ligand, show diverse bioinorganic aspects.

Journal of Photochemistry and Photobiology, 2021

physica status solidi (b), 2021

Herein, optical (e.g., UV–Vis reflectance spectra, infrared and Raman spectroscopy) and magnetiza... more Herein, optical (e.g., UV–Vis reflectance spectra, infrared and Raman spectroscopy) and magnetization properties of the double‐perovskite compounds Ba2DySbO6 (BDS) and Ba2GdSbO6 (BGS) are reported. Optical bandgap is determined to be 4.524 eV for BDS and 4.465 eV for BGS using Kubelka–Munk function fit to the UV–Vis spectra. The lattice dynamical calculations on the force‐field model are undertaken for assigning the observed infrared and Raman modes. The dc susceptibility does not exhibit magnetic ordering down to 2 K for the two compounds. The dc magnetization at T = 2 K does not saturate for BDS even at a magnetic field of 11 T, but saturates at 5 T to a value of 7μB/Gd3+ for BGS. A frequency‐dependent cusp in the ac susceptibility appears at 3.3 K, that is unlikely generic spin‐glass‐like freezing, and may be attributed to the short‐range correlations among Dy3+‐moments in BDS. Crystal‐field (CF) computations are performed to analyze the magnetic data for BDS. The obtained CF level scheme of Dy3+ ions demonstrates the observed susceptibility and isothermal magnetization curves very well. The field‐dependence of the two lowest CF levels along different principal axes does not exhibit any level crossing, consistent with the lack of ordering in Ba2DySbO6.

Acta Crystallographica Section A Foundations and Advances, 2021

Despite the technological significance of the tetragonal PbTiO3 for the piezoelectric transducer ... more Despite the technological significance of the tetragonal PbTiO3 for the piezoelectric transducer industry, its high pressure behaviour is quite controversial as two entirely different scenarios, involving pressure induced (1) morphotropic phase boundary (MPB) like structural transition with concomitant rotation of the ferroelectric polarization vector and (2) antiferrodistortive (AFD) phase transition followed by emergence of a reentrant ferroelectric phase, have been proposed in recent theoretical and experimental studies. We have attempted to address these controversies through a high resolution synchrotron x-ray diffraction study of pressure induced phase transitions in the tetragonal phase of a modified PbTiO3 composition containing 50% BiFeO3, where BiFeO3 was added to enhance the AFD instability of PbTiO3. We present here the first experimental evidence for the presence of the characteristic superlattice reflections due to an AFD transition at a moderate pressure pc1 ~2.15 GPa...

To investigate the possible origin and mechanism of ferroelectricity in polycrystalline spin ices... more To investigate the possible origin and mechanism of ferroelectricity in polycrystalline spin ices Ho2Ti2O7 and Dy2Ti2O7 a detailed dielectric study has been performed. Experimental finding suggests that both materials have two prominent diffuse ferroelectric phase transitions around 90K and 36K. These transitions are distinctly generated by the lattice distortions at the oxygen sites as confirmed by triggered distortions and order of activation energy. Due to the incompatibility of the gyrotropic order with any phonon mode at the Brillouin zone center, observed diffuse ferroelectric phase transitions can have only an electronic origin. Through magnetic susceptibility and previously reported spin relaxation behavior it has been concluded that single ion anisotropy has thermal variation, due to which orientation of rare earth magnetic moment from isotropic non-Ising to Ising spin along local <111>axis takes place. This spin orientation distinctly distorting the both oxygen sites...

Journal of Ayurveda and Integrative Medicine, 2021

BACKGROUND Kajjali is used as a base for Ayurvedic herbo-mineral medicines. It is a combination o... more BACKGROUND Kajjali is used as a base for Ayurvedic herbo-mineral medicines. It is a combination of mercury with sulfur in varying proportions. The ratio of sulfur (S) added to mercury (Hg) directly relates to the therapeutic efficacy of the compound. OBJECTIVE To analyze the physico-chemical characteristics of samaguna gandhaka kajjali (Hg: S = 1:1) and shadaguna gandhaka kajjali (Hg: S = 1:6). MATERIALS AND METHODS X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourier transmission infrared spectroscopy, thermo-gravimetry analysis, and atomic absorption spectroscopy were applied to characterize each type of kajjali. RESULTS It was found that the particle size of the formed kajjali compound increases with a decrease in the mercury to sulfur ratio. The presence of excess sulfur does not change the surface oxidation states as revealed by the XPS analysis. No trace of mercury has been found in both samaguna gandhaka kajjali (SGK-1) and shadguna gandhaka kajjali (SGK-6), indicating a complete Hg reaction with S. CONCLUSIONS Kajjali simulates nanomaterial of the modern era and possesses therapeutic efficacy as mentioned in classical Ayurveda texts. Complete trituration of mercury and sulfur combination ends up with this kajjali formation incorporating the potency of nanotherapeutics.

Journal of Applied Physics, 2019

The role of size reduction on the structural parameters, antiferromagnetic transition temperature... more The role of size reduction on the structural parameters, antiferromagnetic transition temperature (TN), and spin reorientation transition temperature of BiFeO3-0.25PbTiO3 (BF-0.25PT) has been studi...

Meteoritics & Planetary Science, 2002

International Journal of Engineering, Science and Technology, 2011