Brajendra Singh

Synthesis, Characterization, and Electrochemical Cycling Behavior of the Ru-Doped Spinel, Li [ Mn2 − x Ru x ] O4 (x = 0 , 0.1, and 0.25)

Pure and Ru-doped Li[Mn 2-z Ru x ]O 4 (x = 0.1 and 0.25) spinel compounds are prepared by the high temperature solid-state reaction and characterized by X-ray diffraction (XRD), scanning electron microscopy, Brunauer, Emmett, and Teller surface area, density, IR, Raman spectroscopy, and X-ray absorption spectroscopy. XRD studies show single-phase compounds with the cubic spinel structure and a decrease in (Mn-O) and an increase in (Li-O) bond lengths on Ru doping. The electrochemical cycling behavior of Li[Mn 2-x Ru x ]O 4 is examined by galvanostatic cycling and cyclic voltammetry (CV). The CV shows that the Mn 3+ ↔ Mn 4+ couple lies in the range from ~4.0 to ~ 4.2 V and the spinel-to-double-hexagonal (S-DH) transition at ~ 4.5 V in agreement with results in the literature. The Ru 4+ ↔ Ru 5+ redox couple operates at ~4.3 V vs Li in the spinel structure. The S-DH phase transition is suppressed on Ru doping. Charge-discharge cycling, up to 25 or 40 cycles, in the voltage ranges 3.5-4.3 and 3.5-4.6 V vs Li at 0.14C rate gave initial charge capacities ranging from 118 to 139 mAh g -1 for various x. Compositions with x = 0.1 and 0.25 showed smaller initial capacity loss and smaller capacity fading during cycling as compared to x = 0. For x = 0.25 in the voltage range 3.5-4.6 V and 2-25 cycles, the capacity fade is only 9% thereby showing the beneficial effect of Ru doping, by way of suppression of S-DH transition, participation of Ru-redox couple, and better electronic conductivity of the compound during cycling, in comparison to pure LiMn 2 O 4 .

Structural, transport, magnetic and magnetoelectric properties of CaMn1−xFexO3−δ (0.0 ≤ x ≤ 0.4)

The structural, magnetic, transport and magnetoelectric properties of parent and Fe doped CaMn1−xFexO3−δ (0.0 ≤ x ≤ 0.4) manganites are investigated using synchrotron X-ray, Raman, SQUID and nova control impedance analyzer. The Fe doped composition x = 0.3 shows a strong Maxwell–Wagner effect, and quite high positive magnetocapacitance (MC) of ∼ 8.45% at room temperature and at low magnetic field 7.8 kG has been observed. Rietveld refinement of synchrotron X-ray diffraction patterns suggests (i) a structural transformation from orthorhombic to cubic crystal system and (ii) an increase in lattice parameters with the substitution of Fe at the Mn site. Sintering at 1300 °C stabilizes the doping of higher ionic radii Fe+3 (0.645 A)/Fe+4 (0.585 A) atoms at the Mn+4 (0.53 A) site in CaMn1−xFexO3−δ. The Magnetization data show a transformation of the G type of antiferromagnetic arrangement of Mn+4 electrons spins in CaMnO3 into a paramagnetic spin type arrangement with Fe substitution. The AC conductivity of the Fe doped compositions decreases more than two orders of magnitude in comparison to CaMnO3−δ.

Long range ferromagnetic ordering in pulsed laser deposited La0.7Ca0.3Mn1−xRuxO3 thin films

We demonstrate the existence of the metal to insulator transition (TMIT) up to 30% of Ru substitution at the Mn site, while the ferromagnetic transition (TC) is sustained even beyond 30% Ru substitution in the pulsed laser deposited La0.7Ca0.3Mn1−xRuxO3 thin films. The presence of Ru(IV) and (V) states complementing the t2g and eg orbital parentage of Mn(III) and Mn(IV) brings about prolonged double exchange mediated magnetic and electronic interactions, as confirmed by the X-ray absorption spectra and a negative magneto resistance ratio (MR) up to 11% at TMIT even for 30% Ru substitution.

Phase stability of silver particles embedded calcium phosphate bioceramics

In this paper, we report the compositional variation-dependent phase stability of hydroxyapatite (Ca10(PO4)6(OH)2) on doping with silver. The transformation of hydroxyapatite to (β/α) tricalcium phosphate phases during sintering has been explored using Raman spectroscopy and X-ray diffraction techniques. The optical absorption spectroscopy analysis reveals the presence of Ag+ ions at low doping levels. As the doping increases, abundance of Ag particles is enhanced.

Powder neutron diffraction evidence for enhanced inter plane magnetic coupling in La1.2Sr1.8Mn2−xRuxO7 layered manganites

Rotation of Mn spin moments along the c axis enhances interplane magnetic coupling, leading to a steep increase in the Curie temperature (TC) and metal to insulator transition (TMIT) in Ru substituted two dimensional layered manganites, La1.2Sr1.8Mn2−xRuxO7. The temperature dependent neutron diffraction data show the absence of (004) reflection, indicating that the Mn spin moments are not aligned in the a-b plane, and rather the increase in intensity of (110) reflection at 12K shows that the MnO2 layers are stacked ferromagnetically along the c axis. The presence of Ru at the Mn site in pentavalent state assures the required hole carrier density for a sustained double-exchange interaction. The cumulative effect of rotation of Mn spin moments and the variable valence states of Ru lead to a pronounced increase in TC up to ∼50K, which is not observed hitherto in any of the B-site substituted manganites.

Magnetism and electronic transport in Sr4−xLaxRu2−xMnxO9: Interplay of Mn and Ru redox chemistry

Sr4Ru2O9 has a P6¯2C space group and differs from the Ruddelsden–Popper series of the strontium ruthenate family where Ru is distinctly present in Ru+5 state with no Jahn–Teller activity. A significant increase in magnetic moment and coercivity is observed along with lowering of resistivity, especially around a critical concentration of x=0.2 in Sr4−xLaxRu2−xMnxO9. Such an unusual effect is correlated to the mixed magnetic pair effect arising out from the variable oxidation states of Ru as suggested by x-ray photoelectron spectroscopic studies. Our results substantiate the long-range ferromagnetism observed for Ruthenium-substituted three-dimensional manganites.

Evidence for Mixed Valence States of Mn and Ru in LiMn2‐xRuxO4 using X‐ray Photoelectron Spectroscopy

We report the electronic structure of LiMn2‐xRuxO4 (0.0≤x≤0.05) using X‐ray photoelectron spectroscopy (XPS). XPS measurements show the substitution of Ru at the Mn site brings mixed valence of Ru and Mn leading to Ru+4/Ru+5/Ru+6 along with Mn+3/Mn+4. Thermo gravimetric analysis shows more thermal stability for Ru doped samples with better rod shaped surface morphology of the sintered samples suggesting use of Ru doped samples as candidate for cathodic material.

Magnetoresistive behavior in hole doped La1−xPbxMn0.8Ru0.2O3 (0.2⩽x⩽0.4) epitaxial thin films

Epitaxially grown thin films of La1−xPbxMn0.8Ru0.2O3 (0.2⩽x⩽0.4) on LAO (001) substrate using pulsed electron deposition technique shows a systematic decrease in metal to insulator transition from 300to250K when hole concentration varies from 40% to 20%. However, an increase in magnetoresistance ratio by ∼5%–15% is observed for Ru substituted films at the respective Curie temperatures when compared to the parent La0.6Pb0.4MnO3 film. Transport and magnetoresistive properties show that Ru substitution maintains a considerable hole carrier density even for La0.8Pb0.2Mn0.8Ru0.2O3 (8282) composition to stabilize the double exchange interactions.

Electron transport in non-crystalline garnet films

Thin films of yttrium iron garnet (yig) and Gd-substitutedyig of different thickness have been prepared by flash evaporation. The surfacedc andac electrical resistivity and thermopower in these films have been studied. The results are explained on the basis of Mott and Davis model in which narrow tails of localised states exist at the extreme valence and conduction bands and a band of localised levels near the middle of the gap. For the temperature range studied, the main conduction mechanism is on account of excitation of carriers into localised states at the band edges and hopping at energies close to the band tails.

Low temperature neutron diffraction studies showing evidence for charge-exchange-type magnetic ordering in Mn doped SrRuO3

SrRuO3 is a 4d transition metal oxide having a metallic and itinerant ferromagnetic character with TC∼165K. Mn doping in SrRuO3 induces antiferromagnetic interaction, with the equiatomic composition SrRu0.5Mn0.5O3 showing TC∼125K. Temperature dependent zero field cooled and field cooled magnetization data, resistivity data, and neutron diffraction studies on SrRu0.5Mn0.5O3 show the signature of the presence of two ferromagnetic zigzag chains coupled antiferromagnetically, called charge exchange (CE)-type antiferromagnetic ordering; this is contrary to the role of Ru+4, which is reported to remove charge ordering in manganites. The presence of significant amount of Mn+3 ions plays an important role in stabilizing the CE-type antiferromagnetic ordering.