High activity in the dry reforming of methane using a thermally switchable double perovskite and in situ generated molecular level nanocomposite.

This work emphasizes the dry reforming of methane (DRM) reaction on citrate sol-gel-synthesized double perovskite oxides. Phase pure La2NiMnO6 shows very impressive DRM activity with H2/CO = 0.9, hence revealing a high prospect of next-generation catalysts. Although the starting double perovskite phase gets degraded into mostly binary oxide phases after a few hours of DRM activity, the activity continues up to 100 h. The regeneration of the original double perovskite out of decomposed phases by annealing at near synthesis temperature, followed by the spectacular retention of activity, is rather interesting and hitherto unreported. This result unravels unique reversible thermal switching between the original double perovskite phase and decomposed phases during DRM without compromising the activity and raises challenge to understand the role of decomposed phases evolved during DRM. We have addressed this unique feature of the catalyst via structure-property relationship using the in situ generated molecular level nanocomposite.

Correction: Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties.

Correction for 'Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties' by Kamalesh Pal et al., Phys. Chem. Chem. Phys., 2020, 22, 3499-3511, DOI: 10.1039/C9CP05005A.

Components of the interaction energy of the odd-electron halogen bond: an ab initio study.

In the realm of non-covalent interactions (NCI), the odd-electron halogen bond offers a fertile ground to explore the nature of such weak interactions. Here, an ab initio study of odd-electron halogen bonding (XB) is reported. The interactions of five radicals with several freons and interhalogens are studied using the Møller-Plesset (MP2) method. The regioselectivity, interaction energy and the components of the interaction energy of odd-electron XB were tuned by judicial selection of donor-acceptor pairs as revealed by scrutinizing the conceptual DFT parameters, NCI plot and LED-DLPNO-CCSD(T) analysis. The contribution from dispersion interaction is rather high for all XB bonded complexes and it increases when the interacting atom of the XB donor is highly polarizable. Additionally, the polarisation and intermolecular charge-transfer also contribute significantly when both the donor and acceptor atoms are soft species, resulting in a soft-soft interaction. We believe that our finding will not only shed new light on non-covalent interaction of odd-electron XB but will also be able to capture the pnictogen, chalcogen and tetrel bonding interactions. The ability of conceptual DFT parameters to predict the interaction energy and its components shown in this study will be helpful for tuning of substrates for desired products, modelling bio/macromolecules and crystal engineering.

Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties.

The facile synthesis of the Al-doped CaCu3Ti4O12 quadruple perovskite, a well-known and vastly studied material for various technological applications, using the modified citrate combustion route along with structural, microstructural, and X-ray photoelectron spectroscopic (XPS) characterization and magnetic, dielectric and electrical properties has been investigated and reported here. The possible applications of the material as a Schottky barrier diode (SBD) in optoelectronic devices and as a catalyst in methanol steam reforming (MSR) reaction for hydrogen generation, hitherto unreported in the open literature, have also been explored. The compound is crystallized in the cubic body centered Im3[combining macron] space group and the particle size is found to be in nanodimension with rather narrow size distribution. The enhanced resistivity could be attributed to the grain boundary effect, and consequently, it exhibits better performance as a SBD compared to the undoped sample. Desired cationic composition with expected valence states within the probe range is confirmed by XPS analysis. A better catalytic activity towards MSR is noticed for the Al-doped CaCu3Ti4O12 compared to the undoped composition. These new findings, namely MSR activity and applicability in the Schottky device, have highlighted further the multifunctional nature of the material in energy related issues and would thus be of interest to the materials community searching for functional materials.

Non-covalent interactions between epinephrine and nitroaromatic compounds: A DFT study.

Here, we present a density functional theory (DFT) study of hydrogen bonding and π-π stacking interactions between epinephrine and different aromatic nitro-compounds in gas phase as well as in methanol solvent. Detail investigations of hydrogen bonding and π-π interactions are performed and confirmed on the basis of theoretical IR spectra, natural bond orbital (NBO) analysis, non-covalent interaction (NCI), chemical reactivity descriptors and electronic spectra. Among different functionals used for the calculation, the results obtained from ωB97XD functional are found to be more suitable to describe the hydrogen bonding and π-π stacking phenomenon for our considered systems. Weakening of hydrogen bonding and π-π stacking interaction on solvent incorporation is observed. Electronic transition between different orbitals and transition probabilities of epinephrine and nitro-aromatic complexes are described using time dependent density functional theory (TD-DFT) method.

Bi-doped suppression of antisite disordering and associated magnetic properties of La2-x Bi x MnNiO6 (x = 0 and 1).

Here we report synthesis, structure, microstructure and magnetic properties of La2-x Bi x MnNiO6 (x = 0 and 1) double perovskites. Ricciardo et al (2009 Mater. Res. Bull. 44 239) have attempted to synthesize LaBiMnNiO6 (x = 1), but no further characterization was done due to large impurity content in the sample. We have been able to synthesize LaBiMnNiO6 phase at ambient pressure with traces of impurity at 750 °C using sol-gel method. This achievement leads us to compare the structural and magnetic properties of LaBiMnNiO6 with parent phase La2MnNiO6 to highlight the effect of Bi-doping in double perovskite. In contrast to the biphasic rhombohedral (R-3c) and monoclinic (P21/n) crystal structures of La2MnNiO6, LaBiMnNiO6 crystallized in single monoclinic (P21/n) phase. The EDX mapping confirmed the chemical homogeneity of the samples. The electron diffraction confirms the ordered structure of the sample. The microstructure analysis from HAADF-STEM revealed random distribution of misfit dislocations in the structure. Such defects are created to relax the strain due to unusual replacement of Mn/Ni atoms by La/Bi. We observed a decrease in TC with a large increase in magnetic moment of LaBiMnNiO6 compare to La2MnNiO6. There is also large suppression of low-temperature magnetic anomaly in Bi-substituted sample. The lowering of TC can be rationalized to the local structural distortion associated with the stereoactive 6s2-lone pair electron of Bi3+. On the other hand, the increase in magnetic moment and suppression of low-temperature magnetic anomaly for LaBiMnNiO6 can be ascribed to the suppression of antisite disorder in Bi-substituted sample.

Unraveling the regioselectivity of odd electron halogen bond formation using electrophilicity index and chemical hardness parameters.

Odd electron halogen bonding of freons with five different free radicals have been investigated using M06-2X/6-311G++(d,p). Conceptual density functional theory parameters such as Fukui functions, electrophilicity index and chemical hardness have been utilised to examine the regioselectivity and strength of free radical induced halogen bonding. The strong electrophilic radical Cl˙ forms three-electron bonds upon interacting with the nucleophilic ring surrounding the sigma hole or fk- regions. This behavior is also observed for moderately electrophilic radicals OH˙ and OCl˙ for complexation with CHFCl2 only. In other complexes, these radicals along with least nucleophilic radicals ˙NO and ˙Ph form one-electron bond with sigma hole or fk+ regions of freons. This regioselectivity of radicals and the interaction energy upon complexation have been explained in terms of local electrophilicity index and chemical hardness. We hope that this finding will shed light on the understanding of non-covalent interactions in terms of conceptual density functional theory for halogen bonding as well as in pnictogen, chalcogen and tetrel bonding systems. This result may find applications in understanding the radical mediated reactions and ozone depletion in the stratosphere.

A revisit to the effect of annealing temperature on magnetic properties of LaFe0.5Mn0.5O3.

We report an experimental study for the structural and magnetic properties of highly pure LaFe0.5Mn0.5O3 perovskite phase. The impurity free LaFe0.5Mn0.5O3 has been prepared by sol-gel technique at 500 °C and annealed at different temperatures up to 1000 °C. Previous works on LaFe0.5Mn0.5O3 revealed presence of secondary phases along with contradicting magnetic properties. Such as, Bhame et al (2005 Phys. Rev. B 72 054426-7) reported the superparamagnetic or spin-glass like behavior for 200 °C treated sample that persisted even at 700 °C sample. However, Wei et al (2012 Mater. Chem. Phys. 136 755-61) claimed room temperature ferromagnetism in all the samples annealed in the range of 600 °C-700 °C where the saturation magnetization decreases with the increase in temperature. These contradicting results lead us to revisit the effect of annealing temperature on the magnetic properties of LaFe0.5Mn0.5O3. We noticed a gradual increase in magnetization with increase in annealing temperatures without any signature of long range spin ordering for pure single phase samples. The increased magnetic moment with annealing temperatures has been attributed to the suppression of surface contribution of disordered spin. The low temperature magnetic behaviors can be explained by the interacting cluster glass behavior for the pristine as well as for 1000 °C annealed samples.

Ultra-high sensitivity of luminescent ZnCr2O4 nanoparticles toward nitroaromatic explosives sensing.

Here, we report the luminescence based sensing of trace amounts of nitroaromatic explosive organic compounds. The luminescence emission of nanosized spinel oxide ZnCr2O4 with high chemical and thermal stabilities has been used as a potential probe to detect such organic explosives. Low temperature solution combustion synthesized ZnCr2O4 oxide with an average particle size of ∼9 nm exhibits strong luminescence emission at 410 nm upon excitation at 260 nm in an aqueous suspension. The presence of nitroaromatics in ZnCr2O4 suspension dramatically suppresses the luminescence emission providing an opportunity to detect it quantitatively. The detection limit for 2,4,6-trinitrophenol (TNP) is as low as 23 ppb. A number of organic compounds have been investigated for a comprehensive understanding. The astonishing sensitivity of ZnCr2O4 nanoparticles towards nitro explosives is appealing for sensing application. A plausible explanation of such luminescence quenching has been ascribed to a two-fold mechanism. The underling mechanism is further substantiated by a similar study on ZnO nanoparticles.

Room-temperature ferromagnetic Cr-doped Ge/GeOx core-shell nanowires.

The Cr-doped tunable thickness core-shell Ge/GeOx nanowires (NWs) were synthesized and characterized using x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy and magnetization studies. The shell thickness increases with the increase in synthesis temperature. The presence of metallic Cr and Cr3+ in core-shell structure was confirmed from XPS study. The magnetic property is highly sensitive to the core-shell thickness and intriguing room temperature ferromagnetism is realized only in core-shell NWs. The magnetization decreases with an increase in shell thickness and practically ceases to exist when there is no core. These NWs show remarkably high Curie temperature (TC > 300 K) with the dominating values of its magnetic remanence (MR) and coercivity (HC) compared to germanium dilute magnetic semiconductor nanomaterials. We believe that our finding on these Cr-doped Ge/GeOX core-shell NWs has the potential to be used as a hard magnet for future spintronic devices, owing to their higher characteristic values of ferromagnetic ordering.

Impact of Mn3+ upon structure and magnetism of the perovskite derivative Pb(2-x)Ba(x)FeMnO5 (x ∼ 0.7).

On the basis of the Mn(3+) for Fe(3+) substitution in Pb(2-x)Ba(x)Fe2O5, a novel oxide Pb1.3Ba0.7MnFeO5 has been synthesized at normal pressure. Though it belongs to the same structural family, the mixed "MnFe" oxide exhibits a very different structural distortion of its framework compared to the pure "Fe2" oxide, due to the Jahn-Teller effect of Mn(3+). Combined neutron diffraction, high resolution electron microscopy/high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) investigations allow the origin of this difference to be determined. Here we show that the MO6 octahedra of the double perovskite layers in the "MnFe" structure exhibit a strong tetragonal pyramidal distortion "5 + 1", whereas the "Fe2" structure shows a tetrahedral distortion "4 + 2" of the FeO6 octahedra. Similarly, the MO5 polyhedra of the "MnFe" structure tend toward a tetragonal pyramid, whereas the FeO5 polyhedra of the "Fe2" structure are closer to a trigonal bipyramid. Differently from the oxide Pb(2-x)Ba(x)Fe2O5, which is antiferromagnetic, the oxide Pb1.3Ba0.7MnFeO5 exhibits a spin glass behavior with Tg ∼ 50 K in agreement with the disordered distribution of the Mn(3+) and Fe(3+) species.

A comparative study of magnetic and dielectric behaviors for La(1-x)Bi(x)Mn(1-y)Fe(y)O3 series (with x = 0.5, 0.7 and y = 0.3, 0.7).

We have carried out an extensive investigation into the effect of doping on both the A- and B-sites for the multiferroic La(0.5)Bi(0.5)Mn(0.5)Fe(0.5)O(3) in relation to its physical properties. The temperature dependent magnetization and dielectric response are determined for different percentages of Bi- and Fe-substitutions. For La(0.5)Bi(0.5)Mn(0.7)Fe(0.3)O(3), there is a prominent ferromagnetic transition T(C) around 110 K, whereas the other La(0.5)Bi(0.5)Mn(0.3)Fe(0.7)O(3) and La(0.3)Bi(0.7)Mn(0.3)Fe(0.7)O(3) phases fail to exhibit any clear transition. On the other hand, for the Fe-rich phases, the coercive field increases to 2450 Oe compared to 1720 Oe (for the Mn-rich phase). All the compositions exhibit coexistence of ferromagnetic and antiferromagnetic phases at low temperatures. The temperature dependent dielectric constant of the investigated samples varies from 32,000 to 500 at room temperature and the data has been analyzed using the universal dielectric response model.

Electronic phase separation in transition metal oxide systems.

Electronic phase separation is increasingly getting recognized as a phenomenon of importance in understanding the magnetic and electron transport properties of transition metal oxides. The phenomenon dominates the rare-earth manganates of the formula Ln(1-x)A(x)MnO(3)(Ln = rare earth and A = alkaline earth) which exhibit ferromagnetism and metallicity as well as charge-ordering, depending on the composition, size of A-site cations and external factors such as magnetic and electric fields. We discuss typical phase separation scenarios in the manganates, with particular reference to Pr(1-x)Ca(x)MnO(3)(x= 0.3-0.4), (La(1-x)Ln(x))(0.7)Ca(0.3)MnO(3)(Ln = Pr, Nd, Gd and Y) and Nd(0.5)Sr(0.5)MnO(3). Besides discussing the magnetic and electron transport properties, we discuss electric field effects. Rare-earth cobaltates of the type Pr(0.7)Ca(0.3)CoO(3) and Gd(0.5)Ba(0.5)CoO(3) also exhibit interesting magnetic and electron transport properties which can be understood in terms of phase separation.