Degree Of Tetragonal Distortion Research Articles

Size-controlled synthesis of Mn3O4 nanoparticles: characterization and defect chemistry

Identification of defect chemistry and variation of particle size are key factors for understanding many intrinsic properties of materials. Herein, size selective synthesis of Mn3O4 nanoparticles was performed by precipitation of Mn(OH)2 via NaOH or ammonia followed by air oxidation in ethanol/water solution. The amount of ethanol in water affects the average particle size that drops from ~ 25 nm in pure water to ~ 10 nm in 95% EtOH/H2O. TEM studies showed that the Mn3O4 samples have a sphere-cube like morphologies. Calculation of site occupancy factor and unit cell parameters via Rietveld analysis and the excess mass of Mn3O4 samples depending on size by thermogravimetric analysis and IR revealed that the cationic deficiency observed in the structure of the smallest size sample is compensated mainly by anionic vacancies. Ideal spinel structure was found for largest size sample. The degree of tetragonal distortion (c/ $$ \sqrt{2} $$ a) of Mn3O4 unit cell increases with particle size.

Effect of Cobalt on the Crystal Structure and Magnetism of Electron-Doped Sr0.8Ce0.2MnO3 Oxide

The Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.2, 0.3, 0.4, 0.5, and 0.6) complex oxides with the perovskite structure obtained from simple oxides by the solid–state reaction technique have been examined using structural analysis and magnetic measurements. Substitution of Co for Mn in the dilute Sr0.8Ce0.2MnO3 antiferromagnet with a Neel temperature of TN = 210 K leads to a decrease in the degree of tetragonal distortion of the crystal structure and transition to the cubic cell. The degeneracy of the antiferromagnetic interaction (TN = 138 K at y = 0.2) observed at the first stage of the substitution of Co for Mn changes for its gradual enhancement with an increase in the magnetic transformation temperature up to 239 K at y = 0.6. An increase in the Co content weakens the competition between the ferromagnetic and antiferromagnetic couplings and reduces the temperature of the transition to the spin-glass-like state. The magnetic inhomogeneity and formation of Co2+–Mn4+ ferromagnetic clusters in Sr0.8Ce0.2Mn0.4Co0.6O2.69 at 140 K have been observed.

Electronic Degeneracy and Intrinsic Magnetic Properties of EpitaxialNb: SrTiO3 Thin Films Controlled by Defects.

We report thermoelectric power experiments in e-doped thin films of SrTiO3 (STO) which demonstrate that the electronic band degeneracy can be lifted through defect management during growth. We show that even small amounts of cationic vacancies, combined with epitaxial stress, produce a homogeneous tetragonal distortion of the films, resulting in a Kondo-like resistance upturn at low temperature, large anisotropic magnetoresistance, and nonlinear Hall effect. Ab initio calculations confirm a different occupation of each band depending on the degree of tetragonal distortion. The phenomenology reported in this Letter for tetragonally distorted e-doped STO thin films, is similar to that observed in LaAlO3/STO interfaces and magnetic STO quantum wells.

Structural features of copper ferrite-chromites

The structural features and the distribution of cations over crystallographic sites of copperchromite, copper ferrite, and mixed chromite-ferrites with the spinel structure obtained by thermal decomposition at 600°C and 900°C of mixed hydroxy compounds of copper, iron, and chromium with the composition Cu2+/(Fe3++Cr3+) = 1/2 and different Fe3+/Cr3+ ratios are studied using a set of physicochemical methods. It is shown that the spinel-structure phases formed exist in two modifications: cubic and tetragonal, depending on the Fe3+/Cr3 ratio. The crystallographic relationship between the cubic and tetragonally distorted phases of spinel is analyzed. The distribution of cations over crystallographic sites, the character and degree of tetragonal distortion of Cu–Fe–Cr spinel depend on the Fe3+/Cr3+ ratio: when Fe3+/Cr3+ > 1, the ratio of unit cell parameters c*/а* > 1; when Fe3+/Cr3+ < 1, the ratio c*/а* < 1; when Fe3+/Cr3+ = 1, the spinel is cubic irrespective of the temperature of thermal treatment. The oxygen coordination of copper ions in the spinel structure has a significant effect on the catalytic properties of the samples in the low-temperature water gas shift reaction.

Specific features of magnetic properties upon martensitic transition M ↔ L21 in Ni2 + x Mn1 − x Ga alloys

The paramagnetic susceptibility of Ni2 + xMn1 − xGa alloys with x = 0.12, 0.16, 0.19, 0.21, 0.23, 0.24, 0.27, 0.3, 0.33, 0.36, and 0.39 was measured at H = 5 kOe in the temperature range of 400–800 K. It was shown that the degree of tetragonal distortion c/a and the parameters characterizing the electronic (χ0) and magnetic (θP and μeff2) subsystems of the alloys change anomalously sharply in the range 0.27 < x < 0.3 when the number of valence electrons per atom is e/a ∼ 7.7.

Magnetic and structural phase transitions and the tetragonality of thermoelastic martensite in quasi-binary Heusler alloys Ni2 + x Mn1 − x Ga

This paper reports on the results of investigations into the structure of martensite and the electronic and magnetic properties of Ni2 + xMn1 − xGa alloys in the concentration range 0.12 ≤ x ≤ 0.39. The concentration dependences of the critical temperatures and the anomalous behavior of the degree of tetragonal distortion of the martensite have been discussed, as well as the changes in the parameters characterizing the electronic and magnetic subsystems in the thermoelastic martensitic transition L21 ↔ M, which occurs in atomically ordered L21-type alloys upon doping in the range 0.12 ≤ x ≤ 0.39 at different temperatures. The temperature and concentration dependences of the magnetic properties have been determined for the state of the single-phase L21 austenite (at T > TC and Af) and martensite (at T < TC and Mf), which undergoes the transition from a modulated 10M- or 14M-type structure to a nonmodulated (NM) tetragonal structure.

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate

The surface morphology, lattice parameters, and Raman spectra have been investigated at the stages of successive formation of a three-layer heterostructure consisting of epitaxial layers Ba0.8Sr0.2TiO3 (BST) and (Bi0.98Nd0.02)FeO3 (BNFO). The structural distortions arising from sequential deposition of BST/BNFO/BST layers of equal thickness have been determined using X-ray diffraction. It has been found that the degree of tetragonal distortion of the BST film on MgO increases after deposition of the BNFO film on the BST film surface, indicating the appearance of compressive stresses in BST. Based on the analysis of polarized Raman spectra, it has been shown that, in the BNFO layer, there is a new phase state which is not observed in the bulk samples. It has been demonstrated that the degree of tetragonality of the BST film grown on the BNFO surface is higher than that of the BST film grown directly on MgO.

Comparison of the properties of PZTNB-1 + Ni0.9Co0.1Cu0.1Fe1.9O4 − δ magnetoelectric composites manufactured from components synthesized by Sol-Gel processes

Processes were developed for manufacturing mixed magnetoelectric (ME) composites (100 − x) wt % PZTNB-1 + x wt % Ni0.9Co0.1Cu0.1Fe1.9O4 − δ using sol-gel processes to synthesize piezoelectric and magnetostrictive components; these processes provide composite ceramics having different connectivity types with the same composition. When PZTNB-1 prepared by the sol-gel process is used for manufacturing the composites, there is a deep one-way doping of the piezoelectric with ferrite constituents, and this doping considerably reduces the degree of tetragonal distortion of the perovskite unit cell to the point of disappearance. In contrast, when coarse-grained PZTNB-1 powder and a ferrite nanopowder prepared by the sol-gel process are used, undesirable doping is completely inhibited. This type of composite has the highest ME transduction efficiency, by two or more times exceeding the ME ceramics of the same composition manufactured by other processes.

Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni50−xCoxMn32−yFeyGa18

Significantly large inverse magnetic entropy changes (ΔSM) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni50−xCoxMn32−yFeyGa18. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the ΔSM and MR at H=5 T were determined as ≈(+)31 J Kg−1 K−1 and ≈−21%, respectively, for x=8 and y=2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.

Shift in Morphotropic Phase Boundary in La-Doped BiFeO3–PbTiO3 Piezoceramics

Polycrystalline BiFeO3–PbTiO3 (BF–PT) powders with mixed tetragonal and rhombohedral crystal structure (morphotropic phase boundary: MPB) were modified with lanthanum to provide a wide variation in tetragonal distortion. X-ray diffraction from both the powder and the corresponding bulk ceramic demonstrated that the MPB in the bulk is shifted from 1–5 mol % towards the tetragonal PT as compared to the powder. This shift was correlated with the degree of tetragonal distortion as quantified by c/a ratio.

Metal-ligand interplay in blue copper proteins studied by 1H NMR spectroscopy: Cu(II)-pseudoazurin and Cu(II)-rusticyanin.

The blue copper proteins (BCPs), pseudoazurin from Achromobacter cycloclastes and rusticyanin from Thiobacillus ferrooxidans, have been investigated by (1)H NMR at a magnetic field of 18.8 T. Hyperfine shifts of the protons belonging to the coordinated ligands have been identified by exchange spectroscopy, including the indirect detection for those resonances that cannot be directly observed (the beta-CH(2) of the Cys ligand, and the NH amide hydrogen bonded to the S(gamma)(Cys) atom). These data reveal that the Cu(II)-Cys interaction in pseudoazurin and rusticyanin is weakened compared to that in classic blue sites (plastocyanin and azurin). This weakening is not induced by a stronger interaction with the axial ligand, as found in stellacyanin, but might be determined by the protein folding around the metal site. The average chemical shift of the beta-CH(2) Cys ligand in all BCPs can be correlated to geometric factors of the metal site (the Cu-S(gamma)(Cys) distance and the angle between the CuN(His)N(His) plane and the Cu-S(gamma)(Cys) vector). It is concluded that the degree of tetragonal distortion is not necessarily related to the strength of the Cu(II)-S(gamma)(Cys) bond. The copper-His interaction is similar in all BCPs, even for the solvent-exposed His ligand. It is proposed that the copper xy magnetic axes in blue sites are determined by subtle geometrical differences, particularly the orientation of the His ligands. Finally, the observed chemical shifts for beta-CH(2) Cys and Ser NH protons in rusticyanin suggest that a less negative charge at the sulfur atom could contribute to the high redox potential (680 mV) of this protein.

Investigations into Sr 3CaZr 0.5Ta 1.5O 8.75, a novel proton conducting perovskite oxide

A new complex triple perovskite of formula Sr 3CaZr 0.5Ta 1.5O 8.75 has been synthesized. X-ray diffraction indicates a tetragonal unit cell for an as-prepared sample, with a=5.8049(6) and c=8.220(1), although high-resolution neutron diffraction reveals a slight monoclinic distortion. Thermogravimetric analysis (TGA) shows that water is taken up to fill 55% of the vacant oxygen sites when a vacuum-treated sample is subjected to wet 5% H 2/Ar atmosphere at 380°C for 24 h. X-ray diffraction simply shows a decrease in the degree of tetragonal distortion on hydration. In wet H 2 atmosphere (4% H 2/3% H 2O), the bulk and grain boundary conductivities were found to be σ b=4.64×10 −4 S cm −1 ( E act=0.66 eV) and σ gb=7.04×10 −5 S cm −1 at 300°C ( E act=0.88 eV), an improvement over Sr 3Ca 1.18Nb 1.82O 8.73 under the same conditions. Proton conduction has been verified by changing the hydration level of a 5% H 2/Ar atmosphere, while AC impedance conductivity measurements were made. On changing from a dry gas atmosphere to a wet gas atmosphere, bulk proton conductivity increases by a factor of 3.5 when exposed to a wet 5% H 2/Ar mixture for 24 h at 644°C. The process is reversible, i.e. the resistance is seen to increase, on annealing in a dry atmosphere. The half-life of the water (proton) loss process is approximately 72 min at 440°C.

Electronic Structure of Chemically-Prepared LixMn2O4 Determined by Mn X-ray Absorption and Emission Spectroscopies

We have performed Mn K-edge X-ray Absorption and Mn L-edge emission spectroscopies on LiMn2O4, its chemically delithiated and lithiated derivatives (λ-MnO2 and Li2Mn2O4, respectively), and two Mn3+ spinel model compounds. These experiments were undertaken to understand the associated changes in atomic and electronic structure occurring when LiMn2O4 is used in a rechargeable lithium cell. Subtle changes in the Mn K-edge X-ray absorption near edge structure (XANES) occur upon delithiation that are consistent with literature reports of the oxidation of Mn3+ to Mn4+, retention of the cubic phase, and contraction of the spinel lattice. Conversely, when LiMn2O4 is lithiated, the XANES changes dramatically due to the concurrent transformation from a cubic to a tetragonal spinel. The spectrum is different from XANES of tetragonal Mn3+ spinels possessing approximately the same degree of tetragonal distortion as Li2Mn2O4. This spectral difference is attributed to the inserted Li+ imparting an increased degree of co...

The magnetic structure and lattice distortion of YMn 2 and related compounds

YMn 2 is an antiferromagnet with a helical modulation of an extremely long period (430 Å). In order to know the origin of the helical modulation, we have carried out precise neutron diffraction measurements for YMn 2 at several temperatures, and for Y 0.97Tb 0.03Mn 2 and Y(Mn 0.97Al 0.03) 2 at a low temperature. The period of modulation and the degree of tetragonal distortion have been determined and the relation between them are discussed.

Copper(II) Dihydroxybenzoates; Synthesis and Properties

Compounds of composition Cu(2,5-DHB)2 . 4 H2O (DHB, dihydroxybenoate anion) and Cu(2,Y-DHB)2 . 8 H2O (Y = 4 or 6) were prepared as potential antiinflamatory gents with a view of pharmacological screening. The room-temperature magnetic moments allow to classify all compounds into the group of magnetically diluted copper(II) complexes. According the EPR and electronic spectra, the degree of tetragonal distortion increases passing from Cu(2,6-DHB)2 . 8 H2O (III) through Cu(2,4-DHB)2 . 8 H2O (I) to Cu(2,5-DHB)2 . 4 H2O (II. The complexes differ also in the cooperative ordering in their structures. All tested compounds exhibit higher antiinflamatory activities (on dextran edema) than free carboxylic acids. However, their effects were accompanied with relatively high toxicity. Remarkable results were also achieved on evaluating the antipyretic activity.

Influence of the nitrile group on structure properties of pseudohalogeno-copper(II) and nickel(II) complexes with 3- and 4-cyanopyridines

Pseudohalogenocomplexes of the types [CuX2L2] (X = NCO, C(CN)3, N(CN)2; L = 3- or 4-CN-py) and [NiX2L2] (X = C(CN)3, N(CN)2; L as before) have been prepared and investigated by spectroscopic methods. As the results show the [CuX2L2] complexes have pseudooctahedral structures with variable degree of tetragonal distortion and contain bridging pseudohalide ligands. The NCO groups bridge on the end-to-end fashion, forming by their oxygens long axial bondings. The N(CN)2 groups very likely bridge asymmetrically by means of their amide and cyanide nitrogens, the former ones being used for strong equatorial bonds. The [NiX2L2] complexes are octahedral and also exhibit the pseudohalides in bridging function which, however, is realized with dicyanamide symmetrically through both cyanide nitrogens.