Crystallographic and magnetic structures of Y0.8Sr2.2Mn2GaO8−δ: a new vacancy-ordered perovskite structure

The anion-deficient perovskite Y_0.8Sr_2.2Mn₂GaO_8−δ (where δ∼0.1) has been synthesised and the crystal and magnetic structures determined by Rietveld analysis of neutron powder diffraction (NPD) data. The material has body-centred tetragonal symmetry (I4/mmm, a=7.6373(3) Å and c=15.6636(10) Å) and consists of alternating layers of octahedral and tetrahedral polyhedra, the layers being perpendicular to [001]. The octahedral layers are preferentially occupied by manganese and the tetrahedral layers are a mixture of manganese and gallium. The precise cation distribution depends critically on preparative conditions. An unusual structural feature of these materials is the arrangement of oxygen vacancies in the tetrahedral layers: in the basic structure, isolated squares of corner-linked tetrahedra are formed instead of the chains that are observed in brownmillerite phases. Additional oxide ions in this layer probably allow the Mn ions to achieve distorted square pyramidal coordination. Low temperature NPD and magnetisation data indicate antiferromagnetic ordering below 100 K.     

Synthesis,photophysical properties,and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles

Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (perovskite, defect pyrochlore) nanoparticles prepared by a hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO₃·2H₂O) markedly affect the crystal structure of sodium tantalum oxide. At low NaOH concentration and high bismuth precursor (NaBiO₃·2H₂O) content, Bi doped Na₂Ta₂O₆ (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO₃ (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na₂Ta₂O₆) phase was formed and stabilized by the presence of dopant precursor (NaBiO₃·2H₂O). The chemical analysis of the samples confirmed the doping of Bi³⁺ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO₃ samples showed better performance for the photocatalytic degradation of rhodamine B than that of Bi doped Na₂Ta₂O₆, under visible light irritation (λ>420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO₃ based compounds, synthesized by the hydrothermal method.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Structure and magnetic properties of perovskite Sr2CuNbO6−δ

Single phase perovskite Sr₂CuNbO_6−δ with a high proportion of Cu¹⁺ ions and oxygen vacancies was synthesized by solid-state reaction. The structure was determined by Rietveld method with space group Pm3m. Isotropic g value was evaluated from electron spin resonance (ESR) measurements. The ESR result is consistent with that of magnetic susceptibility.     

Thermal stability of A-site ordered PrBaMn2O6 manganites

The crystal structure and electromagnetic properties as well as thermal stability of the A-site ordered PrBaMn₂O₆ manganites have been investigated. These samples have been prepared by using ‘two-steps’ synthesis mode. They have tetragonal structure with no tilt of MnO₆ octahedra and show ferromagnetic metal to paramagnetic semiconductor transition. The most significant structural feature of the A-site ordered manganites is that the MnO₂ sublattice is sandwiched by two types of rock-salt layers PrO and BaO. The different degree of Pr and Ba ions in the A-sublattice is revealed. The A-site ordered PrBaMn₂O₆ sample with maximum degree of the A-site order demonstrates ferromagnetic metallic to paramagnetic insulating transition with the Curie point ∼320 K. The A-site disordered Pr_0.50Ba_0.50MnO₃ sample is ferromagnetic metal below T_C≈140 K. The cation order in these compounds is stable in air up to 1300 °C. For the partly A-site ordered samples the magnetic and electronic phase separation is observed. The magnetotransport properties of the A-site ordered manganites treated under different conditions are discussed in terms of the superexchange interactions and A-site order degree.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3

This paper reports on multiferroic properties of Ho substituted BiFeO₃ (Bi_1−xHo_xFeO₃) ceramics. It is observed that for x=0.15, a prominent ferroelectric loop is seen at 300 K even if the system remains in rhombohedral (R3c) phase without appearance of any observable impurity phases. A well shaped M-H loop is observed at 10 K for x=0.15. However it showed ferromagnetism, confirming the contribution of Ho³⁺ towards enhancement of ferromagnetic properties of BiFeO₃ at 300 K. Suppression of impurity phases of pure BiFeO₃ bulk ceramic favors the reduction of mobile oxygen vacancies and reduces leakage current, due to which ferroelectric properties of BiFeO₃ is enhanced. We argue that Ho substitution at Bi site is likely to suppress the spiral spin modulation and at the same time increase the canting angle, which favors enhanced multiferroic properties. XRD, SEM, magnetization, polarization and chemical bonding analysis measurements were carried out to explain the multiferroic behavior.     

Investigation of the electrical and magnetic properties of electron-doped Ruddlesden-Popper phases, CaO(Pr0.08Ca0.92MnO3)n (n=1, 2, 3 and ∞)

The electrical and magnetic properties of Pr^3±-doped Ruddlesden-Popper manganates with the formula, CaO(Pr_0.08Ca_0.92MnO₃)_n (n=1,2,3 and ∞) have been investigated. The electrical conductivity of the samples increases with the number of perovksite slabs sandwiched between the rock-salt-like CaO layers. Magnetic susceptibility measurements show an anomalous behavior for the n=1 composition which is attributed to the near absence of ferromagnetic correlations in a two-dimensional system. The magnetic behavior of other members can, however, be understood on the basis of a ‘ferromagnetic-droplet’ model. Since the number of electrons per perovskite slab is invariant across the series, the electronic properties are readily correlated to the phase space available for exchange interactions.     

Piezoelectric, dielectric and magnetic properties of (1-x)Pb[Zr, Ti, (Mg1/2W1/2), (Ni1/3Nb2/3)]O3+x(Ni, Co, Cu)FeO4 composites

The phase structure, microstructure, piezoelectric properties, dielectric characteristic and the ME effect of magnetoelectric Pb[Zr_0.23Ti_0.36+0.02(Mg_1/2W_1/2)+0.39(Ni_1/3Nb_2/3)]O₃ (PZT)+xNi_0.8Co_0.1Cu_0.1Fe₂O₄ (NCCF) composite ceramics were prepared by the conventional solid state reaction method. The structural analysis of both the constituent phases and their composites was carried out by X-ray diffraction, energy dispersive spectrometry and scanning electron microscopy. The results showed cubic spinel structure for ferrite phase and tetragonal perovskite structure for ferroelectric phase. The piezoelectric constant, dielectric constant, Curie temperature, remanent polarization and coercive electric field decreased with increase of ferrite content. The coercive field strength, saturation magnetization and remanent magnetization increased with increasing ferrite content.     

Influence of Co, Ni and Zn substitution for Cu on the structural and superconducting properties of (Hg0.7Cr0.3)Sr2CuO4+δ

The effect of Co, Ni and Zn substitutions for Cu on the phase stability and superconducting properties of (Hg_0.7Cr_0.3)Sr₂CuO_4+δ was investigated. X-ray diffraction (XRD) revealed that both Co and Zn are soluble in the (Hg_0.7Cr_0.3)Sr₂CuO_4+δ material up to about 5% of the Cu content, whereas the solubility of Ni is extended up to 10%. Electrical resistivity and magnetic susceptibility measurements show that the value of the superconducting critical temperature T_c decreases linearly with the impurity content. The depression of T_c indicates that the suppression of the superconductivity in Co- and Ni-substituted samples is much stronger than that in Zn-substituted ones. The residual resistivity scales linearly with the doping level as expected from the impurity scattering due to disorder. Some possible explanations for the stronger suppression of T_c by the Co and Ni substitution than by Zn substitution are provided.     

Synthesis and characterization of room-temperature ferromagnetism in Fe- and Ni-co-doped In2O3

Observation of room-temperature ferromagnetism in Fe- and Ni-co-doped In₂O₃ samples (In_0.9Fe_0.1−xNi_x)₂O₃ (0?x?0.1) prepared by citric acid sol-gel auto-igniting method is reported. All of the samples with intermediate x values are ferromagnetic at room-temperature. The highest saturation magnetization (0.453 μ_B/Fe+Ni ions) moment is reached in the sample with x=0.04. The highest solubility of Fe and Ni ions in the In₂O₃ lattice is around 10 and 4 at%, respectively. The 10 at% Fe-doped sample is found to be weakly ferromagnetic, while the 10 at% Ni-doped sample is paramagnetic. Extensive structure including Extended X-ray absorption fine structure (EXAFS), magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phases.     

Nanocrystalline Sr2FeMoO6 prepared by citrate-gel method

Double perovskite Sr₂FeMoO₆ powders with small crystallite size have been synthesised with citrate-gel method. The starting solution pH was varied between 1.5 and 9.0 resulting in large differences in the phase composition and ordering of B^′/B^″ sites. The samples prepared at 975 °C had crystallite sizes under 40 nm whereas crystallite sizes of the samples prepared at 1050 °C were between 78 and 239 nm. The XRD patterns were refined with spacegroup I 4/m, which gave good results for both batches, although clearly better results were obtained with monoclinic P 2₁/n spacegroup for the 975 °C batch. The ordering and the saturation magnetization agreed well with each other after treatment at 1050 °C, but the samples prepared at 975 °C had a strongly reduced saturation magnetization from that given by the ordering.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.     

Ab initio calculations of the stability and lattice dynamics of the MgSiO3 post-perovskite

Using the first-principles computations we have calculated phonon-dispersion relations and the phonon density of states for perovskite and post-perovskite phases of MgSiO₃. From them using the quasiharmonic approximation we have estimated the free energies, bulk modulus and volume expansion coefficients of both structures at various pressures and temperatures. Our calculations indicate that the thermal expansivity of MgSiO₃ changes very little across the phase transition. We have determined the P-T coexisting line of perovskite and post-perovskite phases.     

The role of substitution-induced micro-structural defects on the redox behavior and catalytic activity of LaxTh1−x(VO3−δ)4 mixed oxide catalysts

The catalytic properties and reduction behavior of mixed metal oxides with nominal compositions of La_xTh_1−x(VO_3−δ)₄, where 0.0 ≤ x ≤ 1.0, have been investigated as a function of the value of x. These compositions were synthesized by ceramic route and characterized using powder X-ray diffraction, differential thermal analysis, and temperature-programmed reduction/oxidation/desorption techniques. The substituted samples were comprised of two distinct phases, one corresponding to tetragonal thorium metavanadate with partial substitution at A-sites, i.e. La_xTh_1−x(VO_3−δ)₄, and the other a new phase identified as LaV₄O_11+δ. Both these phases exhibited considerable oxygen non-stoichiometry, depending upon the extent of substitution. Nevertheless, no significant change was observed in their respective crystal symmetry. Further, the substituted samples showed more reproducible redox behavior and also an improved catalytic activity for CO oxidation reaction. A definite correlation is found to exist between the micro-structural defects generated in the above-mentioned two phases because of the oxygen ion vacancies, resultant enhancement in thermal diffusitivity of lattice oxygen, and finally the catalytic activity of the substituted mixed oxides. It is envisaged that a kind of synergism between the micro-structural defects existing in the two inter-grown phases generated simultaneously during the synthesis may control the catalytic properties of composite metal oxide systems.     

Suppression of Tc by Zn impurity in the electron-type LaFe0.925−y Co0.075ZnyAsO system

The effect of non-magnetic Zn impurity on superconductivity in electron-type pnictide superconductor LaFe_0.925−yCo_0.075Zn_yAsO is studied systematically. The optimally doped LaFe_0.925Co_0.075AsO without Zn impurity exhibits superconductivity at T_c^mid of 13.2 K, where T_c^mid is defiend as the mid-point in the resistive transition. In the presence of Zn impurity, the superconducting transition temperature, T_c^mid, is severely suppressed. The result is consistent with the theoretic prediction on the effect of non-magnetic impurity in the scenario of s_± pairing, but it is in sharp contrast to the previous report on the effect of Zn impurity in the F-doped systems. The possible interpretation of the different effects of Zn impurity on superconductivity in different systems is discussed.     

Structural,thermal and electrical properties of the semiconductor system Ag(1−x)CuxInSe2

This paper presents a study of bulk samples synthesized of the Ag_1−xCu_xInSe₂ semiconductor system. Structural, thermal and electrical properties, as a function of the nominal composition (Cu content) x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 were studied. The influence of x on parameters such as melting temperature, solid phase transition temperature, lattice parameters, bond lengths, crystallite size t (coherent domain), electrical resistivity, electrical mobility and majority carrier concentration was analyzed. The electrical parameters are analyzed at room temperature. In general, it is observed that the properties of the Ag_1−xCu_xInSe₂ system for x≤0.4 are dominated by n-AgInSe₂, while for x>0.4, these are in the domain of p-CuInSe₂. The crystallite size t in the whole composition range (x) is of the order of the nanoparticles. Secondary phases (CuSe, Ag₂Se and InSe) in small proportion were identified by XRD and DTA.     

The unusual magnetic and electronic properties of Gd0.5Ba0.5CoO2.9

Magnetic and electrical properties of well-characterized Gd_0.5Ba_0.5CoO_2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln_0.5A_0.5CoO₃ (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd_0.5Ba_0.5CoO_2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.