Anar Singh

Direct evidence for multiferroic magnetoelectric coupling in 0.9BiFeO3-0.1BaTiO3.

Magnetic, dielectric and calorimetric studies on 0.9BiFeO3-0.1BaTiO3 indicate strong magnetoelectric coupling. XRD studies reveal a very remarkable change in the rhombohedral distortion angle and a significant shift in the atomic positions at the magnetic Tc due to an isostructural phase transition. The calculated polarization using Rietveld refined atomic positions scales linearly with magnetization. Our results provide the first unambiguous, atomic level evidence for magnetoelectric coupling of intrinsic multiferroic origin in a BiFeO3-based system.

Nature of ferroelectric to paraelectric phase transition in multiferroic 0.8BiFeO3–0.2Pb(Fe1/2Nb1/2)O3 ceramics

We present here the results of high temperature powder x-ray diffraction study on 0.8BiFeO3–0.2Pb(Fe1/2Nb1/2)O3, which is isostructural with the well known multiferroic BiFeO3 (BF). It is shown that the room temperature ferroelectric phase of 0.8BF–0.2PFN in the R3c space group transforms to the paraelectric/paraelastic cubic (Pm3¯m) phase directly without any intermediate “β” phase reported in the literature for pure BF. This transition is of first order type as confirmed by the coexistence of R3c and Pm3¯m phases over a 100 K range and discontinuous change in the unit cell volume.

A semi-wet route to the synthesis of YBa2Cu3O7-y ceramics

Advantages of a semi-wet route to the synthesis of YBa2Cu3O7-y employing coprecipitation of yttrium and barium as precursor carbonates are presented. It is shown that the precursor carbonate is an intimate mixture of fine barium carbonate and amorphous yttrium carbonate particles. YBa2Cu3O7-y powders prepared under identical conditions by the conventional dry route and the present semi-wet route are analysed for compositional inhomogeneities and coherently scattering domain size (D), using principles of X-ray line profile analysis. It is shown that unlike the powders prepared by the conventional method, which contain compositional inhomogeneities and possess domains of about 2200 AA, powders obtained under identical conditions by the semi-wet route are free from compositional inhomogeneities and possess larger domain size (around 4000 AA). Since the actual particle size in both the cases is several mu m, the loss of coherency in each particle is attributed to the presence of twin interfaces. Better compositional homogeneity and lower density of twin interfaces in specimens prepared by the semi-wet route are believed to be responsible for the improved superconducting behaviour of ceramics prepared by the present method.

High temperature ferroic phase transitions and evidence of paraelectric cubic phase in the multiferroic 0.8BiFeO3–0.2BaTiO3

We present here results of a powder x-ray diffraction study on the multiferroic 0.8BiFeO3–0.2BaTiO3 in the temperature range of 300–925 K. Our results provide unambiguous evidence for paraelectric cubic phase. We do not find any evidence for intermediate β-phase in our studies. The rhombohedral to cubic phase transition is shown to be of first order as revealed by the coexistence of cubic and rhombohedral phases over 100 K range and a discontinuous change in the unit cell volume. An anomaly in the unit cell volume at the magnetic transition temperature indicative of the magnetoelastic coupling is also reported.

Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 − y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 − y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.

Visualization of Bi3+ off-centering in the average cubic structure of (Ba0.70Bi0.30)(Ti0.70Fe0.30)O3 at the electron density level

We present here evidence for the off-centering of Bi3+ symmetrically distributed along the six ⟨100⟩ directions of a pseudocubic composition (Ba0.70Bi0.30)(Ti0.70Fe0.30)O3 at the electron density distribution level. We also calculate the strength of covalent bonding between the oxygen (O2−) anions with A-site (Ba2+/Bi2+) and B-site (Ti4+/Fe3+) cations, which provides evidence for the stabilization of average cubic symmetry. We believe that Bi3+ positional disorder may be ubiquitous in all the lead free piezoceramic solid solutions of the type (Ba1−xBix)(Ti1−xMex)O3, where Me = Fe3+, Sc3+, Al3+, (Mg1/2Ti1/2)3+, and (Zn1/2Ti1/2)3+.

Ferroelectric and antiferrodistortive phase transition in the multiferroic (Bi0.8Ba0.2)(Fe0.8Ti0.2)O3: A high temperature neutron powder diffraction study

Presented here are the results of Rietveld analysis of powder neutron diffraction data, which reveal that both the ferroelectric and antiferrodistortive phase transitions occur together at T ∼ 925 K during heating, leading to a change in crystal structure from room temperature rhombohedral phase in the R3c space group with tilted oxygen octahedra to the high temperature paraelectric and paraelastic cubic phase in Pm3−m space group. The presence of superlattice peaks due to antiphase rotation of oxygen octahedra in the rhombohedral phase until the transition to the cubic phase precludes the possibility of an intermediate R3m space group, conjectured previously on the basis of an x-ray powder diffraction study. Discontinuous change in the unit-cell volume, tilt angle, and bond lengths at the transition temperature TC, and phase coexistence of R3c and Pm3−m over 100 K range across TC, suggest that the transition is of first order. It is argued that the R3c to Pm3−m phase transition is of trigger type involvi...

Evidence for monoclinic distortion in the ground state phase of underdoped La1.95Sr0.05CuO4: A single crystal neutron diffraction study

The existing controversy about the symmetry of the crystal structure of the ground state of the critical doped La1.95Sr0.05CuO4 has been resolved by analyzing the single crystal neutron diffraction data collected between 5 and 730 K. We observed small but significant intensities for “forbidden” reflections given by extinction rules of the orthorhombic Bmab space group at low temperatures. A careful investigation of neutron diffraction data reveals that the crystal structure of La1.95Sr0.05CuO4 at 5 K is monoclinic with B2/m (2/m 1 1) space group. The monoclinic structure emerges from the orthorhombic structure in a continuous way; however, the structure is stable below ∼120 K which agrees with other observed phenomena. Our results on symmetry changes are crucial for the interpretation of physical properties also in other high temperature superconductors with similar structures.

Influence of powder processing procedure on the superconducting behaviour of YBa2Cu3O7−y ceramics

In order to synthesize YBa2Cu3O7−y powder, we first coprecipitate yttrium and barium by adding ammonium carbonate to a solution of barium and yttrium chlorides mixed in the 2:1 ratio at the molar level. Ceramics, prepared from powders obtained by calcining the coprecipitated powder with copper oxide, exhibits a very sharp drop in resistivity near 110 K with Tcmidpoint of 107 K and a resistivity anomaly at 260 K.

HEIMDAL: A novel materials science neutron powder diffractometer at ESS

Developing new materials is the most challenging task for the future demands due to limitations in energy, resources but also environmental damage. Improvements in material performances are reached for example by the incorporation of advanced ceramics and polymers into heterogeneous systems. Their performances usually depend on the interplay between properties defined by the atomic, nano/mesoscopic and microscopic structure. In-situ and in-operandi investigations will be in the focus of such investigations. The instrument HEIMDAL proposed for the European spallation neutron source ESS will offer here perfect prospects, as the instrumental resolution of this powder diffractometer can widely be adapted taking either full advantage of the broad pulse of ESS (2.86ms) offering highest intensity, or using a fraction of the pulse for highest resolution. A thermal and a cold guides pointing to the same virtual source extend the spatial window of the instrumental from an atomic scale (0.3Å-1≤Q≤50Å-1) to a nano/meso scale, 0.002Å-1≤Q≤0.1Å-1 by adding a narrow-band SANS instrument behind. Our chopper system allows switching the different operation modes electronically. Traditionally such structural information has been collected in separated experiments such as powder diffraction (PD), wide angle diffraction scale, small angle diffraction and direct space imaging techniques (sub-micronic to millimeter scale), whereas HEIMDAL can offer these options in its final stage at the same time at therefore for absolutely identical experimental conditions. The top-loading geometry foreseen not only accepts auxiliary from the ESS pool (cryogenics, pressure cells, magnets) but also allows implementing bulky brought-in user equipment. It can be pretested off-line at the instrument, but already fully connected to the HEIMDAL electronics. Figure 1: a) Operation modes of the HEIMDAL b) Layout of the instrument HEIMDAL