Rohini Garg

Long ranged structural modulation in the pre-morphotropic phase boundary cubic-like state of the lead-free piezoelectric Na1/2Bi1/2TiO3-BaTiO3

The nature of the pre-morphotropic phase boundary (MPB) cubic-like state in the lead-free piezoelectric ceramics (1-x)Na1/2Bi1/2TiO3-(x)BaTiO3 at x similar to 0.06 has been examined in detail by electric field and temperature dependent neutron diffraction, x-ray diffraction, dielectric and ferroelectric characterization. The superlattice reflections in the neutron diffraction patterns cannot be explained with the tetragonal P4bm and the rhombohedral (R3c) phase coexistence model. The cubic like state is rather a result of long ranged modulated complex octahedral tilt. This modulated structure exhibits anomalously large dielectric dispersion. The modulated structure transforms to a MPB state on poling. The field-stabilized MPB state is destroyed and the modulated structure is restored on heating the poled specimen above the Vogel-Fulcher freezing temperature. The results show the predominant role of competing octahedral tilts in determining the nature of structural and polar states in Na1/2Bi1/2TiO3-based ferroelectrics

Degenerate rhombohedral and orthorhombic states in Ca-substituted Na0.5Bi0.5TiO3

Neutron powder diffraction and temperature dependent dielectric studies were carried out on Ca-substituted Na0.5Bi0.5TiO3, i.e., (Na0.5Bi0.5)(1-x)CaxTiO3. Stabilization of an orthorhombic phase even at a low Ca concentration (0.05 < x < 0.10) suggests that Na0.5Bi0.5TiO3 (NBT) is susceptible to orthorhombic distortion. The orthorhombic and rhombohedral phases coexist for x=0.10, suggesting these phases to be nearly degenerate. The orthorhombic distortion favoring tendency of Ca assists in promoting the inherent instability with regard to this structure in pure NBT, which was reported recently.

Evolution of texture and grain boundary microstructure in two-phase (α + β) brass during recrystallization

The evolution of texture and microstructure during recrystallization is studied for two-phase copper alloy (Cu–40Zn) with a variation of the initial texture and microstructure (hot rolled and solution treated) as well as the mode of rolling (deformation path: uni-directional rolling and cross rolling). The results of bulk texture have been supported by micro-texture and microstructure studies carried out using electron back scatter diffraction (EBSD). The initial microstructural condition as well as the mode of rolling has been found to alter the recrystallization texture and microstructure. The uni-directionally rolled samples showed a strong Goss and BR {236}⟨385⟩ component while a weaker texture similar to that of rolling evolved for the cross-rolled samples in the α phase on recrystallization. The recrystallization texture of the β phase was similar to that of the rolling texture with discontinuous ⟨101⟩ α and {111} γ fiber with high intensity at {111}⟨101⟩. For a given microstructure, the cross-rolled samples showed a higher fraction of coincident site lattice Σ3 twin boundaries in the α phase. The higher fraction of Σ3 boundaries is explained on the basis of the higher propensity of growth accidents during annealing of the cross-rolled samples. The present investigation demonstrates that change in strain path, as introduced during cross-rolling, could be a viable tool for grain boundary engineering of low SFE fcc materials.

Phases in the (1 - x)Na0.5Bi0.5TiO3-(x)CaTiO3 system

The structures of (1 - x)Na(0.5)Bi(0.5)TiO(3)-(x)CaTiO(3) at room temperature have been investigated using neutron powder diffraction and dielectric studies. The system exhibits an orthorhombic (Pbnm) structure for x ≥ 0.15 and rhombohedral (R3c) for x ≤ 0.05. For x = 0.10, though the neutron diffraction pattern shows features of the orthorhombic (Pbnm) structure, Rietveld refinement using this structure shows a drastic reduction in the in-phase tilt angle (∼4°) as compared to the corresponding value (∼8°) for a neighbouring composition x = 0.15. The neutron diffraction pattern of x = 0.10 could be fitted equally well using a two-phase model (R3c + Pbnm) with orthorhombic as the minor phase (22%), without the need for a drastic decrease in the in-phase tilt angle. The dielectric studies of x = 0.10 revealed the presence of the polar R3c phase, thereby favouring the phase coexistence model, instead of a single-phase Pbnm structure, for this composition.

Competing A-site and B-site driven ferroelectric instabilities in the (1−x)PbTiO3-(x)BiAlO3 system

The system (1-x)PbTiO3-(x)BiAlO3 has been investigated with regard to its solid solubility, crystal structure, microstructure, and ferroelectric transition. The unit cell volume and the tetragonality exhibit anomalous behavior near x=0.10. The Curie point (T-C) of PbTiO3 was however found to be nearly unchanged. The study seems to suggest that the decrease in the stability of the ferroelectric state due to dilution of the Ti-sublattice by smaller sized Al+3 ions is compensated by the increase in the ferroelectric stability by the Bi+3 ions.

Structure and phase transition of Na0.5La0.5TiO3

A temperature dependent neutron powder diffraction study was carried out on Na0.5La0.5TiO3 (NLT) from 298 K up to 1373 K. Rietveld analysis using high resolution x-ray and neutron powder diffraction data confirmed the rhombohedral structure (space group R\bar {3}c ) of this compound. The rhombohedral phase transforms to cubic (Pm\bar {3}m ) above 873 K. The temperature variation of the tilt angle, a continuous change of the specific volume across the transition, and the strain analysis suggest a second order nature of the cubic–rhombohedral phase transition, which is in conformity with the group theoretical prediction based on symmetry considerations.

Complex structural phase transitions in slightly Ca modified Na0.5Bi0.5TiO3

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na(0.5)Bi(0.5)TiO(3) (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: Cc → Cc + Pbnm → Pbnm + P4/mbm → P4/mbm → PmN3m.

Tendency to promote ferroelectric distortion in Pr-modified PbTiO3

Structural, microstructural, and dielectric studies have been carried out on Pr-modified PbTiO3. A comparative analysis with La-modified PbTiO3 suggests that for chemical modification by same amount, the Pr-modified system has larger tetragonal strain and Curie point. No clear feature of relaxor ferroelectric state is observed for Pr concentration as high as x=0.35, suggesting that Pr modification is less effective, as compared to La-modification, in inducing a relaxor ferroelectric state. Results suggest that inspite of increased chemical disorder, Pr modification partly tends to restore the ferroelectric distortion of the lattice through partial occupancy of the Pr4+ ions on the Ti4+ sites.

Magnetic structure and magneto-elastic-structural coupling in Cr-modified SrRuO3: A neutron powder diffraction study

Temperature dependent neutron powder diffraction and magnetization studies have been carried out on a Cr-substituted SrRuO3 close to the solubility limit, that is, SrRu0.88Cr0.12O3 (SRC-12). Evidence of ferromagnetic ordering below 200 K was deduced from magnetization studies as well as from neutron powder diffraction. The magnetic lattice coincides with the chemical one (k = 0) and the major component of the ordered magnetic moment has been found along the c-axis. The system exhibits anomalous variation in the cell volume close to the magnetic ordering temperature. The spontaneous magnetostrictive strain below the ordering temperature, estimated by fitting the temperature dependence of the lattice parameters and the cell volume using Debye–Gruneisen function, revealed that the main contribution to the volume magnetostriction comes from the c-axis. The magneto-elastic behavior is further shown to be coupled to the octahedral tilt about the c axis. The analysis of magnetic structure and the magneto-elastic...

Interrelationship between Interphase Boundaries and Phase Contents near the Critical Compositions of Lead-Free Ferroelectric (Na0.5Bi0.5)TiO3−BaTiO3

Heterophase structures in lead-free perovskite-type ferroelectric solid solutions of (1 − z)(Na0.5Bi0.5)TiO3 − zBaTiO3 are analysed for a few critical compositions near the morphotropic phase boundary (z = 0.05−0.07). Examples of the phase coexistence and elastic matching of the phases from different symmetry groups are considered to find optimum volume fractions of specific domain types and coexisting phases at the complete stress relief in two-phase samples. Some interrelations between these volume fractions are described using variants of the domain arrangement at changes in the composition and unit-cell parameters. The evaluated room-temperature volume fractions of the ferroelectric monoclinic (Cm symmetry) and tetragonal (P4mm symmetry) phases near the morphotropic phase boundary are in agreement with experimental data.