Temperature-time study of the magnetoelectric multiferroic (1-x)BiFeO 3 -(x)PbTiO 3 by x-ray and electron diffraction on the reported morphotropic phase boundary (MPB) compositions revealed that this MPB does not correspond to the equilibrium state. The MPB like state is rather of metastable nature and arise due to kinetic arrest of metastable rhombohedral (R3c) phase, along with the equilibrium tetragonal (P4mm) phase. The life time of the metastable R3c nuclei is very sensitive to composition and temperature, and nearly diverges at x ! 0.27. The MPB like state appears only if the system is cooled before the metastable R3c nuclei could vanish. These findings resolve the long standing controversy with regard to seemingly erratic phase formation behaviour reported by different groups and provides a rational basis for developing genuine equilibrium MPB compositions in this system for better piezoelectric properties. V C 2013 AIP Publishing LLC. [http://dx.

A systematic x-ray and neutron powder diffraction study of the giant tetragonality multiferroic (1−x) BiFeO3-(x) PbTiO3 have revealed that the compositions close to the morphotropic phase boundary present two different structural phase transition scenarios on cooling from the cubic phase: (i) cubic → tetragonal (T2) + tetragonal (T1) → tetragonal (T1) and (ii) cubic →  tetragonal (T2) + tetragonal (T1) + rhombohedral (R3c) → tetragonal (T1) + rhombohedral (R3c). The comparatively larger tetragonality (c/a − 1) of the T1 phase as compared to the coexisting isostructural T2 phase is shown to be a result of significantly greater degree of overlap of the Pb/Bi-6s and Ti/Fe-3d with the O-2p orbitals as compared to that in the T2 phase. The formation/suppression of the minor metastable rhombohedral phase seems to be governed by subtle play of local kinetic factors. In the scenario when the minor rhombohedral (R) phase is formed along with the tetragonal phases it is able to accommodate the large transformation stress in the system due to formation of the tetragonal phases, and prevent the solid from disintegration into powder after sintering. When the metastable rhombohedral phase is not formed, the large transformation strain ruptures the grain boundaries leading to fragmentation of the dense solid to powder.

Neutron powder diffraction and temperature dependent dielectric studies were carried out on Ca-substituted Na0.5Bi0.5TiO3, i.e., (Na0.5Bi0.5)1−xCaxTiO3. 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.

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.

A powder neutron diffraction study was carried out on 0.8BiFeO3-0.2PbTiO3 in the temperature range 27–1000°C. The system exhibits magnetic transition at ∼300°C and a rhombohedral (R3c)-cubic (Pm3m) ferroelectric phase transition at ∼650°C. Anomalous variation in the lattice parameters and the octahedral tilt angle is observed across the magnetic transition temperature. In the magnetic phase, the c parameter is contracted and the octahedral tilt angle is slightly increased. The results suggest coupling between the spin, lattice and structural degrees of freedom.