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Temperature-driven and composition-driven phase transformations in a mixed A-site Pb-free solid solution series of (100−x)Na0.5Bi0.5TiO3–xSrTiO3 (0≤x≤40) were investigated by means of x-ray diffraction, dielectric, and Raman scattering techniques over a temperature range of 20–550 °C. The phonon anomalies observed in the room temperature Raman spectra with increasing Sr concentration revealed a morphotropic phase boundary (MPB) at x∼20. The distinct and discontinuous changes in dielectric properties at x∼20 confirmed the existence of MPB. On heating, the Raman spectral features for different compositions displayed substantial changes at temperatures almost identical to the critical temperatures determined by dielectric measurement, such as ferroelectric–antiferroelectric (Td), rhombohedral–tetragonal (TR-T), and antiferroelectric–paraelectric (Tm) phase transition temperature. However, the two phonon anomalies observed at higher temperatures (∼400 and 500 °C for x=10) suggested the coexistence of rhombohedral–tetragonal and tetragonal–cubic phases, respectively. The transition temperatures determined by the Raman and dielectric measurements gradually decreased as Sr concentration increased. The system showed strong relaxor behavior at x≥20 and the dielectric constant (ε) at room temperature was enhanced significantly from ∼500 (x=0) to ∼3800 (x=40) at 1 kHz.

This work reports the temperature-dependent Raman scattering study of mutiferroic BiFeO 3 (BFO) bulk ceramics in a wide temperature range of 93-843 K. The polycrystalline samples are sintered at four different temperatures and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry, differential scanning calorimetry (DSC), and optical microscopy. The microstructure shows remarkable changes in terms of grain size and domain pattern as the sintering temperature increases. The DSC curves show prominent exothermic peaks at 645 K, the antiferromagnetic-paramagnetic phase transition temperature. The Raman spectra of all the four specimens reveal strong anomalies in the vicinity of the Neel temperature, which can be attributed to the multiferroic nature of BFO. The Raman scattering studies also reveal considerable spectral changes at a temperature range of 140-200 K in all the specimens, which can be inferred to a further spin-reorientation transition exhibited in BFO at a cryogenic temperature.

Lead-free piezoelectric (100-x)Na1/2Bi1/2TiO3-xBaTiO3 solid solution ceramics are investigated by Raman spectroscopy and Xray diffraction over a composition range of 0 ≤ x ≤ 10. Distinct changes in the Raman spectra and X-ray diffraction patterns were discussed in the light of a rhombohedral to tetragonal phase transition. The dominant bands in the Raman spectra were analyzed by observing the changes in their respective peak positions, widths and intensities as the composition increased. Based on these results, it is suggested that the morphotropic phase boundary in the studied system lies at the composition of x ≈ 5.5. This is further evidenced by the X-ray diffraction analysis.

The coarsening behavior of 0.95Na1/2Bi1/2TiO3–0.05BaTiO3 (mole fraction, NBT–5BT) grains has been investigated with respect to the grain shape and compared with a calculation based on crystal growth theories. The powder compacts of NBT–5BT were sintered at 1200°C for various times up to 96 h. The shape of growing grains was a cube with flat {100} faces and rounded edges. The coarsening behavior during sintering up to a few hours was quite normal. However, with an increased sintering time, the relative grain size distribution broadened and apparently abnormal grains appeared. The abnormal grain coarsening behavior intensified and then decreased during extended sintering. This result shows that the coarsening behavior of partially faceted grains changes continuously with sintering time. Grain size distributions were also calculated under the assumption that the growth of grains was governed by the growth of facet planes with a low step free energy. The calculation showed the broadening of relative size distribution, the formation of abnormal grains, and then eventually narrowing of the distribution. This agreement between the measured and calculated distributions suggests the growth of round‐edged polyhedral grains to be governed by the growth of facet planes, which can occur via step formation and growth on the facets.