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Temperature- and electric (E)-field-dependent domain structures and phase transformations in a (001)-cut Pb(Mg1/3Nb2/3)0.6Ti0.4O3 (PMNT40%) single crystal have been investigated by polarizing microscopy and dielectric permittivity. The unpoled crystal has tetragonal phase domains (15–60 μm wide) with polarizations along [100] and [010], which are separated by 90° domain walls. They coexist with a small fraction of the monoclinic phase domains near 200 K. The tetragonal domains increase rapidly at the expense of the monoclinic domains as the temperature increases toward room temperature. This may account for the steplike dielectric anomaly in [variant_greek_epsilon]′ from around 280 K. The whole crystal reaches the cubic phase near 464 K. The E-field-dependent domain structures at room temperature show a significant change near 11 kV/cm when the polarizations rotate to the tetragonal [001] (T001) direction through the monoclinic phase. Microcrackings begin to develop near 12.5 kV/cm. The entire crystal becomes T001 monodomain near 33 kV/cm. After the E field is removed, the crystal does not reestablish the T100 and T010 macrodomains, but instead breaks up into tetragonal microdomains. [ABSTRACT FROM AUTHOR]

Polarization rotations of temperature- and electric-field-dependent phase transformations in a (211)-cut Pb(Mg1/3Nb2/3)0.69Ti0.31O3 (PMNT31%) single crystal were observed by means of polarizing microscopy. Dielectric permittivity was also measured as a function of temperature and frequency. Below 380 K domains are mostly rhombohedral (R) phase mixed with monoclinic (M) domains. As temperature increases above 380 K, polarizations of the rhombohedral domains begin to rotate toward the cubic (C) phase through monoclinic distortions, i.e., R→M→C, associated with a long-range percolation of monoclinic phase near 380 K. However, the tetragonal phase was not observed apparently. Electric-field-dependent domain structures show that a long-range R→M phase transformation takes place in the range of 3–5 kV/cm as electric field increases. The crystal cannot reach a monodomain as the electric field reaches 26 kV/cm. A hysteresis loop of polarization versus electric field was also measured for comparison with electric-field-dependent observation of polarizing microscopy. [ABSTRACT FROM AUTHOR]

This work demonstrates the critical role of orbital hybridizations in the FeO6 octahedral distortion, composition-driven phase transition, and bonding covalency in multiferroic (Bi1-xSmx)FeO3 (x = 0.10-0.20) ceramics in the vicinity of the morphotropic phase boundary (MPB). Sequential composition-driven transitions from the polar rhombohedral R3c to antipolar orthorhombic Pbam and then Pnma phases were revealed as the system crosses the MPB with increasing Sm. A coexistence of ferroelectric (FE) rhombohedral R3c and antiferroelectric (AFE) PbZrO3-like orthorhombic Pbam symmetries was identified by the 1/2{000}, 1/4 {100}, 1/4{110}, 1/4{111}, and 1/4{121} superlattice diffractions at x = 0.12-0.16. In addition to R3c and Pbam space groups, the nonpolar SmFeO3-like orthorhombic Pnma space group becomes the predominant phase at x = 0.20 confirmed by the 1/2{100} superlattice diffractions. The Fe L3-edge and oxygen Kedge synchrotron X-ray absorptions indicate that the O 2p-Fe 3d and O 2p-Bi 6s/6p orbital hybridizations were decreased as the system approaches the MPB. [ABSTRACT FROM AUTHOR]

Phase transition, phonon vibration, and magnetization in (100) BiFeO 3 (BFO) multiferroic single crystals have been characterized as functions of temperature. The BFO single crystal exhibits a weak ferromagnetic hysteresis loop possibly due to appearance of ferromagnetic Fe O Fe orderings and a modulation of the Fe O Fe bond. Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities suggest a weak spin-glass behavior below ∼200 K. High-resolution transmission electron microscopy (HRTEM) suggests a rhombohedral R 3 c symmetry (with tilted FeO 6 octahedra) and lattice distortion in the surface layer. The BFO crystal undergoes a structural transition sequence of rhombohedral−tetragonal−orthorhombic− cubic at ∼620–640, ∼1000, and ∼1100 K, respectively. The spin-lattice coupling accounts for the rhombohedral tetragonal transition upon approaching the Néel temperature (T N ). The low-frequency Raman-active modes ψ 4,II , E (1), A 1 (1), and A 1 (2) reveal clear softening anomalies in frequency shifts and broad peaks in full-width-at-half-maximum (FWHM) around 450 K, which is attributed to the Fe O Fe ordering transition without change of structural symmetry. [ABSTRACT FROM AUTHOR]

The Landau free energy expansion as a function of three-dimensional polarization for perovskite relaxor ferroelectrics is examined to find effects of large electric fields in causing transitions from one ferroelectric state to another. Our examination of field-induced effects includes a fourth order expansion to obtain the Landau parameters for the observed temperature- and field-induced rhombohedral (R) [image omitted] tetragonal (T) transitions in an (001)-cut 0.70Pb(Mg1/3Mn2/3)O3-0.30PbTiO3 (PMN-30%PT) crystal. These parameters enabled us to predict the appearance of an orthorhombic (O) phase in (110)-cut PMN-30%PT crystals under an applied field. These results show that also for field-induced effects, the Landau expansion shows qualitative agreement with experiment for relaxor ferroelectrics. [ABSTRACT FROM AUTHOR]

Glass coating is an important thermal insulation technique used in the aerospace industry that serves as a thermal protection system (TPS) to protect the inner layer at elevated temperatures. This work highlights the superb mechanical and thermal properties of the B 2 O 3 -SiO 2 glass systems as a surface coating for space shuttle. In the B 2 O 3 -SiO 2 system, the glass transition temperature raises from 650 °C to 750 °C with increasing SiO 2 content and devitrification of the α-cristobalite phase. The Al 2 O 3 addition in B 2 O 3 -SiO 2 glass can increase the glass transition temperature from 750 °C to 1000 °C and inhibit the α-cristobalite phase. The phase transition from an unstable α-cristobalite phase to an amorphous phase was achieved at 10 wt% Al 2 O 3 -added specimens. This study demonstrates that the Al 2 O 3 -B 2 O 3 -SiO 2 glass (ABS glass) exhibits thermally stable behavior, making it an effective thermal barrier coating at elevated temperatures. [ABSTRACT FROM AUTHOR]

Phase transformation and bloating behavior of CaMgSi 2 O 6 glass-ceramics with ZrO 2 nucleating agent using two-stage heat treatments were investigated as functions of annealing temperature. The specimens annealed below critical nucleation temperature (T cn ) and sintered at 950 °C exhibit significant bloating phenomena. Kinetic study of CaO-MgO-SiO 2 -ZrO 2 (CMSZ) glasses annealed below T cn reveals high activation energy of crystallization for the ZrO 2 nucleation, and XRD refinement results show the least ZrO 2 contents. The formations of fleeing oxygen and bloating are likely associated with high energy barrier of ZrO 2 phase and zirconia dissolution in the CMSZ glass. Specimens annealed above the T cn and sintered at 950 °C show significant reduction of activation energy of ZrO 2 and enhanced densities. This work demonstrates that the bloating behavior of diopside glass-ceramics is attributed to the activation energy of nucleating agents. [ABSTRACT FROM AUTHOR]