Single-phase insulating BiFeO3 ceramics have been synthesized by a simple but effective method that conventional solid state reaction is followed immediately by quenching processing. At room temperature, the ceramics show a metastable, distorted rhombohedral phase and the refined structure parameters are presented based on x-ray diffraction. It is revealed that the formations of Fe2+ and oxygen deficiency are greatly suppressed by the quenching processing. Awell-saturated ferroelectric hysteresis loop with a large remnant polarization (2Pr=23.5μC/cm2) is observed with an applied field of 155kV/cm. Temperature-dependent magnetic property is investigated and weak ferromagnetism with a remnant magnetization of 4×10−6μB/Fe at 10K is established. These results may have implications for further studies on multiferroics.

Negative refractions of acoustic waves in a two-dimensional (2D) sonic crystal were studied both experimentally and theoretically. By calculating the acoustic band structure and equifrequency surfaces, we theoretically analyzed the acoustic single-beam negative refraction in the first band operating in the ultrasonic regime. A 2D square sonic crystal was constructed with 2.0-mm-diam steel cylinders arranged as square arrays of 2.5mm lattice constant. A scanning transmission measurement of spatial distribution was carried out to establish the acoustic refraction of a single Gaussian beam. It was demonstrated that the negative refraction is strongly dependent on both frequencies and incident angles, which shows great potential in acoustic devices.

The phonon-polariton dispersion relation is calculated in the piezoelectric superlattice (PSL). In this specific case, the coupling could take place not only between the electromagnetic (EM) waves (photons) and the transverse superlattice vibrations (transverse phonons), but also between the EM waves and the longitudinal superlattice vibrations (longitudinal phonons), which gives rise to two types of phonon polaritons. It differs with the case in an ionic crystal, where the phonon polariton could only originate from the coupling between transverse lattice vibrations (transverse optical phonons) and EM waves. The two types of phonon polaritons are experimentally observed in the one-dimensional PSL with a periodicity of micrometer scale, corresponding to the frequency region of microwave. The PSL is based on a congruent lithium tantalite (LiTaO3) crystal and fabricated by the method of electric field poling at room temperature. Both microwave absorption and dielectric abnormality are observed and the sizes of band gaps are measured for the two types of phonon polaritons.

Acoustic negative refractions with backward-wave (BW) effects were both theoretically and experimentally established in the second band of a two-dimensional (2D) triangular sonic crystal (SC). Intense Bragg scatterings result in the extreme deformation of the second band equifrequency surface (EFS) into two classes: one around the K point and the other around the Г point of the reduced Brillouin zone. The two classes can lead to BW negative refractions (BWNRs) but with reverse negative refraction dependences on frequencies and incident angles. Not only BWNR but BW positive refraction can be present at EFSs around the K point, so it is possible to enhance the resolution of acoustic waves with a subdiffraction limit regardless of refractions, which is no analogy in both left-handed material and SCs' first band. These abundant characters make refractions in the second band distinguished.