In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn 3 and Mn 4 via a double-exchange mechanism. The presence of the ZnO around MnO 2 modifies the kinetics of MnO 2 ! Mn 2 O 3 reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.

Ti-doped BiFeO3 ceramics prepared by a mixed-oxide route were structurally characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM), giving evidence of the formation of an inner structure at the nanometric scale. The observed nanograins are separated by Ti-rich areas that originate due to the tendency of the titanium dopant to segregate from the perovskite lattice. Such a peculiar nanostructure is responsible for the changes produced in both the electrical and the magnetic properties of BiFeO3 upon titanium doping: the Ti-rich interfaces act as resistive layers that increase the direct-current (dc) resistivity of the material, while the existence of structural domains in the scale of tens of nanometers causes a ferrimagnetic-like behavior with a huge coercive field (on the order of 20 kOe), even at room temperature.

The obtaining of multiferroic BiFe0 3 as a puré single-phase product is particularly complex since the formation of secondary phases seems to be unavoidable. The process by which these secondary impurities are formed is studied by analyzing the diffusion and solid state reactivity of the Bi 2 03-Fe 2 03 system. Experimental evidence is reported which indicates that the progressive diffusion of Bi 3+ ions into the Fe 2 0 3 particles governs the solid state synthesis of the perovskite BiFe0 3 phase. However a competition is established between the diffusion process which tends to complete the formation of BiFe0 3 , and the crystallization of stable Bi 2 Fe 4 0 9 mullite crystals, which tend to block that formation reaction.

High-temperature piezoelectric ceramics based on W 6+doped Bi 4 Ti 3 O 12 (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800°and 1150°C in air. A rapid densification, >99% of the theoretical density ( th ) at 900°C/2 h, took place in the chemically prepared W 6+ -doped Bi 4 Ti 3 O 12 ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of th , at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W 6+ -doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO 2 produced a further decrease of the electrical conductivity.

Aurivillius oxides family has attracted great interest in the last years due to their promising electrical properties as high temperature piezoelectric materials. Piezoelectric materials that could operate in extreme conditions of use (elevated temperatures and hostile environments) could be of particular interest for different technological applications. Furthermore, increasing concerns for environmental issues have promoted the study of new lead-free piezoelectric materials. This article examines how the development of these materials has taken place from their discovery at the end of the 40's and the existing knowledge about their processing and properties.