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Transition metal oxides with metastable phases: a way towards superior ferroic properties

Periodic Reporting for period 1 - TransFerr (Transition metal oxides with metastable phases: a way towards superior ferroic properties)

Reporting period: 2017-12-01 to 2019-11-30

The growing market of consumer electronics and related industrial applications require a wide spectrum of cost-effective functional materials with controllable properties. Recent ecological regulations stimulate utilizing novel lead-free materials with physical parameters comparable to or exceeding commonly used lead-based piezoeletrics. Compounds based on transition metal oxides are considered as the most promising eco-friendly functional materials to be used in magnetic memory or energy storage devices, energy transfer moduli, tunable microwave resonators, phase shifters and other electronic devices and sensors forming the actual concepts like Internet of Things, Smart home and others. Additionally, magneto-electric coupling specific for ferrites allows to use them as energy converters and energy harvesting devices. Photovoltaic properties of BiFeO3-based materials disclose novel opportunities in solar cells designing.
Declared properties of the materials studied within the Project (complex oxides with perovskite structure) permit to improve the functionality of existing electrical devices via formation of metastable states in the transition metal oxides. Tunability of their electric, magnetic, transport parameters and magnetoelectric coupling provides an effective method to enhance perspectives of their practical applications where special attention is focused on the magnetic field sensors as the market of magnetic memory storage devices permanently grows.
TransFerr Project aims to design and study novel functional materials with multiferroic properties required in electric applications. The project is focused on the two main approaches to create metastable structural states. The first one concerns the design of the compounds based on novel ceramic compositions via chemical substitution and after-synthesis treatment of bulk samples in order to induce nanoscale structure. The second approach exploits chemical routes for the synthesis of thin films and nanoceramics.
The research will permit to design novel functional materials with pronounced and controllable physical properties. The consortium Partners will increase their knowledge and skills in the area of innovative multifunctional materials through results dissemination and extensive transfer of the experience in complementary subjects. It should be also mentioned that innovation should not stay in the laboratory therefore, the Project will be evolved also in the commercialization process and multi-area trainings for researchers.
The research performed during the first two years of the TransFerr project was focused on the preparation of compounds based on bismuth ferrite in form of nanopowders, thin films, and ceramics using different synthesis methods. The important issue was to prepare single phase materials and to perform characterization of their crystal structure depending on the preparation technique, chemical composition and environment (temperature, electric and magnetic field, etc.). The obtained compounds have also been characterized in terms of their functional properties, the correlation between the crystal structure, morphology, and physical properties depending on the preparation methods.
The problem of obtaining pure phase BiFeO3-based compounds is very widely displayed in the scientific literature. Interest to these materials is related to their unique functional properties, and especially to the changes in these properties that occur when the material's crystal structure is modified. Such changes are particularly dependent on the size of the obtained nanocrystals, which influences the temperature of the transformations and the ways they occur.
During realization of the first tasks, the Project participants have consolidated the knowledge about obtaining nanoscale BiFeO3-based compounds and investigated the influence of chemical (organic matrix composition in the sol-gel method and fuel type in the self-combustion method) and physical (temperature and annealing time in both wet chemistry and solid state synthesis methods) factors on particle size, size distribution, and crystal structure of the obtained materials. Several series of single phase nanoscale BiFeO3 powders with different sizes and with different grain morphology were obtained using different synthesis methods.
Using different synthesis methods several other perovskite systems have also been prepared including: Bi(RE)FeO3–BaTiO3, BiFeO3–Ba(Sr)TiO3, Bi(1-x)RE(x)FeO3 (RE=La, Sm); La(1-x)Bi(x)Fe(Mn)O3. Mainly the research was focused on the compounds having chemical compositions near to the morphotropic phase boundary regions where an improvement of functional properties is observed.
The compounds obtained by different preparation methods have been attested by X-ray and neutron diffraction methods, SEM/TEM, DSC/DTA and other research methods. Analysis of the structure and functional properties of the compounds have been implemented within WP2-5.
First two years of the reporting period were closed with three deliverables: (1) Conference calendar, (2) Report on solid state synthesis, (3) Report on synthesis by sol–gel method, (4) Report on phase stability regions and (5) Report on stoichiometry and crystalline structure. The results of the performed investigations have been denoted in 23 scientific articles published in the high-ranged journals, which can be followed through the Project website: http://transferr.eu/dissemination/.
According to the Project’s proposal, the participants expect to obtain the next scientific results:
- to synthesize the compounds (incl. bulk ceramics, films and nanoscale powders) according to the scientific plan as well as some additional compounds which were not planned in the proposal while a necessity to prepare additional compounds (e.g. BiFeO3-BaTiO3 compounds doped with rare-earth elements) is justified by recent scientific results;
- to find out optimal preparation conditions (incl. post-synthesis treatment procedures) for the compounds based on the iron and manganese oxides having prominent and controllable physical properties, the planned preparation procedures are compatible/adaptive with existing industrial requirements;
- to construct solid and reliable physical models which explain the origin of the improved physical properties of the compounds and can predict the chemical compositions of the materials with optimal functional properties.
Because of certain unexpected but very prominent scientific results (viz. rapid increase in polarization and magnetiс response) the project participants have considered to expand the area of the compounds to be study and decided to prepare composites based on ferrite and alkali halides; these unplanned preparation and measurements procedures will assist to clarify the magnetization changes happened in the upper layer of the ferrite grains because of redox reactions occurred at the interface ferrite - alkali halide. Preliminary results obtained for these systems have been published in Scientific Reports and Acta Materialia journals.
The obtained and planned results of the Project will definitely improve the knowledge in the area of tunable functional materials based on complex oxide systems; the already built and expanding network of scientific and social contacts will contribute to a creation of knowledge-based society which is highly important for young researchers for their career development and perspectives.
Scanning electron microscopy image of Bi0.9La0.1FeO3 nanoparticles and magnetization measurement