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Giant polarization in super-tetragonal thin films through interphase strain


Generally speaking, the physical and chemical properties of functional materials are dependent on the changes of crystal lattices. The regulation of lattice strain of materials can promote significant changes in the physical and chemical properties of materials. Therefore, the regulation of lattice strain is widely used in superconductivity, giant magnetoresistance, multi-iron, catalysis and other fields. Researchers usually use the strain engineering method, which is to introduce a certain strain through the lattice mismatch between the film and the matrix to achieve the purpose of regulating the material properties. However, the lattice mismatch between the film and the matrix cannot be increased without limit, and the range of lattice strain regulation is also limited.

Recently, professor xianran xing's team at Beijing university of science and technology has put forward a new Strain engineering method -- "Interphase Strain", which can be simply realized. The research results, titled "Giant polarization in super-tetragonal thin films through interphase strain", were published in Science on August 3, 2018. Dr. Zhang linxing of the university of science and technology Beijing is the first author, and professors Chen jun and xing xianran are the corresponding authors.

Ferroelectric material is an important functional material, widely used in ferroelectric memory, tunable microwave devices, large-capacity capacitors, piezoelectric sensors and other fields. The basic functional elements of ferroelectric polarization are used to enrich the properties of ferroelectric materials. In fact, there is a novel property hidden in ferroelectrics -- negative thermal expansion, that is, abnormal shrinkage of material volume caused by temperature rise. Negative thermal expansion provides an opportunity to regulate and control the thermal expansion property of materials. In the past two decades, the research team has conducted in-depth research in the field of negative thermal expansion, reported a class of negative thermal expansion system of ferroelectric body, and revealed the mechanism of ferroelectric polarization on negative thermal expansion. Based on the research results of negative thermal expansion ferroelectrics, the team recently proposed a method of "phase interface strain" to regulate the lattice strain of ferroelectrics, thus successfully preparing "super strong" ferroelectrics thin films. The assumption of "phase interface strain" is as follows: for example, if two materials with different lattice constants are formed into an epitaxial film with lattice matching, the materials of the small lattice must be subjected to the tensile stress of the material of the large lattice, and then the huge strain is introduced. The team used ferroelectrics PbTiO3 with different lattice parameters and non-ferroelectrics PbO on the SrTiO3 substrate to form an epitaxial film with a perfectly matched lattice. The study showed that this method improved the lattice distortion of PbTiO3 to c/a = 1.238, while the bulk phase was only 1.065. In addition, this kind of ferroelectric thin films is very stable, stable temperature ferroelectric phase from the phase of raised 490 ℃ to 725 ℃. In a word, this "phase interface strain" method provides a new idea for the design of novel ferroelectrics and can also be applied in the design of other multi-functional materials.

The research work was also assisted and cooperated by the university of Texas at Dallas, tel aviv university, the university of st Andrews, tsinghua university and the Chinese academy of sciences.

          Figure 1  Microstructure of 1 PbTiO3/PbO epitaxial film



   Figure 2 Huge iron electrode and temperature stability