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Perovksite oxides, with chemical formula ABO3 (unit cell structure shown above), can display a variety of fascinating properties, including ferroelectricity, large piezoelectric and dielectric responses, multiferroism, as well as high-temperature superconductivity. Chemical and structural complexity of Perovskite oxide has proved to be the critical factor in producing these properties. First principles approaches, on the basis of density functional theory (DFT), allow accurate, quantitative predictions of material properties using only its chemical composition and structure as input. Therefore, first principle calculation has become a vital tool for the discovery, design and development of multi-functional oxide materials.

 

The main research projects in our group are in the use of first principle methods, combining with phenomenological models, symmetry and crystal chemistry principles to predict electronic and ferroic behavior of bulk and heterostructured Perovskite oxides. We are aiming at exploring the basic mechanism underlying Perovskite oxide behavior as well as designing new functional oxide materials for experiment study.

On-going Research Projects

§   First principle simulation of the piezoelectric Perovskite ceramics

    Piezoelectricity refers to the charge accumulating in certain solid materials (notably crystals, certain ceramics) in response to applied mechanical stress. Pb(Zr,Ti)O3 (PZT) ceramic is widely used in piezoelectric applications owing to its excellent electromechanical properties. It is found that the presence of morphotropic phase boundary (MPB) between the tetragonal (T) and rhombohedral (R) phases (see the Figure below) is crucial for easy polarization rotation and high piezoelectric performance. Therefore, predicting the presence and compositional location of     Piezoelectricity refers to the charge accumulating in certain solid materials (notably crystals, certain ceramics) in response to applied mechanical stress. Pb(Zr,Ti)O3 (PZT) ceramic is widely used in piezoelectric applications owing to its excellent electromechanical properties. It is found that the presence of morphotropic phase boundary (MPB) between the tetragonal (T) and rhombohedral (R) phases (see the Figure below) is crucial for easy polarization rotation and high piezoelectric performance. Therefore, predicting the presence and compositional location of MPB for a Perovskite alloy is the key feature in searching for oxide with large piezoelectricity.

The composition and temperature phase diagram of  Pb(ZrTi)O3 solid solution, and Gibbs free energy profile for PZT with Zr/Ti ratio around morphotropic boundary (MPB). Easy rotation path between R and T phases is given by the thick dashed line. Figures are taken from M. J. Haun, et al, Ferroelectrics 99, 63 (1989); B. Jaffe, et al, Piezoelectric Ceramics (Academic, New York, 1971)

 

   To simulate the piezoelectric ceramic, especially alloyed solid solution with the disordered cation arrangement, our first principle calculations will be performed on various oxide supercells with different cation arrangements that can be considered as local snapshots of the overall disordered structure. Modeling of such a "local structure" in the presence of quenched disorder can provide a genuine description of oxide piezoelectric behavior. Using the above-mentioned computational methodology, we will search for oxide solid solution that can meet the following criteria:

1.          1. Characterizes a phase transition between two ferroelectric phases,

2.          2.  High Curie temperature (TC) - piezoelectricity can be preserved at high temperature,

3.          3. Environmentally friendly - elimination of Pb cation from A-site.

 

§   Coupled phase-transition in oxide thin films and heterostructures

        Perovskite oxides, especially the transition metal oxides with the correlated electrons, can exhibit various fascinating properties. These rich behaviors of transition metal oxides arises from the competing interactions - such as electrostatic effect, geometry frustration, charge and orbital ordering, etc (illustrated in the figure below). Therefore Pervoskite oxides can exhibit the strong coupling between electron, spin, orbital and lattice degrees of freedom, making their macroscopic properties highly susceptibilities to small external perturbations.

 

 

 

Figure is taken and reused from P. Zubko, et al, Annu. Rev. Condens. Matter Phys. 2 141–165, (2011)

 

 

   In recent years, owing to advances in experimental deposition techniques, the research interest of Perovskites has moved from bulk to the coherent oxide thin films, heterostructured superlattices. These advancing experimental techniques afford us additional parameters, such as mechanical epitaxial strain, electronic boundary conditions, to realize the designed functionalities that are usually absent in the bulk component. For instance, colossal magnetoresistance (CMR), metal-to-insulator, or insulator-to-superconductor transitions are observed at layered oxides and interface between the heterostructured superlattices. The coupling between mechanical strain, lattice, electron and spin degrees of freedom at oxide thin films and heterostructures will lead to structural and electronic coupled phase transition.

 

   Using first-principle simulation and symmetry principle, we will explore various phenomenons at ultra-thin oxide films and short-period superlattices. Our research will aim at design of materials exhibiting coupled phase transitions, understanding of the mechanism governing the transition behaviors, and exploit the use of coupled phase transitions for novel electronic devices.

 

The  Our research will be carried out in close collaboration with experimental groups, with the theoretical results guiding the experimental searching materials with designed functionality, and also providing theoretical understanding and support to explain experimental findings.

 

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