Interface Structure and Dynamics in Multiferroic Phase Transformations

非技术描述：基于结构和化学复杂的金属氧化物的新兴电子陶瓷的功能特性非常强烈地依赖于这些化合物的晶体结构。最近发现的一系列材料都具有竞争稳定的相，具有不同的晶体结构，并且可以在相间重复和快速转换。这些结构阶段在它们的性质上有很大不同；电子和光学设备中的新功能是通过在阶段之间切换来实现的。目前，这些相变的详细结构机制，包括相间边界的运动，尚不清楚。该项目包括一系列实验研究，旨在探索这种相变的动力学，回答涉及的物理机制的关键问题，并最终为改进的电子陶瓷的设计提供反馈。技术细节：单相复合氧化物铋铁氧体，一种典型的多铁性复合氧化物，可以在具有不同对称性、光学性质和磁性结构的结构相之间重复转换。Evans教授将重点放在这个模型系统上，因为可以使用外延生长技术(如离子溅射、脉冲激光沉积和化学气相沉积)可靠地合成铋铁氧体，使其接近相界，在那里相变可用于机电、光电和磁性设备。埃文斯的初步结果表明，铋铁氧体可以在短至数十纳秒的时间尺度上在结构相之间重复转换，但这些研究没有足够的时间分辨率来探索转换的基本时间尺度。通过这个项目，埃文斯将(1)利用皮秒尺度的光激励和少数纳秒分辨率的电场脉冲来确定铋铁氧体中菱形(类R)和四方(T)相群体的短时动力学，(2)使用X射线纳米束衍射来探测单个R类/T类相界面的应变分布和结构，(3)研究外加电场中单个R类/T类界面的结构和动力学，以确定不同界面结构引起的动力学差异，以及(4)探索这些概念对相关相变复合氧化物的界面动力学的扩展。埃文斯的工作使用了新兴的同步辐射X射线纳米衍射方法，包括在分析由高度会聚的相干X射线光束获得的衍射图方面的最新进展。该项目支持研究生的专业准备、本科生参与研究、通过开发和示范教育活动进行有针对性的推广，以及向更广泛的研究界传播材料表征和X射线纳米束衍射法。

NON-TECHNICAL DESCRIPTION: The functional properties of emerging electronic ceramics based on structurally and chemically complex metal oxides depend very strongly on the crystal structures of these compounds. A series of recently discovered materials have competing stable phases with different crystallographic structures and can be reproducibly and rapidly transformed between phases. These structural phases differ significantly in their properties; new functionality in electronic and optical devices is enabled by switching between phases. At present, the detailed structural mechanism of these phase transformations, including the motion of the boundaries between phases, is not clear. This project comprises a series of experimental studies designed to probe the dynamics of this phase transformation, to answer key questions about the physical mechanism involved, and ultimately to provide feedback to the design of improved electroceramics.TECHNICAL DETAILS: The single-phase complex oxide bismuth ferrite, a prototypical multiferroic complex oxide, can be reproducibly transformed between structural phases with different symmetries, optical properties, and magnetic structures. Prof. Evans focuses on this model system because bismuth ferrite can be reliably synthesized using epitaxial growth techniques (e.g., ion sputtering, pulsed laser deposition, and chemical vapor deposition) such that it is near a phase boundary where phase transitions can be utilized in electromechanical, optoelectronic, and magnetic devices. Evans's initial results show that bismuth ferrite can be reproducibly transformed between structural phases on timescales as short as tens of nanoseconds, but these studies do not have sufficient time resolution to probe the fundamental timescale of the transformation. Through this project, Evans will (1) determine the short-time dynamics of rhombohedral behaviour (R-like) and tetragonal behaviour (T-like) phase populations in bismuth ferrite using picosecond-scale optical excitation and few-nanosecond-resolution electric field pulses, (2) probe the strain distribution and structure of individual R-like/T-like phase interfaces using X-ray nanobeam diffraction, (3) study the structure and dynamics of individual R-like/T-like interfaces in applied electric fields to determine the difference in dynamics arising from differing interface structures, and (4) explore the extension of these concepts to the interface dynamics in related phase-transforming complex oxides. Evans's work uses emerging synchrotron X-ray nanodiffraction methods, including recent advances in the analysis of diffraction patterns acquired with highly convergent coherent X-ray beams. This project supports the professional preparation of graduate students, participation of undergraduate students in research, targeted outreach through the development and demonstration of educational activities, and the dissemination of materials characterization and X-ray nanobeam diffraction methods to the broader research community.