Structures and phase transitions in perovskites - sorting out the subtlelties

Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation
â—‹Christopher J Howard

The perovskite (ABX3) structure comprises a three dimensional corner-linked array of BX6 octahedra with A cations located between. The ideal perovskite is cubic, but most perovskites are distorted variants. The most common distortions are cation displacements and the tilting of BX6 octahedra. Chemical substitutions resulting, for example, in the presence of more than one chemical species on the B site, give rise to further structural variability. Though the distortions may be subtle, the transitions between the different structural variants can result in dramatic changes in physical properties, leading to a variety of applications.

The subtlety of the distortions makes for challenging crystallography. In the approach to be described here [1], group theory is used to enumerate the possible space groups and structures, and to analyse the transitions between them. Computer program ISOTROPY [2] is a valuable aid in this endeavour. Experimental techniques used to elucidate structures include synchrotron-based X-ray diffraction and electron microdiffraction. In studies of temperature-induced phase transitions, the combination of group theoretical analysis with very high resolution neutron powder diffraction patterns recorded (using HRPD at ISIS) at fine temperature intervals has proved particularly effective.

The speaker will outline the methodology, and illustrate with recent applications.

[1] Review article: C J Howard, H T Stokes, Acta Crystallogr. A61, 93-111, 2005
[2] Programs available at: http://stokes.byu.edu/isotropy.html