Polarized light microscopy of ferroelastic domain patterns of orthorhombic perovskite-type B_i-xLa_xFeO₃ (x = 0.50 and 0.57)

Abstract

Flux grown single crystals of the perovskite-orthoferrite system Bi_1−xLa_xFeO₃ with x = 0.50 and x = 0.57, corresponding to space groups C222₁ and Pbn2₁, respectively, have been studied by polarized light microscopy in reflection and transmission (both by orthoscopy and conoscopy). With a view to making magnetoelectric and related measurements on controlled ferroelastic single domains or domain arrangements, the ferroelastic domain structure and the optical indicatrix of the domains have been analysed. In agreement with x-ray data, two of the orthorhombic axes are found to lie along pseudocubic [110]_c-directions. The ferroelastic domains are joined by pseudocubic {100}_c and {110}_c-walls and can be explained on the basis of a cubic prototype with point group m3m. In both phases the optical character is biaxial positive with the plane of the optical axes perpendicular to the orthorhombic c-axis. Elucidation of the ferroelastic domain structure is facilitated due to some properties analogous to certain well known orthorhombic boracites. Strong absorption and strong dispersion of birefringence impede birefringence measurements by classical compensator methods. Therefore Beugnies' method was applied, necessitating path difference measurements over several orders at different wave lengths.