Polarized light microscopy of ferroelastic domain patterns of orthorhombic perovskite-type Bi₁_xLa_xFeO₃ (x = 0.50 and 0.57)

Flux grown single crystals of the perovskite-orthoferrite system Bi1_xLax/sub>FeO3 with x = 0.50 and x = 0.57, corresponding to space groups C2221 and Pbn21, resp., have been studied by polarized light microscopy in reflection and transmission (both by orthoscopy and conoscopy). With a view to making magnetoelec. and related measurements on controlled ferroelastic single domains or domain arrangements, the ferroelastic domain structure and the optical indicatrix of the domains have been analyzed. 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 m.hivin.3m. 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, the Beugnies method was applied, necessitating path difference measurements over several orders at different wave lengths