FORMATION OF THE STRESS-INDUCED MM2 PHASE AT THE FERROELASTIC-ANTIFERROELECTRIC (4)OVER-BAR-3M-(4)OVER-BAR-2M PHASE-TRANSITION IN CR-CL BORACITE

Experimental and crystallographic investigations of the formation of t he mm2 phase induced by internal stress at the <(4)over bar 3m>-<(4)ov er bar 2m> phase boundaries during the first-order ferroelastic-antife rroelectric phase transition in Cr3B7O13Cl boracite crystals are prese nted. Different possibilities are demonstrated, by means of the determ ination of zero-net-strain planes, for the transformation from the tet ragonal or the cubic phase to the orthorhombic one. The formation of t he mm2 phase results in stress relief at the mechanically non-matching <(4)over bar 3m>-<(4)over bar 2m> interfaces. Such a compromise role of the mm2 phase permits us to understand the mechanism of the stress- induced phase transition and the three-phase coexistence.