Experimental and crystallographic investigations of the formation of the mm2 phase induced by internal stress at the - phase boundaries during the first-order ferroelastic - antiferroelectric phase transition in boracite crystals are presented. Different possibilities are demonstrated, by means of the determination of zero-net-strain planes, for the transformation from the tetragonal or the cubic phase to the orthorhombic one. The formation of the mm2 phase results in stress relief at the mechanically non-matching - 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.