Light refraction at the interface between coexisting phases of KHCO3

Polarizing microscope observations of ferroelastic domains which coexist with the monoclinic phase in as-grown KHCO3 crystals below T-c=318 K are reported. The nature as well as the orientation of the phase boundary are predicted in the frame of the theory of Domain-wall orientations in ferroelastics assuming that the domains are triclinic. Specific light refraction experiments at this interface are carried on and analytical expressions between the angles of incidence and of refraction are established using crystal optics. From measurements of these angles, the orientation of the phase boundary is derived with a precision of about 0.2 degree. The excellent agreement between this orientation and the one predicted from the lattice parameters of the two phases shows that the boundary corresponds to a ferroelastic Domain-wall of the W'-type and that the observed domains well consist of a triclinic phase.