LIGHT REFRACTION AT THE INTERFACE BETWEEN COEXISTING PHASES OF KHCO3

Polarizing microscope observations of ferroelastic domains which coexi st 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 bou ndary are predicted in the frame of the theory of Domain-wall orientat ions in ferroelastics assuming that the domains are triclinic. Specifi c light refraction experiments at this interface are carried on and an alytical expressions between the angles of incidence and of refraction are established using crystal optics. From measurements of these angl es, 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 sh ows that the boundary corresponds to a ferroelastic Domain-wall of the W'-type and that the observed domains well consist of a triclinic pha se.