The ferroelastic domains of the orthorhombic phase of YBa2Cu307_ 0 have been observed in polarized light on ceramics and single crystals. By combining polarized light microscopy with the X-ray precession technique, the correlation of the orientation of the orthorhombic a-and b-axes with that of the a-b-plane bireflectance, reflection dichroism, transmission dichroism (at a thickness of about 1 pm), reflection tints generated with compensators and upon uncrossing of polars, as well as the orientation of etch pits has been realized on ferroelastic single domains, bi-domains and more complicated domain patterns. Four ferroelastic orthorhombic domain states have been identified, at variance with former group theoretical considerations, predicting only two states. Ensembles of lamellar domains beyond optical resolution generate strong bireflectance with principal axes rotated by 45 ~ relative to the true a, b-directions.

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