The symmetry of ferroelectric Bi4Ti3O12 is shown to be monoclinic point group m. The spontaneous polarization PS is approx 50 μC/cm2 at 25°C and lies in the monoclinic a-c plane at an angle of less than 5 deg from the plane of the crystal sheets. A complete determination of the optical indicatrix as a function of temperature is given. Switching for fields along the c axis occurs by a unique ``rocking'' of the large PS vector and leads to interesting electrical-optical behavior. The domain structure is quite complex, and observed wall orientations are compared with those predicted theoretically.

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Optical and electrical measurements on thin sections of Bi4Ti3O12 in orientations not previously measured have yielded the following results. The symmetry is monoclinic, point group m. The spontaneous polarization Ps is greater than 30 μC/cm2 and lies in a direction tilted slightly from the major crystal surface. Switching, with fields perpendicular to the major crystal surface, occurs by a small ``rocking'' of this large Ps vector. The large component parallel to the major crystal surface can also be switched with high fields. Thus, there are four possible spontaneous polarization directions in the virgin crystal giving rise to complex domain structures.

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