Spontaneous birefringence in boracites-measurements and applications

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Abstract
The techniques and results of measurements of the spontaneous birefringence versus temperature are discussed for the fully ferroelectric/fully ferroelastic phases of Mg-Cl, Cr-Cl, Mn-Cl, Mn-Br, Mn-I, Fe-Cl, Fe-Br, Fe-I, Co-Cl, Co-Br. Co-I, Ni-Cl, Ni-Br, Cu-Cl, Cu-Br, Zn-Cl, Zn-Br, Zn-I, Cd-Cl, Cd-Br and Cd-I boracites. Three types of phase sequence occur: 43m↔mm2↔m↔3m (Fe-Cl, Fe-I, Co-Cl, Zn-Cl), 43m↔mm2↔3m (Fe-Br) and 43m↔mm2 (all other compositions). All transitions are of first order. The magnetic ones are excluded from discussion.Crystallophysical and potential technological applications of the spontaneous birefringence are commented:-Light gates (display, page composers, holographic storage): the favorable symmetry species 43mFmmm2, 43mFm, 43mF3m and the high spontaneous birefringence of boracites allow light gate devices with longitudinal electrode configuration. Feasibility studies on single crystalline Fe-I and of epitaxial growth of Ni-Cl boracite show that the development of zero shear compositions is mandatory. Potential ways of their realization are discussed.- The study of the statics and dynamics of domains and of their walls.- The obligatory simultaneous optical control of the sample state during physical measurements, which is considered for the time being as one of the most important applications of spontaneous birefringence of boracites.