Crystal optical studies of lithium tetraborate

Using the HAUP-type universal polarimeter and the Senarmont technique, deta iled crystal optical studies of Li2B4O7, lithium tetraborate, are carried o ut. It is shown that the optical indicatrix rotation and the optical activi ty are absent from the crystal, in accordance with symmetry considerations. Measurements of optical birefringence reveal the existence of a regular st aircase-like temperature behaviour in the whole range under investigation ( 290-480 K), a hysteresis character of the birefringence under cycling tempe rature and a pronounced thermooptical memory effect. The origins of the abo ve phenomena are analysed, in particular the possible influence of the pyro electric effect and systematic errors of the optical equipment. A conclusio n is drawn that the main features of the birefringence are well explained b y an incommensurately modulated superstructure which is at present a matter of debate. The peculiarities of the optical properties of lithium tetrabor ate are compared with those of incommensurate crystals known from the liter ature.