Using the HAUP-type universal polarimeter and the Senarmont technique, detailed crystal optical studies of Li2B4O7, lithium tetraborate, are carried out. It is shown that the optical indicatrix rotation and the optical activity are absent from the crystal, in accordance with symmetry considerations. Measurements of optical birefringence reveal the existence of a regular staircase-like temperature behaviour in the whole range under investigation (290-480 K), a hysteresis character of the birefringence under cycling temperature and a pronounced thermooptical memory effect. The origins of the above phenomena are analysed, in particular the possible influence of the pyroelectric effect and systematic errors of the optical equipment. A conclusion is drawn that the main features of the birefringence are well explained by an incommensurately modulated super-structure which is at present a matter of debate. The peculiarities of the optical properties of lithium tetraborate are compared with those of incommensurate crystals known from the literature.

The circular birefringence related to the gyration components along and perpendicular to the optical axis and the circular dichroism of tetragonal a-ZnP 2 single crystals are determined in the temperature range 290-450 K, using universal null-polarimetric techniques. The oscillatory temperature behaviour of the optical rotatory power and the ellipticity of light passed through the crystal in the (001) direction is quantitatively well explained by the influence of multiple reflections of light. The observed thermal hysteresis and dependence of the gyration on sample prehistory testify indirectly the availability of incommensurately modulated structure in a-ZnP 2 .