Abstract
The ferroelectric single crystal of barium titanate exhibits a strong absorption in the ultra-violet region at any temperature including the three transition points. The threshold for the ultra-violet absorption shifts gradually towards longer wave-lengths as the temperature rises, and the temperature coefficient of it is about one Angstrom per degree centigrade. In addition to this ultra-violet absorption, the crystal exhibits another absorption in the visible region. The intensity of natural light with wave-lengths in the neighbourhood of 5000 A transmitted through the crystal is measured by means of a multiplier phototube. It shows quite steep and sudden changes near the transition temperatures, especially in thin crystals. Let I(T), I(O) and I(R) be the intensity of transmitted light in the tetragonal, orthorhombic and rhombohedral phases respectively. Then the following relations can be drawn out of the various curves for the thermal transition of transparency. They are I(O)\gtrsim\frac12I(T) and I(R)\lesssim\frac14I(T). From calculations on optical paths through the adjacent domains in the orthorhombic phase, the former relation is derived. The same considerations in the rhombohedral phase lead to the latter. Furthermore for polarized light, the behaviours are partially different from those for natural light. In the present paper experimental results for the thermal transition of transparency for both natural and polarized light are interpreted from the standpoints of the theories of ferroelectric domains and crystal optics.
