INVESTIGATION OF THE ELECTROGYRATION EFFECT OF FERROELECTRIC PB5GE3O11 BY CIRCULAR-DICHROISM

The circular dichroism of the ferroelectric Pb5Ge3O11 was precisely determined as a function of electric field and temperature in order to clarify the microscopic origin of the electrogyration effect. From the circular dichroic spectrum, which was observed near 410 nm and identified as being due to the intra-ionic 6s --> 6p transition in Pb2+, the sign and magnitude of the rotatory strength could be determined. The rotatory strength exhibited a hysteresis loop under the electric field and its magnitude varied with the spontaneous polarization. The finding confirms our previous proposition that the electrogyration effect originates from the change in the rotatory strength. This conclusion is also examined on the basis of group theory. The optical rotatory power calculated through the Kramers-Kronig relation was compared with the experimental value. The origin of the enormous difference between the calculated and experimental magnitudes is discussed.