FIZIKA A 8 (1999) 1 , 25-34



Department of Physics, Faculty of Science, Alexandria University, Egypt

Received 6 July 1998 ; revised manuscript received 7 December 1998
Accepted 6 July 1999

Studies of the optical and electrical (ferroelectric, electrical conductivity) properties of non- and X-ray-irradiated pentakis undecabromodibismuthate (PMABB) orthorhombic crystals were made. The crystal is anisotropic, weakly positively birefringent and optically biaxial with the optical angle 2V = 72  15. The temperature dependence of linear birefringence D(Dnc) in the temperature range from 295 K to 320 K shows an anomaly at the phase-transition temperature Tc = 312 K, in agreement with the published result. The spontaneous polarization Ps seems slightly large in comparison with those of other ferroelectrics, owing to the strengthened packing along the polar axis. The results show that the microscopic mechanism driving the phase transition has the same influence on the spontaneous birefringence ds(Dnc) and on Ps in the compound. Effects of irradiation on the refractive indices and their dispersions as well as on the optical anisotropy and optical birefingence are studied. The coercive field Ec and the bias field Eb build up during the irradiation, while Ps shows no change with the enhanced doses until the threshold irradiation (D0 = 4104 rad) is reached, then it begins to decay in a fairly rapid manner. The results are discussed in terms of the conventional double-minimum potential energy used for the description of the ferroelectric mechanism. Measurements of the electrical conductivity s in the same temperature range show an anomaly at Tc due to the generation of protonic carriers as a result of rotational motion of cations. The maximum s-value at the threshold irradiation D0 is observed and the variation of s in the vicinity of D0 was studied.

PACS numbers: 78.20.Fm, 63.75.+t, 77.80.Bh
UDC 537.874.32, 536.425
Keywords: pentakis undecabromodibismuthate, spontaneous polarization, refractive indices, optical birefringence, electrical conductivity
Copyright by The Croatian Physical Society
For problems or questions please contact