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FIZIKA A 8 (1999) 1 , 25-34
 
OPTICAL AND ELECTRICAL PROPERTIES OF NON- AND X-RAY IRRADIATED PMABB
M. GAAFAR and S. M. KHALIL
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 =
4×104 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
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