Abstract
The anticipated existence of two mechanisms of ferroelectricity in Ba5Ti2O7Cl4 is now confirmed by studying the variation of spontaneous polarization with temperature by hysteresis loop method, by observing the changes in domain structure, and by using thermoanalytical methods. It is found that at around 80°C the spontaneous polarization begins to fall off rapidly, and this fall continues upto 125°C after which the change in Ps is negligibly small. Also, 79°domains associated with Ti4+ ion begin to disappear at 80°C while 180° and 90° domains associated with Cl1− ion persist beyond 200°C. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) data showed ionic changes at 80°C and 125°C. The dc conductivity measurements showed, however, no phase change in this temperature range. From the observed domain structure variation and Ps variation it is inferred that the ionic changes given by DTA and DSC relate to Ti4+ ion only. It is deduced that at 80°C there is change in Ti4+ displacement from [110] direction to [100] direction, while at 125°C the Ti4+ ion loses its off-center position resulting in the loss of ferroelectric mechanism associated with Ti4+ ion. Since 90° and 180° domain associated with Cl1− ion continue to persist beyond 125°C, it is obvious that this ferroelectric mechanism continues to exist even as the other ferroelectric mechanism disappears at 125°C.
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