a-axis Contribution to the Polarization Value in (117)/(200)-Oriented Bi3.75La0.25Ti3O12 Thin Films Deposited on Pt(111)/Ti/SiO2/Si Substrate

Abstract
The $a$-axis contribution to the polarization value inBi$_{3.75}$La$_{0.25}$Ti$_3$O$_{12}$ (BLT0.25) thin films depositedon Pt (111)/Ti/SiO$_2$/Si was investigated. The films were preparedby using a chemical solution deposition method. In order to examinethe internal vibration modes of the BLT0.25 thin films, we employedRaman spectroscopy at room temperature with a 514.5 nm Ar$^+$ laseras the excitation source. The films annealed at 750 $^circ$C and800 $^circ$C have preferentially (117) and (200) orientations andthe proportion of grains with those two orientations in the filmswas estimated from the X-ray peak intensity. The intensity of the(117) peak increases (over 3.5 times) and the peak position isshifted to lower 2$heta$ angles, largely reflecting in theincreases of the number of (117)-oriented grains and in the latticeconstant $c$ as the annealing temperature increases. The increase inthe lattice constant $c$ can also be supported by the enhancementsof the 566 and the 618 cm$^{-1}$ Raman peaks, which demonstrates anenlargement of TiO$_6$ symmetric stretching mode. In addition, thelattice constant $a$ increases from 5.3675 AA~to 5.3957 AA~as theannealing temperatures is increased from 750 $^circ$C to 800$^circ$C, in spite of the fact that the two polarization curves arequite similar. Thus, the dominant axis contributing to the totalpolarization value is the $a$-axis.
Keywords: Ferroelectric, Bi$_{3.75}$La$_{0.25}$Ti$_3$O$_{12}$, Polarization, $P$-$E$ hysteresis loop, Raman shift

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