Abstract
Thermoanalytical investigations are carried out on the flux-grown BaFCl crystals. An endothermic peak at 725 $\ifmmode^\circ\else\textdegree\fi{}$C and 1.2% weight loss in the temperature range from 730 to 860 $\ifmmode^\circ\else\textdegree\fi{}$C is observed in differential thermal analysis and thermogravimetric analysis measurements. Heating rate dependency of the endotherm peak position and that of weight loss onset temperature indicate that these two processes are independent from one another. Endotherm is attributed to the antiferroelectric to paraelectric transition in BaFCl. High-temperature Raman scattering, high-temperature x-ray diffraction (XRD) measurements and thermomechanical analysis show that BaFCl has no structural transition from 25 to 800 $\ifmmode^\circ\else\textdegree\fi{}$C. Our results indicate order-disorder type transition. We propose the origin of the dipole to be the displacement of a Ba ion center from the nonpolar site, viz. anionic cage charge center. Weight loss is attributed to the evolution of halides with the activation energy of 1.57 eV. The weight loss results in the shrinkage of the unit cell as determined by XRD, leading to the shifting of the endotherm peak to 823 $\ifmmode^\circ\else\textdegree\fi{}$C. The signature of defects produced by the evolution of halide is seen in optical absorption spectra.
Published Version
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