ANTIFERROELECTRIC-TO-PARAELECTRIC TRANSITION IN BAFCL

Thermoanalytical investigations are carried out on the flux-grown BaFC l crystals. An endothermic peak at 725 degrees C and 1.2% weight loss in the temperature range from 730 to 860 degrees C is observed in diff erential thermal analysis and thermogravimetric analysis measurements. Heating rate dependency of the endotherm peak position and that of we ight loss onset temperature indicate that these two processes are inde pendent from one another. Endotherm is attributed to the antiferroelec tric to paraelectric transition in BaFCl. High-temperature Raman scatt ering, high-temperature x-ray diffraction (XRD) measurements and therm omechanical analysis show that BaFCl has no structural transition from 25 to 800 degrees C. Our results indicate order-disorder type transit ion. We propose the origin of the dipole to be the displacement of a B a ion center from the nonpolar site, viz. anionic cage charge center. Weight loss is attributed to the evolution of halides with the activat ion 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 endoth erm peak to 823 degrees C. The signature of defects produced by the ev olution of halide is seen in optical absorption spectra.