Ferroelectric lithium tantalate—1. single crystal X-ray diffraction study at 24°C

Abstract

Ferroelectric LiTaO 3 is rhombohedral, with lattice constants a H = 5.15428 ± 0.00001 and c H = 13.78351 ± 0.00002 A ̊ at 25°C, and space group R3 c . The integrated intensities of 358 independent dispersively-unequal F( hkl̄) and F( hkl̄) were measured, in all forms within a reciprocal lattice hemisphere with (sin θ) λ ⩽ 1.02 Å −1, using PEXRAD. A choice in the location of the Li atom between two nearly equally possible positions was made, based on temperature factor criteria and on a statistical analysis. The final agreement factor was 0.0383. The absolute configuration of the atomic arrangement in this polar crystal has been determined. The sense of the displacement of Ta from the inversion center, and of Li from the oxygen plane, the positions required by the space group assumed in the paraelectric state ( R3̄ c), corresponds to the positive polarization direction. The atomic thermal vibrations in LiTaO 3 are significantly anisotropic. On the basis of several simplifying assumptions, the characteristic Debye temperature is found from the diffraction measurements to be 452°K.