抄録
The crystal structure of LiNbO3 with congruent composition was refined at room temperature by means of the single-crystal X-ray diffraction method under consideration of anharmonic thermal vibration. The space group is R3c with the hexagonal cell dimensions of a=5.15020(6) and c=13.8653(4) Å and Z=6. Large residual electron densities were observed after the ordinary refinement around the Nb5+ ion, which were supposed to be caused by anharmonic thermal vibration of the ion. Coincidence of the observed structure factors with the calculated ones was largely improved with the least-squares refinement containing anharmonic thermal parameters in the variables to be refined. It was revealed that the positional parameters shift significantly with this refinement from those obtained with the harmonic thermal vibration model. The Nb2+ ion in the present crystal has large anharmonicity in the thermal motion at room temperature. However, the magnitude is smaller than that of Ta5+ in LiTaO3 owing to the higher ferroelectric phase transition temperature of this crystal than that of LiTaO3. The coordination octahedra LiO6 and NbO6 show large distributions due to face-sharing as in the corundum-type and other related structures.
