Synchrotron X-ray studies of LiNbO3 and LiTaO3

The structural geometry, vibrations and deformation density Delta rho for lithium niobate, LSTbO3, and Lithium tantalate, LiTaO3, are derived from synchrotron radiation diffraction measurements. Electron density is transferred from the Nb (Ta) atom towards the large O-3 triangle near the Li atom. Spontaneous polarizations of 0.43 (LibNbO(3)) and 0.33 cm(-2) (LiTaO3), calculated with Hirshfeld charges, agree qualitatively with experimental values of 0.71 and 0.50 cm(-2), respectively. Both strong Li ... Nb (Ta) interactions in the structure are markedly shorter than all Li ... Li, Nb ... Nb or Ta ... Ta vectors, as expected if the Li and Nb (Ta) cations carry charges of opposite sign. The Li atom's negative charge plays a pivotal role in the polarization responsible for ferroelectricity. LiNbO3, space group R3c, rhombohedral, M-r = 147.8, a = 5.493 (2) Angstrom, alpha = 55.89 (3)degrees, V = 106.02 (4) Angstrom(3), Z = 2, D-x = 4.629 Mg m(-3), F(000) = 136, mu(0.7 Angstrom) = 5.01 mm(-1), gamma(min) = 0.27, T = 293 K, R = 0.015, wR = 0.015, S = 3.276 (84) for 773 unique reflections. LiTaO3, R3c, rhombohedral, M-r = 235.9, a = 5.471 (2) Angstrom, alpha = 56.16 (3)degrees, V = 105.51 (4) Angstrom(3), Z = 2, D-x = 7.412 Mg m(-3), F(000) = 200, mu(0.7 Angstrom) = 50.03 mm(-1), gamma(min) = 0.32, T = 293 K, R = 0.016, wR = 0.022, S = 1.372 (35) for 773 unique reflections.