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 derive d from synchrotron radiation diffraction measurements. Electron densit y 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 qualitati vely with experimental values of 0.71 and 0.50 cm(-2), respectively. B oth strong Li ... Nb (Ta) interactions in the structure are markedly s horter 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 respon sible 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 An gstrom) = 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, rhombohe dral, 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 = 0.0 16, wR = 0.022, S = 1.372 (35) for 773 unique reflections.