Electric-field-induced ionic displacement and redistribution of bonding electrons in LiNbO₃and LiTaO₃revealed by modulation X-ray diffraction

Abstract

Electric fields with alternating square wave were applied to LiNbO 3 and LiTaO 3 single crystals in [001] directions, and extremely small ionic displacement was directly detected by the measurement of the diffracted intensities of X-rays synchronously with the applied fields. The displacement of Nb and Li ions relative to the oxygen framework were found to be 4.9 x 10 -4 and 13 x 10-4]k at E = 51.5 kV cm -1 for LiNbO 3, and those of Ta and Li to be 6.8 x 10 -4 and 15 x 10 -4 A at E = 50 kV cm -1 for LiTaO 3. The temperature factor was observed to vary with the external field, which is attributed to the change in phonon frequency of the crystal lattice. It was found from the intensity variations in the lower-indexed reflections that the displacements of the bonding electrons of Nb-O and Ta--O bonds are much larger than those of the constituent ions, which may be the origin of the electrooptic effect. The capability of the modulation technique in the absolute structure determination of crystals is demonstrated.

Defect structure dependence on composition in lithium niobate

“…Fujimoto (1982) has reported fi(Li)=0.094(8), t~(Nb)=0.067(8) and t~(O) = 0.085 (7) A in another crystal of uncharacterized composition. The latter values do not differ significantly from the r.m.s, radial displacements found in the present study for stoichiometric LiNbO3.…”

Accurate determination of strain tensors from small shifts of reflections measured on a four-circle diffractometer

“…From X-ray diffraction, the unitcell deformations and thus the strain tensor can be determined by measuring the changes in Bragg angle of a number of reflections. In particular, when combined with the so-called modulation methods (Fujimoto, 1982), very small homogeneous deformations can be accurately measured (Paturle, Graafsma, Sheu, Coppens & Becker, 1991). Barsch (1976) has shown how, in the case that the perturbation is an electric field, the piezoelectric tensor elements are related to small changes in Bragg angle.…”

Principles of differential energy-dispersive X-ray spectroscopy (DEDXS)

“…Additionally in (b) there is the provision of compensation for changes in the Bragg angle caused by the field reversals. Puget & Godefroy (1975) investigated the ionicity of bonds in rutile (TiO2) by this method and attained a precision of about 1 × 10 -4 Fujimoto (1978Fujimoto ( , 1980Fujimoto ( , 1982 developed an alternative approach in which the source remained continuous but the counting periods were synchronized with the field (Fig. 2b).…”