Mechanism of the ferroelectric phase transitions inLiNbO3andLiTaO3

Abstract

The temperature dependences of the nuclear-electric-quadrupole frequency Q of 117 In doped in LiTaO 3 (T C ϭ938 K) and Li 1Ϫx In x/3 TaO 3 with xϭ0.2 (T C ϭ818 K) show that the order-disorder of the Li ions is not the driving mechanism for the ferroelectric instability in LiNbO 3 and LiTaO 3 systems, and imply that the oxygen order-disorder is the driving mechanism. The significantly different temperature dependences of Q of 111 Cd in these materials compared, to those of 117 In, demonstrate that this order-disorder is of dynamic character.

Behavior of Impurities In and Cd in the LiNbO<SUB>3</SUB>-LiTaO<SUB>3</SUB> System

“…12) We observed that the temperature de- pendences of the electric quadrupole frequency ω Q of 117 In in these oxides were similar to that of the 7 Li-nuclear magnetic resonance coupling constant 13) for LiTaO 3 and faithfully reflected the phase transition at the respective T C . These observations suggested that although an In ion was disordered in the paraelectric phase, its order-disorder did not take place at a much lower temperature than that of the Li ion would require.…”

“…It is therefore worth conducting PAC measurements of the temperature dependence of ω Q of 117 In and 111 Cd in Cd-doped LiTaO 3 . In this paper, we report on the results for Li 1−x Cd x/2 TaO 3 (x = 0.167), i.e., Li 0.833 Cd 0.083 TaO 3 in a temperature range between 77 and 1223 K. Using these results, together with our previous PAC results on 117 In and 111 Cd in LiTaO 3 11) and Li 0.8 In 0.067 TaO 3 , 12) we discuss the behavior of In and Cd and the driving mechanism for ferroelectric instability in the LiNbO 3 -LiTaO 3 system.…”

“…The second possibility should be considered since the Cd site of, say, CdO, which is a starting material for the sample and may partly remain as it is, is of zero EFG, because CdO has a cubic structure. This possibility is, however, very small, if any, from the facts that the X-ray-diffraction pattern of the sample shows a singe phase and that the value of A 22 In order to examine the present results with respect to the phase-transition mechanism, we describe in some detail the two opposing interpretations of the role of the order-disorder of Li in the phase transitions and the relevant PAC results on LiTaO 3 11) and Li 0.8 In 0.067 TaO 3 , 12) which were mentioned in the Introduction. The two opposite interpretations mean that in one interpretation the order-disorder of Li is the driving mechanism of the phase transitions, and in the other it is not.…”

“…We therefore considered that the order-disorder of Li is not the driving mechanism of ferroelectric instability. 12) A comparison of the magnitude of ω Q of 117 In in Li 0.8 In 0.067 TaO 3 with that for LiTaO 3 further implied that displacements of oxygen ions were the cause of the ferroelectric instability.…”

“…Additional measurements of ω Q of 111 Cd in the oxides, 11,12) which also occupy those Li sites, 11) showed their temperature dependences to be quite different from those of 117 In. In the case of Li 0.8 In 0.067 TaO 3 , the temperature dependence does not even reflect the phase transition at T C = 818 K. This observation indicates that, different from In, Cd is not disordered in the paraelectric phase.…”