Influence of thermally induced oxygen order on mobile ion dynamics inGd2(Ti0.65Zr0.35)2

Abstract

We report on the influence of oxygen order in the oxygen-ion dynamics in the ionic conductor Gd 2 ͑Ti 0.65 Zr 0.35 ͒ 2 O 7 . The metastable Gd 2 ͑Ti 0.65 Zr 0.35 ͒ 2 O 7 powders prepared by mechanical milling present an anion-deficient fluorite type of structure, stable up to about 800°C. Thermal treatments at higher temperatures facilitate the gradual rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects. Interestingly, metastable pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process. We have found that the ionic conductivity due to mobile oxygen ions increases significantly with increasing sintering temperature from 800 to 1500°C as a result of a systematic decrease in the activation energy for the dc conductivity from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by stretched exponentials of the form ⌽͑t͒ = exp͓−͑t / ͒ 1−n ͔, and the fractional exponent n decreases systematically from n = 0.51 to 0.18 with increasing sintering temperature. These results are explained in terms of weaker ion-ion interactions in the increasingly ordered structure of the samples sintered at higher temperatures, and point to the importance of structural disorder in determining the dynamics of mobile oxygen ions.

Síntesis y propiedades eléctricas de la solución sólida Gd_2-yLa_yZr₂O₇ con estructura de tipo pirocloro

“…This fact also shows in the value of n which decreases significantly when 0.4 < y ≤ 1. These results are consistent with the previous finding of the latter being related to the degree of structural order/ disorder in ionically conducting pyrochlore oxides (14,15). In the present case, a more ordered structure is induced by increasing lanthanum content and n decreases accordingly.…”

Relationships between structural and electrical properties in mixed conductors duplex materials in the ZrO2-Y2O3-TiOColomer1, Díaz‐Moreno2, Boada3 et al. 2014Phys. Rev. B

“…Although, all studies agree that it is with no doubt related to disorder-activated oxygen vibration. For instance, for an ordered Y 2 Ti 2 O 7 pyrochlore, the strongest band is observed near 309 cm −1 , and three less intense bands are clearly seen at 609, 522, and 448 cm −1 [28]. When Zr 4+ ions are partially substituted for Ti 4+ ions, Raman bands become broader, and a new, very broad band appears above 700 cm −1 [28].…”

“…A broad band, which appears at 722 cm −1 , cannot be assigned to either the tetragonal or the cubic phase of ZrO 2 . Literature data show, however, that a broad band above 700 cm −1 is often observed in materials containing RE 2 O 3 (RE = rare earth), ZrO 2 , and TiO 2 due to the presence of a disordered pyrochlore phase [27,28]. Indeed, the high-frequency band has been observed in other Ti containing pyrochlores and has been interpreted in different ways [23,27,[29][30][31].…”

“…For instance, for an ordered Y 2 Ti 2 O 7 pyrochlore, the strongest band is observed near 309 cm −1 , and three less intense bands are clearly seen at 609, 522, and 448 cm −1 [28]. When Zr 4+ ions are partially substituted for Ti 4+ ions, Raman bands become broader, and a new, very broad band appears above 700 cm −1 [28]. Moreover, the intensity of the 522 and 448 cm −1 bands decreases with increasing concentration of the Zr 4+ ions, and these bands disappear for Y 2 Ti 0.6 Zr 1.4 O 7 [28].…”

“…When Zr 4+ ions are partially substituted for Ti 4+ ions, Raman bands become broader, and a new, very broad band appears above 700 cm −1 [28]. Moreover, the intensity of the 522 and 448 cm −1 bands decreases with increasing concentration of the Zr 4+ ions, and these bands disappear for Y 2 Ti 0.6 Zr 1.4 O 7 [28]. Based on the literature data, we assign the 722 and 329 cm −1 bands of 10Ti8h to disordered Y 2 Ti 2−y Zr y O 7 pyrochlore phase.…”