Raman and x-ray absorption spectroscopy study of the phase evolution induced by mechanical milling and thermal treatments inR2Ti2O

Abstract

We report here a study of the local order in R 2 Ti 2 O 7 , (R = Y, Gd, and Dy) pyrochlores obtained by mechanical milling (MM) performed by combining both Raman and x-ray absorption (XAS) spectroscopies. In addition to both as-milled and annealed MM samples, measurements have been also recorded by performing an in situ thermal treatment of the MM pyrochlores in the range from ambient to 1300• C. The analysis of both Raman and XAS data yields a disentangled view of the evolution of the different degree of structural disorder at the Ti and rare-earth sites of the MM samples with the thermal treatment. Moreover, this study allowed us to follow the structural transformation of the MM samples through the region where exothermic processes have been previously observed. Our experiments show that an average fluorite-like structure is never formed in these pyrochlores. As milled-samples are better described as if formed by nanometer-sized pyrochlore domains embedded in a fluorite-like matrix. Thermal annealing induces domain growth, so that the event observed by differential thermal analysis can be attributed to an evolution from medium to long-range pyrochlore ordering, typically from tens to hundreds of nanometers.

XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia

“…Therefore, we demonstrate here that the observed variations in the XANES profile are exclusively related to the extension of the short-range order existing in the milled samples and not to structural transitions, similarly to the case of as-milled R 2 Ti 2 O 7 pyrochlores. 18 The exhaustive XANES and EXAFS analysis support that the ionic conductivity associated to the grain boundary should be independent of the grain size. On the other hand, the not-sintered samples exhibit a highly distorted environment where the cation-cation correlation is lost, and consequently the bulk conductivity can be hindered by the disorder.…”

“…16 The quantitative EXAFS analysis was carried out using the phase and amplitude transferability method. 17,18 EXAFS data fitting were performed by using the VIPER program. 19 In all cases the backscattering amplitude, phase and photoelectron mean free path factor were obtained from the experimental EXAFS spectra of the appropriate reference compounds.…”

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

“…According to the literature, this peak should be very weak or absent in the case of a regular sixfold Ti-O arrangement, as occurring in the Y 2 Ti 2 O 7 pyrochlore reference compound (see inset in Fig. 6) [23], while a very sharp feature should be present in the case of fivefold or tetrahedral structures [23,40,41], with an expected intensity much higher than the observed ones in the systems under study. Indeed, the observed increase of the prepeak intensity in the duplex samples when comparing with the symmetric pyrochlore case is in agreement with the occurrence of a distorted Ti-O arrangement or with an offcenter displacement of the Ti atom within the octahedral TiO 6 local bonding unit [42,43].…”

“…The EXAFS signals were extracted following standard procedures [20] using the program ATHENA within the IFEEFIT package [21]. The quantitative EXAFS analyses were performed by using the VIPER program [22] and the phase and amplitude transferability method [23,24]. The backscattering amplitude, phase, and photoelectron mean free path factors were obtained from the experimental EXAFS spectra of reference compounds; the cubic 10% mol YSZ (c-10) for the Zr and Y K-edges and Y 2 Ti 2 O 7 pyrochlore for the Ti K-edge.…”

“…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]. Although, all studies agree that it is with no doubt related to disorder-activated oxygen vibration.…”