Solubility of ThO₂ in Gd₂Zr₂O₇ pyrochlore: XRD, SEM and Raman spectroscopic studies

Abstract

Solubility of ThO₂ in gadolinium zirconate pyrochlore, a potential host for radioactive materials, has been investigated. The phase relations in Gd_2-xTh_xZr₂O_7+x/2 (0.0 ¤ x ¤ 2.0) systems have been established under the slow-cooled conditions from 1400 ° C. XRD studies reveal that the compositions corresponding to x = 0.0-0.075 are single phasic in nature and beyond x ¤ 0.1 the biphasic region starts. The first biphasic region comprising of pyrochlore and thoria exist from x = 0.1-0.8, and from x = 1.2 another biphasic region consisting of gadolinia stabilized zirconia (GSZ) and thoria appears which persists till x = 1.6. The end member (i.e. x = 2.0) of the series is found to be a mixture of monoclinic ZrO₂ and thoria. Interestingly, gadolinia which has wide solubility in thoria, did not show any miscibility in thoria in the presence of zirconia. Irregular grains of Gd_1.8Th_0.2Zr₂O_7.1 as shown in SEM supports its biphasic nature. Raman spectra of heavily thoria doped (x = 0.1 and 0.2) samples, indicates the presence of Zr-O₇ mode which implies the samples are highly disordered in nature.