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

Solubility of ThO2 in gadolinium zirconate pyrochlore, a potential host for radioactive materials, has been investigated. The phase relations in Gd2−xThxZr2O7+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 Gd1.8Th0.2Zr2O7.1 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.