Phase composition and stability of Gd${}_{2-x}$Th${}_x$Zr${}_2$O${}_7$ under extreme conditions

Introduction of Th into synthetic disordered fluorite-type Gd${}_2$Zr${}_2$O${}_7$ induces a transition to an ordered pyrochlore-type phase at a Th concentration of 10 % at the Gd site (Gd${}_{1.8}$Th${}_{0.2}$Zr${}_2$O${}_7$ composition). The degree of order of the fluorite-type phase reaches 50 % for a Th concentration of 25 % (Gd${}_{1.5}$Th${}_{0.5}$Zr${}_2$O${}_7$ composition). Upon application of high pressure, the Gd${}_2$Zr${}_2$O${}_7$ phase retains the fluorite-type structure until 33 GPa (K${}_0$ = 167(1) GPa), where it undergoes reversible amorphization. The Gd${}_{1.7}$Th${}_{0.3}$Zr${}_2$O${}_7$ phase was found to be stable up to at least a pressure of 25 GPa (K${}_0$ = 169(3) GPa). Upon heating to T${}_{max}$ of 1135 K, the Gd${}_2$Zr${}_2$O${}_7$ phase retains its disordered fluorite-type structural arrangement (α = 3.03 × 10${}^{-5}$ K${}^{-1}$). The excellent stability of the Gd${}_{2-x}$Th${}_x$Z${}_r$2${}_O$7 phases under extreme conditions of temperature and pressure makes Gd${}_2$Zr${}_2$O${}_7$ a promising candidate as a host matrix for radioactive elements for safe long-term underground storage of nuclear waste.