Phase composition and stability of Gd2−x Th x Zr₂O₇ 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 Zr 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.