Pressure-induced order-disorder transition in Gd1.5Ce0.5Ti2O7 pyrochlore.

Jingjing Niu,Shan Qin,Xiang Wu,Haibin Zhang

doi:10.1098/rsos.190842

Abstract

An experimental study on ordered pyrochlore structured Gd1.5Ce0.5Ti2O7 was carried out up to 45 GPa by synchrotron radiation X-ray diffraction and Raman spectroscopy. Experimental results show that Gd1.5Ce0.5Ti2O7 transfers to a disordered cotunnite-like phase (Pnma Z = 4) at approximately 42 GPa. Compared with the end member Gd2Ti2O7, the substitution of Ce3+ for Gd3+ increases the transition pressure and the high-pressure stability of the pyrochlore phase. This pressure-induced structure transition is mainly controlled by cationic order–disorder modification, and the cationic radius ratio rA/rB may also be effective for predicting the pyrochlore oxides' high-pressure stability. Two isostructural transitions are observed at 6.5 GPa and 13 GPa, and the unit-cell volume of Gd1.5Ce0.5Ti2O7 as a function of pressure demonstrates its compression behaviour is rather complex.

Highlights

Pyrochlore oxide, with an ideal chemical formula of A2B2O6O’(or A2B2O7), have attracted substantial attention due to its unique structural properties and its applications in fuel cells [1,2], spin liquid materials [3] and high-level waste disposal materials [4]
The structural stability of A2B2O7 pyrochlores at ambient conditions depends on the ratio of the cation radii, rA/rB [5], only when 1.46 < rA/rB < 1.78, A2B2O7 oxides crystallize in the pyrochlore structure
At the beginning of the experiments, all of the reflections can be indexed as a pyrochlore structure, indicating that the Gd1.5Ce0.5Ti2O7 crystallized in the pyrochlore structure

Summary

Introduction

Pyrochlore oxide, with an ideal chemical formula of A2B2O6O’(or A2B2O7), have attracted substantial attention due to its unique structural properties and its applications in fuel cells [1,2], spin liquid materials [3] and high-level waste disposal materials [4]. The pyrochlore structure (figure 1a) belongs to the Fd3m space group (Z = 8). It can be viewed as A and B cation ordered 2 × 2 × 2 superlattices of ideal fluorite structures (Fm3m) with 1/8 anion deficiency. The larger A cations occupied 16c (1/2 1/2 1/2) with (a) (b) A3+. Eight coordinates located in a distorted cubic polyhedron. The six-coordinate B site located in 16d (0 0 0) is usually occupied by smaller cations centred in an oxygen octahedron. The structural stability of A2B2O7 pyrochlores at ambient conditions depends on the ratio of the cation radii, rA/rB [5], only when 1.46 < rA/rB < 1.78, A2B2O7 oxides crystallize in the pyrochlore structure

Results

Discussion

Conclusion