First-Principles Study on the Stability of Weberite-Type, Pyrochlore, and Defect-Fluorite Structures of A23+B24+O7 (A = Lu3+–La3+, B = Zr4+, Hf4+, Sn4+, and Ti4+)

Crystal structures and energetics of a series of A2B2O7 (A = Lu3+–La3+, B = Zr4+, Hf4+, Sn4+, and Ti4+) compounds were systematically investigated using first-principles calculations. Experimentally, they are known to form either pyrochlore or defect-fluorite structures. Recently, formation of a weberite-type ordering was reported in Ho2Zr2O7. In this study, we performed an exhaustive structure search for composition Yb2Ti2O7 and found that the lowest-energy structure is a triclinic weberite-type structure characterized by alternating Yb and Ti layers. Free energies from first-principles phonon calculations suggest that the transition temperature between the weberite-type and pyrochlore structures of Yb2Ti2O7 is 550 K. The energies of the weberite-type and pyrochlore structures are also compared for a wide range of A2B2O7 compositions. As a general trend, it is found that the stability of the weberite-type structure increases relative to the pyrochlore structure as the A3+ cation radius decreases. Energies of disordered defect-fluorite structures estimated using special quasi-random structures are also reported. The formation of such defect-fluorite structures is found to be more energetically favorable at high temperatures in A2Zr2O7 and A2Hf2O7 when the A3+ cation radius was small. The present results are consistent with previously reported experimental results.