In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A 2 B 2 O 7 pyrochlore (A ¼ Eu, Dy; B ¼ Ti, Zr) up to $50 GPa. Based on X-ray diffraction results, all compositions transformed to the highpressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at $41 GPa for B ¼ Ti

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... $16 GPa B ¼ Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A ¼ Dy, proceeding faster than those with a larger ionic radii, i.e., A ¼ Eu. These results were consistent with previous work in which the radius-ratio of the A-and B-site cations determined the energetics of disordering, and compositions with more similarly sized A-and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B ¼ Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A ¼ Eu than A ¼ Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu 2 Zr 2 O 7 as compared with the initially defect-fluorite structured Dy 2 Zr 2 O 7 .