"High-pressure behavior of A2B2O7 pyrochlore (A=Eu, Dy; B=Ti, Zr)"

Dylan R. Rittman: Katlyn M. Turner, Sulgiye Park, Antonio F. Fuentes, Jinyuan Yan, Rodney C.Ewing, and Wendy L. Mao; Journal of Applied Physics , 01/24/17.

Additional Authors: Katlyn M. Turner, Sulgiye Park, Antonio F. Fuentes, Jinyuan Yan, Rodney C.Ewing, and Wendy L. Mao

Abstract:

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to  ∼ 50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure 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 = Ti and ∼ 16 GPa B = 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 = Dy, proceeding faster than those with a larger ionic radii, i.e., A = 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 = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A =Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu2Zr2O7 as compared with the initially defect-fluorite structured Dy2Zr2O7.