Long- and short-range orders in 10-component compositionally complex ceramics

Abstract

Neutron diffraction and total scattering are combined to investigate a series of single-phase 10-component compositionally complex fluorite-based oxides, [(Pr_0.375Nd_0.375Yb_0.25)₂(Ti_0.5Hf_0.25Zr_0.25)₂O₇]_1-x[(DyHoErNb)O₇]_x, denoted as 10CCFBO_xNb. A long-range order-disorder transition (ODT) occurs at x ​= ​0.81 ​± ​0.01 from the ordered pyrochlore to disordered defect fluorite. In contrast to ternary oxides, this ODT occurs abruptly without an observable two-phase region; moreover, the phase stability in 10CCFBOs deviates from the well-established criteria for simpler oxides. Rietveld refinements of neutron diffraction patterns suggest that this ODT occurs via the migration of oxygen anions from the position 48f to 8a, with a small final jump at the ODT; however, the 8a oxygen occupancy changes gradually (without an observable discontinuous jump). We further discover diffuse scattering in Nb-rich compositions, which suggests the presence of short-range order. Using small-box modelling, four compositions near ODT (x ​= ​0.75, 0.8, 0.85, and 1) can be better fitted by C222₁ weberite ordering for the local polyhedral structure at nanoscale. Interestingly, 10CCFBO_0.75Nb and 10CCFBO_0.8Nb possess both long-range pyrochlore order and short-range weberite-type order, which can be understood from severe local distortion of the pyrochlore polyhedral structure. Thus, weberite-type short-range order emerges before the ODT, coexisting and interacting with long-range pyrochlore order. After the ODT, the long-range pyrochlore order vanishes but the short-range weberite-type order persists in the long-range disordered defect fluorite structure. Notably, a drop in the thermal conductivity coincides with emergence of the short-range order, instead of the long-range ODT.