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, [(Pr0.375Nd0.375Yb0.25)2(Ti0.5Hf0.25Zr0.25)2O7]1-x[(DyHoErNb)O7]x, denoted as 10CCFBOxNb. 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, 10CCFBO0.75Nb and 10CCFBO0.8Nb 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.

Keywords

• Neutron diffraction
• Neutron total scattering
• Order-disorder transition
• Short-range order
• High-entropy ceramics
• Compositionally complex ceramics