Long- and short-range orders in 10-component compositionally complex ceramics
Dawei Zhang, Yan Chen, Heidy Vega, Tianshi Feng, Dunji Yu, Michelle Everett, Jöerg C. Neuefeind, Ke An, Renkun Chen, Jian Luo
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.