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 0.375 Nd 0.375 Yb 0.25 ) 2 (Ti 0.5 Hf 0.25 Zr 0.25 ) 2 O 7 ] 1- x [(DyHoErNb)O 7 ] x , denoted as 10CCFBO x Nb . 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 oxygen anions from the position 48 f to 8 a , 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 C 222 1 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. A Short Text: Neutron diffraction and total scattering were used to characterize long- and short-range orders in 10-component compositionally complex ceramics. An unusual long-range order-disorder transition (ODT) from disordered defect fluorite to ordered pyrochlore occurs, which competes with emergence of weberite-type short-range ordering. Interestingly, reduced thermal conductivity coincides with the rise of short-range ordering, instead of occurrence of the long-range ODT. • Neutron diffraction and total scattering are combined to investigate 10-component oxides. • A long-range order-disorder transition (ODT) from pyrochlore to fluorite is observed. • This long-range ODT occurs abruptly without an observable two-phase region. • Long-range pyrochlore order and short-range weberite-type order can co-exist and interact. • A drop in the thermal conductivity is coincident with emergence of the short-range order.