UC San Diego

Solar thermochemical hydrogen (STCH) generation is a promising approach for eco-friendly H2 production, but conventional STCH redox compounds cannot easily achieve desirable thermodynamic and kinetic properties and phase stability simultaneously due to a rather limited compositional space. Expanding from the nascent high-entropy ceramics field, this study explores a new class of compositionally complex perovskite oxides (CCPOs) (La0.8Sr0.2)(Mn(1-x)/3Fe(1-x)/3CoxAl(1-x)/3)O3 with new non-equimolar designs for STCH. Entropy stabilization may contribute to the phase stability during redox cycling without phase transformation, which enables STCH production for >50 cycles under harsh interrupted conditions.Aliovalent doping on perovskite oxides can tune the oxygen vacancy formation energy. This work discovers normal vs. abnormal aliovalent doping effects on redox behaviors in medium-entropy compositionally complex perovskite oxides (CCPOs) (La1-xSrx)(Mn1/3Fe1/3Ti1/3)O3-δ (LS_MFT) vs. (La1-xSrx)(Mn1/3Fe1/3Cr1/3)O3-δ (LS_MFC). In the LS_MFC series, oxygen non-stoichiometry range Δδ (= δred – δox) linearly depends on the Sr molar ratio x, while the LS_MFT series shows a V-shape dependence of Δδ on x. This unusual observation is investigated and explained based on the analysis of energy loss near edge structure (ELNES) in STEM electron energy loss spectroscopy, along with density functional theory (DFT) calculations. In LS_MFC, Cr-L2,3, Mn-L2,3 and Fe-L2,3 peaks have a similar linear shift to higher energy with increasing x, which indicates higher oxidation states of Cr, Mn, and Fe with lower oxygen vacancy formation energies. In LS_MFT, the V-shape of Δδ vs. x curve is caused by the stable Ti4+ state and a V-shape Mn/Fe valency dependence on x. This study suggests the possible existence of different (including unexpected) coupled aliovalent doping effects in CCPOs with multiple B-site redox active elements. 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 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. Lastly, a redox induced pyrochlore to fluorite transition was discovered in 10CCFBO0.8Nb via annealing in oxidized vs. reduced environments at 1600 C. Notably, the 10-cation oxide remains a homogenous single-phase high-entropy solid solution before and after the ODT in the pyrochlore vs. fluorite structure that can be quenched. In-situ neutron diffraction reveals the oxygen vacancy formation and atomic displacement during this ODT. This study reveals a new pathway to induce ODT via a redox transition to tailor the properties of compositionally complex fluorite-based oxides.