Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

Abstract

Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

Environmentally sustainable facile synthesis of nanocrystalline holmium hafnate (Ho2Hf2O7): Promising new oxide-ion conducting solid electrolyte

“…Pyrochlore oxides have six theoretical Raman active modes [16,[29][30][31][32][33][34][35][36][37] These Raman active modes represent the vibrations of 〈Ln-O〉 and 〈Hf-O〉 bonds [14,[32][33][34][35][36][37][38][39][40]. Raman spectroscopy is more suitable to study anion disorder in pyrochlores as these modes involve motion of oxygen atoms.…”

Modeling Disorder in Pyrochlores and Other Anion-Deficient Fluorite Structural Derivative Oxides

“…Recently, Shamlin et al showed experimentally that the weberite-like structure, which has a random cationic distribution on the 8c site (7-fold coordinated site), best represents the short-range order in the disordered structure (Shamblin et al, 2016). This has inspired DFT calculations of weberite-type disordered structures (Solomon et al, 2016;Finkeldei et al, 2017;Kowalski, 2020;Matsumoto et al, 2020), which show that disordered fluorite has higher energy than disordered weberite. It is argued that this indicates a possibility of forming weberite-type short-range ordering.…”

“…This is a curious result as no 2-2-7 chemistries containing +3 and +4 cations have been experimentally reported to have the weberite structure (Cai and Nino, 2009). On the other hand, calorimetry experiments indicate that the short-range order described by weberite is higher in energy than pyrochlore (Solomon et al, 2016;Finkeldei et al, 2017). The discrepancy between experiments and modeling can be explained for some chemistries, namely A 2 Zr 2 O 7 and A 2 Hf 2 O 7 (A Ho, Er, Yb, Lu), which form δ-phase (see Figure 2A), suggesting δ-phase might be preferred and thus should be examined.…”

“…In the past three decades, atomic scale calculations, by means of molecular dynamics (MD) simulations with interatomic potentials and density functional theory (DFT) calculations, have been extensively used to understand the disordering process in pyrochlores by calculating the intrinsic defects formation energies (Wilde, 1998;Chartier et al, 2002;Minervini et al, 2002;Minervini et al, 2004;Panero et al, 2004;Rushton et al, 2007;Sickafus et al, 2007;Chen et al, 2008;Chen and Tian, 2010;Gunn et al, 2012;Chen et al, 2014;Uberuaga et al, 2014;Li et al, 2015;Li and Kowalski, 2018;Yang et al, 2019), looking at dynamical processes (Chartier et al, 2002;Chartier et al, 2005;Devanathan and Weber, 2005;Todorov et al, 2006;Rushton et al, 2007;Chartier et al, 2009;Devanathan et al, 2010;Xiao et al, 2010;Wang et al, 2013;Uberuaga et al, 2014) (Xiao et al, 2015;Dong et al, 2017), and using models of disordered configurations for DFT calculations Jiang et al, 2009;Solomon et al, 2016;Finkeldei et al, 2017;Pilania et al, 2019;Kowalski, 2020;Matsumoto et al, 2020). The simplest way to study the disorder in a material using atomistic scale calculations is to obtain the formation energies of intrinsic defects in ordered systems, which are generally accessible at a much lower computational cost than completely disordered systems.…”

Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

“…A recent computational study by Solomon et al shows that low-energy, long-range weberite-type congurations are favored at low values of the ionic radius ratio of A-and B-site cations r A /r B . 48 Random averaging of weberite-type local congurations leads to a long-range disordered, defect uorite structure when averaged over many unit cells. 44,46 Heterogeneous disordering processes with different short-and longrange structural motifs have been similarly observed for other ceramics, such as spinel, 49 and different disordering processes through intrinsic (nonstoichiometry and chemical composition 15,50 ) and extrinsic (ball milling and irradiation 43,51 ) means.…”