Abstract
Structural disorder has a profound effect on a material’s physical properties. Transport properties are strongly influenced by crystallographic defects which act as scattering centres for quasiparticles, phonons, photons, and so on. For example, structural disorder alters electron transport in graphene11, photon transport in silicon photonic superlattices12, flux pinning in superconductors13, and heat transport properties in thermoelectric materials14, and thus is used in diverse technological applications. For pyrochlore materials in particular, structural disorder increases catalytic activity in Li–O2 batteries15, increases ionic conductivity to levels comparable to yttria-stabilized zirconia8, and alters radiation resistance, affecting its ability to immobilize radionuclides from spent nuclear fuel3, 4, 5. Key to these applications is a comprehensive understanding of the origin of the order/disorder transformation.
Complex oxides with aliovalent cations often form superstructures of simpler compounds. Pyrochlore2 and weberite-type16 oxides, for example, both form supercells of the fluorite structure with ordered cations and constitutional anion vacancies, but differ in the unit cell size/orientation and symmetry. These highly ordered materials are inherently prone to disordering5, which influences their behavior under extreme conditions. These complex structures cannot incorporate significant defect concentrations without partial or complete loss of long-range order. For pyrochlore, experiments show that a disordered phase forms at equilibrium when cations are similar in size (the ratio of cation ionic radii (rA/rB) < 1.46; ref. 2). This process is governed by the thermodynamics of disordering and the energetics of defect formation, which increases with rA/rB (refs 3,17,18). Conventional diffraction techniques characterizing the average structure demonstrated that the order/disorder transformation involves antisite defects, wherein A- and B-site cations randomly exchange positions, and anion Frenkel pairs leading to randomized oxygen vacancies5, 6, 7. On average, the disordered structure is therefore analogous to the mineral fluorite, with a single cation and anion site. The pyrochlore has essentially lost its superstructure and the unit cell parameter is reduced by 1/2. However, recent studies probing the local structure on disordered fluorite materials have revealed evidence of nonrandom distortions at the nanoscale. For example, whereas the average coordination number (CN) is 7 for both cations in disordered fluorite, reverse Monte Carlo (RMC) analysis from neutron total scattering19 and X-ray absorption experiments20, 21 have shown that the B-site cation CN (6-coordinated in pyrochlore) remains less than that of the A-site cation (8-coordinated in pyrochlore), suggesting that vacancies are still preferentially localized around the B-site cation. It is still unclear, however, from where this local order arises and how it relates to both the fully ordered and disordered phases.
