Structural vs. intrinsic carriers: contrasting effects of cation chemistry and disorder on ionic conductivity in pyrochlores

Abstract

We use molecular dynamics simulations to investigate the role of cation disorder on oxygen diffusion in Gd 2 Zr 2 O 7 (GZO) and Gd 2 Ti 2 O 7 (GTO) pyrochlores, a class of complex oxides which contain a structural vacancy relative to the basic fluorite structure. The introduction of disorder has distinct effects depending on the chemistry of the material, increasing the mobility of structural carriers by up to four orders of magnitude in GZO. In contrast, in GTO, there is no mobility at zero or low disorder on the ns timescale, but higher disorder liberates the otherwise immobile carriers, allowing diffusion with rates comparable to GZO for the fully disordered material. We show that the cation disorder enhances the diffusivity by both increasing the concentration of mobile structural carriers and their individual mobility.The disorder also influences the diffusion in materials containing intrinsic carriers, such as additional vacancies V O or oxygen interstitials O I . While in ordered GZO and GTO the contribution of the intrinsic carriers dominates the overall diffusion of oxygen, O I in GZO contributes along with structural carriers, and the total diffusion rate can be calculated by assuming simple additive contributions from the two sources. Although the disorder in the materials with intrinsic defects usually enhances the diffusivity as in the defect-free case, in low concentrations, cation antisites A B or B A , where A ¼ Gd and B ¼ Zr or Ti, can act as traps for fast intrinsic defects. The trapping results in a lowering of the diffusivity, and causes a non-monotonic behavior of the diffusivity with disorder. By contrast, in the case of slow intrinsic defects, the main effect of the disorder is to liberate the structural carriers, resulting in an increase of the diffusivity regardless of the defect trapping.

Massively enhanced ionic transport in irradiated crystalline pyrochlore

“…In our analysis, we assume that the cation distribution induced by the irradiation is essentially frozen into place at the temperatures of interest while the oxygen defects (beyond oxygen Frenkel pairs, which can be viewed as defects that describe anion disorder), having orders-of-magnitude higher mobility, anneal out, and the oxygen adjusts to that cation structure. 46 It has been reported that local disorder occurs in materials that are nominally of pyrochlore structure, 28 that local order can exist in disordered materials (and vice versa), 8,[29][30][31][32] and also that irradiation can induce local disorder within an otherwise ordered structure. 33,34 As XRD only provides average structural information, transmission electron microscopy (TEM) was utilized to examine crystallinity as a function of lm depth in cross-sections of irradiated lms in order to further investigate the nature of the irradiation-induced disorder.…”

Strain engineered pyrochlore at high pressure

“…Although these defects play a critical role in the diverse applications of the pyrochlore structure-type 14, 33 , the influence of intrinsic defect concentration on the response of pyrochlore to extreme environments has received little attention 34, 35 .…”

Evidence for percolation diffusion of cations and reordering in disordered pyrochlore from accelerated molecular dynamics

“…This is likely a consequence of the high number of carriers that dictate oxygen diffusion in disordered pyrochlore, necessitating larger domains of order and disorder to form a percolation network. Indeed, we observed trapping when intrinsic (non-structural) oxygen defects were simulated 17 . Our results are also contrary to observations in spinel 37 and perovskites 38, 39 , where the oxygen vacancy diffusivity is hindered by cation disorder.…”

“…Ionic conductivity, in contrast, arises from the structural characteristics of the material: the pyrochlore structure is related to the parent fluorite (BO 2 ) structure, only with distinct A and B sublattices and vacant oxygen sites to account for the reduced valence. While these structural vacancies are immobile or very slow in the ordered material, cation disorder activates these carriers, leading to a tremendous increase of the ionic conduction 15–17 . Experimentally, it was notably found that disordered pyrochlores can show an increase in their ionic conductivity by up to four orders of magnitude in the Gd 2 (Ti x Zr 1− x ) 2 O 7 system, with the disorder level increasing with the fraction of Zr introduced 18, 19 .…”