Average and local structure of α-CuI by configurational averaging

Abstract

Configurational Boltzmann averaging together with density functional theory are used to study in detail the average and local structure of the superionic α-CuI. We find that the coppers are spread out with peaks in the atom-density at the tetrahedral sites of the fcc sublattice of iodines. We calculate Cu–Cu, Cu–I and I–I pair radial distribution functions, the distribution of coordination numbers and the distribution of Cu–I–Cu, I–Cu–I and Cu–Cu–Cu bond-angles. The partial pair distribution functions are in good agreement with experimental neutron diffraction—reverse Monte Carlo, extended x-ray absorption fine structure and ab initio molecular dynamics results. In particular, our results confirm the presence of a prominent peak at around 2.7 Å in the Cu–Cu pair distribution function as well as a broader, less intense peak at roughly 4.3 Å. We find highly flexible bonds and a range of coordination numbers for both iodines and coppers. This structural flexibility is of key importance in order to understand the exceptional conductivity of coppers in α-CuI; the iodines can easily respond to changes in the local environment as the coppers diffuse, and a myriad of different diffusion-pathways is expected due to the large variation in the local motifs.

Think locally – linking structure, thermodynamics and transport in grossly non-stoichiometric compounds and solid solutions

“…Following the same approach as for Ba 2 In 2 O 5 in an earlier section, possible local arrangements of the Cu atoms in a-CuI have been considered in detail. 17 A set of initial arrangements are constructed, each configuration is fully optimized 17 using density functional theory 3 and ensemble averages calculated using Boltzmann (configurational) averaging. 1 To start with, 50 random symmetrically distinct arrangements of the 4 Cu atoms over the tetrahedral 8c, octahedral 4b and 'interstitial' 32f sites (e.g., at (x,x,x) with x ¼ 0.375) in a 8-atom unit cell (Fig.…”

“…To generate final results, a further set of optimisations of 25 randomly chosen configurations used a 64-ion cell and ignored this 4b site (see ref. 17 for full details).…”

Superionic Materials: Structural Aspects

“…However, the findings of neutron diffraction studies have indicated that the Cu þ are predominantly restricted to the tet positions within space group Fm 3m (such that the tet sites form a single set in the 8(c) positions) [24,25]. This structural model has been supported by computer simulations [26][27][28], with the Cu þ shown principally to diffuse between the tet positions in h1 0 0i directions. However, while the essential features of a-CuI are now well established, a number of details continue to attract attention, including differences between the partial pair distribution functions derived by computer simulations, neutron diffraction and EXAFS methods (for details, see Refs.…”

“…However, while the essential features of a-CuI are now well established, a number of details continue to attract attention, including differences between the partial pair distribution functions derived by computer simulations, neutron diffraction and EXAFS methods (for details, see Refs. [26,29]). …”

“…These include the use of bond valence formalisms to identify probable diffusion pathways [171], and also the socalled configurational averaging method, which randomly samples local minima on the potential energy surface and has proved valuable for the study of the disordered structure of several ionically conducting systems [26,166]. The MD technique, with its ability to study the dynamics of the ions within the simulation, continues to be widely employed, with an increase in the use of ab initio methods.…”