Diffusion within α-CuI studied using ab initio molecular dynamics simulations

The structure and dynamics of superionic α-CuI are studied in detail by means of ab initio Born-Oppenheimer molecular dynamics simulations. The extreme cation disorder and a soft immobile face centred cubic sublattice are evident from the highly diffuse atomic density profiles. The Cu-Cu pair distribution function and distribution of Cu-I-Cu bond angles possess distinct peaks at 2.6 Å and 60° respectively, which are markedly lower than the values expected from the average cationic density, pointing to the presence of pronounced short-range copper-copper correlations. Comparison with lattice static calculations shows that these correlations and the marked shift in the cationic density profile in the ⟨111⟩ directions are associated with a locally distorted cation sublattice, and that the movements within the tetrahedral cavities involve rapid jumps into and out of shallow basins on the system potential energy surface. On average, the iodines are surrounded by three coppers within their first coordination shell, with the fourth copper being located in a transition zone between two neighbouring iodine cavities. However, time-resolved analysis reveals that the local structure actually involves a mixture of threefold-, fourfold- and fivefold-coordinated iodines. Examination of the ionic trajectories shows that the copper ions jump rapidly to nearest neighbouring tetrahedral cavities (aligned in the ⟨100⟩ directions) following a markedly curved trajectory and often involving short-lived (∼1 ps) interstitial positions. The nature of the correlated diffusion underlying the unusually high fraction of coppers with short residence time can be attributed to the presence of a large number of 'unsuccessful' jumps and the likelihood of cooperative motion of pairs of coppers. The calculated diffusion coefficient at 750 K, D(Cu) = 2.8 × 10(-5) cm(2) s(-1), is in excellent agreement with that found experimentally.

[1]  R. Mcgreevy,et al.  Structure and ionic conduction in CuI: diffuse neutron scattering and RMC modelling , 1998 .

[2]  Wang,et al.  Accurate and simple analytic representation of the electron-gas correlation energy. , 1992, Physical review. B, Condensed matter.

[3]  R. J. Elliott,et al.  Neutron Scattering from a Liquid on a Jump Diffusion Model , 1961 .

[4]  I. Todorov,et al.  Free energy of solid solutions and phase diagrams via quasiharmonic lattice dynamics , 2001 .

[5]  K. Funke,et al.  Jump relaxation in solid electrolytes , 1993 .

[6]  Hafner,et al.  Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. , 1994, Physical review. B, Condensed matter.

[7]  P. Vashishta,et al.  Ionic motion in. cap alpha. -AgI , 1978 .

[8]  S. Wada,et al.  Crystal structure, electron density and diffusion path of the fast-ion conductor copper iodide CuI , 2006 .

[9]  S. Stølen,et al.  Average and local structure of α-CuI by configurational averaging , 2007 .

[10]  K. Ihata,et al.  Structural and dynamical properties of -CuI: a molecular dynamics study , 1997 .

[11]  Andreoni,et al.  Ionic Diffusion in a Ternary Superionic Conductor: An Ab Initio Molecular Dynamics Study. , 1996, Physical review letters.

[12]  R. Mcgreevy,et al.  A molecular dynamics study of ionic conduction in CuI. I. Derivation of the interionic potential from dynamic properties , 1995 .

[13]  A. Trapananti,et al.  Structural disorder in liquid and solid CuI at high temperature probed by x-ray absorption spectroscopy , 2002 .

[14]  G. Jacucci,et al.  Diffusion of F− ions in CaF2 , 1978 .

[15]  S. Stølen,et al.  Order in the disordered state: local structural entities in the fast ion conductor Ba2In2O5 , 2005 .

[16]  S. Kohara,et al.  Comparison of partial structures of melts of superionic AgI and CuI and non-superionic AgCl , 2007, Journal of physics. Condensed matter : an Institute of Physics journal.

[17]  I. Todorov,et al.  Simulation of mineral solid solutions at zero and high pressure using lattice statics, lattice dynamics and Monte Carlo methods , 2004 .

[18]  F. Shimojo,et al.  Diffusion of mobile ions and bond fluctuations in superionic conductor CuI from ab initio molecular-dynamics simulations , 2003 .

[19]  R. Alben,et al.  The Raman spectra of the superionic conductor CuI in its three phases , 1977 .

[20]  R. Mcgreevy,et al.  A molecular dynamics study of ionic conduction in CuI. II. Local ionic motion and conduction mechanisms , 1996 .

[21]  Nicola Marzari,et al.  Dynamical structure, bonding, and thermodynamics of the superionic sublattice in alpha-AgI. , 2006, Physical review letters.

[22]  G. Jacucci,et al.  Vacancy Double Jumps and Atomic Diffusion in Aluminum and Sodium , 1977 .

[23]  S. Hull,et al.  The high-temperature structural behaviour of copper(I) iodide , 1995 .

[24]  R. Mcgreevy,et al.  Disorder in the fast ion conductor CuI , 1997 .

[25]  G. Kresse,et al.  Ab initio molecular dynamics for liquid metals. , 1993 .

[26]  P. Madden,et al.  Nature of the superionic transition in Ag+ and Cu+ halides , 2003 .

[27]  J. Mikkelsen,et al.  Extended-x-ray-absorption-fine-structure investigation of mobile-ion density in superionic AgI, CuI, CuBr, and CuCl , 1981 .