Cooperative excitations within the superionic phase of PbF2

Links between dynamical Frenkel defects and collective diffusion of fluorides in beta-PbF2 are explored using Born-Oppenheimer molecular dynamics. The calculated self diffusion coefficient and ionic conductivity are 3.2 x 10-5 cm2s-1 and 2.4 Omega-1 cm-1 at 1000 K in excellent agreement with results from pulsed field gradient and conductivity measurements: DPFG = 4 x 10-5 cm2 s-1 and sigmad.c. sim 3 Omega-1cm-1. The calculated ratio of tracer diffusion to conductivity diffusion is slightly less than unity (about 0.85), which in previous work has been interpreted as providing little evidence for collective diffusion. In contrast, our molecular dynamics simulations show that diffusion of fluorides is highly correlated and a range of mechanisms play important roles. These include, for example, direct collinear ``knock out'' chains - where a fluoride enter an occupied tetrahedral hole/cavity and pushes the resident fluorine out of its cavity - and `loops'' which generate no net charge ...