  Crystal and electronic structure, lattice dynamics and thermodynamic
stability of little known mixed valent diamagnetic AgIAgIIIF4 beta form of AgF2
is thoroughly examined for the first time and compared with well known
antiferromagnetic AgIIF2 alpha form within the framework of Density Functional
Theory based methods, phonon direct method and quasiharmonic approximation.
Computed equations of state, bulk moduli, electronic densities of states,
electronic and phonon band structures including analysis of optically active
modes and p T phase diagram of the alpha/beta system are presented. This study
demonstrates that alpha is thermodynamically preferred over beta at all
temperatures and pressures of its existance but simultaneously beta is
dynamically stable in much broader pressure range. The beta phase is discussed
in broader context of isostructural ternary metal fluorides and isolectronic
oxides including NaCuO2, the reference compound for existence of CuIII species
in high temperature oxocuprate superconductors.
