While orientational disorder in solid electrolytes is usually discussed in the context of molecular or polyanion orientational degrees of freedom, materials that contain post-transition metals with "stereoactive" lone pairs; such as Sn or Bi; may exhibit electronic orientational disorder . These cations, when in an oxidation state two less than their formal maximum (e.g., Sn II and Bi III ), have a formal electron configuration with a filled s-orbital as their last valence shell. These s 2 states can mix with neighbouring-anion p states to form a bonding state and an antibonding state, and this antibonding state mixes with metal p states to form an asymmetric lone pair state, characterised by an eccentric (off-center with respect to the atomic nucleus)"stereoactive" charge density, that directs the cation coordination geometry and often results in distorted low-symmetry cation coordination environments .