Soft modes and related properties in cubic PbF2

Studies of the temperature and pressure dependences of the static dielectric constant and ionic conductivity of cubic PbF2 have led to the discovery of two important results: (i) the existence of a soft q = 0, TO phonon mode and (ii) an anomalously large initial decrease with pressure in the activation energy for ionic conduction. The TO mode softens only partially with decreasing T, the crystal remaining stable down to the lowest temperatures. Cubic PbF2 is, however, known to undergo a pressure-induced transition to an orthorhombic phase. The possibility of a connection between the ionic conductivity and soft modes in PbF2 is discussed.