Lattice instability and phonon-widths in Pb1-xSnxTe. II

A comprehensive model is given for the lattice instability in Pb 1-x Sn x Te which explains preceding data, especially those in a previous paper (pt.I). The carrier concentration dependence and the composition dependence are included. Some new data from I which exhibit a large and strongly temperature-dependent TO-phonon width are studied. A single perturbative model is given which explains these data and preceding data. In this model the newly observed width is a lifetime effect due to nearest-neighbour anharmonic perturbations in the lattice Hamiltonian. Another more speculative model is given in which the observed width is a modulation effect arising from the presence of intrinsically non-linear solitary waves in the lattice. Possible layering as evidence for solitary waves is discussed. Some future experiments to distinguish among competing models are also proposed.