Theory of the structural phase transition of GeTe

A completely ab initio theoretical investigation of the rocksalt-rhombohedral structural phase transition of GeTe is described. Starting from an anharmonic lattice Hamiltonian, a model Hamiltonian that includes coupling of the order parameter to long-wavelength strain is constructed. The parameters appearing in the model are calculated using the self-consistent ab initio pseudopotential total-energy method. The phase transition in the model system is studied through a momentum-space renormalization-group-theory approach, leading to the prediction of a fluctuation driven first-order transition at 657±100 K. The strain coupling is found to be crucial in determining the first-order character of the transition. A discussion of approximations made in this approach and considerations relevant to its application to structural transitions in other systems is included.