Abstract
The structural phase transition in the ferroelectric semiconductor alloy series SnxGe1xTe has been investigated using high-precision x-ray techniques. The spontaneous strain occurring at the cubicrhombohedral transition has been measured as a function of temperature for 0.53<x<0.87 and a changeover from second-order to first-order transitions is found for x0.72. The behavior in the vicinity of the tricritical point can be analyzed in terms of Landau theory and the composition dependence of the free-energy expansion coefficients has been calculated up to sixth order, the quartic coefficient becoming negative at x0.72. The influence of the electronic structure on the ferroelectric instability is considered and it is argued that when the band gap is small enough, simultaneous instabilities can occur in the electronic structure and the crystal structure, with the cubic semiconductor phase transforming to a rhombohedral semimetallic form. It is suggested that, as the band gap decreases going from SnTe to GeTe, the coupling between these two instabilities grows, leading to the observed line of first-order transitions.