Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

Abstract

GeTe is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic -rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ~30-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ~0.07 eV is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behaviour of GeTe depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted GeTe 6 octahedra, with a rather small activation energy barrier.

Anomalous Behavior of the Elastic and Optical Properties in Bi1.5Sb0.5Te1.8Se1.2 Topological Insulator Induced by Point Defects

“…The result of applying Equation to our data is presented in Figure . The obtained activation energy value V 0 = 78.7 meV is higher than that for point defects in semiconductors, but comparable to the one determined by changing the configuration of short and long Ge‐Te bonds in GeTe …”

Ferroelectric phase transition and the lattice thermal conductivity of Pb1−xGexTe alloys

“…Another controversial issue is whether Pb 1−x Ge x Te alloys undergo a displacive second-order [36,37] or an orderdisorder [50,59,82,83] phase transition with composition and temperature. Resolving these questions is beyond the scope of this work.…”

Magnetoelastic coupling behavior at the ferromagnetic transition in the partially disordered double perovskite La2NiMnO6

“…The simplest interpretation of rounded peaks in Q −1 as a function of temperature is that they are due to a thermally activated loss mechanism which can be described by the Debye equations. A standard fitting procedure for acoustic data makes use of the expression [15,[68][69][70][71][72][73]]…”