Anomalous temperature-induced volume contraction in GeTe

The recent surge of interest in phase-change materials GeTe, Ge2Sb2Te5, and related compounds motivated us to revisit the structural phase transition in GeTe in more detail than was done before. The rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied using high-resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. The results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral-to-cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe, we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.