Competing covalent and ionic bonding in Ge-Sb-Te phase change materials

TL;DR

This paper reveals the competition between ionic and covalent bonding in Ge-Sb-Te phase change materials, explaining their unique physical properties and rapid phase transitions, and suggests pathways for designing improved materials.

Contribution

It uncovers the role of competing ionic and covalent bonds in phase change materials, linking chemical bonding to their unique physical and dynamical properties.

Findings

Strong competition between ionic and covalent bonding in cubic phase.

Bonding differences explain physical property variations between phases.

Bonding insights may guide development of superior phase change materials.

Abstract

Ge2Sb2Te5 and related phase change materials are highly unusual in that they can be readily transformed between amorphous and crystalline states using very fast melt, quench, anneal cycles, although the resulting states are extremely long lived at ambient temperature. These states have remarkably different physical properties including very different optical constants in the visible in strong contrast to common glass formers such as silicates or phosphates. This behavior has been described in terms of resonant bonding, but puzzles remain, particularly regarding different physical properties of crystalline and amorphous phases. Here we show that there is a strong competition between ionic and covalent bonding in cubic phase providing a link between the chemical basis of phase change memory property and origins of giant responses of piezoelectric materials (PbTiO3, BiFeO3). This has important consequences for dynamical behavior in particular leading to a simultaneous hardening of acoustic modes and softening of high frequency optic modes in crystalline phase relative to amorphous. This different bonding in amorphous and crystalline phases provides a direct explanation for different physical properties and understanding of the combination of long time stability and rapid switching and may be useful in finding new phase change compositions with superior properties.