Chemical bonding in phase-change chalcogenides

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Materials Science

Background:

• Phase-change memory (PCM) materials are crucial for data storage technologies.
• The chemical bonding in PCM materials has been debated, with descriptions including 'electron-deficient' (metavalent) and 'electron-rich' (hypervalent, multicentre) bonds.
• Understanding the precise nature of these bonds is essential for optimizing PCM performance.

Purpose of the Study:

• To unambiguously differentiate between 'metavalent' and 'hypervalent' bonding types in PCM materials.
• To determine the accurate electronic structure and bonding characteristics of PCM.
• To clarify the long-standing debate regarding the nature of chemical bonds in PCM.

Main Methods:

• Advanced computational calculations were employed to analyze chemical bonds.
• The study discriminated between electron-deficient and electron-rich bonding scenarios.
• Analysis focused on charge transfer (ET) and shared electrons (ES) between neighboring atoms.

Main Results:

• Calculations definitively showed that PCM materials exhibit electron-rich, 3-center-4-electron (3c-4e) 'hypervalent' bonds.
• The study demonstrated that PCM materials do not possess 'metavalent' bonding and are not electron-deficient.
• Simple charge transfer plots (ET vs. ES) were found insufficient to distinguish between 'metavalent' and 'hypervalent' bonds.

Conclusions:

• The chemical bonding in phase-change memory materials is unequivocally electron-rich and of the 'hypervalent' or multicentre type.
• This finding resolves the debate on PCM bonding, highlighting the role of lone-pair electrons.
• Accurate characterization of bonding in PCM is vital for the development of next-generation electronic devices.