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Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence shell, and an anion (a negative ion) forms when a neutral atom gains one or more electrons in its valence shell. Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions. 
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Chemical bonding theories were pioneered by American chemist Gilbert N. Lewis. He developed a model called the Lewis model to explain the type and formation of different bonds. Chemical bonding is central to chemistry; it explains how atoms or ions bond together to form molecules. It explains why some bonds are strong and others are weak, or why one carbon bonds with two oxygens and not three; why water is H2O and not H4O. 
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The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...
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Updated: Jun 27, 2025

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of ChalcogenidoplumbatesII or IV
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Chemical bonding in phase-change chalcogenides.

P C Müller1, S R Elliott2, R Dronskowski1

  • 1Lehrstuhl für Festkörper- and Quantenchemie, Institut für Anorganische Chemie, RWTH Aachen University, D-52056 Aachen, Germany.

Journal of Physics. Condensed Matter : an Institute of Physics Journal
|May 2, 2024
PubMed
Summary
This summary is machine-generated.

Phase-change memory materials (PCM) utilize electron-rich, hypervalent bonds, not electron-deficient metavalent bonds. Our calculations confirm this bonding nature in PCM, crucial for understanding their function.

Keywords:
chalcogenide materialschemical bondingdensity functional calculationselectron-rich multicentre (‘hypervalent’) bondingmetavalencyphase-change materialsquantum theory of atoms in molecules (QTAIM)

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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.