Incipient Metals: Functional Materials with a Unique Bonding Mechanism

TL;DR

This paper introduces 'metavalent' bonding as a new bonding mechanism in certain solid-state materials, especially phase-change materials, providing fundamental insights and guiding the discovery of novel functional materials.

Contribution

It defines and characterizes 'metavalent' bonding, distinguishing it from covalent, ionic, and metallic bonds, and introduces the concept of 'incipient metals' in solid-state chemistry.

Findings

Identification of 'metavalent' bonding as a distinct mechanism

Differentiation from resonant bonding in pi-orbital systems

Implication for designing new functional materials like PCMs and thermoelectrics

Abstract

While solid-state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. A prominent example is given by phase-change materials (PCMs) for data storage or photonics, which have recently been argued to show 'resonant' bonding; a clear definition of this mechanism, however, has been lacking until the present day. Here we show that these solids are clearly different from resonant bonding in the pi-orbital systems of benzene and graphene. Instead, they exhibit a unique mechanism between covalent and metallic bonding, which we call 'metavalent' bonding. The materials are on the verge of electron delocalization, which explains their exceptional property portfolio, and we therefore argue that they represent 'incipient metals'. This yields deeper, fundamental insight into the bonding nature of solid-state materials, and is expected to accelerate the discovery and design of new functional materials including PCMs and thermoelectrics.