Preface Free

Inorganic solid-state and materials chemistry has long been dominated by, and is synonymous with, the chemistry of oxides. To a large extent, this dominance is a consequence of the natural conditions on Earth and its thermodynamics. Oxygen forms a large part of the gaseous atmosphere of the planet and as a molecule dioxygen is reactive, with a propensity to form the divalent oxide anion, O²⁻. Many of the principles of solid-state chemistry have been established by considering the reactions of one or more elements with oxygen to form binary and higher oxides, which can then be combined together in synthesis (often in the solid state). The design of oxide materials has relied on the relationships between synthesis, structure and properties and the modification and optimisation of materials performance is commonly achieved by the substitution, addition or removal of metal or metalloid cations within the constant environment of an anionic oxide sublattice.

The relative ease with which oxides can be synthesised (and handled) and the ubiquity of oxide compounds (as or from minerals) has perpetuated the emphasis on oxides, yet despite this, many of the most fascinating solids and important materials do not contain oxygen. Some of these are naturally-occurring, but over time scientists have developed increasingly reliable and sophisticated ways to prepare non-oxide materials. In parallel, the adoption of advanced characterisation techniques has enabled us to learn more and more about non-oxides such that compounds previously considered ‘exotic’ have become progressively better understood. Families of non-oxides have become well-established with many as well-known as their oxide counterparts. Moreover, materials design strategies now embrace the chemistry of the anion to a greater extent than at any time previously and the tuning of properties by the (partial) replacement of anions (whether oxide or others) is a central theme of heteroanionic materials chemistry.

This book, Non-oxide and Heteroanionic Materials, assembles many of the most influential classes of inorganic solids beyond the traditional boundaries of oxide chemistry, providing important examples of state-of-the-art and emerging materials within each category. Chapter 1 begins with the familiar territory of Group 16 but with an emphasis on the elements below oxygen in the Periodic Table. This chapter considers some of the vast expanse of chalcogenides currently known to inorganic chemists, first establishing some of the ‘ground rules’ of solid-state chalcogenide chemistry before discussing layered dichalcogenides as archetypal examples of fundamentally important compounds that have developed from important bulk materials to cutting edge nanomaterials. The multitude of methods for the synthesis of chalcogenides are reviewed with the suitability of each to certain compound- and structure types analysed. Finally, a selection of some of the most significant contemporary materials is presented, with an emphasis on sustainable energy applications and how these are influenced by the often singular or unusual properties of sulfides, selenides and tellurides. Chapter 2 covers Group 15 and considers the pnictides from phosphorus to bismuth. The contrast to the chalcogenides is striking and the chapter discusses the unusual and intriguing crystal chemistry of this group of compounds while also discussing in some detail how such compounds can be isolated and stabilised through careful and controlled synthesis. The atypical structure and bonding of pnictides also gives rise to a wide range of interesting electronic and magnetic phenomena from superconductors to topological insulators and semi-metals. Useful chemical as well as physical properties exist however; emerging catalytic and electrochemical applications are among those currently being studied.

Chapter 3 discusses the materials chemistry of carbides, a grouping which represents the most widely studied of the tetrelides of Group 14. From recognised beginnings including both molecular and interstitial compounds, much recent interest in carbides has stemmed from the discovery of MXenes. This chapter provides a detailed overview of these fascinating layered nanomaterials and their origins from the well-established MAX phases. In the space of just over a decade, studies of these two-dimensional (2D) materials have uncovered numerous intriguing and useful physical properties which have been put to use in applications including sensors, actuators, light-emitting diodes (LEDs) and membranes. In Chapter 4 the spotlight falls on solid state halides and hydrides (often themselves considered as pseudohalides). This chapter concentrates especially on applications and the enormous impact that both sets of compounds are having in terms of future sustainable energy. Hydrides are inevitably central to the implementation of a hydrogen economy and inherently linked to the generation, purification and storage of hydrogen. With hydrogen as a potential clean fuel for use in both electricity generation and heating, applications as catalysts (for example in fuel cells) and as thermal storage materials are also potentially very important. Coupled with the ascendency of halide perovskites in solar cells plus the use of both sets of materials as components in secondary batteries, hydrides and halides are arguably at the forefront of future energy conversion and storage technologies.

Finally, the book concludes with an in-depth view of the concept of heteroanionic materials (HAMs) in Chapter 5. In principle, there are endless possibilities when one considers the combinations of simple anions in the solid state. This chapter considers one particularly enticing aspect of the area; namely the interface between oxide and non-oxide chemistry. With a focus particularly on combinations of oxide with its nearest neighbours (the anions of nitrogen and sulfur), the chapter collates some of the many intriguing properties—and potential applications—that can arise when these anions are brought together within a single crystalline structure. Just some of the vast array of structures and bonding permutations are considered and the connections between these structures and properties as diverse as superconductivity and photocatalysis are explained.

Although the exploration of non-oxide materials chemistry is a relatively young venture, there is a rich history of these solids and the catalogue of new and beguiling compounds is rapidly expanding. The aim of this book is to capture this burgeoning area in five distinctive and representative chapters. While this volume by no means covers all the many types of non-oxides (some of which might deserve a tome to themselves) or provides a comprehensive explanation of every one of their countless features, I hope that it serves as a valuable and satisfying introduction to the field.

Duncan H. Gregory