scope:

Preface

Since 1986, when I first started playing around in Arndt Simon's chemical laboratory as a graduate student, I have never regretted entering the fascinating field of solid-state chemistry. Indeed, I have always found that this fundamental brand of the chemical sciences and also its somewhat more applied sister subject, materials chemistry, brings us into contact with a large part of the "real world" surrounding us, and a creative solid-state (or materials) chemist is in true command, in the same way as a molecular inorganic chemist, of the whole periodic table when he or she thinks of making new compounds with often unforeseeable but exciting physical properties. It does not come as a surprise that the extraordinarily broad field of solid-state chemistry is a truly interdisciplinary one. Thus, solid-state chemistry borders with solidstate physics, crystallography, quantum theory, metal science and inorganic chemistry, to name but a few; also, it is one of the rock-solid platforms on which the increasingly popular fields of nanoscience and nanomaterials may be built.

Some of the breathtaking technological advances of the 20th, and also the early 21st century, would have been totally impossible without the fundamental research originating within solid-state chemistry. Here I am thinking of insulators with designed properties such as dielectric ceramics for data transmission, novel ionic conductors for energy storage in hand-held electrical devices, magnetic intermetallics and oxides for data storage applications, advanced nitrides for electro-optical and diverse mechanical purposes, and also superconductors for energy transport and communication applications. This list could easily be made longer, and it surely will get longer as long as solid-state chemists and materials researchers are doing their part by creating the new, particularly when they are not thinking of applications but are concentrating on that wonderful art of curiosity-driven research. When you want to design andmake new

When you want to design andmake new things, however, you must be able to understand (or, at least, describe) the existing ones; thus, sooner or later, theory comes into play. Not too surprisingly, the theoretical tools available in solid-state and materials chemistry are as diverse as the solid-state chemistryrelated fields because this is where they originate. Thus chemists, physicists, crystallographers, and quantum theorists have all contributed to the strange blend of tools for describing, understanding and - which is now becoming increasingly important - predicting solid-state materials. At the present time, it seems that numerical approaches - which I will bravely summarize using the term computational chemistry - have reached a certain maturity which allows usage by the nonspecialist. Of course, computers have become more powerful, too, but this is mostly due to better hardware (solid-state materials) and, to a lesser extent, more user-friendly software.

Despite the ever-increasing importance of (quantum-theoretical) computer programs used in theoretical solid-state and materials chemistry, however, a newcomer will probably have difficulty in seeing thewood for the trees. Thus, I have felt the necessity to briefly present the type of theoretical approaches which might be used to successfully understand existing materials and also to navigate in the search for new solid-state compounds. These approaches purposely include traditional, classical ways of thinking but also quantummechanical approaches, because both are justified. It would be foolish to run high-scale quantum-mechanical calculations unnecessarily if an empirical back-of-the-envelope scheme is almost as predictive; also, a large amount of understanding is based upon classical ways of thinking. This may change but I am afraid it will take some time. On the other hand, why should we restrict ourselves to limited empiricalmethods if a reliable quantum-mechanical alternative is available? There are cases where the predictive power of quantum chemistry is so overwhelming that the experiment no longer has to be performed. Some scientists (like myself) will appreciate this, others will not. As you may have guessed, this book tries to bridge the gap.

As I write, I have imagined an intelligent reader (chemist, physicist, material scientist, crystallographer, etc.) who is already somewhat familiar with solid-state or materials chemistry, at least in terms of structure and also structure determination. However, because of space limitations, crystallographic techniques simply cannot be taught here. Also, some basic knowledge of quantum mechanics would do no harm. The rest of the book, however, is designed to be self-contained. I have tried to address a person working in the laboratory who is trying hard to make sense of his or her new discoveries. What is the bestway to describe your new compound in terms of, say, energetics? What can be learned from a structural discussion using radii concepts and volume considerations? What about more general structure rationalizations? Can one approximate the strength of chemical bonding without using quantum chemistry? If quantum chemistry is needed, what are the most important ingredients? What are these band structures, really? What do you learn from densities-of-states? Can chemical bonding in the solid state be quantified?

What are the pluses and minuses of the diverse quantum-chemical routes? Although it is a crystal, are the atoms really standing still? Can solid-state materials be predicted? Do we have quantum-theoretical access to chemical thermodynamics? Can one design new compounds? (The answer is yes). In general, how do you interpret the quantum-chemical result and how do you transfer the message into a language that can be used in the laboratory? In the end, the book should enable the reader to theoretically handle his or her own materials in the sense of correctly describing and understanding the compounds under study. If it would make you (yes, you) predict and synthesize new compounds, I would be truly happy. To ease the difficult life of the busy synthetic materials scientist, I have tried to make the book an entertaining, extremely light read, and I keep my fingers crossed that the "pure" theorists will kindly agree. For those who want to drill deeper, there are many superb monographs available, stuffed with lots of mathematics, and the appropriate references are also included in this little book.

This book would not be here if I had not gladly accepted the honorable invitation of Tohoku University (Sendai) for a guest professorship in the summer of 2004. I am especially grateful to my colleagues and students at the Center of Interdisciplinary Research, in particular to Professor Hisanori Yamane and his family, who did a spectacular job in making my visit a most memorable one. I am extending my thanks to Professor Shinichi Kikkawa at Hokkaido University (Sapporo) for his hospitality and for the great time I was able to enjoy. Sapporo is a wonderful place for beer, too.

Also, I would like to express my thanks to my own research group at RWTH Aachen University; without them, most of the things I find important to communicate to the readerwould simply be nonexistent. Over the last eight years, it has been a great pleasure to lead a group of experimentalists (challenging theory) and theorists (challenging experiment), and I would like to acknowledge the important contributions of all former and present coworkers. Thank you very much for your scientific and personal input. A good number of improvements to the first version of the manuscriptwere suggested, after careful reading, by Jörg vonAppen, Bernhard Eck, Boniface Fokwa, AndreasHouben, Michael Krings, Marck-Willem Lumey, Paul Müller, and Holger Wolff; whom I also thank. During my stay in Japan, Mona Marquardt did a wonderful job in keeping the group together and taking care of the communication; thank you so much. The manuscript has also profited from the critical remarks of some of my colleagues, namely Peter Blöchl (Clausthal), Lothar Fritsche (Karlsruhe), Karl Jug (Hannover), Gordon J. Miller (Ames), Rainer Pöttgen (Münster), Michael Ruck (Dresden), and Gerhard Raabe (Aachen); thank you for your time. It is fascinating to observe how differently chemists, physicists, experimentalists, and theorists may consider the same subject, and I hope to have come up with a sensible compromise. Birgit Renardy did a nice job in proof-reading the many references; thanks are due to her. AtWiley-VCH, Elke Maase, Linda Bristow, Uschi Schling-Brodersen, and Manfred Köhl provided helpful guidance. I also thank Roger De Souza (Aachen) for consulting his English dictionary on my behalf. If there are scientific or typographical errors left (and I am afraid they are there), I am solely responsible for them.

Finally, my colleagues from the Institute of Inorganic Chemistry at RWTH let me flee from the remarkable teaching burden of our institution for one semester, and I am very grateful to them for having made this possible. Needless to say, a thousand thanks go directly to my family for their inspiration and support. Gabriele was of tremendous help in the final formatting steps.¹

This book appears at a time where the high reputation of Germany's university system and even its top academic institutions are endangered by the thoughtless words and actions of some of our highly political elite. To comment on the recent embarassing large-scale experiments (with whatever future outcome) or to name their political inventors would give these people more honor than they deserve. Instead, I devote this book, with affection, to the University of Münster and its Chemistry and Physics Divisions because this is the great place where I had the pleasure to study in the first half of the 1980s.

1) All text was written on a Linux workstation using my beloved editor vim, and the entire book was processed bymeans of the glorious typesetting system LATEX2e and further handled using xdvi, dvips, ghostscript/ghostview, ps2pdf, and pdftk. The figures were generated using gnuplot, wxdragon, xfig, and xmgrace. A toast to fast, reliable, compact and open-source software!