Z. Kristallogr. 226 (2011) V–VI / DOI 10.1524/zkri.2011.0002 V # by Oldenbourg Wissenschaftsverlag, Mu¨nchen

Preface

This year we celebrate 100 Years of Superconductivity. After having successfully liquefied helium in 1908, Heike Kammerlingh Onnes, a Dutch physicist from Leiden University, studied the resistivity behaviour of mercury in 1911 and observed a sud- den drop to zero at the temperature of 4.2 K. Kammerlingh Onnes named the phe- nomenon superconductivity and was awarded with the Nobel Prize in Physics in 1913 for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium. In the following years supercon- ductivity was found in other metals and in a growing variety of intermetallic com- pounds. In 1933, the German scientist Walther Meißner found that superconductors behave as perfect diamagnets. The repulsion of the interior magnetic field when entering the superconducting state is the famous Meißner-Ochsenfeld effect. It was initially striking that especially highly symmetric compounds and those based on niobium showed superconductivity. In 1967, Bernd Matthias et al. found supercon- ductivity at 23.3 K in Nb3Ge with the cubic A15 structure. Isotypic Nb3Sn is still used today for superconducting coils due to its good mechanical properties. The origin of superconductivity was not known until 1957, when John Bardeen, Leon N. Cooper and John R. Schrieffer proposed the first conclusive theory that explained the phe- nomenon at very low temperatures. The so-called BCS theory won the Nobel Prize in 1972 and explained superconductivity as a quantum mechanical effect of coupled electrons, the Cooper-pairs. These discoveries led to the construction of Josephson junctions and SQUID sensors widely used today. More than 70 years, superconductivity remained a property of metals at very low temperatures, not too far from absolute zero. A breakthrough unique in the history of materials science came in the 1980’s, when Johannes Georg Bednorz and Klaus Alexander Mu¨ller reported on critical temperatures around 30 K in La1–xSrxCuO4 (1986, Nobel Prize in Physics 1987) and Sleight and co-workers on Ba1–xKxBiO3 (1989) at 34 K. Almost no one, least of all Bernd Matthias who discovered more than 1000 superconductors, imagined superconductivity in such materials at that time. One key point to superconductivity in these materials is the mixed valence of Bi and Cu. These compounds structurally derive from the perovskite type, and only shortly later materials with so far unimaginable critical temperatures were discovered. The most prominent is YBa2Cu3O7–d (YBCO) with a critical temperature of 92 K, well above the boiling point of liquid nitrogen. The so far highest transition temperature close to 150 K has been observed for a mercury-cuprate based on mercury, barium, calcium, copper and oxygen. But in spite of the immense scientific efforts on the cuprate based materials with to date more than 100,000 publications, the detailed physical mechanism remains still uncertain. Especially from the view of materials chemistry, one needs to mention at least some of the many other classes of superconductors like rare earth boride carbides and boride nitrides, magnesium boride, the Chevrel phases with high critical fields, superconducting tetrathiafulvallenes, and the recently discovered superconductivity in iron pnictides, pnictide oxides and selenides. The present topic issue of Zeitschrift fu¨r Kristallographie focuses on some recent developments in Superconductivity research and presents nine review articles which provide some insights in his field. The first contribution by Liebau, Klein and Wang gives a crystal-chemical ap- proach to superconductivity via a bond-valence sum analysis of inorganic com- pounds. This empirical approach is followed by an article by Liebau on nonstoichio- metry and bond character in unconventional superconductors. Hackl presents a broad overview on the crystal chemistry and especially the phy- sics of cuprate superconductors, focusing on crystal growth parameters, electronic structure, and topics like metal-insulator transition. The following contribution by Ba¨cker (Zenergy Power) focuses on first applica- tions of high TC materials in the energy sector. Developments in the last few years VI Preface

concern prototypes and first commercial systems like cables, generators, and motors. These systems will contribute significantly to the future key challenges in energy technology by reducing CO2-emmission due to their outstanding efficiency and to the security of energy supply. Niewa, Slykh, and Blaschkowski review the structural chemistry and supercon- ducting properties of borocarbides and nitridoborates with reasonably high critical temperatures up to about 23 K. Particularly the RENi2[B2C] series turned out to exhibit superconductivity next to magnetic order and these compounds have been regarded as ideal materials to study the interplay and coexistence of superconductiv- ity and magnetism. Muranaka and Akimitsu report on superconductivity in MgB2 in terms of crystal and electronic structure, doping studies, electron-phonon coupling, two-gap super- conductivity and possible applications. The layered structures of the metal nitride halides are reviewed in a contribution by Schurz, Slykh, Schleid, and Niewa. These nitrides crystallize in two different polymorphs. The review focuses on the synthesis routes, structural characteristics, and the influence of electron-doping via intercalation on the superconducting proper- ties. Mizuguchi and Takano present recent developments of the PbO-type iron chalco- genides which display one of the simplest crystal structures among the iron-based superconductors and they discuss potential applications of wires and thin films. Finally Johrendt, Hosono, Hoffmann, and Po¨ttgen review the materials chemistry and structure-property relationships of superconducting pnictides and pnictide oxides with layered structures including group-subgroup relations. Although tremendous work on the synthesis and characterization on supercon- ducting materials has been carried out in the last 100 years by solid state chemists and physicists, the mystery and mechanism of superconductivity is not yet solved. The current progress in pnictide superconductors impressively shows that the phe- nomenon of superconductivity still fascinates scientists all over the world. It will arguably be one of the key topics in basic and applied material science in the years to come.

Rainer Po¨ttgen Dirk Johrendt Westfa¨lische Wilhelms-Universita¨t Ludwig-Maximilians-Universita¨t Mu¨nster Mu¨nchen [email protected] [email protected]