Band 226, Heft 4 -

A crystal-chemical approach to superconductivity is described that is intended to complement the corresponding physical approach. The former approach takes into account the distinction between the stoichiometric valence ( stoich V ) and the structural valence ( struct V ) which is represented by the bond-valence sums (BVS). Through calculations of BVS values from crystal-structure data determined at ambient temperature and pressure it has been found that in chalcogenides und pnictides of the transition metals Fe, Co, Ni, Mn, Hf, and Zr the atoms of the potential superconducting units yield values of |BVS| = | struct V | ≥ 1.11 × | stoich V |, whereas the atoms of the charge reservoirs have in general values of | struct V | < 1.11 × | stoich V |. In corresponding compounds which contain the same elements but are not becoming superconducting, nearly all atoms are found to have | struct V | < 1.11 × | stoich V |. For atoms of oxocuprates that are not becoming superconducting and for atoms of the charge reservoirs of oxocuprates that become superconducting, the relation | struct V | < 1.11 × | stoich V | seems also to be fulfilled, with the exception of Ba. However, in several oxocuprates the relation | struct V | = 1.11 × | stoich V | for the atoms that become superconducting units is violated. These violations seem to indicate that in oxocuprates it is the local bond-valence distribution rather than the bond-valence sums that is essential for superconductivity. The present analysis can possibly be used to predict, by a simple consideration of ambient- T , P structures, whether a compound can become an unconventional superconductor at low T , under high P and/or by doping, or not.

The presence of mixed valency in unconventional superconductors is a necessary consequence of the fact that there is chemical equilibrium between the atoms of a transition element having different oxidation states. Consequently, phases described by stoichiometric formulas such as FeSe, Ba(FeAs) 2 , La 2 CuO 4 etc. deviate in fact from their nominal compositions. These deviations can be considered as doping by replacing some of the cationic atoms Fe II and Cu II in the conducting units by small amounts of Fe and Cu in higher, sometimes in lower oxidation state. From a consideration of the electronegativity values of the constituents of a selection of widely different unconventional superconductors and corresponding parent-compounds, it is evident that bonds between atoms of the conducting units have higher covalency than bonds between conducting units and charge reservoirs which are mixed ionic/covalent, and bonds within the charge reservoirs which are strongly ionic. In compounds without charge reservoirs, the bonds between neighbouring conducting units have van der Waals character.

The discovery of superconductors on copper-oxygen basis in 1986 was one of the most celebrated events in condensed matter physics. The maximal tran sition temperature T c close to 150 K was observed in HgBa 2 Ca 2 Cu 3 O 8+ δ at high pressure. In spite of enormous progress in both the experimental knowledge and theoretical understanding, the origin of the high T c remains elusive. Although the materials properties require unexpectedly complicated technical solutions several applications have been commercialized.

More than 20 years after the discovery of high-temperature-superconductors (HTS) first applications in the energy sector are now becoming commercially attractive. Whereas still high end applications of low-temperature-superconductors (LTS) like NMR or MRI systems are the major drivers for industrial production, HTS based systems promise broader use in energy technology over the next few years. Due to their superior performance HTS materials enable more compact and lighter electric devices as well as an unbeatable improvement in energy efficiency. Moreover new devices like fault current limiters (FCL) are under development and field tests exhibit so far unknown properties for future electric grid design. Prototypes and first commercial systems like cables, generators, motors, FCL were successfully commissioned in the last few years. Based on these test results HTS systems will contribute significantly to the future key challenges in energy technology by reducing CO 2 -emmission due to their outstanding efficiency and to the security of energy supply.

Few years after the discovery of superconductivity in high- T c cuprates, borocarbides and shortly after nitridoborates with reasonably high T c s up to about 23 K attracted considerable attention. Particularly for the rare-earth metal series with composition R Ni 2 [B 2 C] it turned out, that several members exhibit superconductivity next to magnetic order with both T c above or below the magnetic ordering temperature. Therefore, these compounds have been regarded as ideal materials to study the interplay and coexistence of superconductivity and long range magnetic order, due to their comparably high ordering temperatures and similar magnetic and superconducting condensation energies. This review gathers information on the series R Ni 2 [B 2 C] and isostructural compounds with different transition metals substituting Ni as well as related series like RM [BC], RM [BN], AM [BN] and R 3 M 2 [BN] 2 N (all with R = rare-earth metal, A = alkaline-earth metal, M = transition metal) with special focus on synthesis, crystal structures and structural trends in correspondence to physical properties.

We review superconductivity in MgB 2 in terms of crystal and electronic structure, electron-phonon coupling, two-gap superconductivity and application. Finally, we introduce the development of new superconducting materials in related compounds.

Two different polymorphs of the metal nitride halides M N X ( M = Ti, Zr, Hf; X = Cl, Br, I) are known to crystallize in layered structures. The two crystal structures differ in the way ∞ 2 { X [ M 2 N 2 ] X } slabs are stacked along the c -axes. Metal atoms and/or organic molecules can be intercalated into the van-der-Waals gap between these layers. After such an electron-doping via intercalation the protoätypic band insulators change into superconductors with moderate high critical temperatures T c up to 25.5 K. This review gathers information on synthesis routes, structural characteristics and properties of the prototypic nitride halides and the derivatives after electron-doping with a focus on superconductivity.

PbO-type Fe chalcogenide has been drawing much attention as the Fe-based superconductor with the most simple crystal structure. Whereas FeSe is an intrinsic superconductor, FeTe, while having a structure analogous to FeSe, exhibits antiferromagnetic ordering. The relationship between antiferromagnetism and superconductivity provides key information to understand better superconductivity in Fe chalcogenides. Furthermore the significant pressure effect on T c in the Fe-chalcogenide superconductors, which is likely to be correlated with the crystal structure and/or magnetism, is important in elucidating the mechanism of Fe-based superconductivity. Due to the simple structure and composition, Fe-chalcogenide superconductor is one candidate for applications to such areas as superconducting wires and thin films.

The basic structural chemistry of superconducting pnictides and pnictide oxides is reviewed. Crystal chemical details of selected compounds and group subgroup schemes are discussed with respect to phase transitions upon charge-density formation, the ordering of vacancies, or the ordered displacements of oxygen atoms. Furthermore, the influences of doping and solid solutions on the valence electron concentration are discussed in order to highlight the structural and electronic flexibility of these materials.