A crystal chemical approach to superconductivity. I. A bond-valence sum analysis of inorganic compounds

Abstract

A crystal-chemical approach to superconduc- tivity is described that is intended to complement the cor- responding physical approach. The former approach takes into account the distinction between the stoichiometric va- lence ( 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 de- termined 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 poten- tial superconducting units yield values of |BVS| ¼ | struct V| � 1.11 � | stoich V|, whereas the atoms of the charge reser- voirs have in general values of | struct V| < 1.11 � | stoich V|. In corresponding compounds which contain the same ele- ments 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 superconduct- ing 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| ¼ 1.11 � | stoich V| for the atoms that become superconduct- ing 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 unconven- tional superconductor at low T, under high P and/or by doping, or not.