Table of contents

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dⁿ configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

Along with pressure and temperature, the magnetic field is one of the physical parameters that are used for the investigation of matter. It is an important tool for investigating magnetic and electronic properties from both the microscopic and macroscopic points of view. The interaction energy of a substance with a magnetic field, i.e. Zeeman energy or Landau energy, is small even at a field of 2 T as can be provided by a conventional electromagnet. Compared with the inner energies of a substance, magnetic fields as high as possible have been required for obtaining fine and detailed information on the internal structure and energy states. Due to the strong Maxwell stress, however, it is quite difficult to obtain a magnetic field higher than 50 T and much effort has been dedicated to the development of magnets ever since the laboratory electromagnet was invented.

The purpose of this paper is to describe the physical effects that have been investigated and the phenomena that have been revealed using high magnetic fields, as well as the development of magnet technology and measurement technology. In addition, application to materials processing is mentioned as an active application of high magnetic fields.

This review summarizes the recent progress in experimental determination of electron densities in inorganic crystals and its contribution to our understanding of bonding and density functional approximations. The experimental techniques for electron density mapping are presented, including the accurate measurement of experimental structure factors by electron diffraction and multipole electron density modelling. Interpretation of electron density maps by combining first-principles calculations and spectroscopic measurements will be illustrated through the example of bonding in copper and cuprite. The significance of experimental electron density is demonstrated by selected examples of materials representative of ionic, metallic and covalent bonding and testing of theory.