Structural instability and superconductivity in A-15 compounds

Structural instabilities are now being found in many high-temperature superconductors. Compounds with the A-15 ($\beta$-W) structure comprise the most important group of these materials, and their properties have been extensively studied for over 15 years. A review is given of some of these results, particularly those relating to the elastic behavior. The empirical relation of the structural instability and the anharmonicity to the high superconducting transition temperature is discussed.