Structural instabilities are now being found in many high-temperature superconductors. Compounds with the A-15 (P-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. CONTENTS

We have measured the Seebeck coefficient and electric conductivity in the air-stable conducting polymers polyaniline and polypyrrole at different doping levels. We find, at 300 K, the general correlation that the logarithm of the electrical conductivity varies linearly with the Seebeck coefficient on doping, but with a proportionality substantially in excess of a prediction from simple theory for a single type of mobile carrier. The correlation is unexpected in its universality and unfavorable in its consequences for thermoelectric applications. A standard model suggests that conduction by carriers of both signs may occur in these doped polymers, which thus leads to reduced thermoelectric efficiency. We also show that polyacetylene ͑which is not air stable͒, does exhibit the correlation with the expected proportionality, and, thus, its properties could be more favorable for thermoelectricity.

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