DEDICATED TO THE MEMORY OF ALEXANDER F. WELLS chemistry under pressure is based, are included, and his book was our ''Bible'' for envisaging new systems to inves-This article comprises four sections. The first introduces the subject of pressure-induced syntheses and phase transitions tigate. In this introduction we cite mainly the work we did taking place in ionic compounds. Examples are described to in the field of high pressure crystallochemistry, but we illustrate the general trends governing these transformations, do this purposely to show how Professor Wells indirectly namely the increase of the cation and anion coordinations under influenced our research; we are honored to pay this tribute pressure and the greater compressibility of anions with respect to him.
to that of cations. In the second section, pressure-induced phase
When ionic compounds are subjected to high pressure, transitions taking place in superconducting layered cuprates the volume, as well as some of the interatomic distances, are discussed and the only example known so far, that occurring decreases. However, the latter cannot decrease below critiin ACuO 2 compounds, is illustrated. In the third section, prescal values at which the structure becomes unstable and sure-induced syntheses are discussed. The examples in which either the compound decomposes or undergoes a structural pressure has been crucial to obtain new phases are numerous. transformation. The changes in the interatomic distances In some cases, like the Hg-based cuprates, the new compounds are very important parameters for understanding the variaare not high pressure phases, but nevertheless the use of prestion of the physical properties. On the other hand, decomsure has greatly helped to optimize the synthesis of the samples.
In other cases, like Sr 2 CuO 3Ψβ¦ , only the synthesis under pressure positions and phase transformations may lead to the synyields superconducting samples. In the fourth section, the in thesis of new phases with known or unknown structures.
situ pressure studies carried out at Houston, Grenoble, Bir-A close-packed array of anions generates two types of mingham, and Argonne are described. The dramatic increases interstices, the tetrahedral and octahedral sites. Specifiof T c for the first four members of the Hg homologous series cally, a close-packed array of n anions contains n octaheare strictly correlated to the shortening of the apical Cu-O dral and 2n tetrahedral sites. If we assume that the anions distance which in these compounds is anomalously large at are spheres of 1.4 A Λin radius, which is about that of ambient pressure. If the structural arrangement responsible for O 2Οͺ , the tetrahedral and octahedral sites can accommodate 164 K superconductivity has to be stabilized by substitution, spherical cations of a diameter around 0.32 and 0.58 A Λ, resecond nearest-neighbor interactions must be taken into acspectively.
count. The synthesis of such compounds should be one of the Let us assume that we have a compound in which the primary future goals.