The Effect of Pressure on Polymorphic Transitions of Solids

Abstract

This note presents in a compact form by means of diagrams many of the essential facts concerning the effect of high hydrostatic pressure on the polymorphic transitions of 30 substances. Five of these diagrams have been previously published,l eleven are to be published with greater detail in a forthcoming number of the Proceedings of the American Academy, and the rest await detailed publication. The experimental methods have been fully described previously. The investigation has been assisted in great measure by generous grants from the Bache Fund of the National Academy of Sciences and from the Rumford Fund of the American Academy of Arts and Sciences. The diagrams show the transition curves on the pressure-temperature plane of the several solid phases, which are indicated by Roman numerals. The liquid phase, where it occurs, is denoted by an L. Notice that the temperature scale is changed in the diagrams for KHS04 and H20. NH4NO3 has one transition line not shown, of the ice type, beginning at -16?, and RbNO3 has one at 219?, probably of normal type. The arrows on the transition lines indicate the directions in which the difference of volume of the two phases decreases numerically. An a, f or Cp placed on one side of a curve indicates that the phase on that side of the curve has the larger compressibility, thermal expansion, or specific heat. In the detailed presentation of data, the numerical values of all these factors, as well as of the latent heat of transition and change of energy are given. The number of substances is perhaps sufficient to justify an enumeration of the relative frequency of different types of behavior. The first impression is one of bewildering complexity, it is obvious that the phenomena of polymorphism, even under high pressures, do not tend to any simple type. The apparent complexity is rather increased when one considers the diagrams of chemically related substances, such as the six nitrates, the four iodides, the two sulfocyanides, and acid sulfates. It is possible, however, to detect traces of regularity among the nitrates and sulfocyanides by putting into correspondence the phases which belong to the same crystalline system. The variety of shapes possible for the individual curves is in striking contrast with the case for melting curves. Every rising melting curve