Structure and thermal properties of crystals, VI. The role of hydrogen bonds in Rochelle salt

Abstract

X-ray measurements have been made of changes in the lattice spacings of Rochelle salt between —90 and +40° C. Rough visual estimates have also been made of changes in the intensities of X-ray reflexions with temperature. The thermal properties of Rochelle salt have been correlated with its crystal structure. As with other crystals containing hydrogen bonds, thermal expansion is largest in the direction of these bonds, but above the upper Curie point this predominance largely disappears. Between the two Curie points the thermal expansion shows anomalies and is subject to hysteresis. Two main theoretical problems are discussed in the light of the observations. In the first place, the thermodynamic changes at the lower and upper Curie points have hitherto appeared to be transitions ‘of the second kind’, occurring within single crystals without change of phase. Fresh X-ray evidence shows that over the transition range of temperatures measurable differences can be detected between subcrystalline domains within a ‘single’ crystal. These differences reach a maximum value in certain directions in the crystal, but appear to be insufficient to cause it to break up into powder, with change of phase, at the two Curie points, so that most of the properties of a single crystal are retained over the whole range of temperatures. The experimental conclusions throw light on the general theory of phase transitions of the second kind. Secondly, the onset of anomalous dielectric properties at the lower Curie point is ascribed to the stretching of short hydrogen bonds owing to the thermal expansion of the crystal. This changes the bonds from a non-polar to a polar character and is correlated with thermal effects in other crystals containing hydrogen bonds. In extension of apparatus previously described a new controlled temperature chamber has been devised, which has been used with various X-ray cameras for single crystal work at temperatures down to — 90° C.