Surface tension in ionic crystals

Abstract

The calculation of surface tensions in crystals of NaCl type is described, with particular reference to the (100) planes. In the cases investigated, the surface tension was greater than the corresponding surface energy by a factor of about 5. An account is given of experimental work carried out to confirm the presence of these surface forces. The basis of the method used was the measurement of the unit-cell dimensions of small crystals compared with those of normal size. As the particle size is diminished, surface forces of the order of those calculated should cause a gradual change in the unit-cell dimensions which becomes measurable by standard X-ray powder technique before line broadening masks the effect. Samples of small crystals of magnesium oxide and sodium chloride, whose particle size was determined by means of the electron microscope and by measurement of line broadening, were prepared and were found to show changes in the unit-cell dimensions of the sign and order of magnitude expected, provided that surface layers of adsorbed gases were absent.