Unit cell parameters of 32S8 – 34S8 solid mixtures and their extremal behavior

Abstract

The behavior of isotopic mixtures of polyatomic molecular compounds can have a critical character. Orthorhombic sulfur α-S8 is a typical representative of polyatomic molecular compounds among simple substances. Samples comprised of orthorhombic sulfur of different 32S and 34S ratio have been prepared and examined with powder X-Ray diffractometry in the temperature region from −100 to +90 °С. Unit cell parameters of sulfur samples with intermolecular isotopic substitution, containing 0.4 to 0.5 parts of 34S8 molecules, differ radically from the samples of the isotopic composition close to pure α-32S8 and α-34S8. Maximal deviation of unit cell volume −0.17% at −100.0 °C is observed for the sample with a part of 34S8 molecules being 0.45. Such a behavior could well be attributed to the ordering between 32S8 and 34S8 molecules in the crystal that can be triggered by lowering temperature.