Thermal behavior of realgar As4S4, and of arsenolite As2O3 and non-stoichiometric As8S8+x crystals produced from As4S4 melt recrystallization

Abstract

An in situ high-temperature X-ray powder diffraction study of the thermal behavior of realgar (α-As 4 S 4 ) has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the realgar → β-As 4 S 4 phase transition starts at 558 K and is completed at 573 K due to kinetics. Melting starts at 578 K and is completed at 588 K. Thermal expansion of realgar is significant and fairly isotropic. In fact, the a -and b -parameters expand almost at the same rate, whereas the c -parameter is slightly softer against heating. Moreover, the β-angle contracts as temperature is raised. The geometry of the As 4 S 4 molecule is largely independent from heating. The lengthening of a few As-S and As-As contacts above or near the sum of the As,S van der Waals radii represents the driving force of the phase transition. In addition, the thermal behavior of arsenolite As 2 O 3 and non-stoichiometric As 8 S 8+x crystals produced from As 4 S 4 melt recrystallization has been investigated. Two members located along the β-As 4 S 4 -alacranite (As 8 S 9 ) series joint were identified at RT: a term close to the β-As 4 S 4 end-member (As 8 S 8+x : x = ca. 0.1) and one term of approximate As 8 S 8.3 composition. The thermal expansion of β-As 4 S 4 is significantly anisotropic following the α b > α a > α c relationship. This is clearly the result of the different packing scheme of the As 4 S 4 cages in β-As 4 S 4 with respect to realgar. The dependence of cell parameters and volume of As 8 S 8.3 is more complicated. In fact, a strong discontinuity on the dependence of cell parameters and volume is observed in the 403–443 K thermal range, i.e., that at which As 8 S 8.3 converts partly to realgar. A significant volume expansion is observed as a result of a change of composition to As 8 S 8.7 .