Refinement of Unit Cell Parameters on Modern Single-Crystal Diffractometers. Study of the Anisotropy of the α-33S Single Crystal Thermal Expansion

A technique for refining unit cell parameters of crystals belonging to arbitrary systems on modern single-crystal diffractometers is described. The technique is based on the 2D-detector position calibration. The elementary orthorhombic unit cell parameters of the α-³³S single crystal have been refined. The anisotropy of change in the parameters in the range of 90–350 K has been analyzed. The relative increase in the parameter c is shown to be 6.4%. The obtained dependences are approximated by second–third degree polynomials. The absolute and relative increments in the unit cell volume were found to be 138.4 ų and 4.3%, respectively. The temperature dependences of the thermal-expansion tensor elements have been refined. The coefficients of thermal expansion of α-³³S at room temperature turned out to be α₁₁ = 15.35 × 10^–5, α₂₂ = 8.56 × 10^–5, and α₃₃ = 9.12 × 10^–5 K^–1.