The development of an approach for the precision determination of thermal strain tensor elements for single crystals using the example of t-Ag0.8Li0.2InSe2

Abstract

An original approach for the determination of unit cell parameter temperature dependence using a conventional single-crystal X-ray diffractometer is proposed. The approach is based on the calibration of the goniometer equatorial circle segment using the position of several Kα₁/Kα₂ doublets from reference single crystals. It allows for the refinement of lattice parameters of single crystals with a relative error of less than 10⁻⁴. Its capabilities are demonstrated using an example of tetragonal single crystals. The space group, crystal structure, and composition of t-Ag_0.8Li_0.2InSe₂ were determined using single-crystal X-ray diffraction analysis of three small (linear sizes ∼0.1 mm) fragments from a larger (linear sizes up to 10 mm) single-crystal grown using the Bridgman method. The dynamics of the crystal structure changes in the temperature range of 90–500 K were studied. The obvious anisotropy of t-Ag_0.8Li_0.2InSe₂ thermal behavior was shown with an overall increase in the volume of the tetragonal unit cell by 0.51%—parameter a increased by 0.34, while parameter c decreased by 0.17%. The nonlinear dependence of the thermal expansion tensor elements on temperature was determined: α₁₁ × 10⁶ = −4.28 + 0.08T − 1.95 × 10⁻⁴T² and α₃₃ × 10⁶ = −5.59 + 0.01T − 1.97 × 10⁻⁵T².