The development of an approach for the precision determination of thermal strain tensor elements for single crystals using the example of t-AgLiInSe

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 Kα 1 /Kα 2 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 0.8 Li 0.2 InSe 2 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 0.8 Li 0.2 InSe 2 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: α 11 × 10 6 = −4.28 + 0.08 T − 1.95 × 10 −4 T 2 and α 33 × 10 6 = −5.59 + 0.01 T − 1.97 × 10 −5 T 2 . A new approach to the precision determination of single-crystal unit cell parameters is proposed and demonstrated using tetragonal Ag 0.8 Li 0.2 InSe 2 .