Investigation of the homogeneity of BGO single crystals, a promising X-ray diffraction standard

Abstract

This article demonstrates the application of an original technique for single crystal unit cell parameter determination during the certification of a new X-ray diffraction standard Bi₄Ge₃O₁₂ (BGO) using a conventional laboratory diffractometer equipped with a 2D detector. A BGO crystal weighing 51.86 kg and over 50 cm in length was grown using the LTG CZ method. The dynamics of structural characteristics was studied in the temperature range of 140–480 K by single crystal X-ray diffraction analysis. The cubic unit cell parameters of nine BGO single crystals taken from different areas of the initial sample were determined using the original technique based on the calibration of the goniometer equatorial circumference of small segments according to external standard doublets. The measurements were carried out in the diffraction angle range of 2θ ∼ 120° (MoKα-radiation), and the relative error was no more than 5 × 10⁻⁵. It was found that the initial BGO sample is homogeneous, and its unit cell parameter values are in the range of 10.5176–10.5183 Å, 〈a〉 = 10.5180(5) Å. The thermal expansion of BGO was studied in the range of 100–500 K. It was shown that the absolute increase in cell volume is 8.1 ų with a relative increase of 0.7%. The thermal expansion coefficient of BGO at room temperature is 6.39 × 10⁻⁶ K⁻¹. The potential use of BGO single crystals as an external and internal standard is demonstrated in the test refinement of Si and Ge poly- and single crystal unit cell parameters.