Geometry-Dependent Nature of Microstrain in Single-Phase NIR Upconverting Crystals

The broadening of Bragg’s peak in X-ray diffraction (XRD) due to lattice irregularities is assessed by the Williamson–Hall (WH) equation and accounts for the concept of microstrain in the crystal lattice. Diffraction data obtained from the synchrotron source possess a superior ability in resolving closely lying Bragg peaks through minimum angular beam divergence and thus provide a reliable method for the estimation of microstrain in a lattice. Since the optical properties of near-infrared (NIR) upconverting (UC) crystals are linked to the perturbation of local symmetry and local disorder, an extensive probing of microstrain using synchrotron X-ray diffraction (SXRD) on all seven possible types of UC host crystals was conducted. Irrespective of the composition, crystal size, and deliberate incorporation of defects, positive (tensile) and negative (compressive) microstrains appeared in these crystal systems, where all three lattice parameter angles were equal and unequal, respectively. The results indicate the presence of two opposing types of basic lattice irregularities in the UC crystals based on geometry. A similar trend can also be observed in some non-UC host lattices. The report herein indicates the possible presence of a fundamental relationship between the crystal symmetry and lattice irregularities, requiring further understanding.