Ordering of tysonite structure in an as-grown Er0.715Ca0.285F2.715 crystal and in a component of annealed two-phase crystal of the Er0.67Ca0.33F2.67 composition

Abstract

The structures of the as-grown tysonite phase (TP) Er0.715Ca0.285F2.715as grown and the tysonite component in a crystal of the Er0.67Ca0.33F2.67 composition annealed at 760°C have been determined by X-ray diffraction methods from the main reflections in the sp. gr. P63/mmc, Z = 2. Ca2+ cations in the Er0.715Ca0.285F2.715 structure occupy the 2c site on the 63 axis, thus confirming the conclusion about the symmetrizing effect of alkaline earth cation, which was made previously for the Y0.715Ca0.285F2.715 compound. Er3+ cations are disordered over 6h sites around this axis. The Er0.67Ca0.33F2.67 composition contains Er0.67+δCa0.33−δF2.67+δ TP and inclusions of distorted fluorite phase rhβ-Ca8Er5F31. The tysonite and two fluorite lattices have no “correct” relationship. Ca2+ cations and a half of TP Er3+ cations are fixed in the 2c sites, while the other Er3+ cations are disordered over the 6h sites. Superstructural ordering occurs in Er0.715Ca0.285F2.715 crystals; however, the process is not completed, as follows from the character of the diffraction pattern. Weak satellite reflections in the diffraction pattern of the Er0.67Ca0.33F2.67 composition are indicative of the development of TP ordering processes but remain unfit for structural calculations.