Crystal and local structure refinement in Ca2Al3O6F explored by X-ray diffraction and Raman spectroscopy

Abstract

We present a combined structural analysis on the powder of the Ca₂Al₃O₆F phase using X-ray diffraction (XRD) and Raman spectroscopy techniques. The crystal structure of Ca₂Al₃O₆F has been refined in the rhombohedral system, R space group, a = 17.3237(7) Å, c = 7.00017(4) Å, V = 1819.38(2) ų, Z = 6. The Ca₂Al₃O₆F phase consists of almost ideal AlO₄ tetrahedrons linked through corners, Ca²⁺ ions in voids, and F⁻ ions disordered over 6 sites around the Ca2 ion. The two different Ca sites have also been verified by the photoluminescence spectrum and decay curves using Eu²⁺ as the probe ion substituted onto the Ca²⁺ sites. A lattice dynamics simulation based on the simplified version of the Born–Karman potential model has been produced. Calculated Raman phonon modes agree qualitatively well with the experimental data. The calculations show that the strong line at 538 cm⁻¹ (A_g) corresponds to the vibrational mode of a six-membered AlO₄ tetrahedrons ring, and the line at 572 cm⁻¹ (A_g) corresponds to the full symmetric vibration of fluorine atoms in the ab crystal plane.