Ab-initio Structure Determination of the New Ion Conductor K2Al2O3F2 from Powder Diffraction Data

The crystal structure of K2Al2O3F2, prepared at 800 °C by ceramic methods, was determined from conventional laboratory X‐ray powder diffraction data. The compound crystallizes in the monoclinic space group C2/m (No. 12) with a = 11.21675(8), b = 8.16351(6), c = 6.12301(5) Å β = 88.8108(6) ° and Z = 4. Unit cell and space group suggestions were found by using the TOPAS program. Starting positional parameters for potassium and aluminum atoms were obtained from 548 reflections by the methods implemented in the FOX program. Oxygen and fluorine atom positions were determined by structural considerations. The structure was refined using the fundamental parameter approach of TOPAS, which gave the final residuals (%) RBragg = 1.6 and Rwp = 4.6. The crystal structure consists of individual layers of oxygen connected AlO3F tetrahedral groups with the fluorine atoms separating the layers. Empirical energy calculations agree with the crystal structure refinements and suggest no or little oxygen–fluorine exchange. Between and perpendicular to the sheets, the potassium ion can move without steric hindrance. Conductivity measurements from room temperature to 300 °C show decent potassium ion conductivity.