Neutron diffraction study of the β′ and γ phases of LiFeO2

The β, β′, γ and α phases of LiFeO2, synthesized as powders, were annealed at different temperatures and characterized by X-ray measurements. The β′ and γ modifications were also studied by time-of-flight neutron diffraction (ISIS Facility, UK). The structure of the β′ phase was refined in the monoclinic C2/c space group (a=8.566(1), b=11.574(2), c=5.1970(5) Å, β=146.064(5)°) to wRp=0.071–0.080 (data from four counter banks). Fe and Li atoms are ordered over two of the four independent sites, and partially disordered over the other two. The ordered Li has a distorted tetrahedral coordination. The γ structure was refined at RT (a=4.047(1), c=8.746(2) Å) and at 570 °C (a=4.082(3), c=8.822(6) Å) in the I41/amd symmetry, showing full order with Li in octahedral coordination at RT, and in a split-atom configuration at high temperature. On annealing, the β′ polymorph was found to transform to γ at 550 °C, thus suggesting that it is a metastable phase. Electrostatics is discussed as the driving force for the α→β′→γ ordering process of LiFeO2.

Crystal structure of β′-LiFeO2 (monoclinic C2/c). Lithium and iron atoms are both ordered (blue and yellow balls) and partially disordered (green balls) over four independent sites. The β′ phase transforms to fully ordered γ (tetragonal I41/amd) at 550 °C.Figure optionsDownload as PowerPoint slide