Search

NaFePO4 is known as a promising intercalation cathode material for sodium-ion batteries. During the electrochemical reaction, an intermediate phase forms with a composition close to Na≈2/3FePO4, whose crystal structure is defined with a supercell that results from both Na/vacancy and charge ordering. In this work, we present a detailed study of this superstructure through synchrotron powder X-ray diffraction and electron microscopy studies.

International audience; A solid solution (1 - x)LiCdVO4 - xLi1/3Cd 1/3□1/3CdVO4 has been observed for 0 ≤ x ≤ 1. From the cell parameters evolution, two domains of the solid solution have been distinguished with a discontinuity at x ≈ 0.6. The first one for 0 ≤ x ≤ 0.6 corresponds to the LiCdVO4 substructure of the Na2CrO4-type. For 0.75 ≤ x ≤ 1 satellite peaks are observed which correspond to a modulated structure. The first end-member LiCdVO4 (x = 0) crystallizes in Cmcm space group with a = 5.911, b = 8.975, and c = 6.513 Å. The structure consists of infinite edge-sharing chains of [CdO6] octahedra linked together by [VO4] tetrahedra and [LiO4] tetrahedra. According to the general formula Li1-2x/3Cdx/3□x/3CdVO4 of the solid solution, when x increases cadmium replaces lithium in the [LiO 4] tetrahedra leading to [(Li/Cd/□)O4] disordered tetrahedra. All the phases in the domain 0.7 ≤ x ≤ 1 are one-dimensionally modulated with wavevector q = γc* and γ values of 0.75, 0.733, and 0.722 for x = 3/4, 6/7, and 9/10, respectively. The superspace group is Xmcm(00γ) where X stands for (1/2, 1/2, 0, 1/2) centering. The main structural result is a strong occupation modulation of lithium and cadmium in the tetrahedral site.

We report the structure determination on a single crystal of Ba 12 Co 11 O 33 , which belongs to the family of 1-dimensional structures related to the 2H hexagonal perovskite. The structure can be derived from the hcp stacking of [Ba 8 Co 2 O 18 ] and [Ba 8 O 24 ] layers. Assuming a similitude in the building principle with that present in the homologue rhombohedral series based on [A 3 A'O 6 ] and [A 3 O 9 ] layers, a generic model within the superspace formalism of the whole family is proposed and used successfully for Ba 12 Co 11 O 33 . Two different strategies are presented. The first one considers the structure as a commensurate modulated composite containing two subsystems [CoO 3 ] and [Ba]. With the [CoO 3 ] subsystem chosen as the reference, the superspace group is Fddd(00γ c )0s0 with a = 11.4129(2) A, b = 19.7677(2) A, c 1 = 2.4722(1) A, and q = (6/11)c 1 * (Z = 8 for the formula Ba 12/11 CoO 3 ). The final global R value is 4.46% for 4625 independent reflections (at a I/σ(I) > 3 level) and only 123 refineable parameters. The second model follows the natural description of the structure as formed by the uniform hcp stacking of the two types of layers [Ba 8 Co 2 O 18 ] and [Ba 8 O 24 ]. The structure is formulated as BaCo 11/12 O 33/12 (Z = 16) and considered to be a commensurate modulated phase with crenel occupational modulations. The superspace group can be denoted as Xdcd(00γ L )qq0 with a = 11.4129(2) A, b = 19.7677(2) A, c = 4.5324(1) A, and q = (1/6)c*. The final global R value is 4.36% for the same number of independent reflections and 151 refineable parameters. Both approaches yield similar results, demonstrating for the first time the feasibility of refining these structures as layerlike modulated structures, instead of composites. The chains consist of polyhedra sharing faces formed by 10 consecutive CoO 6 octahedra followed by one trigonal prism. Previous models based on simulated and high-resolution electron microscopy images are refuted.