Structure of the Monoclinic-Form Misfit-Layer Compound, (Ca_0.85OH)_2αCoO₂ (α ≈ 0.57822)

Abstract

The incommensurate modulated crystal structure of the new misfit-layer calcium cobalt oxide (Ca0.85OH)2alphaCoO2 was investigated using a superspace-group formalism with synchrotron X-ray diffraction data. The compound is a kind of composite crystal that consists of two interpenetrating subsystems, [CoO2]infinity layers containing triangular lattices formed by edge-sharing CoO6 octahedra, separated from each other by [2Ca0.85OH]infinity double-layered rock-salt-type slabs. Both the subsystems are monoclinic lattices with the unit cell parameters, a1 = 2.8180(4) A, b = 4.8938(6) A, c = 8.810(1) A, alpha0 = 95.75(3) degrees , and alpha(=|q|=a1/a2) = 0.57822(8), viz., a2 = 4.8736 A, with Z = 2. A possible superspace group is C2/m(alpha10)s0-C21/m(alpha(-1)10) for the respective subsystems. The atomic positions deviate from the average positions of the fundamental structure due to the incommensurable periodic interaction between the subsystems. A significant structural modulation was found in the [2Ca0.85OH] subsystem, whereas the modulation in the [CoO2] subsystem is less than in [2Ca0.85OH], due to the tight bonding of the close-packed CoO6 octahedra. The degree of modulation in the CoO2 layers, i.e., the potential modulation, is almost the same as those of other compounds of the misfit-layer cobalt oxides. Flattened CoO6 octahedra indicate hole doping into the CoO2 layers. The [2Ca0.85OH] blocks act as the charge reservoir layers, and the defect Ca ions are presumably the source of the holes.

Role of Structure and Defect Chemistry in High-Performance Thermoelectric Bismuth Strontium Cobalt Oxides

“…13 The predicted concentration of the Co 4+ in COL for [Bi0.87SrO2]2[CoO2]1.83 (12% Bi sites defective, corresponding to the experimentally determined stoichiometry of these samples) is 29.4%. A similar ratio of Co 4+ /Co 3+ was found for the other misfit layer cobalt oxides 14,19 and this amount of Co 4+ is in good agreement with the clear experimental evidence from the EELS, as much smaller concentrations would have a less visible impact on the L3/L2 ratio method used to determine the Co valence experimentally.…”

Structure and microstructure of the high pressure synthesised misfit layer compound [Sr2O2][CrO2]1.85

“…Apart from this oxide, in other cobaltites, despite being shorter when compared to sulphides, the shortest intrasubsystem distance alternates from shorter to longer than the intrasubsystem interaction along t [29,30,38,39]. However, in this Sr-Cr misfit oxide, both shortest distances become shorter for the same t Regarding the modulation in the distances of the RS-type layer of the Sr-Cr misfit, the largest difference between the maxima and minimum Sr-O1 distance for the same bond is that observed for O1 1 30,40]. These hydroxides present very short Ca-OH distances, with very large in-plane deformations in the CaOH layers, which are attributed to the HP employed in the synthesis.…”

Equivalence of superspace groups

“…Other incommensurate composite crystals, including misfit layer sulfides (Wiegers, 1996), misfit layer cobalt oxides (Isobe et al, 2007) and urea inclusion compounds (van Smaalen & Harris, 1996), also exhibit a pairing of two inequivalent superspace groups. A detailed analysis of this feature is outside the scope of the present overview and will not be discussed further here.…”

“…For example, for the case of [PbS] Isobe et al, 2007), M Ã has been chosen as One difference is the stronger inclination for employing non-standard settings in the case of composite crystals, because the set M Ã of reciprocal basis vectors is preferably chosen to contain reciprocal basis vectors of the basic structures of the subsystems.…”