Structural chemistry of NaCoPO₄

Abstract

Sodium cobalt phosphate, NaCoPO4, occurs as two different polymorphs which transform reversibly at 998 K. The crystal structures of both polymorphs have been determined by single crystal X-ray diffraction. The lowtemperature form c~-NaCoPO4 crystallizes in the space group Pnma with cell parameters: a = 8.871 (3), b = 6.780 (3), c = 5.023 (1) A, and Z = 4 [wR(F 2) = 0.0653 for all 945 independent reflections]. The o~-phase contains octahedrally coordinated Co and Na atoms and tetrahedrally coordinated P atoms, and is isostructural with maracite, NaMnPO4. The structure of high-temperature fl-NaCoPO4 is hexagonal with space group P65 and cell parameters: a = 10.166 (1), c = 23.881 (5)/~, and Z = 24[wR(F 2) -0.0867 for 4343 unique reflections]. The /3-phase belongs to the large family of stuffed tridymites, with the P and Co atoms occupying tetrahedral sites and the Na atoms located in the cavities of the tetrahedral framework. The long c axis corresponds to a 3 x superstructure of the basic tridymite framework (c _~ 8/~) and is caused by the displacement of the Na atoms, tetrahedral tilts and strong distortions of the CoO4 tetrahedra. A bond-valence analysis of these phases reveals that the polymorphism in NaCoPO4 is due in part to over-/underbonding of the Na atom in the low-/high-temperature structures, respectively.

Hydrothermal Synthesis, Crystal Structure, and Characterization of Tetrasodium Tricobalt(II)-bis-Phosphate-bis-Hydrogenophosphate Octahydrate, Na4Co3H2(PO4)4·8H2O, with a New Structure Type

“…This makes the octahedron slightly compressed along the pseudo-fourfold O(3)}O(3) apical axis. Similar compression was observed in -NaCoPO (15). The average distance of Co (1)}O is 2.094 A > , very close to the 2.096 A > in NaCo (PO )(HPO ) (6) but smaller than the value 2.111 A > derived by Wildner (16).…”

Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phosphates in Alkaline Solution

“…NaCoPO 4 and Na 2 CoP 2 O 7 crystallize into orthorhombic framework with different space groups [Pnma (#62) and Pna2 1 (#33) respectively], which arises due to different coordination of O and Co-transition metal as well as varied coordination of PO 4 tetrahedra around the CoO 6 /CoO 4 polyhedral units. [26] NaCoPO 4 , assuming Pnma symmetry, was first reported by Hammond and Barbier [34] in 1996 (PDF 82-752 [28] Rietveld refinement of XRD patterns of combustion synthesized end-members are shown in Figure 1. The corresponding crystal structures are depicted in the inset images.…”

“…A well-defined two pairs of redox peaks were observed in the CV curves of both cobalt phosphates hinting at the involvement of Co redox reactions in the OER that is associated with Co 3 + /Co 2 + and Co 4 + /Co 3 + conversion as observed in various Co-based OER catalysts. [34,40] The peak current values of Na 2 CoP 2 O 7 catalyst are higher than the NaCoPO 4 for both 0.1 as well as 1 M of NaOH, which is due to the open framework structure of pyrophosphate. Na 2 CoP 2 O 7 as well as NaCoPO 4 catalyst are performing better in 1 M NaOH than 0.1 M NaOH with slight shift in onset potentials towards higher value and higher current.…”

Disodium tris(dioxidomolybdenum) bis(diarsenate)

“…In these materials, the association of XO 4 (X = P, As) tetrahedra and MO 6 (M = transition metal) octahedra forms covalent hybrid structures that delimit tunnels, see: Linnros (1970) ;Hammond & Barbier (1996). For details of the preparation, see: Zid & Jouini (1996a,b); Zid et al (1997Zid et al ( , 1998; Hajji et al (2004); Hajji & Zid (2006);Ben Hlila et al (2009).…”

“…In these materials, the association of XO 4 (X=P, As) tetrahedra and MO 6 (M= transition metal) octahedra forms covalent hybrid structures that delimit tunnels (Linnros, 1970;Hammond & Barbier, 1996) or interlayers favorable to the migration of cations. For instance, the junction between these polyhedra can develop new materials that could have properties associated with the ion mobility of metal cations.…”