Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO4)2
Adam Bouraima1,2, Said Ouaatta1, Jamal Khmiyas1
1Laboratoire de Chimie Appliquée des Matériaux, Centre des Sciences des Matériaux, Faculty of Science, Mohammed V University in Rabat, Avenue Ibn Batouta, BP 1014, Rabat, Morocco.
Abstract:
Single crystals of β-KCoFe(PO4)2, potassium cobalt(II) iron(III) bis-(ortho-phosphate), were grown from the melt under atmospheric conditions. This phosphate crystallizes isotypically with KZnFe(PO4)2 in space group C2/c, adopting a zeolite-ABW type of structure. The structure of the present phosphate is distinguished by an occupational disorder of the two transition-metal sites with ratios Fe:Co of 0.5725:0.4275 for the first and 0.4275:0.5725 for the second site. In the crystal structure, PO4 and (Co,Fe)O4 tetra-hedra are linked through vertices to form elliptical rings with the sequence DDDDUUUU of up (U) and down (D) pointing vertices. Each eight-membered ring is surrounded by four other rings of the same type, delimiting inter-stices with rectangular shape. This arrangement leads to the formation of [(Co/Fe)(PO4)]- ∞ sheets parallel to (001). Stacking of the sheets into a three-dimensional framework results in the formation of two types of channels. The first one is occupied by potassium cations, whereas the second one remains vacant. Calculations of bond-valence sums and charge distribution were used to confirm the structure model.
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