Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO4)2.

Single crystals of β-KCoFe(PO₄)₂, potassium cobalt(II) iron(III) bis(orthophosphate), 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 transitionmetal 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 tetrahedra 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 interstices with rectangular shape. This arrangement leads to the formation of [(Co/Fe)(PO₄)]^-_∞ sheets parallel to (001). Stacking of the sheets into a threedimensional 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. [ABSTRACT FROM AUTHOR]