Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO 4 ) 2 .

Single crystals of β-KCoFe(PO 4 ) 2 , potassium cobalt(II) iron(III) bis-(ortho-phosphate), were grown from the melt under atmospheric conditions. This phosphate crystallizes isotypically with KZnFe(PO 4 ) 2 in space group C 2/ 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, PO 4 and (Co,Fe)O 4 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)(PO 4 )] - ∞ 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.