KNi0.93FeII0.07FeIII(PO4)2: a new type of structure for a compound of composition MI MII MIII(PO4)2.

The solid solution KNi_0.93Fe^II_0.07Fe^III(PO₄)₂ {potassium [nickel(II)/iron(II)] iron(III) bis(orthophosphate)} has been prepared by the flux method. The compound shows a new type of structure for a phosphate with the general composition M^I M^II M^III(PO₄)₂. The framework is formed by [(Ni/Fe)O₆] polyhedra [Fe site occupancy = 0.069 (14)] linked via shared oxygen vertices forming a cis-like parallel chain stretching along a and [FeO₅] polyhedra (located on alternate sides of the chains) connected via two types of PO₄ groups into a three-dimensional structure. The K atoms are disordered between two sites, denoted K1 A and K1 B, with occupancies of 0.930 (9) and 0.070 (9), respectively, and reside inside channels along the a axis. Calculations of the Voronoi-Dirichlet polyhedra of the K atoms give a coordination scheme for K1 A of [9 + 3] and for K1 B of [10 + 2]. The most remarkable feature of the structure is the splitting of the K-atom site and the population of the K1 A and K1 B positions due to substitution of Ni by Fe in the (Ni/Fe) position. [ABSTRACT FROM AUTHOR]