Structural investigations on the fertilizer component K2Ca2Si2O7

Polycrystalline material as well as single crystals of the fertilizer component K₂Ca₂Si₂O₇ have been prepared by solid state reactions and melting experiments, respectively. The compound is hexagonal (space group P6₃/m, a = 9.8020(5) Å, c = 13.8781(5) Å, V = 1154.76(9) ų, D_calc = 2.815 g/cm³, Z = 6). The crystal structure was solved by direct methods using a sample that was subject to twinning by merohedry and subsequently refined to a residual of R(|F|) = 0.044 (551 independent observed reflections (I >2σ(I)), 59 parameters). K₂Ca₂Si₂O₇ can be classified as a sorosilicate based on disordered [Si₂O₇]-moieties. The dimers are arranged in 5.2 Å wide layers perpendicular to [001] centered at about z ≈ ¼ and z ≈ ¾. Charge compensation within the structure is accomplished by K⁺- and Ca²⁺-cations distributed among a total of six independent sites within the asymmetric unit. Basically, two types of coordination environments of the non-tetrahedral cations can be distinguished: M(2) and M(4) are surrounded by six nearest oxygen neighbours in form of distorted octahedra. The coordination spheres around M(1), M(3) and M(5) can be approximately described as trigonal prisms. In case of M(5) the prism is capped by three additional oxygen atoms. By sharing common faces the octahedra and the prisms are linked into one-dimensional columns running parallel to [001]. The relative alignment of these non-intersecting columns corresponds to a hexagonal rod packing with parallel rods. The [Si₂O₇]-dimers provide linkage between the columns, thus the whole structure can be considered as a mixed polyhedral framework where the two fundamental building units are connected via common corners. The remaining irregularly coordinated M(6)-atoms in turn are situated in cavities of the network. The structural relationship of the present compound with a family of rare-earth silicates with general composition A₃BSi₂O₇ is discussed.