Cobalt substitution in a nano-porous zinc phosphate: Hydrothermal synthesis and crystal structure

New open-framework of zinc phosphate materials, [CH3NH3][(CoxZn1−x)4(PO4)3], were synthesised under hydrothermal conditions in the presence of the organic additive 3-dimethylamino-1-propylamine, (CH3)2NH(CH2)3NH2. This amine was found to decompose to methyl amine and a brown oil upon hydrothermal heating to T > 200 °C. Thereby the structure-directing agent (methyl amine) is prepared in situ prior to crystallisation of the product. This approach is further explored by systematic syntheses performed in order to study the phase stability of the different phosphate materials found in the system, 1Zn(CH3COO)2:mH3PO4:2.0(CH3)2NH(CH2)3NH2:∼140H2O. Furthermore, this new approach is utilised to substitute cobalt into the framework of zinc phosphate materials by a study of the synthesis system, nCo(CH3COO)2:(1 − n)Zn(CH3COO)2:3.5H3PO4:2.1(CH3)2NH(CH2)3NH2:∼140H2O. The amount of Co is varied in the range 5-100 mol%, i.e. 0.05 ⩽ n ⩽ 1. One to four phases are identified in each synthesis product and the phase distribution as a function of the relative amount of cobalt and zinc is determined. Energy dispersive X-ray fluorescence is used to measure the amount of Co incorporated into the framework as a function of the amount of cobalt present in the synthesis mixture, n. A relatively broad substitution range was observed, 0 < x < 0.35. The structure of [CH3NH3][Co1.35Zn2.65(PO4)3] was determined by single-crystal X-ray diffraction: orthorhombic, space group Pbca (no. 61), a = 14.687(1), b = 10.111(1), c = 16.612(2) Å, V = 2466.7(4) Å3, Z = 8, μ = 7.37 cm−1, Dcal = 3.071 g cm−3. The structure has a three dimensional open framework structure built from PO4 and (Zn/Co)O4 tetrahedra and (Zn/Co)O5 trigonal bipyramids. The methyl ammonium template was found in the one-dimensional seven-ring channels and interacts with the inorganic framework by hydrogen bonds. Cobalt has a preference for the higher coordination numbers demonstrated here where the trigonal bipyramid sites are found to have significantly more Co(II) than the tetrahedral sites.