Two new cobalt–zinc orthophosphate monohydrates: hydrothermal synthesis, crystal structures and thermal investigation

Abstract

Two new cobalt zinc orthophosphate hydrates with similar chemical formula, (Co_xZn_1−x)₃(PO₄)₂·H₂O, but different composition and structure, have been prepared by systematic hydrothermal synthesis from the system nCo(CH₃COO)₂ ∶ (1 − n)Zn(CH₃COO)₂ ∶ 3.5H₃PO₄ ∶ 2.1(CH₃)₂NH(CH₂)₃NH₂ ∶ 144H₂O (0 ≤ n ≤ 1). The material Co_2.59Zn_0.41(PO₄)₂·H₂O 1 has a three-dimensional structure that can be considered to be built from layers of edge-sharing CoO₆ octahedra joined by edge-sharing (Co/Zn)O₅ trigonal bipyramids, which also share edges with PO₄ tetrahedra. Compound 2, Co_0.72Zn_2.28(PO₄)₂·H₂O, is isostructural with a known phase of Zn₃(PO₄)₂·H₂O: its structure contains corner-sharing (Zn/Co)O₆ octahedra, (Zn/Co)O₄ tetrahedra and PO₄ tetrahedra, forming channels into which the coordinated water molecules project. Magnetic susceptibility measurements for 1 and 2 are consistent with the chemical compositions determined by the single-crystal X-ray analyses and with the presence of Co²⁺. The range for possible Co/Zn substitution in 1 and 2 (assessed by EDX analysis) is relatively small: x lies in the range 0.74–0.80 (± 0.05) for 1 and 0.23–0.28 (± 0.05) for 2. Thermal investigation of 1 and 2 by thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) shows that both materials transform to γ-(Co_xZn_1−x)₃(PO₄)₂ when heated to 518 and 435 °C, respectively, with enthalpy changes for complete dehydration of ΔH = 41.9 and 53.5 kJ mol⁻¹, respectively. Dehydration of 1 occurs in a single irreversible step, while that of 2 occurs over a greater temperature range and proceeds via several steps. A new phase, (Co_xZn_1−x)₃(PO₄)₂·0.27H₂O, is formed when 2 is heated to 357 °C.