A new Co-substituted aluminophosphate (CoAPO-14): crystal structure and thermal behaviour by in situ single crystal X-ray diffraction

Open-framework aluminophosphates (AlPO₄) containing divalent atoms are of interest because of their enhanced redox, acidic, and magnetic properties. We have synthesized a new Co-substituted AlPO₄-14, with a mixed polyhedral framework containing MO₅ units. The compound (H₂O)_1.9(C₃H₁₂N₂)[Co_1.10Al_2.98P₄O₁₆OH] crystallizes in P-1 space group, where Co²⁺ mostly occupies a five-fold coordinated site in a trigonal-bipyramid geometry. By tracking in situ the structural modifications as a function of temperature, we could identify a structural transformation, which lead to the formation of a continuous four-connected net with AFN topology, between 100 and 150 °C. This is induced by the loss of OH groups belonging to the framework and originally coordinated to the CoO₅ and AlO₆ units. This transformation is accompanied by a sudden expansion of the unit-cell volume, which at 150 °C is approximately equal to that measured at room temperature. The structure starts loosing its crystallinity at 300 °C.