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

Open-framework aluminophosphates (AlPO 4 ) containing divalent atoms are of interest because of their enhanced redox, acidic, and magnetic properties. We have synthesized a new Co-substituted AlPO 4 -14, with a mixed polyhedral framework containing MO 5 units. The compound (H 2 O) 1.9 (C 3 H 12 N 2 )[Co 1.10 Al 2.98 P 4 O 16 OH] crystallizes in P -1 space group, where Co 2+ 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 CoO 5 and AlO 6 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. • We synthesized a new Co-Aluminophosphate with mixed polyhedral units • Co 2+ is five-fold coordinated forming a trigonal bipyramid • Upon heating the structure releases H 2 O and OH groups • At 150°C the polyhedral framework transforms to a tetrahedral framework with AFN topology