Proton positions in and thermal behaviour of the phase 4 CaO · 3 Al₂O₃ · 3 H₂O and its thermal decomposition to |(OCa₄)₂|[Al₁₂O₂₄]-SOD, determined by neutron/X-ray powder diffraction and IR spectroscopic investigations

The proton positions in the phase 4 CaO · 3 Al₂O₃ · 3 H₂O (C₄A₃H₃) were determined by neutron diffraction exper iments at room temperature and 5 K. Two crystallographically distinct proton sites, 16g and 8f in space group Abma, were located from the evaluation of difference Fourier maps. Additionally, IR spectroscopic measurements were carried out to further characterise the observed OH groups. On the basis of this knowledge and due to topo logical considerations, we propose to write C₄A₃H₃ according to the IUPAC nomenclature for microporous mate rials as an interrupted framework host structure with the formula |Ca₈(OH)₄|₂ [Al₄^[4][1;4]Al₈^[4][1;3]. O₂₀^[2](OH)₈^[1]]_2h{3; 4-ring}_p{0[4⁶6⁸]} (Abma) – -SOD. X-ray diffraction experiments in the temperature range from 28 K to 973 K give an insight into the thermal behaviour of the phase C₄A₃H₃, and its topotac tic condensation to |Ca₈O₂|[Al₁₂^[4][1;4]O₂₄^[2]]_h{3; 4-ring}_p · {0[4⁶6⁸]} (I-43m)-SOD (C₄A₃) + 6 H₂O. The questioned fractional coordinates of the framework oxygens in the decomposition product C₄A₃, as described in the first structure de termination, were corrected by a structure refinement using the Rietveld method. It is shown unambiguously that |(OCa₄)₂| · [Al₁₂O₂₄]-SOD has a sodalite-type structure.