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.
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