Abstract
In SrC4O4-3H2O (type I), Sr2+ has CN 8. It is surrounded by 3 water molecules and 5 oxygen atoms of 4 different squarate dianions. The C-O and C-C bond lengths are typical of extensive delocalization of the π-electron system. The connection of Sr2+ and C4O4 2- leads to infinite layers, obviously interlinked by hydrogen bonding. The oxygens of the squarate behave differently: two are Sr-chelating, but simultaneously bound to two additional Sr2+, one is connected with one Sr2+ only, and the fourth is not bound to any Sr2+. The single crystal character remained in principle unchanged during dehydration to SrC4O4 · 1 H2O and subsequent rehydration to the trihydrate. Grinding of a crystal treated in this way led to a new modification of SrC4O4 ·3 H2O.

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