Abstract
Deuterated potassium orthophosphate hepta-hydrate, K3PO4·7D2O, crystallizes in the Sohnke space group P21, and its absolute structure was determined from 2017 Friedel pairs [Flack parameter 0.004 (16)]. Each of the three crystallographically unique K+ cations is surrounded by six water mol-ecules and one oxygen atom from the orthophosphate group, using a threshold for K-O bonds of 3.10 Å. The highly irregular coordination polyhedra are linked by corner- and edge-sharing into a three-dimensional network that is consolidated by an intricate network of O-D⋯O hydrogen bonds of medium strength.
Highlights
Following projects devoted to studying the formation and crystal chemistry of hydrous arsenate and phosphate phases of monovalent metals, viz. NaH2AsO4 (Ring et al, 2017), K2HAsO4(H2O)2.5 and K2HAsO4(H2O)6 (Stoger et al, 2012), M2HXO4Á2H2O (M = Rb, Cs; X = P, As; Stoger & Weil, 2014), and several acidic thallium(I) arsenate phases (Schroffenegger et al, 2019), we became interested in the system K3PO4/H2O
Taking 3.1 Aas the upper limit of K—O bond lengths in the first coordination sphere, each of the three crystallographically independent potassium cations is surrounded by six water molecules and one oxygen atom of the phosphate group (Fig. 1)
All other hydrogen bonds are directed towards the O atoms of the phosphate group, with O1 being twofold, O2 threefold, O3 fourfold and O4 threefold acceptor atoms, respectively (Fig. 3)
Summary
Hydrate phases of potassium orthophosphate have been known for a very long time to exist for the 3-hydrate and the 7-hydrate (Gmelin, 1938), crystal-structure determinations of these two phases or of any other hydrate of K3PO4 have not been reported so far. Previous investigations on the trihydrate revealed that the crystal structure of K3PO4Á3H2O is incommensurately modulated below 300 K (Stoger, 2020). To better elucidate the role of hydrogen bonding in this structure with the aid of single-crystal neutron diffraction, we started crystalgrowth experiments to obtain the deuterium analogue K3PO4Á3D2O. The title compound, K3PO4Á7D2O, was the unexpected product of such a crystallization attempt at temperatures below the freezing point of pure water, and its crystal structure is reported here
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