Rebecca Volkmann , Marie Münchhalfen, Liane G. Benning

The magnesium phosphate mineral struvite (MgNH4PO4·6H2O) is of interest for the recovery of phosphorus from wastewaters and for use as a fertilizer in agriculture, yet its structure is still debated.
The structure of synthetic single crystals of struvite was characterized through refinement of a single-crystal X-ray diffraction pattern acquired at 100 K with the positions of all hydrogen atoms being identified through subsequent difference Fourier analyses. We show that the ionic interactions between the Mg2+ and NH4+ cations and the phosphate anions are reinforced by a three-dimensional network of hydrogen bonds of moderate strength.
In contrast to the previously published struvite structure determined at room temperature, the ammonium groups do not rotate but are locked by three hydrogen bonds to neighboring oxygen atoms. It is likely that the rotation of the NH4+ groups that occurs when approaching room temperature plays an essential role in the known decomposition of struvite to newberyite (MgHPO4·3H2O), ammonia, and water.