Magnesium dichloride decahydrate (MgCl10HO) and its deuterated counterpart (MgCl10DO) are identified for the first time by in-situ powder synchrotron X-ray and spallation neutron diffraction. These substances are crystallized from a previously unidentified nanocrystalline compound, which originates from an amorphous state at low temperature. A combination of a recently developed autoindexing procedure and the charge-flipping method reveals that the crystal structure of MgCl 10HO consists of an ABCABC... sequence of Mg(HO) octahedra. The Cl anions and remaining water molecules unconnected to the Mg cations bind the octahedra, similar to other water-rich magnesium dichloride hydrates. The D positions in MgCl10DO, determined by the difference Fourier methods using the neutron powder diffraction patterns at 2.5 GPa, show the features such as bifurcated hydrogen bonds and tetrahedrally coordinated O atoms.
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