Crystal structure and transitions for monoclinic KH2PO4 crystal

A monoclinic form of potassium dihydrogen phosphate, KH2PO4, crystal has been crystallized. X-ray diffraction and differential scanning calorimetry measurements have been performed on this compound. The space group symmetry (monoclinic P2(1)) and the crystal structure of this compound are determined at room temperature. It has been found that there are ten possible O-H-O hydrogen bonds with O-O distances in the range from 2.561(3) to 2.642(3) angstrom. Five transitions are found at temperatures around 432, 483, 512, 544, and 668 K. The transition at 432 K is suggested to be produced by a chemical reaction of thermal decomposition accompanied with a dynamic disorder of the H atoms on the hydrogen bonds.