Nature of the Hydrogen Bond in Tetragonal KDP (KH₂PO₄) Observed by Maximum Entropy Method

In order to explore the nature of the hydrogen bond, the electron density distribution of KH₂PO₄ (KDP) at room temperature was obtained by the maximum entropy method (MEM) using synchrotron radiation X-ray powder diffraction data. In the obtained electron density distribution maps, the contour lines linked the two oxygen atoms of neighboring PO₄ groups and a hydrogen-bonding network was clearly seen. However, no local maxima of electron density due to hydrogen atoms were recognized. This figure of hydrogen bonding in KDP is similar to that in ice (I_h), which is the only hydrogen-bonded compound previously analyzed by MEM. In both cases, the hydrogen atoms are believed to be disordered between the two oxygen atoms. The present study shows that the character of the hydrogen bond in crystalline materials can be described clearly by MEM, even in the case where heavier atoms such as K and P exist besides O atoms.