Structure and thermal properties associated with some hydrogen bonds in crystals. IV. Isotope effects in some acid phosphates

The isotope effect has been measured for (NH$_{4}$)H$_{2}$PO$_{4}$. Expansion of the crystal lattice takes place on substituting deuterium for hydrogen. The expansion is in the direction of the hydrogen bonds, and is of the same magnitude as in the isomorphous KH$_{2}$PO$_{4}$. A contraction of the lattice at right angles to the plane containing the hydrogen bonds is considerably larger for (NH$_{4}$)H$_{2}$PO$_{4}$ than for KH$_{2}$PO$_{4}$. From an application of the calculus of errors, the accuracy of the results is estimated to be $\alpha _{11}=+0\cdot 00269\pm 0\cdot 00006$, $\alpha _{33}=-0\cdot 00123\pm 0\cdot 00009$. Further investigations on the monoclinic form of KD$_{2}$PO$_{4}$ has shown the presence of curious diffuse reflexions in moving-film photographs. Attempts to obtain (NH$_{4}$)D$_{2}$PO$_{4}$ or KH$_{2}$PO$_{4}$ in the monoclinic structure were unsuccessful.