THEORETICAL GROWTH AND EQUILIBRIUM FORMS OF ADP (NH4H2PO4)

The Hartman-Perdok theory enables one to establish the F forms of ammonium dihydrogen phosphate (ADP) as left brace 100 right brace and left brace 011 right brace . Although the left brace 0kl right brace forms are theoretically expected to be S faces, they nevertheless exhibit K face characteristics for k less than l. The forms left brace 0kl right brace , with k greater than l, which are typical of the so-called tapering of the ADP crystals, are S faces. The attachment energy of a particular face, which is assumed to be directly proportional to its growth rate, can be computed in an electrostatic point charge model. As the ionic charge distribution in the crystallizing units is not known exactly, the calculations have been made in 36 models, with P-O and N-H bond varying from purely ionic to completely covalent. The results are presented and discussed.