Theoretical growth and equilibrium forms of ADP (NH4H2PO4)

Abstract

The Hartman-Perdok theory enables one to establish the F forms of ammonium dihydrogen phosphate (ADP) as {100} and {011}. Although the {0kl} forms are theoretically expected to be S faces, they nevertheless exhibit K face characteristics for k < l. The forms {0kl}, with k > 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 theoretical growth forms comprise always the {011} and sometimes {100}. The presence of the latter is a function of the ionic charge of the oxygen, q₀, and that of the hydrogen H₀ belonging to the [H₂PO₄]- group, qH₀. The form {100} is present, if -0.9 < q₀ < 0, when qH₀ varies accordingly between 0 and 0.8. When halving of the elementary growth layer d₀₁₁ is applicable, the theoretical growth forms of all the models show {100} and {011} resulting in a very pronounced prismatic habit. The presence of impurities can explain the occurrence of {031} on the growth forms of all the models, which therefore resemble the so called tapered ADP crystals. The equilibrium forms show always {100} and {011}. The prismatic habit is more pronounced for models with a low q₀ and qH₀. The hydrogens H0₀are situated just on the boundaries of the elementary growth layers. The effect of ordering of these H₀ ions on the theoretical growth and equilibrium forms is not substantial.