Crystallization behaviors of KDP and ADP

Abstract

Crystallographically, KDP and ADP have familiar (H2PO4-)n framework, which is formed via hydrogen bonding between H2PO4- groups along main crystallographic axes. The common characteristics of both KDP and ADP crystallization behaviours are studied from the viewpoint of (H2PO4-)n framework. On the basis of chemical bonding theory of single crystal growth, the anisotropic hydrogen bonding in (H2PO4-)n framework and the chemical bonding between K+/NH4+ and (H2PO4-)n dominate the anisotropic thermodynamic equilibrium morphology of KDP and ADP. The thermodynamic equilibrium morphology is used to find out the possible preferential growth directions for the dendrite growth of both KDP and ADP, i.e., 〈100〉 and 〈101〉 directions. IR spectroscopy of both KDP and ADP solutions with different concentrations has testified our theoretical results, which shows that H2PO4- ions are initially form (H2PO4-)n framework, and then constituent cations inset into the (H2PO4-)n framework during both KDP and ADP crystallization processes. The present work can provide helpful basic information for the study of crystallization of MH2XO4 crystal family.