Abstract
The objective of this paper is to highlight how understanding solution thermodynamics of aqueous systems can aid in choosing process parameters for hydrothermal synthesis reactions of phase pure materials. The steps necessary for generating equilibrium diagrams from thermochemical data and fundamental thermodynamic relationships are discussed. A case study is presented to illustrate how data from equilibrium diagrams aid in selecting appropriate hydrothermal synthesis conditions for optimisation of doping rare earth ions into lanthanum phosphate hosts. Thermochemical engineering for hydrothermal synthesis has broader applications for optimisation of dopant incorporation in other systems, as well as for reducing the number of empirical experiments necessary to synthesise phase pure products. Thus, thermochemical engineering is an important tool to maximise the potential of hydrothermal synthesis to be an industrially relevant process.