Investigating the Reductive Phosphatization Reaction Pathway in the Synthesis of Transition Metal Phosphates: A Case...

Reductive phosphatization is an original synthesis approach to the formation of transition metal phosphates (TMPs). The approach enables the synthesis of known TMPs, but also new compounds, especially with transition metals in a low-valent state. However, to exploit the enormous potential of this synthesis method, it is necessary to identify and characterize all of the potential intermediates and final synthesis products. Here, we report on in situ synchrotron X-ray powder diffraction experiments to unravel the temperature-dependent formation pathway of TMPs using TiO₂-NH₄H₂PO₂ as an example. The pathway consists of several consecutive steps, including the melting of NH₄H₂PO₂, which acts as a reducing agent and a reaction medium. A reduction in the ratio of TiO₂ to NH₄H₂PO₂ decelerates the reaction and causes increased impurity formation. The hypophosphite melt reduces Ti⁴⁺ in TiO₂ to Ti³⁺, and a previously unknown compound, denoted as Ti(III)po with chemical composition (NH₄)H_1-Ti(HPO₄)₂, is formed. In a subsequent step, (NH₄)H_1-Ti(HPO₄)₂ reacts in a polycondensation reaction to form monoclinic NH₄TiP₂O₇, denoted as Ti(III)p in our earlier work.