Abstract
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 TiO(2)-NH(4)H(2)PO(2) as an example. The pathway consists of several consecutive steps, including the melting of NH(4)H(2)PO(2), which acts as a reducing agent and a reaction medium. A reduction in the ratio of TiO(2) to NH(4)H(2)PO(2) decelerates the reaction and causes increased impurity formation. The hypophosphite melt reduces Ti(4+) in TiO(2) to Ti(3+), and a previously unknown compound, denoted as Ti(III)po with chemical composition (NH(4))(x)H(1-x)Ti(HPO(4))(2), is formed. In a subsequent step, (NH(4))(x)H(1-x)Ti(HPO(4))(2) reacts in a polycondensation reaction to form monoclinic NH(4)TiP(2)O(7), denoted as Ti(III)p in our earlier work.