We request 3 days of beam-time on HRPD to identify the substitution mechanism of Ti3+ and Ti4+ in synthetic hibonites [CaAl12O19]. The experiment forms part of a larger programme in which we are using the Ti3+/Ti4+ ratio as a redox sensor to monitor the evolution of the oxygen fugacity conditions in the early solar system. This work is dependent upon analysis of XAS data from a set of synthetic Ti substituted hibonites that have been prepared under known oxygen fugacity conditions. To perform the XAS analyses successfully we require good structural characterisation of these standards, and we believe that neutron diffraction is the most suitable method available.