In this proposal we request 3 days of beam time on HRPD to study the structure and phase transitions of XNbOP2O7 (X = Rb and ND4). These materials have been shown to undergo displacive phase transitions presumably from complex superstructures to simpler structures at higher temperatures on warming. Preliminary data suggest the possibility of an unusual re-entrant transition in the Rb case. High resolution powder diffraction patterns will help to confirm the orientation changes of the ND4 cation.